Short answer: when radiant floor heating is not heating one room, start with two numbers: how much heat that room loses per square foot of floor, and how much water its loop gets. In the HeatAlgo reference house - 1,100 ft², a -4 °F design day, tubing at 6 in on center - 104/95 °F water covers the heat loss in all 8 rooms. Turn the water down to 100/91 °F and the hall goes cold first: it loses 17.2 Btu/h·ft², and its tile floor puts out 16.4 Btu/h·ft². The same hall also goes cold with 104 °F water if its flow meter drops below 0.29 gpm.
When one zone stays cool and the rest of the house is warm, the quick advice is to raise the water temperature or "open every flow meter all the way". Sometimes that works, but it does not tell you what the cause was. Here 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: tile in the kitchen, bathroom, hall and mechanical room, engineered wood in the other rooms, a 9 °F 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 a room-by-room load calculation, in Btu/h. Divide it by the floor area and you get the Btu/h·ft² the floor has to put out. What it actually puts out depends on the water temperature, the tubing spacing and the flooring. If the first number is bigger than the second, the room will be cool however you set everything else.
| Room | Heat loss | Floor at 104/95 °F | Floor at 100/91 °F |
|---|---|---|---|
| Living room (wood) | 15.2 Btu/h·ft² | 17.2 Btu/h·ft² | 15.2 Btu/h·ft² |
| Bedroom (wood) | 14.8 Btu/h·ft² | 17.2 Btu/h·ft² | 15.2 Btu/h·ft² |
| Rooms 1 and 2 (wood) | 12.0 Btu/h·ft² | 17.2 Btu/h·ft² | 15.2 Btu/h·ft² |
| Kitchen (tile) | 12.1 Btu/h·ft² | 18.6 Btu/h·ft² | 16.4 Btu/h·ft² |
| Bathroom, 75 °F (tile) | 12.2 Btu/h·ft² | 17.6 Btu/h·ft² | 14.9 Btu/h·ft² |
| Hall (tile) | 17.2 Btu/h·ft² | 18.6 Btu/h·ft² | 16.4 Btu/h·ft² |
| Mechanical room, 61 °F (tile) | 22.1 Btu/h·ft² | 22.9 Btu/h·ft² | 20.7 Btu/h·ft² |
At 100/91 °F two rooms come up short: the hall (17.2 against 16.4 Btu/h·ft²) and the mechanical room (22.1 against 20.7 Btu/h·ft²). The living room only just makes it, by a fraction the table rounds away. The hall loses the most heat per square foot of any room heated to 68 °F. 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: gpm = Btu/h ÷ (500 × ΔT °F). For the hall that is 1,887 Btu/h ÷ (500 × 9 °F) = 0.42 gpm - the most of any loop in the house. HeatAlgo's radiant floor module also counts the heat the floor loses downward, so its figure for the loop is slightly higher.
What happens when the hall's flow meter reads less (104 °F supply):
| Hall loop flow | Hall floor puts out | Hall heat loss |
|---|---|---|
| 0.42 gpm | 18.4 Btu/h·ft² | 17.2 Btu/h·ft² |
| 0.40 gpm | 18.2 Btu/h·ft² | 17.2 Btu/h·ft² |
| 0.26 gpm | 16.9 Btu/h·ft² | 17.2 Btu/h·ft² |
| 0.21 gpm | 15.9 Btu/h·ft² | 17.2 Btu/h·ft² |
| 0.16 gpm | 14.6 Btu/h·ft² | 17.2 Btu/h·ft² |
Less water cools down more in the loop, so the floor's average temperature falls. Below 0.29 gpm the hall stops keeping up, even though the water going in is still at 104 °F. Setting every loop to the same number happens to be enough for this hall - but a flow meter turned down during balancing, or left low after startup, is enough to make it cold. How to work out each loop's flow is in a separate article: what radiant floor flow meters should read.
3. Is the water temperature the one in the design?
The manifold gives every loop the same water temperature. If the reset curve is set lower than the design assumes, the rooms with the highest heat loss per square foot go cold first:
- at 104/95 °F the floor covers the heat loss in 8 of 8 rooms,
- at 100/91 °F in 6 of 8,
- at 95/86 °F in 3 of 8.
If several rooms are cool at once, start with the water temperature, 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 radiant floor heating water temperature guide.
4. What is on the floor?
Flooring is a thermal resistance that all the heat has to pass through. With 104/95 °F water the hall floor puts out 18.6 Btu/h·ft² under tile, 17.2 Btu/h·ft² under engineered wood, and 15.6 Btu/h·ft² under carpet - less than the 17.2 Btu/h·ft² 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 warm-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 neighbors with its flow meter open. Ask your installer to purge it.
- An actuator or thermostat - the loop does not heat at all, or only sometimes. Check with your installer that the actuator on the manifold opens.
- A new slab or pour - the first warm-up follows the contractor's schedule. Ask your installer which stage it is at.
Radiant floor heating not heating? Check it on your own house
Steps 1 and 2 are two numbers per room, and both come from the design. In HeatAlgo's radiant floor 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.
Check floor output and flow for every loopA HeatAlgo report follows EN 12831-1 and EN 1264, not ACCA Manual J, and is not accepted for permits or rebates.