Guide

Boiler pressure keeps rising: what to check, in order

Rising pressure is a short list of causes and one useful question that splits it in half before you touch anything.

Ask this first

Does it climb only when the heating runs, or all the time?

Pressure that rises as the system heats and settles back as it cools is expansion with nowhere to go. That points at the vessel.

Pressure that creeps up regardless, including overnight with the heating off, means water is getting in from somewhere. That points at the filling loop or a heat exchanger.

1. The filling loop

Start here because it is the most common and the quickest to rule out. A loop that is not fully closed, or one whose washer is passing, will add a little water continuously. Check it is properly shut, and check it is not weeping. On a flexible loop, disconnecting one end settles the question for good.

2. Expansion vessel pre-charge

If it climbs with heat, drain the system down and check the vessel pre-charge with the water side empty. It should match the system cold-fill pressure. Charged too low, or lost its charge entirely, and the water has nowhere to expand into, so the pressure goes up instead.

3. Vessel sizing

A vessel with the right pre-charge can still be too small for the system volume. That is common where rads have been added over the years and nobody revisited the vessel. Work it out from the system volume, the cold-fill pressure and the safety-valve setting, rather than assuming the original was right. The expansion vessel calculator does the sizing.

Also worth checking on an external vessel: isolation valves that are partly shut, and waterlogging.

4. Plate heat exchanger

On a combi, a plate heat exchanger passing between the mains side and the heating side will push pressure up, and it often shows as pressure that rises when hot water is used rather than when the heating runs.

5. The safety valve

Not a cause, but check it. A safety valve that has been lifting and reseating on debris will have been discharging, and a system that keeps losing water and being topped up brings in fresh oxygenated water, which is its own problem later.

The customer has usually had parts fitted already

By the time this gets to a second visit, the heat exchanger and the vessel have often been changed. That does not rule either out. A new vessel charged to the wrong pressure behaves exactly like a failed one, and it is worth checking the pre-charge on a part that is three weeks old before accepting it as innocent.

Common questions

Why does my boiler pressure keep rising when the heating is on?

Pressure that climbs as the system heats and settles back when it cools points at the expansion vessel. Pressure that climbs whether or not the heating has run points at water getting in, which usually means the filling loop is passing.

Can a filling loop cause pressure to rise?

Yes, and it is the most common cause. A filling loop that is not fully closed, or one with a washer that is passing, will creep the pressure up slowly over days.

What pressure should a boiler sit at?

Around 1.0 to 1.5 bar cold on a typical domestic sealed system, rising modestly when hot. The manufacturer's instructions give the figure for that appliance.

Flint is a reference tool for competent, Gas Safe registered engineers. Everything here informs your judgement and never replaces it. Always work to the manufacturer's instructions and the current standards, and check every figure against the appliance data badge and manual.