Boiler Losing Pressure? What Causes It and How to Put It Right

If your boiler is losing pressure, water is leaving a system that is meant to be sealed — either through a leak, or through a pressure relief valve that is dumping it outside. Topping the gauge back up is a five-minute job you can do yourself, but a boiler that needs topping up more than once or twice a year has a fault, and that fault will not fix itself.

It is one of the most common calls our engineers take across Deal, Dover, Sandwich and Thanet, and one of the few boiler problems where a homeowner can genuinely do the first step. Here is how to tell a system that needs a top-up from one that needs an engineer.

What the pressure gauge is actually telling you

A sealed central heating system runs at a set water pressure so the pump can push hot water round the radiators. On most combi and system boilers you want somewhere between 1 and 1.5 bar when the heating is cold. Many gauges have a green band marked on the dial — if the needle sits in it cold, you are fine.

One thing that is normal and often mistaken for a fault: pressure rises when the heating is on, because hot water expands. Going from 1.2 bar cold to around 1.8 bar hot is expected.

What is not normal is a needle that keeps falling. Below roughly 0.5 bar most boilers lock out and refuse to fire — which is why “no heating and no hot water” so often turns out to be nothing more dramatic than a pressure fault. Several manufacturers give it a dedicated code, and our boiler fault code library will tell you what the display means before you ring anyone. Low pressure lockouts are especially common on Glow-worm units — the Glow-worm fault code pages cover the ones we see most.

Why is my boiler losing pressure?

Water only leaves a sealed system in three ways. Working out which one you have is the whole job.

1. A leak somewhere on the system

The obvious one, and still the most common. It is rarely a dramatic burst — usually a weep you would never notice: a radiator valve gland, a compression joint under a floorboard, a towel rail fitting, or a pinhole in pipework buried in a screed floor. Walk round with a dry tissue and press it against every radiator valve, both ends, top and bottom, check the pipework where it comes out of the floor, and look under the boiler. Anything damp, discoloured or crusted with white or green residue is your answer.

Older properties are the usual suspects. Much of our patch is Victorian and Edwardian housing — the terraces behind Deal seafront, the converted lodging houses in Ramsgate and Margate — where the heating has been extended and re-routed several times over the decades, and every one of those joints is a candidate.

2. The pressure relief valve is discharging

Every sealed system has a safety valve that opens if pressure gets too high, sending water out through a copper pipe on an outside wall. If that pipe drips, or there is a rust or limescale stain on the brickwork below it, the valve has been operating. Sometimes the valve itself has failed and no longer seats properly — a small piece of debris is enough. Often, though, it is doing exactly what it was designed to do, because of the third cause.

3. A failed expansion vessel

This is the one that catches people out, and it is the answer to the query we get asked most: my boiler is losing pressure but there is no leak.

The expansion vessel is a sealed chamber with a rubber diaphragm and a cushion of air behind it, and it absorbs the extra volume when the water heats and expands. When the diaphragm perishes or the air charge behind it leaks away — and after ten years or so, they do — there is nowhere for that expansion to go. Pressure spikes every time the heating fires, the relief valve opens to protect the system, and a little water goes out of the discharge pipe. Repeat that daily and the gauge creeps down all week.

The tell is the pattern: pressure that swings wildly between cold and hot — say 1 bar cold and well past 2.5 bar hot — then sits lower each morning. Recharging or replacing the vessel fixes it, and it is not a job that needs a new boiler.

Boiler losing pressure overnight or every few days: what the timing tells you

How fast it drops narrows it down more reliably than anything else:

• Within hours, heating on or off — an active leak, and one big enough to find. Turn the heating off and start looking.
• Only when the heating has been running — expansion vessel or relief valve, nearly every time.
• Slowly over weeks or months — a weep on a joint or a valve gland, or a slightly tired vessel.
• Straight after a radiator was bled or removed — you have simply let water out. Top it up and watch it; if it holds, there was never a fault.

Note the reading each morning for a week before you call. It sounds fussy, but that list saves diagnostic time on the visit — and because repairs are charged by the hour, saved time is saved money.

How to repressurise a boiler safely

This part you can do yourself. Check your manual first, but the principle is the same on almost all boilers.

1. Turn the boiler off and let the system cool — filling it hot gives a false reading.
2. Find the filling loop: usually a braided silver hose with a valve at each end underneath the boiler, or a built-in key or lever on newer units.
3. Open both valves slowly. You will hear water flowing in.
4. Watch the gauge and close the valves as it reaches 1 to 1.5 bar. It fills faster than you expect.
5. Close both valves fully, and disconnect a hose-type loop if your manual says to.
6. Turn the boiler back on. If it had locked out, it may need a reset.

Two warnings worth taking seriously. Do not overfill — overshoot past 2.5 bar and you will need to bleed a radiator to let some back out, and leaving it high stresses the whole system. And do not treat weekly topping up as a way of living with the problem: every top-up brings fresh, oxygenated, mineral-rich mains water in, and in our hard-water area that means more limescale and more corrosion sludge in the radiators and the heat exchanger. We have condemned heat exchangers killed by nothing more than a homeowner faithfully topping up every Sunday for three years. If you are repressurising more than a couple of times a year, stop topping up and get it looked at.

When it needs a Gas Safe engineer

Anything inside the boiler casing is a Gas Safe job by law — the expansion vessel, the relief valve, the heat exchanger and the internal pipework all included. Ring us on 01304 381999 if you cannot find the leak but the pressure keeps dropping, the discharge pipe outside is dripping or stained, pressure swings sharply between cold and hot, the boiler locks out and will not stay in operation after a top-up, or you can see water inside the casing or any sign of scorching.

We trace the cause rather than guess at it — pressure test the system, check the vessel charge, and tell you plainly whether it is a valve, a vessel or a leak under the floor. Repairs and leak tracing are charged by the hour, first hour billable and half-hour increments after that; current rates are on the pricing page. Same-day visits are usually available within working hours, and a real person answers the phone. More on how we handle faults is on our boiler breakdown repairs page.

Stopping it happening again

Most pressure faults give warning long before they leave you cold. An annual service catches a tired expansion vessel or a relief valve that has started passing, and it is £90 + VAT — considerably less than a January callout. Our boiler servicing page explains what the visit includes.

BOYLA HomeCare plans start at £9 a month and include the annual service, with no call-out fees on the paid plans and emergency response within 24 hours, Monday to Friday, for members. Plans start after a £108 inc VAT inspection so we know what we are taking on.

Twenty-five years on East Kent heating systems has taught us that a dropping pressure gauge is almost never the emergency people fear — but it is always a message. Read it early and it is a cheap fix; ignore it and you are eventually replacing something expensive.

Boiler losing pressure and you would rather someone just sorted it? Call 01304 381999.