Boiler losing pressure: what to check before you ring

Why the pressure matters

Most boilers fitted in the last twenty years, combi and system boilers alike, run on a sealed system. The water in the radiators is under a little pressure so that it circulates properly. The gauge on the front of the boiler, or on the pipework under it, shows that pressure.

When the pressure falls too low, most boilers shut themselves down and show a low pressure fault code. That is why a pressure problem so often turns up as no heating and no hot water on a cold morning.

Too much pressure is a problem as well. Above about 2.5 to 3 bar the pressure relief valve opens and lets water out through a copper pipe on the outside wall. If that pipe drips, the system is over pressure or the valve has failed.

How to top it up

Your boiler manual shows where the filling loop is and how yours works. The general steps are the same on most boilers.

1. Find the filling loop

   Usually a braided silver hose with a small valve at each end, under the boiler. Some makes use a key or a lever built into the boiler instead.

2. Open the valves slowly

   Watch the gauge. Stop when it reads between 1 and 1.5 bar. Going past 2 is too far.

3. Close both valves fully

   Remove the key if there is one. A valve left slightly open keeps filling the system and pushes the pressure too high.

4. Reset the boiler

   If it locked out on low pressure, press reset. The heating should come back on within a minute or two.

If the pressure was low because you bled the radiators, that is normal. Letting air out of a radiator lets water volume out of the system, and a single top up puts it right.

Why it keeps dropping

If you have topped up twice in a month, water is leaving the system somewhere. These are the usual reasons.

• A small leakA weeping radiator valve, a joint under a floor, or a damp patch on a ceiling below a pipe. Small leaks can lose a lot of water over a week without leaving an obvious puddle.
• The expansion vesselA rubber diaphragm inside a red or grey cylinder near the boiler. When it fails, the pressure climbs high as the system heats up, the relief valve dumps water, and the gauge reads low again once it cools. That pattern is the giveaway.
• The pressure relief valveOnce it has opened, it can fail to seal again. Water then drips from the pipe outside even at normal pressure.
• The automatic air ventA small brass vent on top of the pump or boiler that should let air out and nothing else. A worn one lets water out as well.
• The heat exchangerRarer, and more serious. A pinhole inside the boiler lets water into the combustion side. Signs include fizzing or spitting when the boiler fires, and a smell of damp from the case.

When to stop topping up and ring

Every top up puts fresh water into the system, and fresh water brings oxygen and limescale with it. Keep doing it and the radiators corrode from the inside, which is how sludge starts. Ring rather than top up again if any of these apply.

• You have topped up more than twice in a month.
• The gauge climbs above 2.5 bar when the heating is on.
• Water drips from the copper pipe on the outside wall.
• There is water inside the boiler case, or a damp patch below it.
• The boiler shows a fault code that a reset does not clear.

If you can smell gas, this is not a heating problem. Turn the supply off at the meter if it's safe to, open the windows and ring the National Gas Emergency Service on 0800 111 999 before anyone else.

What Ian does on the visit

Ian checks the pressure cold and hot, tests the expansion vessel charge, looks at the relief valve and the air vent, and checks the visible pipework and radiator valves for weeps. Once he has found where the pressure is going, he tells you what it needs and what it will cost before doing anything further.

Questions