Boiler pressure low: what it should be and how to top it up

Safety first

Topping up boiler pressure is a normal homeowner task, and most manufacturers explain how in the user manual. It does not involve opening the boiler case or touching anything gas or oil related.

Stop and call an engineer instead of topping up if:

• water is leaking from the boiler itself, or from inside the casing;
• the pressure is too high (close to or above 3 bar);
• you have to top up more than once every few weeks;
• you cannot find the filling loop, or the valves will not turn without force.

If you smell gas, do not touch the boiler or any switches. Turn off the gas at the meter if you can do so safely, open doors and windows, leave the property and call the NI Gas Emergency Service on 0800 002 001, free, 24 hours a day.

What pressure should my boiler be?

Most modern boilers in Northern Ireland, gas or oil, are on a sealed system: the heating water is held under pressure and there is a gauge on the front or underneath the boiler.

• Cold system: about 1 to 1.5 bar is typical.
• Heating on: it will rise a little, often by around half a bar.
• Below about 0.5 to 1 bar: many boilers will show a fault and stop working.
• Close to 3 bar: too high. The pressure relief valve may open and discharge water outside through a small copper pipe.

Your boiler manual gives the exact range. Some gauges have a green zone or a red marker set by the installer.

If you have an older open-vented system with a small tank in the roof space, there is no pressure to top up in this way, and this guide does not apply.

Why boiler pressure drops

Pressure falls when water leaves the sealed system. The usual causes:

1. You have bled radiators. Bleeding lets out air and a little water, which lowers the pressure. This is normal; top up afterwards. See bleeding radiators.
2. A small leak. A weeping radiator valve, a joint under the floor or a pinhole in a pipe. Look for damp patches, stains or corrosion around valves.
3. Pressure relief valve. If it has opened once because of high pressure, it may not seal properly afterwards and keeps dripping. Check the small copper pipe outside, near the boiler, for drips or staining.
4. Expansion vessel. The vessel absorbs the expansion of the water as it heats. When it loses its air charge, pressure rises sharply when the heating is on and the relief valve lets water out. Then it falls again when cold. A pressure that swings a lot between hot and cold is the clue.

How to top up boiler pressure

Check your boiler manual first, because the method varies. Most boilers use one of two types of filling loop.

External filling loop (a braided hose with two valves)

1. Turn the heating off and let the system cool. You will get a more accurate reading cold.
2. Find the filling loop, usually under the boiler: a flexible silver hose with a valve at each end, or two valves on the pipework.
3. Make sure both ends of the hose are connected securely.
4. Open one valve fully, then slowly open the second. You will hear water flowing.
5. Watch the gauge. When it reaches about 1 to 1.5 bar (or the figure in your manual), close both valves.
6. If the manual says so, disconnect the hose and refit the caps. Some manufacturers advise leaving it disconnected so it cannot let water in by accident.
7. Turn the boiler back on and reset it if it is showing a fault.

Built-in filling loop (a key or lever under the boiler)

Some boilers have an internal filling device: a key you insert and turn, or a lever or tap underneath. The manual will show where it is and which way to turn it. Fill slowly, watching the gauge, and close it at the right pressure.

If you overfill

If the gauge goes above about 2 bar cold, you can bring it down by bleeding a radiator a little, with a cloth ready to catch the water. Check the gauge as you go.

What to do if the pressure keeps dropping

Topping up once after bleeding radiators is normal. Needing to top up every week or two is not, and topping up repeatedly can cause problems of its own: every fill brings fresh water and oxygen into the system, which speeds up corrosion and sludge.

Book an engineer if:

• the pressure drops again within a few weeks;
• it swings a lot between cold and heating on;
• you can see water dripping from the pipe outside;
• there is a damp patch, stain or drip anywhere on the system.

An engineer will check the expansion vessel charge, the pressure relief valve and the system for leaks. Expansion vessel and relief valve problems are common and usually straightforward to put right.

What a service has to do with pressure

On every gas boiler service, and on oil boilers with sealed systems, our engineers check the system pressure and the expansion vessel. Catching a vessel that is losing its charge on a service visit is much cheaper than dealing with it as a breakdown.

Low pressure versus other faults

If the pressure is fine and the boiler still will not work, low pressure is not the problem. Other common causes:

• Frozen condensate pipe on condensing boilers in cold weather, often with a gurgling sound.
• Oil supply on oil boilers. See oil boiler not firing.
• Noises such as kettling or banging, covered in boiler noises.

Get it looked at

A repair visit is £85 for the first hour, then we quote before doing anything more. If your boiler is due a service, book that instead and we will check the pressure, vessel and relief valve as part of it: gas £79, oil £89. Book online or email [email protected].