A pressure gauge means the heating system is sealed. Combi and system boilers are sealed. An open-vented system, with a feed and expansion tank in the loft, does not hold this pressure and usually has no gauge. A system boiler has an expansion vessel inside the case, usually red or grey. A large system may have another vessel as well.
The working band is 1 to 2 bar. For a typical house or flat, up to three or four bedrooms and three floors, set 1.2 bar when the system is cold. Engineers often set anywhere from 1 to 2 bar. The right figure depends on how many floors there are and how high the boiler sits. A taller house, or a gauge low in the building, needs a little more cold pressure so the top of the system is still above the lockout point.
When the heating is on
Pressure rises as the water expands. From the cold fill, the rise should be about 0.5 bar. Filled to 1.2 bar, it should settle around 1.7 bar while the heating is on, and fall back when the system cools. Filled to 1 bar, it should not go past about 1.5 bar if the expansion vessel is the right size and charged.
A rise of more than 0.5 bar, or a climb past 2.5 bar into the red, means the vessel is short of air, undersized, or failed. The charge is about 0.75 bar with the system empty. A bigger property, or a vessel at the top of the house, needs a different charge. Repressurising it means the system is drained first. That is an engineer’s job. Blockages can cause the same swing.
Reading the gauge
Gauges may be a numbered dial, a green and red band, or a digital display. Most domestic dials run up to 4 bar. The red zone is the high end, often from about 2.5 bar. The pressure-relief valve is set to open at 3 bar and dump water outside. If the gauge goes past 3 bar and keeps rising, the valve may be stuck. That is unsafe.
Most boilers stop between about 0.5 and 0.8 bar. Below that there is not enough water to carry heat away from the heat exchanger.
If the system has more than one gauge, readings at the same height should match. A difference at the same height means one gauge is faulty or stuck. A gauge higher up the house will read a little lower.
Pressure that keeps falling
A sealed system should need topping up once a year at most. Microscopic weeps at valves are normal over a long time. If you are topping up every few weeks, water is leaving the system: a radiator, valve, or joint, or the relief valve letting by because debris is sitting on the washer. Check the discharge pipe outside.
- Top the pressure back up, then find the leak. Look for water marks or debris at valves and joints, and have those nuts tightened.
- Leak sealer is only for a very slow weep, not a system that drops in a day or two.
- Each top-up dilutes the inhibitor and adds fresh minerals. Fix the leak rather than keep filling.
Pressure that keeps rising
If the gauge climbs toward 3 bar while the heating is off, mains water is getting into the heating circuit. The filling loop has been left open, or its valves are letting by. A rapid rise is almost always the loop. Shut both valves. If it still rises, disconnect the loop.
To bring a high reading down, bleed a radiator into a container, or use a drain-off point. Some boilers have their own drain. Replacing a faulty filling loop is an engineer’s job.
How to fill
Find the filling loop for your boiler. On many combis the valves are under the case. Open them only until the cold gauge reads about 1.2 bar, then close both. This is not a substitute for a heating engineer when the pressure will not stay put.