Radiator output at low flow temperatures
Are the existing radiators enough with a heat pump? Free check to EN 442: real output in watts, load coverage and the lowest usable flow temperature.
Checks existing radiators to EN 442 for a heat-pump retrofit — real output and the lowest flow temperature that suffices.
Radiator output at low flow temperatures — method and FAQ
Radiators with a heat pump: when the existing emitters are adequate
An emitter rated "1 500 W" gives that much only at the catalogue condition — 75/65 °C with the room at 20 °C. With a heat pump and a flow temperature of 45 °C, the same emitter gives roughly one third. The calculation is to EN 442, using the emitter's exponent and the arithmetic mean excess temperature — the difference between the average of flow and return and the room temperature.
The return temperature is not asked for — it is solved. With the flow rate fixed, the return rises when the emitter gives out less, and that changes the answer: an emitter at a flow temperature of 45 °C gives 552 W, not the 531 W that the naive 45/40 assumption produces.
The lowest flow temperature that covers the room
The main answer is not "how much does the emitter give" but "how low can the system run" — every degree lower on the flow temperature improves the heat pump's COP. A full output table from 35 to 65 °C is printed.
In a real retrofit the flow temperature is set by the worst room in the house — the tool checks one emitter and one room at a time, and says so.
Frequently asked questions
Do existing radiators work with a heat pump?
Often yes, but not at their nominal output. At 55 °C an emitter gives roughly half its rated output, at 45 °C roughly one third. If the emitters were installed oversized — common in older installations — the margin is often enough. The calculator checks ONE emitter and its room: in a real retrofit you repeat the check for every room, because the system flow temperature is set by the worst one and not by the average.
What does the emitter's exponent n mean?
It describes how non-linearly output falls with temperature: panels and cast iron around 1.30, convectors higher (≈1.38), underfloor lower (≈1.10). The higher the exponent, the more the emitter is penalised at low temperatures.
Related tools
- Heat pump sizing for a dwelling — Heat loss, cooling load and required capacity — always with the operating conditions stated, so quotations become comparable.
- Buffer tank sizing — Minimum volume from calorimetry — run time, defrost and hydraulic separation, not a litres-per-kW rule.