Short answer: yes, underfloor heating can be the main heat source in a well-insulated house - but not at every water temperature. In HeatAlgo's reference house, 102.2 m² with 15 cm spacing under tiles, the floor covers the heat loss of 3 of 8 rooms at 35/30 °C, 5 of 8 at 38/33 °C and all of them at 40/35 °C. The hall and the plant room decide the water temperature for the whole house.
"We were told more than once that separate heating upstairs was not required," writes one self-builder on a UK forum, and the replies go both ways: some say their floor copes on its own, others add radiators. Both are true for someone's house. What settles it is a room-by-room comparison: what each room loses, and what its floor can give.
Every room has its own water temperature
A floor gives more the warmer the water. So each room has a lowest flow temperature at which its floor covers its heat loss. The manifold gives all loops one temperature, so the whole house gets the highest of them.
Lowest flow temperature at which the floor is enough - reference house
35/30 °C
38/33 °C
40/35 °C
Plant room16 °C · 71.5 W/m²40/35 °C
Hall56.2 W/m²40/35 °C
Bathroom24 °C · 40.6 W/m²39/34 °C
Living room49.8 W/m²38/33 °C
Bedroom48.8 W/m²38/33 °C
Kitchen40.1 W/m²35/30 °C
Room 139.8 W/m²35/30 °C
Room 239.8 W/m²35/30 °C
Scale: flow temperature from 30 to 42 °C. By each room: its temperature if not 20 °C, and its heat loss per m² of floor.
Room by room
The table shows what the floor gives in each room at three flow temperatures, beside what the room needs. All figures in W/m².
| Room | Need | 35/30 °C | 38/33 °C | 40/35 °C |
|---|---|---|---|---|
| Living room | 49.8 | 41.5 ✗ | 51.8 ✓ | 58.5 ✓ |
| Kitchen | 40.1 | 41.5 ✓ | 51.8 ✓ | 58.5 ✓ |
| Bedroom | 48.8 | 41.5 ✗ | 51.8 ✓ | 58.5 ✓ |
| Room 1 | 39.8 | 41.5 ✓ | 51.8 ✓ | 58.5 ✓ |
| Room 2 | 39.8 | 41.5 ✓ | 51.8 ✓ | 58.5 ✓ |
| Bathroom (24 °C) | 40.6 | 27.8 ✗ | 38.1 ✗ | 44.9 ✓ |
| Hall | 56.2 | 41.5 ✗ | 51.8 ✗ | 58.5 ✓ |
| Plant room (16 °C) | 71.5 | 55.1 ✗ | 65.3 ✗ | 72.1 ✓ |
| Enough in | - | 3 of 8 | 5 of 8 | 8 of 8 |
The hall comes out high here, at 56.2 W/m², because it is calculated the way HeatAlgo's form takes it: without the air that, in a house with heat recovery, flows into the hall from the rooms. That is a cautious assumption. A design should check how much heat that air really brings.
Underfloor heating downstairs, radiators upstairs?
A common setup in a two-storey house is underfloor heating on the ground floor and radiators upstairs, all on one heat pump. The same rule applies: the heat pump makes one flow temperature, and whichever room needs the warmest water sets it. A radiator's catalogue output is rated at 75/65 °C, and on 35/30 °C water it gives only a fraction of that, so the radiators upstairs usually set the temperature for the whole house.
There are two ways out. Radiators sized for a low flow temperature - larger, or more of them - let the whole system run cool. Or the radiators get warmer water and the floor takes it through a mixing valve: the floor is safe, but the heat pump makes the radiators' temperature all season. The reference house is single-storey, so this page does not calculate the split. The method is the same: take each upstairs room's heat loss and check its radiator's output at your flow temperature, from the manufacturer's low-temperature data.
How to check your own house
- Take each room's heat loss from the heat loss calculation and divide it by the floor area that can be heated. Fitted wardrobes and the bath do not count.
- Read what the floor gives for your covering and spacing from the table in the guide underfloor heating flow temperature. Mind the room temperature: a bathroom is at 24 °C.
- The highest flow temperature any room needs is the temperature of the whole house. Before you accept it, check whether that one room can be dealt with on its own.
What to do about it
The whole house at 40/35 °C works, but the heat pump then runs less efficiently in every room, including the ones that manage at 35 °C. It usually pays to deal with the hardest rooms on their own:
- The plant room: it is heated only to 16 °C, but has two outside walls, a window and an outside door over 6 m² of floor. A better door or an electric heater, rather than warmer water for the whole house.
- The hall: 10 cm spacing instead of 15 cm gives 57.4 W/m² at 38/33 °C, more than the 56.2 the hall needs.
- The bathroom: it needs 39/34 °C. A towel radiator with an electric element takes it off the list. Why an ordinary towel radiator on the same water barely helps is in the answer is underfloor heating enough in a bathroom.
With those three changes the house can run at 38/33 °C, which is what the living room and bedroom need. The guide what an underfloor heating design should include shows how to check that in a design.
The figures are for the reference house in Warsaw at -20 °C with wet underfloor heating in a screed. The UK and Irish design days are milder, so the same house would need less heat; a less insulated house needs warmer water or a second heat source. Only a room-by-room heat loss calculation answers it for your house. A HeatAlgo report is not an MCS heat loss calculation.
Check your rooms