Boiler Losing Pressure? Why It Happens and What to Do

What the pressure should be

Modern combi and system boilers run on a sealed system. Instead of a water tank in the loft, an expansion vessel soaks up the rise in pressure as the water heats. The gauge on the front of the boiler (or near the hot water cylinder on some system and conventional boilers) shows the pressure in bar.

• As a guide, most sealed systems sit at about 1 to 1.5 bar with the heating off (Viessmann, British Gas).
• It rises a little when the heating is on. Vaillant, for example, gives an operating range of 0.7 to 2.5 bar for its boilers, and says below 1 bar means pressure has been lost.
• Many gauges mark the right range, and modern boilers often show an error code when pressure is low. Your boiler's manual is the one to follow.

Low pressure won't hurt you, British Gas points out, but it can stop the boiler giving you heating or hot water, and running low makes it less efficient. The opposite problem is covered by a safety device: every sealed system has a pressure relief valve that lets water out if the pressure gets too high.

Why it keeps dropping

The makers' own guidance points to the same handful of causes:

• You've bled a radiator. Letting air (and a little water) out lowers the pressure. Check the gauge every time you bleed one.
• A leak on the system. Most often at radiator valves and connections, especially where a radiator can move on its brackets. Pipes and fittings corrode or loosen over time, and pipework under floors can leak out of sight.
• The pressure relief valve. It discharges through a small pipe that usually runs from the boiler through an outside wall. If that pipe is wet or dripping, the valve may be faulty. A failed expansion vessel can damage it, Viessmann notes.
• The boiler itself. Parts inside can corrode or loosen and leak, usually as a drip from the underside, or a component can simply fail.

Finding a leak

Before you call anyone, walk round the house with a torch:

• Look at every radiator's valves and connections. Wipe them dry and put a tray or a piece of kitchen roll underneath, then come back to see if a regular drip appears. A wet pipe on its own can just be condensation.
• Look for the signs a slow leak leaves: flaking or bubbling paint on pipes or skirting boards, rust marks, skirting that has swollen or lifted, stains on the ceiling below.
• Check the pressure relief pipe where it comes out of the outside wall.
• Look under the boiler for drips, without opening it.

Found a leaking joint? Don't try to remake it unless you're trained, Vaillant advises. A plumber can fix it, and if the pressure then holds, that was the problem.

How to top up the pressure

First check your boiler's manual says you can do it yourself (you can usually find it online if you've lost the paper copy). Then switch the boiler off and let it cool. Worcester Bosch describes three kinds of filling loop:

• External filling loop: a flexible braided hose with a valve at each end, one on the cold mains and one on the heating. If the hose isn't already attached, fit both ends, open the valves and you'll hear water flowing in. Watch the gauge, close both valves once it's up to the right pressure, then remove the hose (expect a few drips).
• Internal filling loop with a key: the key fits a port under the boiler. Turn it, watch the pressure rise, turn it back and remove it.
• Keyless filling loop: a lever under the boiler. Hold it until the pressure is up, then let go.

Keep your eye on the gauge the whole time: it's easy to overshoot, and you may need someone to help if you can't see the gauge from the valves. If you do overfill, bleed a radiator to bring it back down. Then switch the boiler back on (press reset if it needs it).

When to call someone, and who

Topping up is a stop-gap. If the pressure keeps falling after you've topped it up, something needs fixing, and the makers advise getting it looked at early to avoid more damage.

• A leaking radiator valve, joint or pipe: a plumber or heating engineer. See plumbing repair costs.
• The pressure relief valve, the expansion vessel or anything inside the boiler: a Gas Safe registered engineer. See boiler repair costs.
• Losing pressure on an old boiler that's had a run of faults: that's when it's worth pricing a new boiler alongside the repair.

Work inside a gas boiler must be done by a Gas Safe registered engineer: it's the law, not advice. Ask for their Gas Safe ID card: the back lists the kinds of gas work they're qualified for. See checking a gas engineer.

If it's only a slow loss, we'd still mention it at your next boiler service, with your note of how often you've topped up.

Frequently asked questions