Boiler pressure is too high: what causes it & how to fix

Increases in boiler pressure are absolutely normal. The system typically goes from 1 – 1.5 bar when cold, to at least 2. However, if your boiler pressure is too high, it may trigger the pressure release valve or even stop the boiler from working completely. Read on to learn more about this fault, and how to deal with it.

Boiler pressure is too high

However, anything over 2.5 is usually too much. Anything over 3 is likely to activate the Pressure Release Valve (PRV), and discharge water outside via the pressure release pipe. And anything higher than that is likely to trigger a pressure safety switch, and stop the boiler from working altogether.

When water exits via the PRV, the pressure in the system drops. Enough pressure may be released so that when the system is cold, the pressure may have dropped low enough to trigger a low pressure fault. In this case, the boiler won’t come on at all. You may often find yourself having to top up the pressure via the filling loop.

If this keeps happening, it’s really bad for the health of your central heating system. Fresh mains water is rich in oxygen. This reacts to the metal inside the system to form sludge, which reduces the system’s efficiency. It can block your radiators and even damage your boiler.

The system has been overfilled

The most common cause of this problem is the system was overfilled when trying to fix a low pressure fault.

The optimum pressure setting depends on the boiler’s model and manufacturer. However, generally speaking, the ideal range for most combi boilers when the heating is off is between 1 and 1.5 bar. Anything higher than 1.5 is almost certainly too much and may result in excessive pressure when the system is on.

When topping up the pressure, keep a close eye on the pressure gauge, and open the filling loop a really small amount.

Another cause for this problem is that the filling loop has been left open after it was used to top up the pressure. Even when the boiler is on, mains water is usually at a much higher pressure than the water in your heating system. If the filling loop is still open, it will eventually result in too much pressure.

Another possible reason concerning the filling loop is that its valves are leaking at either end. However, the likelihood of both valves failing is very small.

How to fix

You can reduce the pressure by bleeding a radiator. You can only do this when the system is off and has completely cooled down.

If you’re still getting this fault but the filling loop closes properly and the pressure level when the system is cold is OK, one of the components in the boiler may be faulty. The usual suspect is a leaking expansion vessel. A classic symptom of this is the need to keep constantly topping up the pressure. In this case, you should call a Gas Safe engineer to take a look at it.

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