Boiler losing pressure but no sign of a leak?

Why a boiler loses pressure

A sealed central heating system should hold its pressure. If the gauge keeps dropping, water is escaping somewhere, or a part of the system isn't doing its job. The most common causes are:

• A hidden leak in the heating pipework — very often under floorboards, beneath a solid concrete floor, or behind a wall where you'll never see it.
• The pressure relief valve letting water out through the small pipe that runs outside, usually near the boiler.
• The expansion vessel losing its charge, which makes the pressure swing up and down as the heating comes on and off.
• Small weeps at radiator valves, towel rails or old joints that evaporate before you notice them.

What you can safely check yourself

Please don't open up the boiler itself — anything inside the casing is a job for a Gas Safe registered engineer.

• Look under every radiator and towel rail, and around the valves, for staining, rust marks or damp.
• Find the small copper or plastic pipe that comes out of the wall near the boiler. If it drips or has marks underneath, the relief valve may be the problem.
• Watch the gauge. Note how far it drops and how quickly. It helps to know if it drops faster when the heating is on.
• Check ceilings below bathrooms and the floor near the boiler for damp patches or warm spots.

When it's time to call a leak detection specialist

If the boiler engineer has ruled out the boiler and the pressure still drops, the leak is in the pipework. This is where most people get stuck, because a plumber's usual next step is lifting floors until they find it.

We find it without that. Using pressure testing to confirm and isolate the leaking circuit, thermal imaging to see where warm water is escaping under floors and behind walls, and tracer gas to pinpoint the exact spot, we can usually mark the leak to a precise point so only a small, targeted area needs opening.

Hidden heating leaks also cause damp, swollen floors and higher energy bills, so it pays to find them early.

Is it OK to keep topping up my boiler pressure?

As a short-term fix, yes, but it isn't a solution. Every top-up brings in fresh water and oxygen, which speeds up corrosion inside the system, and the leaking water is still going somewhere in your home.

Can a leak under a concrete floor cause my boiler to lose pressure?

Yes, and it's one of the most common causes we find. Heating pipes are often run through solid floors, and a pinhole leak there can go unnoticed for months. Thermal imaging and tracer gas let us locate it without breaking up the whole floor.