Low Boiler Pressure but No Leak? How to Find the Cause

What the gauge should read, and what counts as a real drop

On most sealed systems, cold pressure should sit between 1.0 and 1.5 bar, and it will climb by roughly 0.3 to 0.5 bar once the heating is up to temperature. That daily rise and fall is normal expansion, not a fault.

A genuine problem looks like a steady one way slide. Topping up once a year when the boiler is serviced is fine. Topping up monthly means you are losing perhaps a litre or two over that period. Topping up weekly, or watching it fall below 0.5 bar in a few days, means a fault worth chasing now, because most boilers lock out around 0.3 to 0.5 bar and will leave you without hot water.

Where the water goes when there is nothing to see

Water that escapes a sealed system does not always land on your floor. The four usual routes, roughly in order of how often we find them:

• The pressure relief valve. A small copper pipe exits through an outside wall, often near the boiler or above a flat roof. If it drips or the brickwork below it is stained and damp, your system is dumping water outside every time it heats up.
• A failed expansion vessel. The vessel absorbs expansion; when its internal air charge goes, pressure spikes hard when hot, forces water out of the relief valve, then reads low when cold. This is by far the most common cause of the classic no visible leak case.
• Pipework under solid floors or buried in walls. Common in the converted Victorian and Edwardian terraces round Balham, Tooting and Putney, where microbore has been run under screed or chased into plaster. A slow weep can evaporate for years without a mark.
• Radiator valves and towel rail joints. A weep at a gland nut often dries on a warm pipe and leaves only a faint white or greenish crust rather than a drip.

A twenty minute check you can do yourself

Note the cold pressure, then run the heating for an hour and read it again. A rise above about 2.5 bar strongly suggests the expansion vessel, not a leak. Go outside and feel the discharge pipe while the system is hot; if it is warm or wet, you have found your loss.

Then check the filling loop, the braided silver hose or keyed valve under the boiler. If a valve is not fully closed it can let mains water in and quietly push pressure up, masking a real leak or causing repeated over pressurising. Finally, run a hand along the bottom of each radiator and around every valve nut, and look at ceilings below upstairs pipework for faint brown ring stains.

Repressurising safely, and when to stop doing it

With the boiler off and cold, open both filling loop valves slowly until the gauge reads 1.2 bar, then close them fully and remove the loop if it is the detachable type. Do not chase 2 bar; overfilling just pushes water back out through the relief valve.

Repeated topping up is not harmless. Every refill brings fresh oxygenated mains water into the system, which accelerates internal corrosion and sludge build up, and in the hard water we get from Thames Water it also adds scale to the heat exchanger. If you are refilling more than every couple of months, get the cause found rather than living with it.

Getting it fixed properly

A Gas Safe registered engineer will pressure test the system, check the expansion vessel charge with a gauge and pump, and replace the relief valve or recharge or replace the vessel as needed. Vessel recharge is usually a straightforward visit; an external replacement vessel is a bigger job but often the sensible answer where the internal one is buried inside a wall hung boiler.

For losses under floors, thermal imaging or acoustic tracing avoids lifting an entire floor. Costs vary a great deal depending on access, boiler make and whether floorboards or tiling need lifting, so any honest quote should follow a look at the system rather than a phone call. If your boiler is over twelve to fifteen years old and needs a vessel, a valve and a heat exchanger flush, ask for the repair and replacement figures side by side before deciding.