Why isn't underfloor heating warming one room?
If the whole underfloor heating system were cold, finding the cause would be simple. A far more common and confusing situation is when every room is warm except one — and that one is usually the bedroom or bathroom, where the cold is most noticeable.
A single cold loop almost always means the wrong amount of water is flowing through that circuit. There are four possible causes, and they can be ruled out one by one.
Below we go through them starting at the manifold, since that's where 80% of the answers are found.
Start at the manifold
Open the manifold cabinet and look at the flow meters (rotameters). In every loop, the float should be visible and at the level specified in the design.
- The float sits at the bottom — no water at all is flowing in that loop
- The float is noticeably lower than in the others — the loop is underfed
- All the floats sit uniformly low — the problem is with the pump or pressure, not one room
- The flow and return pipes of the loop feel the same temperature by hand — the water isn't circulating
A hand check is reliable
Touch both pipes of each loop. In a working loop, the flow pipe is clearly warmer than the return. If there's no difference, no water is moving.
The thermal actuator, or electric valve head
Each loop's return usually has a small white thermal actuator controlled by the room thermostat. If the actuator doesn't open, the loop stays cold even though everything else is working.
Check that the room thermostat is set higher than the current temperature and that the actuator feels warm to the touch. A cold actuator while heating is being called for points to a fault or a broken wire.
The actuator can be temporarily forced into its open position by hand to check whether the loop then warms up. If it does, the cause lies in the controls, not the pipework.
Air in the loop
In a long loop, an air pocket can stop circulation completely. This typically happens after filling the system, after a repair, or if pressure has ever dropped too low.
The solution is to bleed that specific loop through the manifold, rather than bleeding the whole system.
Loop length and balance
If one room has been cold ever since installation, it isn't a fault — the system is unbalanced or the loop is simply too long.
In a loop that's too long, the water has already cooled by the time it reaches the end. No amount of adjustment fully solves this; it requires redistributing flow rates and sometimes splitting the loop.
When it's a more serious problem
If a loop used to be warm and suddenly went cold, and flow is at zero even with the valve fully open, there could be a blockage or physical damage in the pipe.
If system pressure is dropping at the same time, a leak under the floor should be suspected. In that case, tracing is done with a thermal camera, without breaking up the floor — see leak detection services.
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