What does boiler pressure actually mean?
Most homes in Birmingham with a combi or system boiler have a sealed heating system. The water in the radiators and pipes is kept under a small amount of pressure, shown on a gauge on the front of the boiler or underneath it. The boiler needs that pressure to circulate water safely, so when it falls too low the boiler shuts down and usually shows a fault code, such as F22 on a Vaillant or F1 on an Ideal.
Pressure can only fall if water leaves the system. The question is always where it is going.
What are the common causes of a boiler losing pressure?
- A small leak on the heating system. Radiator valves, compression fittings under floors and towel rails are the usual suspects. Look for damp patches, rust marks or white crusty deposits around valves.
- The pressure relief valve letting by. This safety valve releases water if the pressure gets too high. Once it has opened a few times it may not seal properly. A copper pipe from the boiler to the outside wall that drips or is damp at the end is a strong clue.
- A flat expansion vessel. The vessel absorbs the expansion of water as it heats. If it loses its air charge, the pressure swings high when the heating is on, the relief valve opens, and the pressure then drops when the system cools.
- Recently bled radiators. Bleeding releases air and a little water, so a small drop afterwards is normal and only needs one top-up.
- A leak inside the boiler. Pump seals, the heat exchanger or internal pipe joints can weep, and this is the cause that most needs attention quickly.
What can you safely check yourself?
- Read the gauge with the heating cold. Note the figure, top up if your user manual shows you how, and write down the date.
- Watch it for a few days. If it holds, the drop may have been a one-off after bleeding radiators.
- Look for visible leaks. Check every radiator valve, the pipework you can see and under the boiler. Do not remove any covers.
- Check the pipe outside. Find the copper pressure relief pipe where it leaves the wall. If it is wet or dripping, say so in your enquiry.
- Note any fault code and the boiler make and model.
When is a falling pressure gauge more serious?
If you need to top up more than once a fortnight, if water is coming from inside the boiler, or if the pressure climbs above about 2.5 bar when the heating is on, the system needs an engineer. Repeatedly topping up a leaking system brings fresh oxygen into it, which speeds up corrosion and sludge, so it is worth fixing the cause rather than living with it.
The engineer will usually test the expansion vessel charge, check the relief valve and pressure-test the system to find the leak. Many causes are a single-part repair.