Short answer: one room sets the flow temperature for the whole house - the room whose radiator has the least to spare over its heat loss. In the HeatAlgo reference house (102.2 m², 4.38 kW) with radiators picked from a catalogue, that room is the living room: it loses 1340 W and its radiator gives 762 W at 55/50 °C, so the house needs 73 °C. To heat the whole house at 55 °C you swap 5 of the 8 radiators, not all of them.
A heat pump sends water at one temperature to every radiator. When one room needs warmer water, the whole house gets it. So the question "are my radiators low temperature radiators?" has a room-by-room answer. Every figure in this article comes from HeatAlgo's Radiators module (the EN 442-2 characteristic), from room-by-room heat loss calculated to EN 12831-1.
What a low temperature radiator is
There is no separate standard or class called "low temperature radiator". Every radiator has a datasheet output at 75/65/20 and gives less when the water is cooler. A type 22 panel radiator 600 × 1000 mm rated 1694 W gives 953 W at 55/50 °C and 266 W at 35/30 °C - see how much is left at 55, 45 and 35 °C.
So a low temperature radiator is a radiator sized for cool water: bigger (longer, taller, a type 22 or 33 instead of a type 11) or with a fan that raises its output. Only one thing counts: does it cover its room's heat loss at your water temperature?
The reference house: radiators picked from a catalogue
This is an example: each room of the reference house got the smallest type 22 panel radiator 600 mm high that covers its heat loss at 75/65/20 - the way a radiator is picked from a catalogue for a boiler. The Radiators module then checked these radiators for a heat pump, with a 5 K difference between flow and return:
| Room | Heat loss | Radiator type 22, 600 mm | Datasheet output 75/65/20 | Output at 55/50 °C | Enough from |
|---|---|---|---|---|---|
| Living room | 1340 W | 800 mm | 1355 W | 762 W | 73 °C |
| Kitchen | 381 W | 400 mm | 678 W | 381 W | 55 °C |
| Bedroom | 753 W | 500 mm | 847 W | 476 W | 69 °C |
| Room 1 and Room 2, each | 450 W | 400 mm | 678 W | 381 W | 60 °C |
| Bathroom, 24 °C | 311 W | 400 mm | 678 W | 320 W | 55 °C |
| Hall | 553 W | 400 mm | 678 W | 381 W | 66 °C |
| Plant room, 16 °C | 418 W | 400 mm | 678 W | 445 W | 54 °C |
"Enough from" is the lowest flow temperature at which the radiator covers the room's heat loss. The highest of them, 73 °C in the living room, is the flow temperature for the whole house.
Why do the rooms differ so much? The module sizes a radiator's length in 100 mm steps, from 400 mm. The kitchen, bathroom and plant room need little heat, so their shortest radiator has plenty to spare. The living room got a radiator that covers its heat loss at 75/65/20 almost to the watt, and with cooler water it falls shortest.
One room sets the temperature for the whole house
When you swap the radiator in the room that sets the temperature, the house's flow temperature drops to the next room. This is the reference house, swapping at each step the radiator the module proposed:
| Radiators swapped | Flow temperature for the house | Set by |
|---|---|---|
| none | 73 °C | living room |
| Living room: 800 → 1000 mm type 33 | 69 °C | bedroom |
| + Bedroom: 500 → 800 mm | 66 °C | hall |
| + Hall: 400 → 600 mm | 60 °C | room 1 and room 2 |
| + Room 1 and room 2: 400 → 500 mm, each | 55 °C | - |
Five swaps out of eight radiators bring the house from 73 to 55 °C. The kitchen, bathroom and plant room keep their radiators. The living room alone is worth 4 K.
Below 55 °C is a different sizing. At 45/40 °C, after these five swaps, none of the eight rooms is covered. How many radiators the whole house needs at 55, 40 and 35 °C is in underfloor heating flow temperature with a heat pump.
How to find the room that sets it in your house
- Calculate every room's heat loss to EN 12831-1 in the heat loss calculation. An estimate from the floor area shows the direction, but it cannot tell you which room sets the temperature.
- In the Radiators module, enter the house's radiators: kind, type, height and length, or the datasheet output. Algo AI can enter a radiator from a photo.
- Read "Enough from" beside every room and the house's flow temperature. The module proposes which radiator to swap to go lower.
The result is a check of the radiators, not an installation design. The output is calculated without installation factors (a niche, a cover) and without heat from the pipes. Heat pump sizing in HeatAlgo is preliminary.