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Updated 01 November 2020   Boiler Services Ltd 2009-2020

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Combi Boiler System Pressure — Problems with Central Heating System Pressure

For another article we wrote about central heating system pressure problems go to:

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In a sealed system you have to allow for this expansion. This is done by adding an expansion vessel to the system pipework and it's often inside the boiler. The expansion vessel is just a chamber with a rubber diaphragm across the middle. One side of the expansion chamber is connected to the system pipework. The other side has a car tyre type valve so it can be pumped full of air (or nitrogen when they’re manufactured).

In manufacture the air side is pumped up, typically to about 1 bar (about 15 psi). This forces the diaphragm right across to the opposite wall of the chamber. When the central heating system is filled with water and pressurised (to about 1 bar) the water pushes the diaphragm back to about the middle.

When the system is heated up the water expands a little but this expansion is accommodated by pushing the diaphragm across partway and compressing the air pocket. The pressure gauge on the boiler or on the pipework will go up by maybe a bar or bar.

If the air pocket has been largely or completely lost, the pressure will rise sharply as the system is heated, maybe by more than 1 bar.

To stop the pipework bursting, the boiler or pipework is fitted with a 3 bar pressure relief safety valve (usually with a red cap). If the pressure goes above 3 bar this valve is forced open and water passes out of the system along a safety pipe and is dumped outside the building.

Though a failed or de-pressurised expansion vessel is the most likely culprit when water passes out through the safety pipe it is not the only possible cause. If the plate heat exchanger (secondary heat exchanger) fitted to most combi boilers is leaking internally water will probably be forced out through the safety valve and the pressure gauge will read consistently around 3 bar.

There are 3 ways the air pocket from an expansion vessel can be lost:

If the heating system contains a lot of water (lots of radiators in the central heating system) an extra expansion vessel will be needed from the start, or you will consistently get a big rise in pressure.

If, however, your system pressure has normally changed by only bar or bar as it heats and cools but the pressure now swings much more widely, I would check the expansion vessel…

With the heating turned off I would check the pressure at the Schraeder valve, using a tyre pressure gauge. If water comes out of the valve when you try to do this, the expansion vessel is damaged and cannot be repaired. If there's no obvious water and the tyre pressure gauge reads near zero, and the system pressure gauge (boiler pressure gauge) still shows pressure on the water side, you may simply need to pump the expansion vessel up again.

First you must lose the pressure on the central heating water side by draining from a drain cock until the system pressure gauge (boiler gauge) shows zero. Don't use the 3 bar pressure relief valve to dump the pressure because, once operated, the seating may become contaminated and it may leak water through the safety pipe afterwards. If the system has been regularly dumping water through the safety valve you will probably have to replace the safety valve anyway.

With the water pressure down to zero you use a foot pump or large bicycle pump to pump up the air side of the expansion vessel to 1 bar (15 psi). The valve on the expansion vessel is the same as the valve found on car tyres or some bicycle tyres so you can use the same pump.

If the air pressure takes a long time to rise and you hear air bubbling into the water system, the diaphragm in the expansion vessel is probably ruptured. If, however, the vessel pumps up correctly, you re-fit and tighten the Schraeder valve cap and then fill the system water side again to increase the pressure. Typically you want  between 1 and 1 bar in the heating system when the system is cold. You then heat the system up and the pressure rise should not be excessive. A rise of between bar and bar is typical. This pressure should drop back down as the system cools.

Note that the pump on some combination boilers and system boilers can cause a change in pressure when the pump kicks in but this pressure change is immediate and generally small.

If the expansion vessel is damaged you need to change it if you can get at it. If it's inside the central heating boiler it's not always simple to remove. If it's going to be very difficult to remove (and some boilers have to be removed from the wall) you can opt to leave it in place inside the boiler and fit an external, auxiliary expansion vessel instead. Make sure the cap on the Schraeder valve on the old expansion vessel is tight.

New expansion vessels come already pumped up to about 1 bar. If you need to fit an external expansion vessel you tee it into the central heating return pipe. Most plumbers' merchants carry expansion vessels for sealed systems and a typical size is 12 litres.

If your expansion vessel diaphragm is ruptured there is a temporary "fix" you can try.  Drain water from a drain cock and release a radiator bleed point (upstairs) to allow air in to the radiator. Allow quite a lot of air in so the radiator is only full of water (you'll have to guess)…

Close the radiator bleed and the drain cock and re-pressurize the system. Make sure that at least one of the radiator valves is open. The air in the radiator will now act as the expansion pocket. (This won't work if the radiator is piped from the top but most aren't.)

This should only be used as a temporary fix because the air will progressively dissolve into the system water, oxygenating it and making it much more likely to cause corrosion.

Finally, check that the system is not losing pressure through the safety valve. If the valve is letting by, without the pressure becoming excessive, you will need to change the valve. Don't be tempted just to keep topping up. In hard water areas (much of the UK) you will be bringing in lots of dissolved lime-scale with the water. This will precipitate out in the hottest place in the heating system, the boiler heat exchanger, and will make the boiler kettle loudly.

So from where does the system lose pressure?

If you have a lot of air in the system when you set the system up to your chosen pressure (say 1 bar), the system may expel this air through an automatic air vent (on combi boilers or system boilers there’s usually an automatic air vent on or very near the circulating pump). If it’s not expelled via an automatic air vent, air will usually end up in a radiator or towel rail from where you bleed it out. Either way, the pressure will drop if you let the air out. All that’s needed then is to restore the system pressure using the filler loop.

System pressure also drops if you lose water via the 3 bar safety valve and safety pipe, in this case the water is dumped (or slowly dripped) outside the building. The causes of this are dealt with above.

If the pressure drop is not being caused by air being expelled or by water being lost via the safety pipe, the system must be leaking somewhere. Slow drops in pressure (over several months) are common and not a huge problem. They are caused by slow seepage from joints in the boiler or external pipework or from radiator valve glands. If they are very slow the water will often evaporate as it is lost, just leaving a crusty deposit behind.

If the pressure drop is rapid (over minutes or hours) there is a significant leak somewhere. If the leak is in pipework upstairs it will almost always become visible very quickly, showing downstairs as staining on a ceiling. If the staining is not visible it is more likely that the leakage is into the floor at ground floor level. What you can be sure of is that if you are having to top up the pressure, and the water’s not being lost outside through the safety pipe, it must be being lost inside. You have to find the leak!

The volume of water lost when the pressure drops from 1 bar down to zero is typically about 5 or 6 litres (about half a bucketful). If this is going into the oversite of a house (the space below the ground floor floor-boards in many houses) it may not seem to be a big problem; however, every time you top the pressure up you bring in fresh water and in much of the UK that’s hard water with lots of dissolved limescale. The limescale is precipitated out mainly where the system is hottest, in the main heat exchanger of the boiler and it causes noisy kettling problems if you top up frequently.

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Combination boilers and system boilers are used in sealed central heating systems. These are systems without a header tank (feed and expansion tank). All sealed central heating systems need a pressure gauge, expansion vessel and a pressure relief safety valve. With combi boilers and system boilers these components are part of the boiler, along with the pump and diverter valves. Regular boilers can also be used in a sealed system; in this case there will be a separate pressure gauge, expansion vessel and pressure relief safety valve.

We're often asked about the pressure shown on these gauges so let me explain a bit further:

If you heat water it expands. In an open vented central heating system (with a header tank) some of the water moves back up into the header tank. The expansion does not increase the overall volume of water by a huge amount (maybe 2 to 4 litres, depending on the size of the heating system) but it would be more than enough to burst the pipes if it had nowhere to go…

Expansion vessel which has been removed from a Baxi Combi HE boiler. This expansion vessel was replaced as it had failed. Dial gauge reading pressure in a sealed central heating system Brass pressure relief safety valve with a red cap, fitted to the pipework of a sealed central heating system.

3 bar safety pressure relief safety valve

Sealed system pressure gauge

Baxi Combi HE expansion vessel

Call out to a pressure relief safety valve letting by

We were called out to a house where water was passing from a safety valve into a tun dish and then down through the safety pipe within the  building before passing out at ground level. A tun dish is a type of funnel which allows you to see that there is water passing along the pipe.

Water from the safety valve runs down through the pipe at the top of the tun dish and passes across an air gap where it can be observed. It drips or trickles into the funnel at the bottom of the air gap. The tun dish is almost always located close to the pressure relief valve.

We turned off the gas central heating boiler before starting to work.

In this case the tun dish was connected the safety pipework from an unvented hot water cylinder. There was a regular drip of water passing across the air gap. There were three safety valves connected to the pipe feeding in to the tun dish:

Photo of a black plastic tun dish. A tun dish is a funnel arrangement which allows water from a safety valve to run vertically down into the funnel via an air break.

Sometimes the raised temperature of the safety pipe passing away from a relief valve indicates that the valve may be letting by warm or hot water. There was no difference in temperature at the outlets of the 3 valves so we disconnected the outlet connection from each safety valve in turn to see which valve was letting by. The 6 bar pressure relief valve from the cylinder and the TPR valve were both dry at the outlet.

When we disconnected the 3 bar safety valve protecting the heating system from over-pressure we found it was dripping regularly from the outlet. Safety valves will be forced open if the charge pressure in the central heating expansion vessel has been lost. In this case the expansion vessel was quite new and the charge pressure was normal at about 1 bar.

A new boiler had been recently fitted and it’s possible that the safety valve had been used to dump pressure from the system. This is a bad idea as, quite commonly, the valve will not reseat properly and will then drip. Since we had to replace the valve anyway, we used it to dump the five or so litres or excess water via the safety pipe.

With the pressure down to zero we removed the pressure gauge from the valve and removed the valve from the pipework. The red plastic head on the new 3 bar valve was a different shape but the three valve connections were identical to the old valve so it was a straightforward replacement.

We taped the threaded connection on the pressure gauge with PTFE and screwed it into the new valve. It’s tempting to hold the black body of the pressure gauge and screw it in by hand but this is likely to break the gauge. There is commonly a square brass section, between the black plastic and the brass thread, designed to allow the use of a spanner.

Using the braided metal filler loop we raised the pressure of the central heating system to 1 bar with the system cold.

We switched on the boiler and the central heating and ran the system. The small amount of air entrained in the pipework was flushed along the pipework and into a radiator from where it could easily be bled out.

This 3 bar pressure relief valve had failed and was slowly letting by even though the pressure had not reached 3 bar

The photo above shows the old 3 bar safety valve and the photo below shows the new replacement

The new safety valve had identical pipe connections but a different style of plastic knob.