Potterton Boiler Pressure: Correct Range and How to Top Up

What pressure a Potterton boiler should be at

Aim for 1 to 1.5 bar with the heating off and the system cold, and expect it to rise a little when the radiators are hot. Potterton's figures vary slightly between ranges, so it helps to know what your own guide says.

The figures come from the Potterton Assure Combi 25 and 30 user guide (reference 7685721) and the Gold Combi 24, 28 and 33 user guide, both published by Baxi Heating UK, which makes Potterton boilers.

Reading the gauge or the display

Most Potterton boilers have a dial gauge on the underside or front of the boiler, with a needle against a scale marked in bar. The Assure can also show the pressure as figures on its display, and its filling steps refer to watching either "the figures on the display or needle on the pressure gauge". Always take your reading with the heating cold, because that is the figure the guides compare against. A reading taken with the radiators hot will be higher and can make a low system look healthy.

Low pressure fault codes: 118, E118 and E119

When the pressure falls too far, the boiler stops and shows a code. Which code you see depends on the age of the boiler:

• 118 on the Assure, described as "primary system water pressure too low". The Promax Ultra uses the same code and refers to it as E118.
• E119 on the Gold Combi, Promax Combi HE Plus A and earlier Promax models, described as "water pressure switch not operated". The Gold guide says it shows when pressure is below 0.5 bar. Our Potterton E119 page covers it in full.

Both codes have the same first fix: top up the system. The Assure guide says the boiler resets automatically once the correct pressure is restored.

How to repressurise a Potterton boiler

Potterton's guides describe a filling loop made of two taps and a separate filling pipe. On the Gold Combi the taps have blue handles. If your installer fitted a filling arrangement built into the boiler or pipework instead of a removable hose, follow the steps in your own user guide or ask the installer to show you. Our general filling loop guide shows the common types.

1. Turn the heating off and let the system cool, then read the pressure.
2. Check both taps are closed. If your loop has blanking caps, remove them and connect the filling pipe, using the washers at each end.
3. Open one tap fully. Then, watching the gauge, open the second tap slowly. You will hear water entering the system.
4. Close both taps when the reading reaches the right level. The Assure guide says between 1 and 1.5 bar. The Gold guide says to stop between 1 and 2 bar, but as it expects a normal cold reading of 1.0 to 1.5 bar, stopping at the lower end leaves more room for expansion.
5. Disconnect the filling pipe. A small amount of water may run out, so have a cloth ready.
6. Refit the blanking caps, hand tight, and store the pipe somewhere you will find it next time.
7. If the boiler does not restart by itself, reset it as your guide describes. On the Assure, press Reset for about one second, as holding it for more than two seconds will not reset it.

Correct valve positions after filling

Both filling taps must be fully closed once you have finished. A tap left slightly open lets mains water creep into the heating system, and the pressure keeps climbing until the safety valve opens at 3 bar. Check again an hour later: if the cold reading has crept up since you filled, one of the taps is not shut. Disconnecting the pipe, as the Assure and Gold guides both tell you to, is the best protection, because it physically separates the mains from the heating circuit. Leave the gas isolation valve under the boiler alone during this job; the filling taps are separate.

Pressure too high: causes and how to bring it down

A cold reading above about 2 bar, or a hot reading close to 3 bar, is too high. The usual reasons are:

• overfilling, or a filling tap that has been left open or is letting by;
• an expansion vessel that has lost its air charge, so the water has nowhere to expand into when hot.

If overfilling is the cause, you can lower the pressure by bleeding a radiator. With the heating off and cool, put a cloth under the bleed valve, open it with a radiator key and let water out slowly while someone watches the gauge. Close it at 1 to 1.5 bar. If the pressure is normal when cold but shoots up every time the heating runs, bleeding will not fix it, because the expansion vessel is the likely problem.

Boiler keeps losing pressure, and how often topping up is normal

Topping up once or twice a year, or after bleeding radiators, is normal. Both Potterton guides say that needing to repressurise regularly means "a fault or leak is indicated", and that you should get advice from your installer. The common sources are:

• Leaks on the system: radiator valves, towel rails and joints under floors. Look for damp, stains or crusty deposits.
• The pressure relief valve: if it opens when the system gets too hot, or does not reseal fully, water drips from the discharge pipe outside.
• The expansion vessel: once flat, it lets the pressure swing high when hot, the relief valve lets water go, and the cold reading ends up lower each time.
• Bleeding radiators: every radiator you bleed takes a little pressure with it, so check the gauge afterwards.

Write down the cold pressure and the date each time you top up. A drop of a few tenths of a bar over months is a minor weep. A full bar in a day or two is a significant leak or a relief valve discharging.

When to call a Gas Safe registered engineer

Book an engineer if you are topping up more than every few weeks, if water drips from the discharge pipe or from under the boiler, if the pressure climbs to 3 bar when hot, or if the gauge reads correctly but the low pressure code will not clear.

For a step-by-step look at low pressure on any make, see our guide on boiler pressure that is too low.