0333 567 2639

Heating losing pressure: is the leak under the floor?

If you keep topping up the boiler and there is no drip anywhere, water is leaving the system somewhere you cannot see. A pipe under the floor is a common answer, but not the only one. A few checks rule out the usual boiler causes, and a survey finds the rest.

Call 0333 567 2639

Monday to Friday, 7:30am to 6:30pm. The office will explain how the survey would work at your property.

Excellent 4.7 out of 5 from 1,618 reviews of ADI Leak Detection on Trustpilot

What the gauge should read

Most sealed heating systems with a combi or system boiler sit at around 1 to 1.5 bar when cold and rise a little when hot. The boiler manual gives the figure for yours. A system that stays near that level for months is fine. One that needs topping up every week, every day or, as on our Torquay job, every three to four hours, is losing water.

Causes that are not a floor leak

Before looking under the floor, rule out the parts that can lose pressure without a hidden leak. A Gas Safe registered engineer should check anything inside the boiler.

  • The pressure relief valve. It lets water out of a small copper pipe, usually through the wall outside. A drip or damp mark under that pipe means the valve is opening. It can be caused by over-filling or a failed expansion vessel.
  • The expansion vessel. If it has lost its charge, pressure rises sharply when hot and the relief valve lets water out.
  • Radiator valves and joints. Check every radiator, towel rail and valve with a dry tissue.
  • The boiler itself. Internal leaks happen. On one of our reviews, the customer’s plumber confirmed the boiler was working normally before they called ADI about a leak.
  • The filling loop left slightly open, or a cylinder coil leaking into the hot water.

Signs it is under the floor

If the boiler and radiators are sound, a buried pipe becomes the main suspect. It is more likely if:

  • Your heating pipes are not visible downstairs, which usually means they run in the floor.
  • There is a warm patch on a floor, or a floor that feels warmer since the problem started.
  • Laminate or wood has buckled, or there is damp at the skirting.
  • You have underfloor heating, or radiators that were moved or removed in the past. In Torquay a radiator taken out years ago had left live pipes under the kitchen.

Sometimes there is nothing to see at all. In Burnham the heating was losing pressure and the kitchen floor looked and felt normal; the pipe had failed inside the screed.

What to do in the meantime

  • Top up only to the normal cold pressure. Over-filling can make the relief valve open.
  • If you are topping up very often, turn the heating off and use other heat until it is traced. Every top-up adds fresh water and oxygen to the system, and the escaping water is soaking into the floor.
  • Do not rely on a leak sealer. It will not show where the water has gone.
  • Note how often you top up and what the gauge reads. It helps the engineer judge how fast the leak is.

How the leak is traced

The engineer splits the heating into sections, such as the underfloor loops, each radiator circuit and the boiler, and pressure tests each one. That finds which part is leaking. Then thermal imaging maps the pipe runs while the system is warm, and tracer gas, acoustic listening and moisture readings pinpoint the spot on the pipe.

Boiler pressure gauge reading low, in the red
Boiler pressure low within the hour. Burpham job: the leak was a coupler that had backed off a buried heating pipe.

After the repair the system is refilled, inhibitor is added and the pressure is watched to make sure it holds. What happens on the day.

Pressure loss jobs we have traced to the floor

Published ADI Leak Detection jobs, with the engineer’s own photos. All case studies.

  • Motherwell, Lanarkshire. Floorboards

    Buckled laminate and a pressure drop nobody could place

    Another engineer had confirmed the system was leaking but not where. Thermal, moisture, acoustic and tracer gas readings agreed on the living room before a single board was lifted.

    Pinhole found; section replaced in copper

  • Torquay, Devon. Concrete slab

    Topping up the heating every three hours

    Pipes to a radiator that had been removed years before were still live under the kitchen. The cold feed had also worn through where it rubbed against them.

    Two leaks found and repaired; pipes protected

  • Burnham, Buckinghamshire. Screed floor

    Nothing visible, just a heating system losing pressure

    Nothing showed on the surface. Thermal imaging mapped the pipes, tracer gas found a strong signal under the kitchen within ten minutes, and only the laminate and screed above it came up.

    Leak found; 22mm copper section replaced; held at 2 bar

Reviews from people whose boiler kept losing pressure

Reviews of ADI Leak Detection on Trustpilot, quoted word for word. More reviews that mention floors.

Questions people ask

More answers on the questions page.

Will a leak sealer fix an underfloor heating leak?

We would not rely on one. A sealer may slow a very small weep, but it does not tell you where the leak is or whether water has already soaked the screed, and it can affect valves and pumps. If the pressure keeps falling, the water is going somewhere under the floor.

My underfloor heating keeps losing pressure. Is it the pipes in the floor?

Possibly, but check the manifold first. Look for drips or white crust at the valves, actuators and connections, and at the pressure relief valve outlet. If the manifold is dry, the leak may be in a circuit, on a radiator pipe, or at the boiler.

Each underfloor circuit can be isolated at the manifold and pressure tested on its own, which tells you which loop is leaking before anyone searches the floor. Underfloor heating leaks.

Can you find the leak without lifting the floor?

The finding is done from the surface: pressure testing, thermal imaging, acoustic listening, tracer gas and moisture readings. None of them needs the floor up.

To repair the pipe, the floor above the leak has to be opened, but only there. On our cases that has meant a few tiles, one strip of floorboards or a small patch of screed rather than a whole room.

Is there a no find, no fee guarantee?

ADI’s no find, no fee terms apply to a single leak on a hot or cold water supply at one property losing at least half a litre a minute, and to water mains up to 30 metres long. They do not apply to central heating, which includes wet underfloor heating. Ask on the phone whether your job qualifies.

Call 0333 567 2639

Mon to Fri, 7:30am to 6:30pm