What the Manifold Is Doing
Nearly all of the faults on this page live at or near the manifold, so it helps to know what it does. Warm water arrives from the boiler, far hotter than a floor can take. A blending valve mixes it down with returning water to a much lower flow temperature, typically somewhere between 35C and 50C depending on the floor construction. A pump pushes that water out through the flow bar into each loop, round the floor, and back into the return bar.
Each loop has a flow meter, one of the small clear gauges in a row, used to balance how much water each circuit gets. Loops are different lengths, and without balancing the shortest ones take more than their share while the longest rooms stay cool.
Flow and Circulation Faults
One room stays cold while the rest of the floor is fine
The system is working, but that particular loop is not getting water through it.
- Most likely cause
- Air trapped in that loop. Long, small-bore circuits hold air stubbornly and it will not always clear itself.
- Also possible
- That circuit throttled right down or closed at the flow meter, so it never receives its share of the flow.
- Also possible
- A partial blockage in the loop, or an actuator on that port not opening, which is a controls fault rather than a flow one.
- What the fix involves
- Purging that loop individually at the manifold to drive the air out, then rebalancing every circuit so the flow is properly shared.
The rooms never reach the temperature they are set to
The system runs, but every room settles short of its target and stays there. Usually described as the floor not getting warm, though the floor temperature is the symptom rather than the problem. Rooms being affected equally points at the water going into the floor rather than at any individual loop.
- Most likely cause
- The blending valve is set too low or has partly failed, so the water reaching the floor is cooler than it should be.
- Also possible
- The manifold pump has failed or is running on too low a setting, so circulation is weak everywhere.
- Also possible
- The boiler flow temperature has been turned down far enough that there is not enough heat arriving to blend from.
- Also possible
- The system was underspecified or the floor poorly insulated, in which case it was never going to reach temperature. This shows up as a system that has never worked properly rather than one that has changed.
- What the fix involves
- Measuring the actual flow and return temperatures at the manifold, then correcting whichever of those is out.
Gurgling, trickling or bubbling from the floor or manifold
Audible water movement is close to conclusive: there is air in the system where there should only be water.
- Most likely cause
- Air in the loops, often after the system has been drained, topped up, or worked on.
- Also possible
- The system pressure has dropped low enough to draw air in, which points at a leak or an expansion vessel fault.
- What the fix involves
- A proper loop-by-loop purge rather than a quick bleed, then finding out how the air got in so it does not come back.
The floor feels genuinely hot underfoot
A correctly running floor sits only a few degrees above room temperature. Noticeable heat through your feet is a fault, and one worth acting on.
- Most likely cause
- The blending valve has stuck or failed, letting water at close to boiler temperature into the floor.
- Why it matters
- Sustained overheating can damage floor finishes: engineered timber can shrink and move, and adhesives and vinyl can be affected.
- What the fix involves
- Checking the actual flow temperature at the manifold, then servicing or replacing the blending valve and confirming the limit is holding.
The system keeps losing pressure
The boiler gauge falls over days or weeks and the system needs topping up to keep working.
- Most likely cause
- A small leak. Manifold joints, valve glands and connections are much more common culprits than the buried pipework.
- Also possible
- A failed expansion vessel, so the system dumps pressure through the pressure relief valve every time it heats up. A tell-tale sign is water discharging from the outside overflow pipe.
- Worth knowing
- Repeated topping up is not a fix. Every refill brings fresh oxygenated water into the system, which accelerates corrosion and sludge.
- What the fix involves
- Pressure testing to establish whether it is losing water at all, then tracing the loss, including thermal imaging on the floor if a loop is genuinely suspected.
Dark, dirty water or poor flow across every loop
Several circuits underperforming at once, with dirty water when anything is opened up.
- Most likely cause
- Magnetite and sludge. Underfloor heating suffers more than radiators do, because the bore is narrow and the water moves slowly enough for debris to settle.
- Root cause
- Usually no inhibitor, inhibitor never topped up, or no magnetic filter on the system.
- What the fix involves
- Flushing the system properly, fitting or servicing a magnetic filter, and dosing with inhibitor. After that it needs checking periodically rather than being left for another decade.
The flow meters read zero or wildly different to each other
The gauges on the manifold are the clearest window into what is actually circulating.
- All reading zero
- Nothing is circulating: suspect the pump, a closed isolation valve, or no call for heat reaching the pump at all.
- Wildly uneven
- The system has never been balanced, or has been knocked out of balance by someone adjusting it.
- One reading zero
- That single loop is blocked, air-locked, closed off, or its actuator is not opening.
- What the fix involves
- Establishing whether the pump is running and the valves are open, then balancing every circuit against the loop lengths so each room gets what it needs.
Pump and Mixing Unit Setup, and Why Overheating Matters
The pump and mixing unit on the manifold is the part of a wet underfloor system that most often gets set up wrongly, and it is the part where getting it wrong does lasting damage rather than just causing discomfort.
Its job is straightforward. It takes hot water from the boiler, typically arriving at 60C to 75C, blends it down with cooler water returning from the floor, and circulates the result round the loops at the much lower temperature the floor was designed for — usually somewhere between 35C and 50C depending on the floor construction and what is laid on top of it. Set up correctly, the floor never sees water hotter than it was designed for. Set up wrongly, the floor sees something close to boiler temperature, and the floor is what pays for it.
The setup faults we find repeatedly are these:
- The mixing valve set too high at commissioning, or left on a factory default suited to a screed floor when the floor is actually timber, which needs a lower temperature
- A failed or seized mixing valve that has stopped controlling altogether and is passing whatever the boiler sends it
- The mixing valve fitted the wrong way round, so it never blends properly in the first place
- No high limit safety thermostat fitted, or one fitted but never wired in, which removes the only backstop the system has
- The underfloor circuit connected straight to the boiler flow with no blending at all, which we do still find
- Pump speed set wrong, giving weak circulation and uneven heat at one extreme, or noise and wasted energy at the other
Why overheating a floor is a real problem
A floor run above its design temperature is not simply uncomfortable to stand on. Overheating causes damage that is expensive to put right and often not noticed until it is done:
- Timber floors move. Engineered and solid wood shrinks, cups, gaps at the joints and can delaminate. This is the most common and most costly outcome, and it is not reversible.
- Vinyl, LVT and adhesives soften. Many coverings are rated to a maximum surface temperature of around 27C, and adhesive failure shows up as lifting and movement.
- Screed can crack, particularly where a new screed was heated too soon or brought up to temperature too quickly during commissioning.
- Floor covering warranties are voided. Manufacturers state maximum surface temperatures, and exceeding them is a straightforward way to lose your cover on an expensive floor.
- The room becomes hard to control. A floor that is too hot overshoots the target temperature and cannot be reined back quickly, because all that heat is already stored in the slab.
As a rule, the floor surface should sit only a few degrees above room temperature, broadly around 27C in living areas and a little higher in bathrooms and around the edges of rooms. If you can obviously feel warmth through your feet, the flow temperature is worth checking properly rather than living with.
The safety backstop that is often missing
A correctly built manifold has a high limit thermostat on the flow going into the floor, wired so that it shuts the pump down if that water exceeds a set temperature. It exists for exactly the failure described above: if the mixing valve sticks or fails, the limit stat stops hot water reaching the floor before any damage is done.
It is one of the most commonly omitted components on the systems we are called out to, particularly on installations done in a hurry or by someone who does not fit underfloor heating regularly. On a timber floor construction it matters even more, because there is less thermal mass to absorb a fault and the covering is the thing most easily damaged. If you have had a floor that ever felt hot, it is worth having both the mixing valve and the limit stat checked.
Why Balancing Is Not Optional
Balancing gets skipped more often than any other part of an underfloor installation, and it is the reason a great many systems are described as having never worked properly. Loops serving different rooms are rarely the same length, and water takes the easiest path, so without deliberate balancing the short circuits run hot while the long ones stay cool. No amount of turning thermostats up fixes it.
It is set at the flow meters, using the design flow rate for each loop, and it is part of commissioning any system properly. It is also one of the more satisfying repairs, because a system that has been uneven for years can often be transformed in an afternoon without a single new part.
Related Faults
If a whole room is cold and its flow meter shows no flow at all, check controls and actuator faults first, since an actuator that never opens produces exactly this symptom. If the floor is cold but the radiators are hot, see boiler and underfloor heating problems.
Flow and Cold Spot FAQs
Why does my underfloor heating have cold spots?
Uneven heat almost always comes down to one of three things: air trapped in a loop so water cannot circulate through it, a loop throttled down at the flow meter so it never gets its share of the flow, or a partial blockage from sludge. All three are diagnosed at the manifold and fixed without lifting the floor.
Can air get into underfloor heating?
Yes, and it is one of the most common causes of a cold room. Because underfloor loops are long, small bore and full of gentle bends, air collects easily and does not always shift on its own. Clearing it properly means purging each loop individually at the manifold rather than simply bleeding the system.
How do I know if my underfloor heating is blocked?
The usual signs are one or more loops staying cold while others work, flow meters reading much lower on the affected circuits, and dark or dirty water when the system is drained. Underfloor heating is more vulnerable to sludge than radiators because the pipework is narrow and the water moves slowly.
Does underfloor heating need inhibitor?
Yes. Inhibitor protects against the corrosion and sludge that block narrow underfloor loops, and it should be checked and topped up periodically rather than added once and forgotten. A magnetic filter fitted on the system catches debris before it reaches the floor.
Why does my underfloor heating keep losing pressure?
A system that needs regular topping up is losing water somewhere, or its expansion vessel has failed and pressure is being dumped through the pressure relief valve. Leaks are far more often found at the manifold, its joints and valves than in the buried pipework.
Can a leak in a buried underfloor heating loop be repaired?
Yes, though it is rare to need it. A leak in a loop can be traced with thermal imaging and pressure testing, and repaired by opening a small area of floor at the leak rather than lifting the whole room. Most suspected loop leaks turn out to be at the manifold instead.
Why is my floor too hot?
That points at the blending valve, which is the component that mixes hot boiler water down to the lower temperature underfloor heating needs. If it sticks or fails, near full boiler temperature reaches the floor. It is worth acting on promptly because sustained overheating can damage floor coverings and, with timber floors, cause shrinkage and movement.
Why is my underfloor heating overheating?
Overheating almost always traces back to the pump and mixing unit on the manifold. The mixing valve may be set too high, may have seized or failed so it is no longer controlling, may have been fitted the wrong way round, or on some installations there is no blending at all and the floor is connected straight to the boiler flow. A missing or unwired high limit thermostat is what allows any of those faults to reach the floor rather than being stopped.
What temperature should an underfloor heating mixing valve be set to?
It depends on the floor rather than on a single universal figure. Wet underfloor heating generally runs between about 35C and 50C, with screed floors towards the higher end and timber floor constructions needing lower temperatures. The correct setting comes from the floor build-up, the insulation and the covering manufacturer's maximum surface temperature, which is why a factory default left untouched at commissioning is a common cause of overheating.
Can underfloor heating damage a wooden floor?
Yes, if it is run too hot. Engineered and solid timber will shrink, cup, gap at the joints and can delaminate when the floor is taken above its design temperature, and the damage is not reversible. Most floor coverings are rated to a maximum surface temperature of around 27C. This is why the mixing valve setting and a working high limit thermostat matter more on timber floors than on screed.
What is a high limit thermostat on an underfloor manifold?
It is a safety thermostat on the flow going into the floor, wired to shut the pump down if the water exceeds a set temperature. It is the backstop that stops a failed mixing valve from sending boiler-temperature water into your floor. It is one of the most frequently omitted parts on underfloor systems, and worth checking on any installation where the floor has ever felt hot.