How A Toilet Silent Fill Valve Works

silent fill valve

Introduction

This article is in effect an addendum to the post Toilet Silent Fill Valve Repair which explains how to access and dismantle such a valve and inspect or change the diaphragm that sits in the valve. That post also explains what is, and is not, silent about a Silent Fill Valve. This is now complemented by the post Replacing A Toilet Fill Valve.

In order to satisfy the requirements of Internet SEO (Search Engine Optimisation) the valve is referred to, in this article, specifically as a Toilet Silent Fill Valve or more generally Silent Fill Valve but its full title is a Silent Fill Toilet Cistern Inlet Valve since it is:

  1. A Valve.
  2. An Inlet Valve.
  3. A Cistern Inlet Valve.
  4. A Toilet Cistern Inlet Valve.
  5. A Silent Fill Toilet Cistern Inlet Valve.

This type of valve is complex and deploys a built-in mechanism which uses the inherent water pressure in the Mains Water to drive the valve from the closed to open position and then from the open to closed position. The mechanism is controlled by a much smaller valve in the form of a stopper covering a pinhole which requires a much smaller effort to operate it than the main valve.

The article endeavours to explain to the reader how the complete valve works so that they understand the order in which physical processes take place and why they follow on from each other as they do. The theory behind the workings of the valve are developed from the basic principles of forces and pressures acting on the individual parts but without the detail of too many mathematical formulae.

CONTENTS

Feature Image

Basics About The Valve

The Advantages A Diaphragm Silent Fill Valve Has Over A Ball Cock

The Fill Cycle Explained

1. The Static State

Fig. 1 – Silent Fill Valve Closed, Cistern Full & Water Static.

2. The Fill Cycle Starts

Fig. 2 – Silent Fill Valve Opens During Flushing (Cistern Emptying).

3. The Silent Fill Valve Begins Closing

Fig. 3 – Silent Fill Valve Starts Closing Near The End of A Fill Cycle.

4. The Fill Cycle Ends

Fig. 4 – The Cistern Is Full & The Silent Fill Valve Closes.

The Diaphragm In Detail

1. Valve Closed – A Static State

Fig. 5 – Silent Fill Valve Diaphragm In The Closed Position.

2. Valve Open – A Dynamic State

Fig. 6 – Silent Fill Valve Diaphragm In The Open Position.

3. Diaphragm Grommet Detail

Fig. 7 – Diaphragm Grommet Detail.

4. Calculation of The Diaphragm Areas

Fig. 8 – Diaphragm Pressure Area Dimensions.

Referrers


Basics About The Valve

silent fil valve
Example similar to mine.

The Silent Fill Valve described here is mounted on a support tube moulded so that it can be mounted in the bottom of a cistern (usually a porcelain toilet cistern about 1cm thick) where the water feed comes from below.

The screw thread moulded on the end of the support tube is ½” BSP (British Standard Pipe), the same as that on a standard ½” (15mm) tap connector.

The support tube fits through a hole in the bottom of the cistern and is fixed with a plastic nut and washer. Then a ½” (15mm) tap connector, often flexible these days, can be connected to it from a water supply underneath the cistern where the pipes are out of sight.

NOTE: The rubber washer should be on the inside of the cistern with the cone shape pressing into the hole and the ridge on the nut should go into the hole from the outside to keep it centred.

The valve has one internal moving part – the Diaphragm which moves up to open the valve and down to close it.

Externally the moving parts consist of a rocking Arm driven by an attached Float (blue in this picture) which can be adjusted to determine the height of water in the Cistern. These two parts open and close a small pinhole valve to control the actions of the Diaphragm.

The anti-syphoning collapsible output pipe can be seen dangling in front of the float in this picture of a later version which is similar to the one described here. It has a rigid plastic support inside it to keep it in position. Else air in it would cause it to float on top of the water and then the water flowing through it would spray in all directions.

The Advantages A Diaphragm Silent Fill Valve Has Over A Ballcock

Ballcocks (float valves) can be very basic – a brass piston with a rubber bung on the end connected by a brass lever to a rising spherical float slowly closes the hole at the end of a pipe until no more water can get through the pipe. The water initially flows fast but as the float rises the flow of water is restricted and so the float rises ever more slowly. The restriction reduces ever more slowly as a consequence. So the rate at which the cistern fills goes down and down. Who knows it might never get to the fill line and just drip into the cistern for ever.

silent fill valve
Ballcock or Float Valve without a float. (Viewed looking up at it.)

People want a cistern to fill quickly so it can be flushed (emptied) again soon. In less than one minute perhaps. I’ve timed my silent fill valve cistern and it fills in about 30 seconds after a short flush and 45 seconds after a long flush. How does it do that? Well it stays full open for most of the time and then completely shuts off the flow of water rapidly but not too quickly, since there is no banging in the pipes (a sign of a very rapid shut off). How? By using mains water pressure to rapidly move the diaphragm and close the valve as soon as it starts closing. This short fill time helps it to be a silent fill valve because after 30 to 45 seconds there is no more noise. With a ball cock the filling sound can sometimes go on for several minutes.

Below is an assumed Cistern Fill Speed graph comparing fill speeds of a ball cock, taking around 180 to 200 seconds to fill, with the diaphragm of a silent fill valve taking only 45 seconds to fill, after a long flush, when the initial (maximum) flow rates are the same at (2.2%)/s:

silent fill valve
Select the graph to enlarge it.

The Fill Cycle Explained

The explanation here describes an Ideal Standard Silent Fill Valve mounted in a toilet cistern.

The four stages of the Fill Cycle depicted here:
  1. The Static State – where the Silent Fill Valve is closed, the Cistern is full and Cistern Water is in a static state (neither filling nor emptying);
  2. The Fill Cycle Starts – when the Silent Fill Valve opens during flushing;
  3. The Silent Fill Valve Begins Closing – when the Cistern is nearly full and the Top Cover Pinhole is closed;
  4. The Fill Cycle Ends – when the Cistern is full and the Silent Fill Valve closes.
Key to water colours used in Figs. 1 to 4 below where the water is coloured according to its pressure:
  • Water at Mains Pressure.
  • Water at Cistern Pressure, i.e. at or near atmospheric pressure.
  • Water pressure dropping from Mains Pressure down to Cistern Pressure.
  • Water pressure rising from Cistern Pressure up to Mains Pressure.

NOTE: The Diaphragm, Diaphragm Support and Pinhole Cone are drawn in simplified form in Figs. 1 to 4. Refer to ‘The Diaphragm In Detail’ to see enlarged more realistic diagrams.

This description begins with the cistern in a static state, i.e. no water is flowing in or out of the cistern. The water supply is turned on and the full cistern is waiting to be flushed (emptied).

1. The Static State

Refer to Fig. 1.

Fig. 1, below right: A labelled diagram of a section through the closed Silent Fill Valve. It’s closed by the Diaphragm sealing off the Mains Water at the top of the Silent Fill Valve Support Tube. The Cistern is filled with water up to the Fill Line and no water is flowing anywhere.

silent fill valveThe force keeping the Silent Fill Valve closed is generated by a mechanism, within the valve, which is activated when water escaping through a pinhole in the valve is stopped. The pinhole is closed by the upward force of the Float’s buoyancy forcing a rubber stopper down onto the Top Cover Pinhole Cone.

The dark red arrows in the diagram show the direction of static forces and pressures acting on various parts thus:

  • The Float is buoyant in the full Cistern and pushing upward against the Adjusting Screw.
  • The Adjusting Screw is pushing up on one end of the Arm.
  • The other end of the Arm is levering the Rubber Stopper down onto the Top Cover Pinhole Cone and keeping the pinhole closed.
  • The Diaphragm is held down, by the pressure difference across, it  keeping the valve closed.

The state of the Cistern Water and Silent Fill Valve in detail is:

  • The water level is up to the Fill Line.
  • The Float is buoyant in the water and applying an upward force to the Float end of the Arm via the adjusting screw.
  • The Arm can rock as required on the Fulcrum Pivot so that the other end, containing the rubber stopper, bears down on the Top Cover Pinhole Cone. This stops water coming out of the pinhole under mains pressure.
  • The cross-sectional area of the pinhole is so small that the small float, with leverage from the arm, can easily apply the necessary force to counter the mains pressure.
  • The 1mm hole in the Diaphragm Grommet, although restricted by a plastic pin, allows the water pressure above to rise until it equals the mains pressure from the Support Tube below. (The Pin in the Diaphragm hole may drop down.)
  • The Diaphragm is in a relaxed position, holding the natural shape it is moulded to, and all of it is under mains pressure from above which presses the centre of the Diaphragm down onto the Support Tube. In this position it cuts off the Mains Water so it cannot pass from Inlet to Outlet.
  • The water in the Outlet Pipe is at Cistern Pressure (approximately atmospheric pressure) because it is cut off from the main supply by the closed diaphragm but is connected to the water in the cistern via the Outlet Pipe. This low Cistern Pressure extends all around the red annular Diaphragm Support underneath the Diaphragm, even though there is not much of a gap.
  • It’s the pressure difference between both sides of the Diaphragm (high above, low below), in the area of the red annular Diaphragm Support, that keeps the Diaphragm forced down to shut off the Mains Water going to the Cistern.
  • The centre thick rubber area has the same pressure on both sides (Mains Pressure) so the forces in that area are neutralised.

NOTE: Only the surface of the water in the Cistern is at atmospheric pressure. Water below the surface is above atmospheric pressure due to the weight of water above. The water in the Outlet Pipe above the Cistern Water has reduced pressure according to its height above the Cistern Water surface. That water is trying to fall out of the Outlet Pipe due to gravity but is supported by atmospheric pressure acting on the cistern water while air cannot get into the Outlet Pipe. Remember Atmospheric Pressure can lift a column of water 10.3m (33.8 feet).

2. The Fill Cycle Starts

Refer to Fig. 2.

Fig. 2, below:

A labelled diagram of a section through the open Silent Fill Valve with water flowing from Inlet to Outlet.

silent fill valveIn this state the Cistern has just been flushed and the water level is dropping rapidly. The falling Float has tilted the Arm and lifted the Stopper off the Top Cover Pinhole Cone allowing water to escape through the pinhole. The escaping water has allowed the pressure above the Diaphragm to drop. The mains Pressure below the Diaphragm has then pushed it up and opened the valve rapidly. Mains Water is rushing through the valve to the Outlet Pipe where it can fill the cistern.

The green arrows in the diagram show the direction of movement of water and various parts thus:

  • The Cistern Water level falls.
  • The Float drops maintaining its buoyancy until it hangs from the adjusting screw.
  • The Arm rocks to the open position.
  • The rubber Stopper in the Arm goes up to open the pinhole.
  • Water goes up through the pinhole in the Top Cover Pinhole Cone.
  • The Pin in the Diaphragm hole may be pushed up.
  • The Diaphragm goes up opening the valve.
  • Water rushes through the Silent Fill Valve Support Tube as it flows from inlet to Outlet and into the Cistern.

The state of the Cistern Water and Silent Fill Valve in detail is:

  • The water level is falling rapidly as the Cistern discharges into the toilet.
  • The Float is no longer buoyant and is suspended on its Adjusting Screw which is hanging from the Arm tilted by the weight of the Float.
  • Tilting the Arm has lifted the Rubber Stopper off the Top Cover Pinhole Cone so that water can escape from above the Diaphragm through the pinhole.
  • The Diaphragm is pushed up under mains pressure as the water above the Diaphragm escapes.
  • Once the Diaphragm has begun to lift, mains pressure from the inlet tube pushes water into the Outlet Pipe chamber from where it can get between the Diaphragm and its support adding to the area of the Diaphragm being forced upward so it opens quickly. This is the mechanism at work opening the valve.
  • Water continues to flow from the Inlet to the Outlet  until the end of the fill cycle.
  • During the fill cycle water continues to escape out of the pinhole via the restricted Diaphragm hole.

3. The Silent Fill Valve Begins Closing

Refer to Fig. 3.

Fig. 3, below:

A labelled diagram of a section through the Silent Fill Valve after it has started closing and the Cistern is nearly full of water.

silent fill valveIn this state the Cistern has been filling for some time and the water level has risen. In the later stages the Float has become buoyant again and applied an upward force to the Arm and pressed the Rubber Stopper down on the Top Cover Pinhole Cone shutting off the flow of water through the pinhole and commencing the closure of the Silent Fill Valve. The Diaphragm starts to press down and Mains Water flow into the Cistern decreases.

The dark red arrows in the diagram show the direction of static forces acting on various parts thus:

  • The Float is buoyant and pushing upward against the Adjusting Screw.
  • The Adjusting Screw is pushing up on one end of the Arm and rocks it to the closed position.
  • The other end of the Arm pushes the Rubber Stopper down onto the Top Cover Pinhole Cone to close the pinhole.

The green arrows in the diagram show the direction of dynamic movement of water and various parts thus:

  • The Cistern Water level continues rising.
  • The Pin in the Diaphragm hole may continue to be dragged up by the water flowing through the hole.
  • The Diaphragm goes down due to the increase in pressure above it and a drop in pressure below it.
  • Water in the Silent Fill Valve Support Tube slows down as it flows from Inlet to Outlet.

The state of the Cistern Water and Silent Fill Valve in detail is:

  • The water level is nearly up to the Fill Line. (Observe THE GAP between the water and the Fill Line in the diagram.)
  • Water still passes through the Silent Fill Valve while the Diaphragm is partly open.
  • The Diaphragm closes slowly as the mains pressure builds on the top side of it by leaking through the restricted hole in the Grommet. This is the mechanism at work closing the valve.
  • The natural compliance of the Diaphragm forces the centre down as the difference in water pressure above (high pressure) and below (low pressure) allows it.
  • As the Diaphragm closes the water flow from Inlet to Outlet diminishes and the Outlet Pipe pressure falls. This same low pressure region is connected to the underside of the Diaphragm above the red annular Diaphragm Support and a pressure difference builds up slowly between the top and bottom surfaces of the Diaphragm (high above, low beneath). This helps to force the Diaphragm down until it rests on top of the Inlet Tube and red annular Diaphragm Support. Then it completely stops the water flow from Inlet to Outlet thus closing the Silent Fill Valve.

These two things (listed below) work together to slowly close the Silent Fill Valve without any hammering in the pipework of the building:

  1. Slow build up of pressure above the Diaphragm as it rises towards mains pressure.
  2. Slow reduction of pressure at the Outlet Pipe and in the associated chamber within the Silent Fill Valve Body causing slow reduction of pressure under the Diaphragm above the red annular Diaphragm Support.

4. The Fill Cycle Ends

Refer to Fig. 4.

Fig. 4, below:

A labelled diagram of a section through the Silent Fill Valve after it has just closed with the Cistern full of water again.

silent fill valveIn this state the Cistern has been filling for some time, the water level has risen to the Fill Line. The Silent Fill Valve is closed as the Diaphragm has pressed all the way down onto the Silent Fill Valve Support Tube and has stopped the Mains Water from over filling the Cistern. A small amount of water squeezes through the restricted hole in the Diaphragm Grommet until the pressure above it has risen to mains pressure.

The dark red arrows in the diagram show the direction of static forces acting on various parts thus:

  • The Float is buoyant and pushing upward against the Adjusting Screw.
  • The Adjusting Screw is pushing up on one end of the Arm.
  • The other end of the Arm is levering the Rubber Stopper down onto the Top Cover Pinhole Cone and keeping the pinhole closed.
  • The Diaphragm is held down, by the pressure difference across, it  keeping the valve closed.

The green arrows in the diagram show the direction of movement of water and various parts thus:

  • The last drop of water goes up the Support Tube to raise the pressure above the Diaphragm to mains pressure.
  • The Diaphragm beds down onto its support due to the increase in pressure above and decrease below it.

The state of the Cistern Water and Silent Fill Valve in detail is:

  • The water level is up to the Fill Line. (Observe NO GAP between the water and the Fill Line in the diagram.)
  • The Float is buoyant in the water and applying an upward force to the Float End of the Arm via the adjusting screw.
  • The Arm can rock as required on the Fulcrum Pivot so that the other end, containing the rubber stopper, bears down on the Top Cover Pinhole Cone. This stops water coming out of the pinhole under mains pressure.
  • The cross-sectional area of the pinhole is so small that the small float, with leverage from the arm, can easily apply the necessary force to counter the mains pressure.
  • The 1mm hole in the Diaphragm Grommet, although restricted by a plastic pin, allows the water pressure above to rise until it equals the mains pressure from the Support Tube below. (The Pin in the Diaphragm hole may drop down.)
  • The Diaphragm is in a relaxed position, holding the natural shape it is moulded to, and all of it is under mains pressure from above which presses the centre of the Diaphragm down onto the Support Tube. In this position it cuts off the Mains Water so it cannot pass from Inlet to Outlet.
  • The water in the Outlet Pipe drops to Cistern Pressure (approximately atmospheric pressure) because it is cut off from the mains supply by the closed diaphragm but is connected to the water in the cistern via the Outlet Pipe. This low Cistern Pressure extends all around the red annular Diaphragm Support underneath the Diaphragm, even though there is not much of a gap.
  • It’s the pressure difference between both sides of the diaphragm, in the area of the red support (395mm2) plus the downward pressure on the thick rubber area resting on the Support Tube (32mm2), that keeps the diaphragm forced down to shut off the Mains Water going to the cistern.
  • The centre thick rubber area in contact with the water inside the Support Tube has the same pressure on both sides (Mains Pressure) so the forces in that area (63mm2) are neutralised.

The Diaphragm In Detail

The diaphragm and its housing work as a valve due to some very fine holes and gaps that allow the water pressure to equalise on both sides of the diaphragm but don’t allow water to pass through quickly. The diagrams below (Fig. 5 and Fig. 6) of the Silent Fill Valve Diaphragm show an enlarged cross-section of the valve with the important working parts to a scale of approximately 3:1 on a 94 dots per inch PC monitor. It’s only at this scale that the fine holes and gaps, through which the water passes, and the shapes of the parts can be seen.

Key to water colours used in Figs. 5 to 7 below where the water is coloured according to its pressure:

  • Water at Mains Pressure.
  • Water at Cistern Pressure, i.e. at or near atmospheric pressure.
  • Water pressure between Mains Pressure and Cistern Pressure.

1. Valve Closed – A Static State

Refer to Fig. 5 and Fig. 6.

Fig. 5, below, shows a section through the Silent Fill Valve Diaphragm in The Closed Position. This is the state depicted in section ‘1. The Static State’ and section ‘4. The Fill Cycle Ends’ in ‘The Fill Cycle Explained’ above.

Waters at A and B are at mains pressure. The Rubber Stopper, forced down by the Arm, is preventing water escaping through a pinhole in the Pinhole Cone on the Top Cover. The cone shape containing the pinhole allows it to dig into the Rubber Stopper and make a good seal.

Water at B is connected to A through the restricted hole in the Red Grommet, mounted in the centre of the Diaphragm, allowing the static pressure of these two volumes to equalize.

Water at C is at a low pressure because it is trying to fall out of the Outlet Pipe and into the cistern due to gravity. The Red Annular Plastic Diaphragm Support is not sealed against the central bulge of the Diaphragm so the pressure at C extends all the way under the Diaphragm to the point where it is clamped to the valve body.

This difference in static pressure between B and C keeps the valve shut so that water cannot travel from A to C and into the cistern.

The effective area of downward pressure at B is the area above C  = 395mm2 (see ‘4. Calculation of The Diaphragm Areas’).

Fig. 5.

Select this image to enlarge it.
The Dynamics of Lowering The Diaphragm

This is the state depicted in section ‘3. The Silent Fill Valve Begins Closing’ in ‘The Fill Cycle Explained’ above.

Once the pinhole in the Top Cover Pinhole Cone is closed and water can no longer escape from B  to D in Fig. 6. The pressure at B begins to build up to mains pressure. Then as the difference in pressure between (A + C + T) and B diminishes the natural compliance of the Diaphragm forces its centre down onto the top of the Support Tube.

While the Diaphragm is moving down the flow from A to C is reduced and the pressure at C, which extends under the Diaphragm Support, starts to fall increasing the downward force.

When the Diaphragm arrives on top of the Support Tube water at A is cut off from C and the pressure at C drops to Cistern Pressure. The area under the Diaphragm A + C + T (490mm2), which was subject to mains pressure is reduced to Area A (63mm2), but the area with a differential (and downward) force applied is Area C (395mm2) (see ‘4. Calculation of The Diaphragm Areas’). Arguably Area T (32mm2) also has a downward force acting on it once the diaphragm is closed and that part of it rests on the Support Tube. The difference in pressure between the top of the Diaphragm and the bottom is then at its maximum and the Diaphragm is forced down by this mechanism.

2. Valve Open – A Dynamic State

Refer to Fig. 6, and Fig. 7.

Fig. 6, below, shows a section through the Silent Fill Valve Diaphragm in The Open Position. This is the state depicted in sections ‘2. The Fill Cycle Starts’ in ‘The Fill Cycle Explained’ above.

The Rubber Stopper has been lifted off the Top Cover Pinhole Cone by the tilting Arm and water B above the Diaphragm is escaping under pressure (between mains and atmospheric) to D where it drains into the cistern.

The pressure difference between waters (A + C) and B lifts the Diaphragm. Once lifted the area under the diaphragm increases to include T. The lifting pressure difference operating on the diaphragm then becomes that between waters (A + C + T) and B. While there is this pressure difference the valve stays open so water flows from A through T & C to the Outlet Pipe and into the cistern.

NOTE: The area under the diaphragm (A + C + T) = the area above the diaphragm B until the diaphragm is forced up against the Top Cover. Once it seals against the Top Cover the effective area of B is temporarily reduced to the area above the Red Grommet.

Fig. 6.

silent fill valve
Select this image to enlarge it.
The Dynamics of Raising The Diaphragm

Although water at B is slowly replaced by water from A under mains pressure, through the restricted Diaphragm Grommet hole, B is not at mains pressure once it becomes dynamic because it can escape through the Top Cover Pinhole Cone faster than it can be replaced. So the pressure difference between water at A and B lifts the Diaphragm initially.

Waters at A and C become directly connected when the Diaphragm is lifted initially and the effective lifting area increases from 63mm2 (Area A) to 490mm2 (Area A + Area C + Area T). This is equal to Area B. (see 4. Calculation of The Diaphragm Areas) so water at C begins to rise towards mains pressure. The Red Annular Plastic Diaphragm Support is not sealed against the central bulge of the Diaphragm so the pressure at C extends all the way under the Diaphragm to the point where it is clamped to the valve body.

The increased area upon which the mains pressure acts as the Diaphragm lifts speeds up its lifting. That is the mechanism at work.

The Restricting Pin in The Red Grommet

When fully inserted (raised in Fig. 6) the head of the Restricting Pin doesn’t seal the hole in the Diaphragm Grommet because it is spaced away by four circular studs moulded onto the grommet allowing water to pass (see Fig. 7 which shows the Silent Fill Valve Diaphragm correctly oriented). The Restricting Pin serves to limit the flow through the grommet hole.

3. Diaphragm Grommet Detail

Refer to Fig. 7.

Fig. 7, below, shows the Silent Fill Valve Diaphragm correctly oriented and at an approximate scale of 8:1 on a 94 dots per inch PC monitor. The Restricting Pin is shown hanging down as gravity requires.

In Fig. 7, the Red Grommet in the centre of the Diaphragm is clearly visible with its four moulded cylindrical studs equally spaced around the central 1mm diameter hole. The red plastic Restricting Pin can be seen to be loose in the hole with a very narrow gap around it through which water can pass. The movement of the pin in the hole, and the materials from which the grommet and pin are made, probably prevent calcium deposits forming which might otherwise block the hole.

The four cylindrical studs that prevent the pin head from blocking the hole completely can clearly be seen in Fig. 7. The studs can be seen to be spaced apart leaving Channels for water to pass between them on its way to or from the central hole.

On the other side of the hole (not shown) the Restricting Pin is squashed flat to make it bulge out on two opposite sides. The bulge prevents the pin dropping out through the hole but at the same time allows water to pass easily through the hole on the flattened sides. The pin can move up and down the hole freely covering a distance of 2mm.

Fig. 7.

silent fill valve
Select this image to enlarge it.

4. Calculation of The Diaphragm Areas

Refer to Fig. 5, Fig. 6, Fig. 7 and Fig. 8.

Fig. 8, below, shows the bottom of the Diaphragm with three diameter dimensions used to calculate various Diaphragm Areas thus:

  1. In the formulae below let d1 = 9mm which is the diameter of the area which covers the hole at the top of the Support Tube when the valve is closed.
  2. In the formulae below let d2 = 11mm which is the outer diameter of the contact area of the rim of the Support Tube (approximately 2mm thick).
  3. In the formulae below let d3 = 25mm which is the diameter of the area of the whole Diaphragm. From these three dimensions the areas of the Diaphragm covering Waters ‘A’, ‘B’ & ‘C’ in Fig. 5 and the area ‘T’ where the Diaphragm comes into contact with the rim of the Support Tube can all be deduced as shown here with reference to Fig. 6 & Fig. 7:
Calculating The Area of The Diaphragm Above Water ‘A’ in Fig. 5.

\text{(1) . . . Area }A=\pi {{\left( \frac{d1}{2} \right)}^{2}}

\text{(2) . . . Area }A=\pi {{\left( \frac{9}{2} \right)}^{2}}

\text{(3) . . . Area }A=63\text{m}{{\text{m}}^{2}}

Calculating The Area of The Diaphragm Below Water ‘B’ in Fig. 5.

\text{(1) . . . Area }B=\pi {{\left( \frac{d3}{2} \right)}^{2}}

\text{(2) . . . Area }B=\pi {{\left( \frac{25}{2} \right)}^{2}}

\text{(3) . . . Area }B=490\text{m}{{\text{m}}^{2}}

Calculating The Area of The Diaphragm Above Water ‘C’ in Fig. 5.

\text{(1) . . . Area }C=\text{Area }B-\pi {{\left( \frac{d2}{2} \right)}^{2}}

\text{(2) . . . Area }C=490-\pi {{\left( \frac{11}{2} \right)}^{2}}

\text{(3) . . . Area }C=395\text{m}{{\text{m}}^{2}}

Calculating The Area of The Diaphragm Above The Rim of The Support Tube ‘T’ in Fig. 5.

\text{(1) . . . Area }T=\pi {{\left( \frac{d2}{2} \right)}^{2}}-\text{Area }A

\text{(2) . . . Area }T=\pi {{\left( \frac{11}{2} \right)}^{2}}-63

\text{(3) . . . Area }T=32\text{m}{{\text{m}}^{2}}

Fig. 8.

silent fill valve
Select this image to enlarge it.

Referrers

  1. Toilet Silent Fill Valve Repair.

Author: Helpful Colin

I have a background in telecommunications and a fascination with all things scientific and technical - from physics to electronics, and computing to DIY.

47 thoughts on “How A Toilet Silent Fill Valve Works”

  1. Really helpful and detailed diagrams.Best description of all parts and workings I have ever seen.Carry on the good work.

  2. Brilliant description – helped me realise why at first the replacement diaphragm that I fitted wouldn’t work. It was upside down. Thanks very much

  3. This is one of the best descriptions that I have encountered on a topic that is far from intuitive! Impressive and very he[pful. I did managed to somehow correct my ‘ever flowing’ cistern before I read the article, and now I understand the why and how which is very satisfying!

    1. Hi Prash,
      I have looked at the picture on Screwfix and another picture of a side entry valve also at Screwfix – http://www.screwfix.com/p/side-entry-fill-valve/64191. Both valves look as if they have the same part capping the diaphragm with the fulcrum for the arm built on it. The side entry valve gives me another viewing angle of the four slightly curved slots in that piece. I suspect that piece is held on with clips of some sort. so it just clicks together. I suspect they might be released by using a blade down some or all of those slots to prise the clips. Check it out and use a magnifying glass if needs be to examine it close up. Good luck.

  4. Silent fill valve explained perfectly. Mine was overfilling and wasting water constantly. Found this website and took it apart, cleaned the diaphragm in warm water washing all the gunk off, re assembled and works perfectly now:-) This could have cost me a huge amount in plumbers fees. Thanks for your expert help.

  5. Hi I’m having the same problem as prash and can’t seem to get the clips undone with a fear of snapping the clips

    1. Hi Chris,
      I suspect some valves are made to use and throw away when broken. If you can visit Screwfix you might see if they have one the same. If they do you can risk breaking the clips on yours knowing you can at least replace the whole valve.
      Regards, Colin.
      P.S. I presume you saw my reply to Prash.

  6. Excellent detail. i opened a Mira shower solenoid valve and couldn’t figure out how the delicate spring in the solenoid could stop mains water pressure. Your diagrams sung the answer.,Nice one!!

  7. Great explanation and graphics. But I have a related question. My toilet fill valve is of a different design, with no pin or pin hole. It has a rubber plug that is pushed down over the inlet opening, and three separate rubber washers of different diameters that mate with three different plastic cylinders. All the washers are mounted on the same shaft between the top, which is pushed down by the float arm, and the bottom plug that stops the inlet flow. I’ve convinced myself this device uses line pressure to finish the closing of the water supply, similar to the device you have explained, but I can’t figure out how. Can you help?
    Jim

        1. Hi Jim,
          Please send your PDF as an attachment to an email in the same way you would send a picture file. Send the email to helpful.colin@btinternet.com (the address I use for blog related emails). I will then save it on the server and reply again in these comments with a link to your PDF to make it publicly available. Discussion regarding the content of the PDF will then follow in further comment(s).

  8. Hi Colin, thanks for your explanation – I just could not fathom it out how it worked. The problem I have is similar to others (slow leak when cistern is full), but having taken the diaphragm out to examine it – looks OK so cleaned and put back – I am now in a worse position because the leak is now running at full power,just as if the pinhole is permanently open. Any ideas?

    1. Hi Brent,
      If it is now worse and you really can’t find a hole. I have to ask if you have now put the diaphragm in upside down by mistake (I’m not sure it’s possible) after you looked at it? A new diaphragm is only £1.57 at B&Q in the UK. Plumbsure Diaphragm Washer (Dia)30mm
      Else is the diaphragm sitting correctly and is the clamping nut now cross threaded. If you can remove the arm attached to the float so you can put your finger over the pinhole you will find out of the problem is internal or external. If you can’t stop the water with your finger over the pinhole then something inside the valve is a problem or it is incorrectly assembled. However it won’t stop filling if the float lets the water rise until it goes down the overflow. Then you must adjust the float. Good luck.

  9. Thanks Colin
    Prior to your reply, I bought a new diaphragm from B&Q and tried that. First of all it’s a slightly different design. The original has an extra flange which sits in a recess on the top, but I tried it anyway by sitting it on the inlet part. Referring to your diagram mine does not have a “red annular diaphragm support”. Replaced the top and this time I got back to the original problem ie. slow leak once main fill is complete implying that the pinhole and arm mechanism is OK. Since then your reply has been received and to confirm the pinhole theory I removed the arm and placing finger over the pinhole – the water continued at full power. I tried this with both the original diaphragm and the B&Q one but to no avail. So the B&Q one now does not work at all ie water at full pressure. As for cross threading top, this is quarter turn snap lock fitting which sits correctly. Looks like a new inlet valve. This has happened before (6 months ago) on a different WC and in that instance I just replaced the lot.

    1. Hi Brent,
      I have to say that with these modern fill valves the replacement diaphragm must be exactly the correct one the manufacturer intended. I don’t believe any are interchangeable. I suspect you may be using the wrong type. B&Q do sell similar but different diaphragms, e.g. Plumbsure Float Valve Diaphragm Washer (Dia)25mm and Plumbsure Ballvalve Diaphragm Washer (Dia)30mm, Pack of 2
      I have not had experience of all types so I am sorry if I have not been very helpful. Good luck.

      1. Thanks again Colin
        Neither of those suggestions are compatible. I have now replaced the whole unit.
        Thanks for your help anyway.

  10. Best ever description of something simple yet complex that I have ever read.Thank you very much for your effort

    1. Thanks Eric,
      Your comment is much appreciated. I have spotted room for improvement and the recently introduced Fill Speed graph is part of that. I have previously implied that the valve closes slowly when in fact it doesn’t start closing until late in the fill cycle but it closes rapidly once it starts closing.

  11. hi colin i am looking for a spare part that u r holding in your hand,it is inlet valve that is not functioning.water coming in to the pan constant

    1. Hi Lalji,

      The part you can see me holding, in the feature image, is the diaphragm. This is a flexible rubber part which is held firm at the edge, but the thick centre part can move up and down to either shut off the water supply or let the water through to fill the cistern. Because the centre moves up and down every time the cistern fills with water the thin rubber attaching the centre to the edge eventually rips and lets the water through all of the time. That is when you have to replace the diaphragm with another of exactly the same type.

      Diaphragms that look different are different and cannot replace yours. If yours is like mine with a red pin in the middle then I advise you to replace it with one with a red pin. If yours is not like mine and has a white pin in the middle I advise you to search for an exact same one with a white pin in the middle. If it doesn’t have any pin at all then buy one without any pin at all.

      I got my replacement diaphragm from B&Q. If you read my other post entitled, “Toilet Silent Fill Valve Repair,” you will see I spoke about it there. If you go to the B&Q website http://www.diy.com/ and search for ‘diaphragm’ you should be able to find it. (I just tried it.)

      Good luck.

  12. Thanks very much for the excellent description of how these things work! I have a system with low input (mains) pressure and it never seems to seal… filling to the overflow and then wasting water. Would I be better off with another fill valve system?

    1. Hi David,

      Sorry to hear you have low pressure. Perhaps you should get your supplier to check it. It may be below the level they guarantee to provide or they may be able to give you advice. I don’t know if another type of fill valve would be better or not. Maybe there is a problem with the one you have got. Does the water shut off if you pull the float up and hold it up. You could use a piece of string and an elastic band to pull it up and tie it to something above your cistern. If the water does stop flowing try adjusting the float. If it doesn’t change the diaphragm.

  13. Hi colin. Fitted a new univalve filler yesterday similar to your pic ..the water seems to trickle out of the top and nothing through the fill arm/plastic slieve and is taking half hour to refil the cistern….any ideas or is it a faulty one? Thanks

    1. Hi Fiona,
      Why not take it back to the store and have it replaced? If you’ve fitted it correctly then it’s not fit for purpose. Alternatively dismantle it and try to work out how it should work and why the water is not passing through correctly. If it has a diaphragm like mine (or similar) has it been assembled incorrectly with the diaphragm upside down? Are there some holes missing in the plastic mouldings?
      Good luck.
      Regards HC.

  14. Hi Colin,
    I have a question regarding my AMSTD silent filling valve that works on this principle of using mains pressure to move the diaphragm controlled by the opening or closing of the weep hole. The only difference I see is the valve is mounted horizontally so there is no gravity assist for the small restrictor valve in the diaphragm.

    If during the filling cycle I lift the float, the flow into the cistern is not diminished no matter how soft or hard I lift float or for how long.
    However if I let go of the float and allow the level rise to the fill level the valve works correctly and halts the flow so all is well.

    It was only through exercising the routine maintenance precaution of fitting the right new rubbers parts correctly to the cisterns of rental properties that I noticed this phenomenon.

    It was after fitting the new rubbers that I turned the water on and lifted the float to check the flow would be stemmed when to my dismay it did not.
    I double checked my installation and tried again several times to no avail even after fitting the old rubbers again.
    I removed the lever holding the weep hole seal to try and block it with my finger but unfortunately the pivot supports prevent this, but I feel sure the new rubber is effective.

    After a while in desperation I allowed the cistern to fill to the fill level and to my surprise the valves worked correctly stemming the flow.

    So no real problem, except that having thought I understood the principle of operation and having had it confirmed by your excellent article I fear I must be missing something.
    But what?

    This phenomena is consistently repeatable and I have run out of ideas.

    Any ideas you have would be very welcome.

    Thank you for your time,
    Steve Bradford.

    1. Hi Steve,
      Sorry for my late reply.
      I read your comment afresh and I can’t understand how it works either. So I ask, “Can you control it manually by lifting the float when it’s very nearly full?” If you can that would suggest that its workings are affected by the water pressure in the cistern caused by the depth of water in it. I would have to handle this type of valve to get a better idea.
      Regards, Colin.

      1. Hi Colin,
        Thank you for your reply.

        The cistern valve is a Dudley Hydroflo where the valve mechanism is horizontal and the small float works via a crank based on the principle you describe so well.
        A quick search on e-bay would show you the design and the replacement diaphragms available that I fitted.

        I did try the method you describe lifting the float just before the fill level is reached and it will work, just not always repeatable.

        Allowing the system to fill from empty in it’s own time works reliably and has done so since fitting the new diaphragm before Christmas.

        I am at a loss in understanding the dynamics of the system as the principle of pressure equalisation across the diaphragm as the float rises and unseating the control valve forcing the filling valve to close due to the difference in effective area seems sound.

        The cistern is in a rental property and fitting a new diaphragms is good preventative maintenance, but I would not have fitted a new diaphragm if the system had not failed the manual lift test. Of course several days passed before I recieved the new diaphragms and you can imagine my surprise when the new ones fitted to both cisterns both failed the manual lift tests.
        I admit it took me a while to find the anomaly where both would work well when allowing a full fill.

        So it is not a problem except for the frustration of not understanding the difference. I am sure it is not mechanical and cannot reconcile a back pressure effect up through the flexable anti-syphon silent fill tube.

        The good news is I am not losing so much sleep any more but it would be nice to know what I am missing.
        Thanks again,
        Steve Bradford.

  15. Cracked it- but only with your help Colin,old diaphragm completely nuked in as much the outer edge had become detached which I had not realised. Went to a local supplier who said “My that’s taken a hammering, it should be much bigger than that”. So on investigation found the outer remnants still in situ in the valve, replaced said diaphragm and behold, didn’t work. Stupid me had inserted it upside down, and now it works fine. Let the incoming water shut the valve off, as lifting the float by itself won’t stop the incoming water not like the old fashioned systems.
    Once again Colin thanks.

    1. Hi Gordon,
      I’m glad you had success.
      Maybe other people have had difficulty when not realising that a piece of the original diaphragm was left in situ when a new one was inserted.
      Regards, HC.

  16. The most useful description of a diaphragm I have ever seen. Thanks for making it understandable.

  17. Very explicit diagram which removes the mystery of these valves. My problem is very slow filling which has been the case for a few months. After reading your blog I decided to fix it. I changed the diaphragm (more than once!) and checked the pinhole was clear etc. All to no avail. On first fill after inserting the diaphram everything works but then it resumes to slow filling via the pinhole. I am assuming that the diaphragm is not in the closed position on the first fill but subsequently retains the closed position with the pin dropped to allow water through the pinhole. Any clues?
    Mal

    1. Hi Mal,
      This sounds like there might not be enough water pressure underneath the diaphragm to lift it up and push water into the cistern. The diaphragm has a natural tendency to take a flat closed position when there is no water pressure.
      You could turn off the water supply, uncouple the pipe from the inlet valve, point it into a container and turn the water back on to check if there is plenty of pressure to drive water along the pipe. If there is little or no pressure fix it. Are your pipes blocked with calcium (this happens in hard water areas)?
      If there is plenty of pressure then check there is not a blockage in the pipe of the inlet valve leading up to the diaphragm. A blockage could be caused by debris driven along the pipe by the flowing water. If there is a built in restriction in the pipe to slow water flow in high pressure areas the small gap left for water may be blocked.
      With the diaphragm and inlet pipe removed it may be possible to poke a wire down the inlet valve pipe or shine a light up it to check it is clear. The alternative would be to remove it from the cistern so you could inspect it on a bench.
      I hope this is helpful.
      Regards, Colin.

  18. Apologies for my appalling typing – should check before send! Should have said ” I am assuming that the diaphragm is not in the closed position on the first fill but subsequently retains the closed position with the pin dropped to allow water through the pinhole.

      1. I have come to the conclusion that there is inadequate pressure to “open” the diaphragm. I have approx 4metres of head feeding various items. If I tweak the inline valve to induce a kind of hammer effect this causes the valve to open so I deduce that there is almost enough head. I cannot find any reliable information on the minimum pressure required to operate the valve so am reluctant to commit to a replacement valve such as the SIAMP 99B Low Pressure unit

        1. Hi Mal,
          It’s nice to know you’ve identified the problem.
          Water pressure can be increased using a pump. Obviously you don’t want a pump using electricity continuously. There are pumps which turn on when a flow commences and then they boost the flow. That may not work in this case due to the initial flow being very slow to zero.
          You could consider a basic ball valve. Modern ones are plastic and can be mounted in the bottom of a cistern, as well as the side, and can have small floats. You would have to carefully select one. A builder’s or plumber’s merchant may be able to advise you and show you different ones. I’m suggesting this because they use a simple valve which doesn’t resist the water flow until it’s forced shut when the float rises sufficiently.
          Regarding the SIAMP 99B: I looked at it on http://www.siamp.co.uk/compact-99b/ and the minimum pressure at which it will work is 0.5bar. I then went to http://www.convertunits.com/from/bar/to/meters+head and it converted to a 5m head. So that makes it unsuitable for you.
          Good luck with this one Mal.

          Regards, Colin.

          1. Thank you for your advice Colin. I have very little room in the cistern for a ball valve & arm. I have spoken to SIAMP and they tell me that the 99T will operate at 0.2 bar which should work for me.. Cross fingers.
            Regards
            Mal

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