Short answer: when underfloor heating is not working in one room, start with two numbers: how much heat that room loses per square metre of floor, and how much water its loop gets. In the HeatAlgo reference house - 102.2 m², Warsaw, -20 °C, 15 cm pipe spacing - a 40/35 °C flow covers the heat loss in all 8 rooms. Turn the water down to 38/33 °C and the hall goes cold first: it loses 54.2 W/m², and its tiled floor gives 51.8 W/m². The same hall also goes cold with 40 °C water if its flow meter drops below 1.09 l/min.
When one room stays cool and the rest of the house is warm, the quick advice is to turn up the heating curve or "open the flow meters all the way". Sometimes that works, but it does not tell you what the cause was. Below are five things to check, in this order - the first two need only the figures from the design and a look at the manifold.
Every figure in this article was calculated by the HeatAlgo engine for the reference house: tiles in the kitchen, bathroom, hall and plant room, engineered wood in the other rooms, a 5 K drop across each loop, EN 1264-2:2021.
1. Can the floor in this room cover its heat loss at all?
Every room has its own heat loss from the heat loss calculation, in watts. Divide it by the floor area and you get the W/m² the floor has to give off. What it actually gives off depends on the water temperature, the pipe spacing and the floor covering. If the first number is bigger than the second, the room will be cool however you set everything else.
| Room | Heat loss | Floor at 40/35 °C | Floor at 38/33 °C |
|---|---|---|---|
| Living room (wood) | 47.8 W/m² | 54.4 W/m² | 48.1 W/m² |
| Bedroom (wood) | 46.8 W/m² | 54.4 W/m² | 48.1 W/m² |
| Rooms 1 and 2 (wood) | 37.8 W/m² | 54.4 W/m² | 48.1 W/m² |
| Kitchen (tiles) | 38.1 W/m² | 58.5 W/m² | 51.8 W/m² |
| Bathroom, 24 °C (tiles) | 38.4 W/m² | 55.6 W/m² | 47.2 W/m² |
| Hall (tiles) | 54.2 W/m² | 58.5 W/m² | 51.8 W/m² |
| Plant room, 16 °C (tiles) | 69.7 W/m² | 72.1 W/m² | 65.3 W/m² |
At 38/33 °C two rooms come up short: the hall (54.2 against 51.8 W/m²) and the plant room (69.7 against 65.3 W/m²). The living room only just makes it: 48.1 against 47.8 W/m². The hall loses the most heat per square metre of any room heated to 20 °C. That is the typical "one cold room": nothing is broken, the floor simply has nothing more to give.
2. Is the loop getting the water it needs?
The second number is the flow. The water in a loop carries heat to the room, and how much of it you need follows from that heat and how much the water is meant to cool: l/h = 0.86 × W / K. For the hall that is 0.86 × 552.88 W / 5 K = 95.1 l/h, or 1.58 l/min - the most of any loop in the house. HeatAlgo's underfloor heating module also counts the heat the floor loses downwards, so its figure for the loop is slightly higher.
What happens when the hall's flow meter reads less (40 °C flow):
| Hall loop flow | Hall floor gives off | Hall heat loss |
|---|---|---|
| 1.58 l/min | 57.9 W/m² | 54.2 W/m² |
| 1.5 l/min | 57.4 W/m² | 54.2 W/m² |
| 1.0 l/min | 53.2 W/m² | 54.2 W/m² |
| 0.8 l/min | 50.3 W/m² | 54.2 W/m² |
| 0.6 l/min | 46.0 W/m² | 54.2 W/m² |
Less water cools down more in the loop, so the floor's average temperature falls. Below 1.09 l/min the hall stops keeping up, even though the water going in is still at 40 °C. "1.5 on everything" happens to be enough for this hall - but a flow meter turned down during balancing, or left low after commissioning, is enough to make it cold. How to work out and set each loop's flow is in a separate guide: underfloor heating flow meter settings.
3. Is the flow temperature the one in the design?
The manifold gives every loop the same water temperature. If the heating curve is set lower than the design assumes, the rooms with the highest heat loss per square metre go cold first:
- at 40/35 °C the floor covers the heat loss in 8 of 8 rooms,
- at 38/33 °C in 6 of 8,
- at 35/30 °C in 3 of 8.
If several rooms are cool at once, start with the heating curve, not the flow meters. If it is one room, go back to steps 1 and 2. Why every room needs different water is covered in the guide to underfloor heating flow temperature.
4. What is on the floor?
The floor covering is a thermal resistance that all the heat has to pass through. At 40/35 °C the hall floor gives off 58.5 W/m² under tiles, 54.4 W/m² under engineered wood, and 49.3 W/m² with a carpet's resistance (0.15 m²K/W) - less than the 54.2 W/m² the hall loses. A thick rug over much of the floor, or carpet laid years later, can turn a warm room into a cold one on its own.
5. The rest: air, actuators, thermostat, first heat-up
These causes are real, but no figure from the design can check them:
- Air in the loop - the loop is clearly cooler than its neighbours with its flow meter open. Ask your installer to bleed and flush it.
- An actuator or room thermostat - the loop does not heat at all, or only sometimes. Check with your installer that the actuator on the manifold opens.
- New screed - the first heat-up follows the screed contractor's schedule. Ask your installer which stage it is at.
Underfloor heating not working in one room? Check it on your own house
Steps 1 and 2 are two numbers per room, and both come from the design. In HeatAlgo's underfloor heating design tool every room shows the floor's output next to its heat loss and flags a shortfall where the floor does not cover it, and the Flow meters table gives the flow to set for each loop on the manifold.
These figures come from the same engine that calculated our reference house, with every input published for checking. See the method: a reference house calculated in full.
Check floor output and flow for every loop