Boiler Pressure

Your boiler pressure or central heating system pressure may stay normal for years but when it drops (or rises too far) you need to know what to do next!

Boiler Pressure or Central Heating System Pressure?

Boiler pressure and central heating system pressure are very much the same thing. Homeowners tend to call it boiler pressure and heating engineers tend to call it central heating system pressure.

Where to check boiler pressure and why it changes

In central heating in the UK and Europe, boiler pressure or central heating system pressure is usually measured in bar. One bar is 14.5 psi (pounds per square inch).
For years, central heating systems were “open vented,” with a feed and expansion tank (known as an F&E tank or a header tank), usually in the loft. Nearly all of these header tanks were fitted with a float operated valve (ball valve) which automatically topped up the central heating system water as necessary.

Boiler pressure gauge with scale marked between 1 and 4 bar
Analogue pressure gauge. The red pointer is only a suggested pressure indicator and may have been moved. The black pointer shows the actual system pressure. With the heating system cold, the pressure should be between 1 and 1½ bar

With the heating system turned off, the only pressure in the boiler and radiators was the pressure from the the weight of water above that point (the static pressure). The higher the header tank was above the radiators, boiler and pipework, the higher the static pressure. In a typical house, the water level in the header tank might be about 18 feet (5½ metres) above the lowest pipe in the heating system, so the highest pressure would be a little over 0.5 bar (half a bar).

In the last 20 years, more and more combi boilers have been fitted in houses, as have a number of non-combi, sealed system boilers. With very rare exceptions, central heating systems with combi or system boilers don’t use a header tank. The central heating system is sealed (closed) and pressurised, usually to between 1 and 1.5 bar (cold). This pressure is the boiler pressure.

When the system heats up, the water expands, taking up more space. The boiler pressure will rise a little, normally by up to half a bar. The position of the circulating pump in the system can slightly alter the pressure at the gauge too.

Occasionally there are two pressure gauges in the same heating system. The central heating system may have the boiler downstairs and a separate expansion vessel and filler loop upstairs. In that case there might be a boiler pressure gauge downstairs built into the boiler and a separate gauge upstairs, near the expansion vessel and filler loop. They are both measuring the pressure in the central heating system.

If the filler loop is upstairs, the upstairs gauge is there so you can read the pressure while you are filling the system. Otherwise you would have to have someone downstairs reading the boiler pressure gauge.

Where there are two gauges they may not read identically but they should be reasonably close, within about 0.3 bar. The reason they don’t read the same may be partly because they are cheap and not very accurate but also because they are at different heights in the system. You would expect the gauge which is lower in the system to read slightly higher, because there is more weight of water above it. If the gauges are 5 metres apart (vertically), the difference between them should be about 0.5 bar.

Round Worcester Greenstar boiler pressure gauge
Worcester Greenstar boiler pressure gauge. The pressure should never be in the red zones whether the system is hot or cold

Sealed central heating systems must always have a pressure gauge (usually a round, analogue, dial gauge but sometimes a digital gauge with the display on the boiler).

Some gauges have two pointers, one red and one black. Where there are two pointers it is usually the black pointer which shows the actual boiler pressure. The red pointer is just set to show a normal setting pressure. If the red pointer can be turned to alter its position, ignore it. It might not be pointing in the right place anyway. It’s the pointer you can’t directly alter that matters here (usually the black one).

Many pressure gauges, like the one on a Worcester Greenstar combi boiler, have only one pointer. The pointer should be in the green zone. If the green zone is narrow, it should be OK for the pressure to go a little above it, but not above 2.5 bar.

The green zone in the Worcester Greenstar boiler pressure gauge in the picture is a wide zone. In this case the pointer should always be in the green. The boiler may stop working if the system pressure drops into the red zone towards zero.

If boiler pressure rises into the opposite red zone, towards 3 bar or above, there is a problem which needs to be found and rectified. There’s more on that below.

Combination (combi) boilers and sealed system boilers must always have a pressure relief safety valve. This valve opens to dump water out of the system via a safety pipe if the boiler pressure (central heating system pressure) goes too high.

Brass 3-bar safety valve connected to pipework
3 bar pressure relief safety valve with a system pressure gauge attached but facing away

These safety valves are typically set to release water if the pressure rises above 3 bar. There are other safety valves in houses which are not part of the central heating system. Those valves may operate at higher pressures and, in domestic premises, they are usually fitted to unvented hot water cylinders. However, for safety valves in boilers and domestic central heating systems in the UK and Europe, the norm is 3 bar.

Most 3 bar safety valves (pressure relief valves) have a red cap (they may occasionally be black). Twisting the cap lifts the valve off the internal seating and releases water along the safety pipe. This lowers the pressure but you shouldn’t use this method to deliberately lower the pressure unless it is an emergency. Find a drain cock in the system instead.

If the 3 bar valve is operated, either automatically because the boiler pressure has risen too high, or manually by twisting the red cap, the valve seat or the rubber valve seal may become contaminated with debris carried in the central heating system water.
If the seat or seal becomes contaminated, the safety valve will not be able to re-seal properly and will then need to be replaced.

In an emergency however, where water is leaking from the boiler or the central heating system, the quickest way to drop the system pressure may be to twist the red cap. You will probably have to pay out to to have the safety valve replaced but you may be preventing more significant damage.

The amount of water which needs to be drained away to drop the boiler pressure from 3 bar down to zero is usually no more than 10 or 15 litres. Once the pressure has gone, most leaks will only drip very slowly and maybe not at all.

New brass boiler pressure safety valve
Another pattern of 3 bar pressure relief safety valve. Twisting the red cap manually opens the valve and allows heating system water to pass out of the safety pipe

The red cap may vary in shape but it will usually be knurled (ribbed) to make it easier to grip. Some 3 bar safety valves use a lever, which is usually red. You lift the lever to open the valve and release the pressure.

Normal boiler pressure changes

In normal operation, the boiler pressure changes. When the system is cold and switched off we’d expect the pressure to be between 1 and 1.5 bar. A little below this or a little above is not a problem. When the pump kicks in, the pressure may rise or fall slightly. This effect is immediate and it’s not a problem. Whether it is a slight rise or slight fall depends on the direction of the pump and where it is located, relative to the pressure gauge.

As the system heats up, the pressure should slowly rise. The rise will normally be up to half a bar by the time the system is fully hot. That rise should be slow and progressive, taking as long as it takes for the system to reach full temperature, typically 15 to 30 minutes.

The size of the normal pressure rise varies from system to system. It is linked to the size of the system (how much water it contains) and the size of the expansion vessel which must be present in all sealed systems. We would not expect boiler pressure rise, between cold and hot, to be more than half a bar.

If the pressure rise from cold is rapid and more than half a bar, there is usually a problem. If the rise is more than 1 bar, there is almost certainly a problem. We’ll come to that in a bit but first it’s helpful to understand why the normal pressure rise happens.

How central heating expansion vessels work

Water expands when it is heated. In an old-fashioned, open-vented system with a header tank, the expanding water simply backed up into the header tank a little, raising the level slightly. In a sealed system, the expanding water would burst the pipes or radiators or boiler so the expansion has to be accommodated. This is done by fitting an expansion vessel into the system. (Expansion vessels are also called expansion chambers.)

An expansion vessel is basically a chamber with a flexible butyl rubber diaphragm across the middle. On one side of the diaphragm is the central heating system water; on the other side is a gas, either air or nitrogen (for convenience, we’ll assume it’s air). During manufacture, the air side is pumped up to about ¾ bar for an expansion vessel fitted inside a boiler, or up to 1½ bar for auxiliary expansion vessels (these are additional, external expansion vessels).

The expansion vessel has a car-tyre-type valve on it (Schraeder valve) to allow the air side to be pumped up. When the air side of the expansion vessel is pumped up in manufacture, the rubber diaphragm is pressed right across to the other side of the expansion vessel, so the metal vessel is full of air. The expansion vessel is then connected to the central heating system.

The connection to the central heating system is on the water side of the rubber diaphragm.
When the system is filled with water and pressurised to 1½ bar, the rubber diaphragm in a boiler’s internal expansion vessel is pushed back to about the middle of the expansion vessel (this is possible because gases like air can be compressed). The air pocket is squashed to about half its volume and its pressure doubles, to about 1½ bar, to match the water pressure.

As the system is heated up, the water expands and more water is pushed into the expansion vessel. The pressure rises slowly and steadily as the diaphragm moves towards the air side, squashing the air pocket a little more. Because the diaphragm is flexible, it moves so that the air pressure and water pressure stay matched to each other. The fact that the air side can be squashed prevents a large rise in pressure on the water side, which would burst the system.

Where to top up the boiler pressure

If you can find the Installation and Servicing Instructions manual for the boiler it will usually contain diagrams and details telling you how to top up the system. The filling method may be given in the User Instructions too. If you have the instructions, follow them rather than the method below. If you don’t have a copy, it’s worth trying a Google search for your boiler manual. You can often find a free copy.

Boiler manufacturers do not want you to use softened water to top up the central heating system. (We don’t know why either.) They are referring to water softened with an ion exchange (salt regenerated) water softener. We don’t think they have any objection to inline magnetic or electrolytic scale inhibitors. If you have a water softener, turn it to hard water (bypass mode) while you top up your central heating system.

Start by checking the pressure when the system is cold. If the pressure is below ¾ bar (0.75 bar) we’d top it up, taking it to somewhere between 1 and 1½ bar.

Braided central heating filler loop assembly
Flexible metal braided filler hose connected between two valves

Sealed systems are topped up using a filler loop or filling link of some kind. Unfortunately there are many different kinds and they can look quite different. Originally they took the form of a flexible, metal braided hose connected between two valves. They were usually close to the boiler but not actually a part of it. These flexible braided hoses are still around.

More commonly now, filling links are built into the boiler as an integral part, usually accessed from the underside of the boiler. Many have two valves, one at each end of the link. Some have a “key” which must be fitted before opening the filling valves and removed after they have been closed. Others have a spring-loaded lever which, when lifted, allows water to enter the central heating system.

The water regulations have strict rules about connecting the stored water in central heating systems to the clean mains drinking water supply. The regs are designed to prevent any risk of contaminated water passing back into the mains water supply. The filler loop or link assembly is only allowed to be connected during the actual filling (or top up) process and must be disconnected immediately afterwards.

Besides being capable of disconnection, the assembly must incorporate an isolation valve at each end and a double check valve. A double check valve is two non-return valves in line, with a test point between them. The filler link arrangements which are incorporated into boilers must be designed to comply with these regulations.

Glow-worm Ultracom 2 boiler pressure filler link assembly
Blue filler tools attached to valves under an Ultracom 2 cxi boiler. The green collar is part of the filler assembly but the yellow lever is not; it isolates the gas

There was a heavy fine for leaving the filler loop connected. Back around 1990 this fine was £400 and a number of installers were fined for leaving the loop in place. We never heard of any householders being fined but the rules applied equally to them. Some years later the fine was increased to £1000 but we have not heard recently of the rules being enforced.

While we understand the need to protect the drinking water supply, the rules have always seemed to be well over the top. With the isolating valves at each end turned off but the loop left connected, the contaminated water would have to pass back through 2 closed valves and 2 non-return valves (against the normal direction of flow).

It would also almost invariably be trying to go from a lower pressure system to a higher pressure system (which is impossible without additional force being applied). It’s more than a little unlikely but if you don’t want to risk the fine, disconnect the filler loop!

It would take far too long to go through all the different manufacturers’ filling methods. If you don’t have a external braided filler loop you will need to consult your boiler manufacturer’s Installation and Servicing Instructions. The information might also be in your boiler’s User Instructions.

To fill the central heating system (or to top up the boiler pressure) with the braided loop you must first connect the braided hose at both ends. There may be a screwed cap on the valve at each end in the place where the hose would connect. In this case you would have to ensure that the 2 isolation valves were both turned off before removing the caps and connecting the hose. Once the hose is connected you open one valve (it really doesn’t matter which). Then, while watching the pressure gauge, you slowly open the other valve.

You should hear water hissing into the system. When the system pressure is high enough, you close the second valve and then the first valve. When both are closed, you disconnect the braided filler hose and refit any caps.
If the central heating system is already full of water and the air has been bled out, it takes surprisingly little extra water to raise the pressure from zero bar to 1½ bar. This varies with the size of the central heating system but in most houses it only about 10 litres.

If your central heating system uses an external expansion vessel, rather than one built into the boiler, the amount of water needed to raise the boiler pressure from zero bar to 1½ bar may be as little as a litre. This assumes that the central heating system was already full of water and the air had been bled out. The reason less additional water may be needed with an external expansion vessel is that external vessels are usually manufactured with a pre-charge pressure of 1½ bar.

If the boiler pressure is only raised to 1½ bar, with the system cold no water enters the expansion vessel as the water pressure is not enough to compress the air within the expansion vessel.
However, as the system heats up, the water pressure will increase and water will start to enter the expansion vessel, compressing the air pocket on the other side of the diaphragm. The boiler pressure will rise a little at the same time.

Where an external expansion vessel is fitted, and the cold boiler pressure is 1 bar, you may see a rapid rise in boiler pressure as the system heats up. This is because, with a pre-charge pressure of 1½ bar, the expansion pocket in the external vessel is only available once the boiler pressure reaches 1½ bar. Once the pressure in the system reaches 1½ bar, the pressure will rise much more slowly as the large air pocket in the external expansion vessel starts to be compressed.

If an external expansion vessel has been fitted to replace a failed boiler expansion vessel, we’d prefer the pre-charge pressure in the new vessel to be reduced to between ¾ bar and 1 bar. This would help to keep the maximum system pressure (in the boiler, radiators and pipework) a little lower.

Boiler pressure stays too high

If your boiler pressure stays too high despite water being drained out of the system via a drain cock, there is a fault. In this case you should check where the safety pipe from the 3 bar pressure relief valve dumps its water. This is usually directly outside through a wall or into an internal tun dish.

A tun dish is a funnel arrangement with water passing downwards through the pipe at the top, across an air gap, before following another pipe out of the building or into an internal drain pipe. These tun dishes are typically about 6″ long (15cm) and made of black plastic.

If system water passes through the safety pipe, and the boiler pressure gauge continuously reads over 3 bar despite water draining away, there are 2 likely causes:

One is that your filler loop has been left connected and the isolating valves have been left open or not perfectly closed. In this case the system is continuously topping up. The filler valves would need to be closed and the loop disconnected. Water would need to be drained out until the system pressure (cold) was down to about 1½ bar. If water continued to pass through the 3 bar safety valve and the boiler pressure dropped away, the safety valve would need to be replaced.

If the filler loop is disconnected but the boiler pressure still stays too high despite some water having been drained out of the central heating system via a drain cock, the likely culprit is the plate heat exchanger. Plate heat exchangers are secondary heat exchangers. Non-combi system boilers do not have secondary heat exchangers but nearly all combi boilers do.

This fault, that causes continuously high pressure, is an internal leak in the secondary heat exchanger (plate heat exchanger). This is a heat exchanger in the boiler which has system water separated from clean tap water by a series of metal plates.

Effectively, hot system water (heated by the boiler) is pumped through a pathway in the secondary heat exchanger. Clean cold mains water passes through a separate pathway on its way to the hot taps. These pathways are long and “folded” round each other so that their walls are always in contact. Heat passes through the walls separating the pathways so that the central heating water indirectly heats the tap water.

If there is a leak through the wall between the two pathways, high pressure mains water passes into the heating system water pathway, raising the pressure in the heating system. The pressure typically rises to a little over 3 bar, causing the safety valve to open.

Since the heating system pressure is being constantly topped up by the leak, the pressure gauge stays at around 3 bar and water continuously passes through the safety valve and out of the safety pipe. In this case the secondary heat exchanger (and probably the safety valve) need to be replaced.

Big swings in boiler pressure

Big swings in boiler pressure are caused by a failing or failed expansion vessel. You can find more in our Expansion Vessel Faults article. There is also a temporary workaround to keep your central heating running if your expansion vessel has failed, at least until you can have the vessel replaced.

Boiler pressure drops but doesn’t swing high

If you’re topping up and keep losing boiler pressure but the pressure doesn’t swing abnormally high, it’s likely to be the 3 bar safety valve. Once the valve has been operated, either automatically by a rise in pressure or manually, it may not close perfectly. Since the valve would have to be replaced if it continues to leak, there’s no harm in now operating it manually again, to see if that will flush it clean.

Twisting the red cap will lift the internal valve off its seating, allowing system water to rush through. Twisting it further will cause it to clunk as it drops back onto its seating. You can repeat this, topping the boiler pressure up as necessary. If the pressure then holds, you’re lucky. If not, you’ll need to change the valve.

Boiler pressure keeps dropping

If your boiler pressure keeps dropping despite being topped up, there are 3 places to look:

Tun dish with air gap which allows water to be seen passing through
Water is visible as it passes down through the tun-dish from the upper to the lower pipe

First is water passing out through the safety valve and safety pipe. This usually dumps the water outside where it is visible. Follow the route of the safety pipe out from the red-capped safety valve.
Sometimes the safety pipe terminates in the waste water pipework within the house. In this case there should be a tun dish fitted.

A tun dish is a funnel arrangement which allows you to see the water passing. Water passing out via the safety valve and safety pipe is the commonest cause of pressure loss.

The second cause of boiler pressure loss is very rare. With condensing boilers, if there is a leak in the internal wall of a heat exchanger it is possible for system water to pass into the condensate sump and out via the condensate pipe. We’ve seen this happen once, with a Greenstar Ri boiler which was over 8 years old. The manufacturers, Worcester Bosch, were excellent and replaced the heat exchanger free of charge.
It is possible for a non-condensing boiler to leak in this way. If there is a very slow leak from the heat exchanger into the combustion chamber it could evaporate away, without showing externally. The system pressure would gradually reduce. Inspection of the burner chamber should show this up.

The third cause of pressure loss is an internal leak in the system, from the boiler, controls, radiators or pipework. If the boiler pressure drops and you can’t see where it’s going you need to search! Leaks upstairs usually show on the ceilings downstairs. However, if the upstairs floors are concrete, leaking water may pass to the edges and down through a wall cavity, without showing.

If you can’t see it, a leak is most likely under a floor. If the floors are solid or can’t be lifted, this is a big problem but there are leak detection companies who specialise in locating them.
Even if you can see that the water is going safely outside, don’t be tempted to just keep topping up. It’s a bad idea.

Fresh water dilutes the corrosion inhibitors in the system. It also brings in fresh dissolved oxygen which attacks the boiler and radiators.
In hard water areas, fresh water brings in fresh dissolved limescale too. This precipitates out in the hottest point in the heating system, your boiler.

The boiler can very quickly become alarmingly noisy as the limescale causes the boiler to kettle.

If topping up won’t raise the system pressure enough

Very occasionally we come across systems where the filling link (filler loop) won’t top the boiler pressure up as high as we’d like. We hear the water hissing in but it slows and stops, even though the filler loop valves are open. If this happens, it is usually because the mains cold water pressure in your house is too low to take the pressure up higher.

As long as you can get the pressure above ¾ bar (0.75 bar) with the system cold, it shouldn’t be a problem. If it doesn’t even go up that high, try topping up at night, when mains water pressure may be higher. Mains pressure is often higher at night because fewer people are drawing water from the main.

If you still can’t get the pressure high enough, check that you don’t have a pressure reducing valve, which is set too low, fitted into the mains. Beyond that, ring your water supply company and ask. They may take a few days to come out but they are usually helpful.

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