Why is my boiler losing pressure? Honest answers from a London plumber

If your boiler keeps losing pressure, the question is not really "how do I top it up". You can find that on YouTube in two minutes. The real question is "why does it keep dropping". That has only a handful of answers, and one of them you cannot ignore.

I have repaired thousands of low-pressure boilers across London since 2004. Here is the short version, then the longer one if you want to know more before you call.

The short version

A combi boiler should sit between 1 and 1.5 bar of pressure when the heating is cold. If yours is dropping below 1 bar regularly, the cause is one of:

1. You bled the radiators recently. Normal. Top it up once and you are done.
2. A small internal leak inside the boiler (often the pressure relief valve or expansion vessel). Repair, not replacement.
3. A leak in the heating pipework somewhere in the house. Could be tiny, could be hidden under floorboards.
4. A leak from a radiator or radiator valve you have not noticed. Often a slow drip behind furniture.

The first one is fine. The other three need a plumber. The third one in particular gets worse the longer you leave it.

How to check the pressure

The pressure gauge is on the front of your boiler. Modern boilers have a digital readout, older ones have a round dial with a needle and a green band somewhere between 1 and 2 bar.

With the heating off and cold: should read 1 to 1.5 bar.

With the heating on and hot: can rise to 2 or 2.2 bar. This is normal expansion.

Above 2.5 bar hot or 3 bar: the pressure relief valve will open and dump water from a small pipe outside the house. Not dangerous, but a sign of a different problem (usually a failed expansion vessel inside the boiler).

Below 0.8 bar cold: the boiler will refuse to fire. You need to top it up.

How to top it up properly

Underneath every combi boiler is a filling loop. It is usually a silver braided hose with a tap or lever at one or both ends. Some are integrated and only need one lever turned. Others are external and need attaching first.

The procedure for an external filling loop:

1. Turn the boiler off at the front panel
2. Locate the filling loop (silver braided hose)
3. Open the first valve, then the second, slowly
4. Watch the pressure gauge climb
5. Close both valves when the pressure reaches 1.2 bar (do not overshoot)
6. Disconnect the loop fully if your model requires it (read the manual)
7. Turn the boiler back on

If pressure drops back below 1 bar within a week, you have a leak and need to find it. Repeated topping up is treating the symptom, not the cause.

Where leaks hide

This is where the diagnostic part starts. Most pressure-loss leaks fall into one of three places:

Inside the boiler

The pressure relief valve is a safety valve that opens when pressure gets too high. When it fails, it leaks slowly all the time. The water goes out through a small copper pipe (usually 15mm or 22mm) that exits the building, often visible high on an external wall above a path or driveway. If you see water coming out of that pipe regularly, the valve is failed and needs replacing. Standard parts job, around £150 to £220 in 2026 London.

The expansion vessel inside the boiler is a small pressurised tank that absorbs the expansion of hot water. When the diaphragm inside it fails, the boiler cycles between normal and overpressure repeatedly. Repairable in some boilers, requires a new vessel in others. Costs vary by model.

Under the floorboards

The most stressful one to find. Heating pipes that run under floors can develop pinhole leaks where they were soldered decades ago, where they pass through joists, or where rodents have chewed insulation off and they have corroded.

You will not see this water. It soaks into floorboards, into ceilings below, and shows up as a damp patch in a downstairs ceiling weeks after it started. By then you have a bigger problem than the boiler.

How a plumber finds it:

• Pressure-test the system to confirm a leak
• Isolate the boiler from the heating circuit to confirm it is not internal
• Inspect visible pipework for moisture or staining
• Use a thermal imaging camera or moisture meter to narrow down the location
• Lift floorboards in the suspect area

This is not a five-minute job. Expect £200 to £500 for diagnosis, plus the repair cost once the leak is found.

Radiator valves

The valves on each end of every radiator (one for control, one for balancing) can develop slow drips. You usually do not see this because the drip happens behind a sofa or under a bed.

Pull each radiator forward and check underneath both valves. If you see any water staining on the valve body, the spindle, or the connection to the pipework, that is your leak.

Fix is usually replacing the valve (£60 to £120 per valve, parts and labour), or in some cases just tightening the gland nut and renewing the packing.

When pressure loss means urgent action

If you have to top up your boiler more than once a week, you have an active leak that is putting water somewhere it should not be. Each top-up is fresh, oxygenated water entering your heating system. That oxygen accelerates corrosion of every radiator and pipe in the system. Within a year or two you will have rust and sludge problems that cost far more than fixing the leak in the first place.

Same goes if you keep seeing a damp patch on a wall or ceiling. Call someone the same day. Water does not improve with patience.

What I do on a pressure-loss visit

The first job is to confirm whether the leak is internal to the boiler or external in the system. That changes everything about what comes next.

I will:

1. Check the current pressure and how recently you topped up
2. Inspect the pressure relief discharge pipe outside for evidence of a recent dump
3. Pressurise the system to slightly above normal and watch how fast it drops
4. Isolate the boiler from the heating circuit and watch each side independently
5. Inspect every visible joint and radiator valve
6. If the leak is in pipework, narrow down the location before lifting floors

Almost all of this is included in a standard diagnosis visit. The repair is quoted separately, in writing, before any work begins.

Things not to do

A few things I see customers try, mostly from internet advice, that make the problem worse:

• Adding a leak-sealer product to the system. These can clog narrow heat exchanger channels and make the next plumber's job much harder. Sometimes they work, often they cause more expense than the leak itself. I would only ever consider one as a last resort on a system about to be replaced anyway.
• Topping up the pressure to 2.5 or 3 bar to "buy time". Overpressure stresses every weak point in the system and will create new leaks.
• Assuming the boiler is broken and replacing it. A new boiler will lose pressure too if the leak is in the radiators or pipework. The old boiler is rarely the cause.
• Ignoring it because the heating still works. It does, until the pressure drops below the cut-off and you have no heating in February.

Call me if

• You are topping up more than once a week
• You see water anywhere in the house that should not be there
• The pressure relief pipe outside is dripping
• A damp patch has appeared on a ceiling under a heating pipe run
• You have already had two plumbers out and they have not found it

I cover every London borough, and pressure-loss diagnostics is one of the most common jobs I do. Most cases I find within an hour on site. Send a WhatsApp with a photo of your pressure gauge and the boiler make and model, and I can usually tell you whether it is a same-day visit or something that can wait.