The one question that decides everything: how often do you top it up?
Repressurising a boiler is a five-minute job and there is nothing wrong with doing it. What matters is the frequency, because a sealed system should not lose water at all.
| How often you top up | What it means |
|---|
| Once or twice a year | Normal. Do nothing. |
| Every month or two | A weeping valve, joint or towel rail, or an expansion vessel that has lost its charge. Worth finding. |
| Weekly | A live leak, usually under a floor, in a wall, or inside the boiler case. |
| Daily, or it will not hold at all | A significant loss, or a pressure relief valve discharging outside. |
The reason weekly topping up is worse than it sounds: every refill puts fresh, oxygenated mains water into the system. That accelerates internal corrosion, which produces more pinholes, which means more topping up. Meanwhile the water that left is going somewhere, and in a London flat that is usually into a floor, a ceiling void or a neighbour's home. Our boiler pressure hub covers the top-up procedure itself and the fault codes.
Three different faults that look identical on the gauge
Before anyone lifts a floor, these have to be separated, and a pressure test does it in under an hour.
- A genuine leak. The system loses water and the gauge falls whether the heating is running or not. This is the one that needs tracing.
- A failed expansion vessel. The pressure climbs steeply when the heating runs and drops below normal when it cools. The system is not losing water; it has nowhere to put the expansion, so the relief valve lifts and dumps it. The symptom is identical from the gauge, and the fix is a recharge or a replacement vessel rather than a detection job. Ruling this out before opening a floor is basic, and it is skipped surprisingly often.
- A discharging pressure relief valve. If the discharge pipe outside drips after the boiler fires, water is leaving through the safety valve. Sometimes that is the valve itself passing, sometimes it is the vessel above. Either way it looks like a leak and is not one.
Low-pressure fault codes differ from brand to brand, so the code on your display tells you the boiler has noticed, not what is wrong. What is wrong is one of the three above. The symptom pages cover the pattern in more detail: a boiler that keeps losing pressure and pressure dropping with no visible leak.
How the visit runs
- Read the system. Gauge behaviour cold and hot, expansion vessel charge checked at the schrader valve, relief valve and discharge pipe inspected, filling loop checked for a valve left cracked open — which quietly does the opposite and pushes pressure up.
- Pressure-test the sealed side. The system is isolated from the fill and put under test, so the loss is proven and measured rather than assumed.
- Section it. Circuits are valved out one at a time — upstairs, downstairs, the underfloor loop, the towel rails — until the failing one stops the drop. On a two-storey terrace this frequently ends the search on its own.
- Trace the run. Heating pipework is hot, which is exactly what thermal imaging reads best. The camera follows the run under screed, behind plasterboard and along joist lines to the point where the pattern breaks.
- Tracer gas if needed. The circuit is drained and a hydrogen and nitrogen mix pushed in; the gas escapes through the defect and is picked up at the surface. Non-invasive, and it works where thermal has two candidates and no way to choose.
- Open one access point and repair.
- Refill, vent, pressurise and hold with you watching the gauge, then make the opening good.
Where heating leaks actually are, in London properties
Victorian and Edwardian terraces
Microbore or 15mm copper run under suspended timber floors, clipped to joists and notched through them. Notches hold damp and the pipe corrodes at the notch. Radiator tails dropping through the floorboards are the other repeat offender, because the tail is the one part of the circuit that moves every time the radiator is knocked. Where a kitchen extension was added, the heating was usually extended with it, and the new run went into the new slab.
Purpose-built blocks
Solid floors mean pipework runs in the screed or in a boxed duct. Older blocks have a communal heating system, in which case the pressure is not yours to manage and a leak may be outside your demise entirely. Towel rails in bathrooms are a standing weak point across all of them.
Converted flats
Heating added as a retrofit takes the shortest route, which is usually a long horizontal run above a ceiling in the flat below. A pinhole there produces a ceiling leak downstairs and a falling gauge upstairs, and the two are often reported as separate problems by two different people.
New builds
Screed floors with underfloor heating and push-fit manifolds in a cupboard. A weeping manifold connection runs quietly into the floor build-up for months. See underfloor heating leak repair for how those are handled.
What the repair involves
A pinhole in a copper heating run is cut out and the section replaced. A weeping compression joint is remade, or replaced if the olive has bitten into a scored pipe. A corroded radiator tail is renewed. A failed valve is changed. A run under a solid floor that has failed more than once is better re-routed than repaired again, and where that is the right call we will say so and explain what re-routing involves before you commit to it.
Work inside the boiler case is a different thing from a pipework repair. If the loss turns out to be an internal appliance component rather than the system around it, we will tell you exactly what has failed and what the next step is, rather than dismantling it and presenting you with the news afterwards.
After any heating repair the system is refilled, vented properly at every radiator and towel rail, and brought back to pressure. Inhibitor displaced by the drain-down is replaced, because a system refilled without it corrodes faster than it did before.
Making good
Heating leaks live under floors, which means the access is a floor. We have handymen and builders in-house, so the floorboards come up and go back down, the cut in the screed is filled and levelled ready for the covering, the engineered board or the tile is relaid, the chased wall is filled and plastered, and the decoration is finished. That is the whole reason to use one firm for this rather than a detection company and then whoever you can find.
Before you book: what you can rule out yourself
- Check the discharge pipe outside. If it drips after the boiler fires, you are looking at a relief valve or an expansion vessel, not a buried pipe.
- Check the filling loop. Both valves closed, loop detached if it is removable. A loop left cracked open lets mains pressure creep into the system and produces the opposite symptom.
- Look at every valve. Run a dry tissue around each radiator valve spindle and each tail nut, and around the towel rail. A weep that is invisible will mark tissue.
- Look under the boiler. Staining or a green-white deposit on the pipework under the case is a slow weep that has been going for a while.
- Feel the floors. An unexplained warm patch that stays warm after the heating goes off is a hot pipe losing water below it.
If all of that is dry and the gauge still falls, the loss is concealed and that is what we are for.
Terms
£150 per hour, fixed before we attend, unchanged for a difficult property or an evening visit. The total depends on the hours the job takes and the materials it needs — we will not quote you a total for a leak nobody has located yet. No find, no fee: if we do not find it, you do not pay for the time spent looking. Written report for a trace and access claim included.
