How to top up low boiler pressure
Low pressure is one of the most common reasons a boiler stops working. The good news is that you can usually fix it yourself. It takes about five minutes, and you don’t need any tools.
What boiler pressure means
Most modern homes have a “sealed” heating system. The water in your radiators and pipes is kept under a little pressure. The boiler needs that pressure to work. If it drops too low, the boiler stops to protect itself.
Combi boilers and system boilers have a sealed system. They have a pressure gauge on the front or underneath. Older “regular” systems often fill from a small tank in the loft instead. Those usually don’t need topping up.
What the gauge should read
Most boiler makers say the pressure should be between 1 and 1.5 bar when the heating is cold. Vaillant, Glow-worm and Ideal all give this range. Your user guide may give a slightly different number, so check it if you can.
The reading goes up a little when the heating is on, because hot water expands. That’s normal.
If the gauge is under 1 bar, the boiler may show a fault code. For example:
- Vaillant and Glow-worm: F.22
- Ideal: F1
- Baxi: E118, or H.02.07 on newer models
Why the pressure drops
A small drop now and then is normal. Common reasons are:
- You bled a radiator. Letting air out also lets out a little water.
- A small leak. Look for drips under radiators, around valves and below the boiler.
- The safety valve let water out. Every sealed boiler has a pressure relief valve. It sends water outside through a small pipe if the pressure gets too high. If that pipe outside is dripping, tell your engineer.
How to top it up
Most boilers use a filling loop. It’s usually a silver braided hose with a small tap at each end. You’ll find it under the boiler. Some boilers have a built-in lever or key instead. Your user guide shows you where yours is.
- Turn the boiler off and let the heating cool down. You’ll get a truer reading this way.
- Find the filling loop. If it’s a loose hose, connect both ends firmly.
- Open the first tap, then slowly open the second. You’ll hear water flowing.
- Watch the gauge. Stop when it reaches about 1 to 1.5 bar, or the level in your user guide.
- Close both taps firmly.
- Remove the hose if your boiler has a loose one. Keep a towel handy for a few drips.
- Turn the boiler back on. If it still shows a fault code, press reset.
Don’t overfill it. If you go too high, you can bleed a radiator to let a little water out.
When to call an engineer
Topping up once in a while is fine. Call a Gas Safe registered engineer if:
- you have to top up more than once every few weeks, as there may be a hidden leak;
- the pipe outside the house keeps dripping;
- the pressure climbs close to 3 bar when the heating is on;
- the gauge doesn’t move when you top up.
A part called the expansion vessel can lose its air charge over time. Then the pressure rises a lot when the boiler heats up. The safety valve lets water out, and the pressure drops again when it cools. An engineer can check and recharge it.
Related guides
If you smell gas, leave the building and call the National Gas Emergency Service on 0800 111 999.