Boiler Pressure Keeps Going Up When the Heating Is On

How much should the pressure rise when the heating is on?

Water expands as it heats. A home heating system holds many litres of water, and warming it from cold to running temperature adds a noticeable amount of volume. On a sealed system that extra volume has to go somewhere, and a healthy expansion vessel takes it, so the gauge moves only a little.

As a rule of thumb, start at 1 to 1.5 bar cold and expect to see 1.5 to 2 bar once the radiators are hot. When the heating goes off and the water cools, the needle should come back to where it started. That round trip, up by a few tenths and back again, is a working system.

What is not normal is a swing of a bar or more, a reading that reaches 2.5 to 3 bar every time the heating runs, or a starting pressure that is higher each morning than the morning before.

The one test that tells you which fault you have

You can narrow this down without touching anything inside the boiler. It takes a day, and it separates the two families of fault that cause a rising pressure.

• Morning, heating still off: write down the cold pressure
• Heating on for an hour: write down the hot pressure
• Heating off, left to cool for two to three hours: write down the reading again
• Next morning, before the heating comes on: write it down once more

If the pressure climbs when hot and falls back to the same cold figure, the expansion vessel is the suspect. If the cold figure itself is higher each morning, water is getting into the system from the mains.

When it rises with the heating and falls back: the expansion vessel

This is by far the most common reason the pressure goes up only when the heating is on. The expansion vessel is a steel tank, usually red or grey, split inside by a rubber diaphragm. One side holds heating water, the other holds air or nitrogen at a set charge, typically around 0.75 to 1 bar on a domestic combi. The air side acts as a cushion.

Over years the charge seeps out through the valve or the diaphragm. With no cushion left, the vessel fills solid with water and the system behaves as if it had no expansion space at all. The pressure then jumps with every heating cycle. On combi boilers from most makers, including Worcester Bosch, Vaillant and Baxi, the vessel sits inside the casing, often at the back, so this is a job for an engineer rather than something to test yourself.

Left alone, a flat vessel usually ends the same way. The pressure hits 3 bar, the relief valve opens and dumps water through the copper pipe outside, and the system then reads low when cold. People top it up, the cycle repeats, and the relief valve starts to drip permanently. If you have already seen that pipe dripping, read our page on a boiler pressure relief valve leaking as well.

When it rises even with the heating off: water is getting in

A cold pressure that creeps up from one day to the next has a different cause. Heating water does not multiply on its own, so something is feeding it from the mains, which runs at a higher pressure than the heating circuit.

Check the filling loop first. Close both valves firmly. On a braided hose loop, disconnect one end once the valves are shut, which guidance under the Water Supply (Water Fittings) Regulations 1999 expects anyway for a temporary filling connection. If the pressure stops climbing, the valves were passing. Some boilers have a built-in filling valve or key underneath instead of a hose. It can wear the same way and needs replacing rather than disconnecting. To confirm mains water is the source, close the stopcock overnight with the heating off. If the cold pressure stops creeping up, it is.

On a combi, the other route in is the plate heat exchanger. A crack between its hot water side and heating side lets mains water, which sits at a higher pressure, push into the heating. It shows as a slow creep over hours or days, even with the heating switched off, because the mains side is under pressure all the time, not only while a tap runs. If the filling loop is disconnected and the stopcock test still shows water getting in, the plate heat exchanger is the prime suspect.

What you can safely do right now

If the reading is under 2.5 bar hot, you can carry on using the heating while you arrange a repair. If it touches 3 bar, turn the heating off and let the system cool.

Once it is cold, bring it back to 1 to 1.5 bar. Bleeding a radiator works, or you can use a drain-off valve, as set out in our guide to lowering boiler pressure without bleeding radiators. Starting cold at the bottom of the range gives a failing vessel as much headroom as possible, which can keep the relief valve shut for a few more days.

Do not open the boiler casing to reach the expansion vessel, and do not open the relief valve by hand to let pressure out. Both are engineer work.

What the engineer does and what it costs

On a boiler pressure repair visit the engineer isolates the boiler, drains the heating side of the boiler to zero, and checks the vessel charge with a gauge on its Schrader valve. If the charge is low they pump it back up to the figure on the data plate, refill the system and run the heating to confirm the rise is back to normal. If the charge will not hold, or water comes out of the air valve, the diaphragm has split and the vessel is replaced. Some installers fit an external vessel on the pipework instead of stripping the boiler, which is a recognised fix when the internal one is hard to reach.

Where the fault is water getting in, they rule out the filling loop, then test the plate heat exchanger by isolating the cold mains to the boiler and watching whether the pressure stops climbing. Gas work is carried out by Gas Safe registered engineers.

The £60 call-out gets an engineer to you, and the repair price is agreed on site before any work starts. The details are on our emergency plumber cost page. As typical UK market ranges, a vessel recharge is a short job, a replacement vessel is usually about £150 to £350 fitted, and a combi plate heat exchanger about £300 to £600.