Where this starts
Underfloor heating is a set of loops leaving a manifold, running the room at a spacing, and returning. Heat is delivered along each loop and the water cools as it goes, so the floor's temperature is a map of where the pipes are and how much flow each is getting.
MAP IT WITH YOUR HAND, methodically, across the whole room on a morning when the heating has been running. Distinct warm and cold STRIPES at the pipe spacing mean some loops are flowing and some are not. A gradient from warm at one side to cold at the other means the loop is delivering its heat before it gets there. Uniform but only mildly warm means the whole system is running cool rather than any loop being blocked.
AT THE MANIFOLD, the same information is available directly and more precisely. Each loop has a flow and a return connection; feel the return pipes. A loop that is working has a return noticeably cooler than the flow but still warm. A loop not flowing has a return at room temperature. A loop taking too much flow has a return almost as hot as the flow — which means it is short-circuiting and starving its neighbours.
BALANCING IS THE COMMONEST CAUSE and the least understood. Loops of different lengths present different resistances, so without adjustment the short ones take most of the flow and the long ones get very little. Every manifold has flow meters or adjusters for exactly this, and a system that has never been balanced, or was balanced and then disturbed, produces precisely this pattern. It is an adjustment, not a repair.
AIR IS THE SECOND, and it is easy to confirm. Air trapped in a loop stops circulation in that loop completely — the return goes cold, the floor above it goes cold, and everything else works. A system that has been drained, topped up, or had work done on it is the usual history.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
The loops have never been balanced
Most commonSome rooms or areas warm and others barely, with all loops apparently flowing, and the coldest areas served by the longest loops.
How to check: Feel the return pipes at the manifold. Big differences between loops with nothing blocked is a balancing question.
Air trapped in a loop
Most commonOne area completely cold in clear stripes, with that loop's return at room temperature while others are warm, often after the system was drained or worked on.
How to check: Which returns are cold. A loop at room temperature is not circulating at all.
A loop too long for its own good
CommonA gradient across a room, warm near one wall and cold at the far side, in a large space or one served by a single circuit.
How to check: Whether the cold is at one END rather than in stripes. Water cools as it travels and a long loop runs out.
Flow temperature set too low, or the mixing valve mis-set
CommonThe whole floor uniformly mild rather than patchy, with every return warm and no loop standing out, often after a boiler or control change.
How to check: Whether any loop is clearly worse. Uniform mildness is the supply temperature, not the pipework.
The floor covering is throttling the output
CommonOne room noticeably cooler than its neighbours with the same pipework and the same flow, and a thick carpet, a heavy underlay or a rug over the area.
How to check: What is on the floor. A thick carpet and underlay can halve the heat a floor delivers into the room.
An actuator or zone valve not opening
Less commonOne zone permanently cold with its loops all cold together, regardless of the thermostat, often with an actuator head that has failed or been fitted to the wrong port.
How to check: Whether a whole zone rather than individual loops is out. That is a control fault rather than a hydraulic one.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
Stripes, a gradient, or uniform mildness are three different faults, and the floor draws the diagram for you if you feel it methodically rather than in one spot.
- 2
It gives the same information more precisely: a return at room temperature is a loop not flowing, and a return nearly as hot as the flow is a loop taking more than its share.
- 3
Loops of different lengths need adjusting or the short ones take the flow, and it is an adjustment rather than a repair — the commonest cause and the cheapest fix.
- 4
Draining, topping up or any work introduces air, and a single loop completely cold shortly after is almost always that.
- 5
A thick carpet, a heavy underlay or a large rug throttles output substantially, and the floor is working exactly as designed underneath it.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where balancing is needed, since it is done with flow meters at the manifold against a design schedule and guessing at it makes the distribution worse.
- Where a loop will not clear of air after venting, which may mean a restriction or a damaged pipe within the floor.
- Where the system shares a boiler with radiators, because the mixing arrangement that drops the flow temperature for the floor is where these faults usually originate.
- Where a whole zone is dead, which is a control or actuator fault and involves wiring rather than water.
- Where a leak is suspected within the floor — falling system pressure, a persistently warm damp patch — since locating it means thermal imaging rather than lifting the floor.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.