Heat pumps with low-temperature radiators
Many radiator systems can run on a heat pump, but not automatically. A radiator sized for 70 °C boiler water gives far less heat at heat pump temperatures, so each room needs checking.
What sets radiator output
Radiator output depends on the difference between the mean water temperature and the room temperature, and on the radiator's size. Catalogue ratings under EN 442 are given at 75/65/20 °C, a mean-water-to-room difference of 50 K. At other temperatures the output scales approximately as:
Two worked examples for a 20 °C room:
- 45/40 °C water: ΔT = 22.5 K, so output is about (22.5/50)^1.3 ≈ 35% of the catalogue rating.
- 55/45 °C water: ΔT = 30 K, so output is about (30/50)^1.3 ≈ 51% of the catalogue rating.
These are illustrations of the method, not a sizing for any particular room. The exponent n comes from the radiator manufacturer's data.
Checking an existing system
- Calculate the heat loss of each room at the local design outdoor temperature.
- Record every radiator's type and size, and find its rated output.
- Choose a flow temperature and ΔT, and calculate each radiator's actual output.
- Upsize or add radiators in rooms where output falls short, rather than raising the flow temperature for the whole house.
Flow temperature and efficiency
The RV DC inverter series delivers water up to 55 °C. Running close to that limit works, but lowers the COP and leaves no margin. Designs that reach the room temperatures at 45 °C or below use the heat pump far better.
Radiators and cooling
Radiators cannot cool a room. Where cooling is wanted, add fan coils in the rooms that need it; they share the same water circuit.
Related: Underfloor heating · Weather compensation · Cold-climate operation
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