Short answer: yes, if the house is well insulated - but not at every water temperature. In HeatAlgo's reference house, a 1,100 ft² (102.2 m²) house calculated for a Warsaw design day of -4 °F (-20 °C), with tubing at 5.9 in (15 cm, the usual 6-inch spacing) under tile, the floor covers the heat loss of 3 of 8 rooms at a 95/86 °F (35/30 °C) supply and return, 5 of 8 at 100/91 °F (38/33 °C) and all of them at 104/95 °F (40/35 °C). The hall and the mechanical room decide the water temperature for the whole house.
A homeowner on HeatingHelp put the worry plainly in a thread title, asking whether they just could not get enough radiant floor output and would need supplemental heating. On Green Building Advisor, another asked why a radiant slab would still need supplemental heat. Both questions have the same answer: the comparison has to be made room by room, between 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 supply water temperature at which its floor covers its heat loss. The manifold sends all loops one temperature, so the whole house gets the highest of them.
The room loads below are stated inputs from the European reference house, not a HeatAlgo sizing of a US house. Floor output is EN 1264-2:2021 for tubing in a slab, 6 in on center, tile, a 9 °F (5 K) drop between supply and return.
| Room | Need | At 95/86 °F | At 100/91 °F | At 104/95 °F | Lowest supply that is enough |
|---|---|---|---|---|---|
| Living room | 15.8 | 13.2 no | 16.4 yes | 18.5 yes | 100 °F |
| Kitchen | 12.7 | 13.2 yes | 16.4 yes | 18.5 yes | 95 °F |
| Bedroom | 15.5 | 13.2 no | 16.4 yes | 18.5 yes | 100 °F |
| Room 1 | 12.6 | 13.2 yes | 16.4 yes | 18.5 yes | 95 °F |
| Room 2 | 12.6 | 13.2 yes | 16.4 yes | 18.5 yes | 95 °F |
| Bathroom (75 °F) | 12.9 | 8.8 no | 12.1 no | 14.2 yes | 102 °F |
| Hall | 17.8 | 13.2 no | 16.4 no | 18.5 yes | 104 °F |
| Mechanical room (61 °F) | 22.7 | 17.5 no | 20.7 no | 22.9 yes | 104 °F |
| Enough in | - | 3 of 8 | 5 of 8 | 8 of 8 |
All figures in Btu/h·ft² of heated floor. Rooms are at 68 °F (20 °C) unless shown. Metric originals: the bathroom is 24 °C, the mechanical room 16 °C, the bathroom's lowest supply 39 °C.
The hall comes out high, at 17.8 Btu/h·ft² (56.2 W/m²), because it is calculated the way HeatAlgo's form takes it: without the air that, in a house with balanced ventilation and heat recovery (an HRV or ERV), flows into the hall from the rooms. That is a cautious assumption. A design should check how much heat that air really brings.
How to check your own house
- Take each room's heat loss from the contractor's room-by-room load calculation and divide it by the floor area that can be heated. Cabinets, closets with built-ins and the tub do not count.
- Read what the floor gives for your covering and spacing from the guide to radiant floor heating water temperature. Mind the room temperature: a bathroom is usually kept warmer.
- The highest supply water 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.
Supplemental heat for the hardest rooms
The whole house at 104/95 °F works, but a heat pump then runs less efficiently in every room, including the ones that manage at 95 °F. It usually pays to deal with the hardest rooms on their own:
- The mechanical room: it is kept at only 61 °F, but has two exterior walls, a window and an exterior door over about 65 ft² (6 m²) of floor. A better door or a small electric baseboard heater, rather than warmer water for the whole house.
- The hall: 10 cm (3.9 in, about 4-inch) spacing instead of 6 in gives 18.2 Btu/h·ft² (57.4 W/m²) at 100/91 °F, more than the 17.8 the hall needs.
- The bathroom: it needs 102/93 °F (39/34 °C). A towel warmer with an electric element takes it off the list. Why a towel warmer on the same water barely helps is in the answer is radiant floor heat enough for a bathroom.
With those three changes the house can run at 100/91 °F, which is what the living room and bedroom need. In a less insulated house the gap can be in every room, and then the supplemental heat is a second system: forced air, baseboard or a ductless mini-split. The radiant floor heating design guide shows how to check this in a design.
These figures are for a European house with radiant floor heating in a slab, on a Warsaw design day. Many US locations have a milder design day and some a colder one, and the same house would need correspondingly less or more heat. Only a room-by-room heat loss calculation answers it for your house.
Check which of your rooms set the water temperatureA HeatAlgo report follows EN 12831-1 and EN 1264, not ACCA Manual J, and is not accepted for permits or rebates.