The good news about underfloor heating leaks
An underfloor system is mostly one continuous loop of plastic pipe per room, buried in screed, with no joints in it. That is the point of the design, and a pipe with no joints is hard to make leak — it takes a nail, a screw, a floor fixing or a genuine manufacturing fault.
Everything that can loosen, perish, wear or be done up wrong is at the manifold. Every loop is terminated there twice, once on flow and once on return. Every loop has a flowmeter. Every zone has an actuator. There is an isolation valve at each end, an air vent at the top, and a fill and drain point at the bottom. On a ten-loop manifold that is more than fifty sealing surfaces standing in a cupboard, against zero in the floor.
So when the heating pressure drops and someone starts talking about pulling up your kitchen tiles, the honest first move is to open the cupboard. If the manifold is the source, the floor never has to come up at all.
The parts that actually leak
The compression fittings where each loop enters
This is the most common single point. Each pipe tail is terminated into the manifold bar with a compression fitting: a brass body, an olive or a split ring, and a support sleeve pushed inside the plastic pipe. Get any of that slightly wrong at installation and it does not leak on day one; it weeps two winters later, once the pipe has crept under heat and the joint has relaxed.
The tell is a dull green or white crust on the brass, or a dried mineral ring below one tail while its neighbours are clean. Wipe it. If it comes back, that joint is the one.
The flowmeter O-rings
The clear plastic tubes with a floating indicator, usually on the flow bar, sit in the manifold on O-rings and a screw collar. They are the weakest part of most budget manifolds. The plastic goes brittle after years of hot water and the O-ring flattens. The leak is typically a slow bead running down the outside of the tube onto the bar below, which makes it look like the tail beneath is at fault.
The actuator seats
Actuators are the small white or grey heads screwed onto the return bar that open and close each zone. The actuator itself does not carry water. Underneath it is a spring-loaded valve pin with a seal around it. When that seal fails you get water appearing from under the actuator body, often only when the zone calls for heat and the pin is lifted. An intermittent drip that follows the thermostat is nearly always this.
The isolation valves
The ball or gate valves at each end leak in two places: the body joint, and the spindle where the handle passes through. A spindle leak often starts the day someone turns the valve for the first time in six years — so a leak that appears right after a service visit is usually a gland nut needing a quarter turn, not a fault in the floor.
The auto air vent
The brass cap at the top of the manifold vents air automatically as the system heats. Inside is a float and a small seal that debris and scale eventually stop from closing, and when it fails it vents water instead of air. It is a slow spit rather than a flow, and because it sits at the top, the water runs the full height of the manifold and pools at the bottom — the evidence appears as far as possible from the cause.
The fill and drain points
The two hose connections at the bottom are used once at commissioning and then forgotten. They leak when the blanking cap is loose, when the washer inside it has perished, or when a filling loop has been left permanently connected. A loop left attached is also a backflow problem in its own right: the Water Supply (Water Fittings) Regulations require fittings to be installed so water cannot return from a heating system into the drinking supply, which is why a temporary loop is meant to come off after use.
How to inspect it properly
Looking at a manifold and saying it is dry is worth very little. A weep of a few millilitres an hour will drop your boiler pressure over a month and evaporate off warm brass in minutes. You need to give it somewhere to show up.
- Dry everything first. Kitchen roll over every fitting, every flowmeter, both bars, under each actuator, both valves, the air vent and the drain points. Take the moisture off, do not just wipe the visible drip.
- Put paper underneath. Lay plain kitchen roll or a sheet of newspaper across the full floor of the cupboard, edge to edge, and under the manifold bars if you can get it there. Water marks paper permanently. It cannot dry off and hide.
- Run it hot, and run every zone. Turn all the thermostats up so each actuator opens. A manifold that is dry at rest can weep only under the expansion and the pump pressure that come with a hot, flowing system, and a zone that never calls for heat never shows its leak.
- Leave it, and come back. Two hours minimum, overnight is better. This is the step people skip, and it is the one that produces the answer.
- Read the paper, not the metal. One wet patch under one tail is your leak. A general dampness across the whole sheet usually means the air vent at the top, or condensation on a cold fill pipe rather than a leak at all.
- Photograph it before you touch anything. If this is going to an insurer, the marked paper and a note of the pressure gauge before and after are worth more than any description.
Check the boiler or system pressure at the same time. A manifold weep and a pinhole in the screed drop the gauge in exactly the same way, so the gauge tells you there is a loss, and the paper tells you where.
Why a cupboard hides this for months
Manifolds live in the worst possible place for early detection: a utility cupboard, under the stairs, in a plant room, behind a hinged panel nobody opens. There is no carpet to stain and no ceiling below to mark. The manifold is warm, so a small weep evaporates as fast as it arrives and leaves only a chalky trace. And the loss is small enough that you top the system up, blame air in the radiators, and carry on.
By the time it announces itself, the damage is usually not the leak — it is what the leak has been doing quietly. Chipboard flooring under a manifold swells and crumbles. A joist end sitting damp for a heating season starts to rot. In a first-floor plant cupboard the first symptom is often a stain on the ceiling below, which sends everyone looking at the wrong floor entirely.
If you have underfloor heating and a cupboard you never open, open it twice a year. It costs nothing and it is the highest-value check in the system.
When the manifold is clear
If the paper stays dry overnight with every zone hot, the manifold is not your problem and the loss is in the floor or elsewhere on the heating circuit. At that point the manifold stops being the suspect and becomes the tool: because each loop can be isolated and pressure-tested individually at the bars, we can prove which single loop is losing water before anything is opened. That narrows a whole-floor search to one room, and often to one run of pipe, which thermal imaging then pins down through the screed.
Two things that look like a manifold leak and are not. Cold or lukewarm sections of floor with no pressure loss are a balancing, air or actuator problem, not a leak — see cold spots in underfloor heating. And a manifold that is wet only on the cold fill pipe in a humid cupboard is condensation. Both get diagnosed and sent away without a repair, and we will tell you that rather than sell you a detection survey you do not need.
What repairs look like
Most manifold faults are same-visit work. A compression tail is remade with a new olive and sleeve. A flowmeter takes a new O-ring, or a whole new body where the plastic has gone brittle. Actuator seats, air vents and valve glands are all replaceable parts. Where a budget manifold has several flowmeters weeping at once, replacing the bar is cheaper than chasing it seal by seal.
The circuit is drained, the part is changed, and the system is refilled, vented and re-pressurised properly — with the filling loop taken off afterwards rather than left connected.
We charge £150 per hour, agreed before we attend, with no call-out fee. If a detection element is needed and we do not find the leak, you do not pay for the time we spent looking. Where a floor does have to be opened, our own handymen and builders put the screed, tile or board back, and the written report is prepared to support a trace and access claim — what the policy covers is your insurer's decision, not ours.
If your pressure is dropping and you have underfloor heating, start at I think my underfloor heating is leaking, or go straight to underfloor heating leak repair. The rate is set out in full on pricing.
