[MEP Center Engineering]

Starter question — a frequently asked question posted by the MEP Center team to open this forum. Share your own experience below.

We are replacing a gas boiler with an air-to-water heat pump in a detached house, and in a second project designing a new house with a heat pump from the start. The existing house has panel radiators sized for 70/50 °C.

How much does the flow temperature really affect heat pump efficiency? Can existing radiators work, and how much larger would they need to be at low temperature? When is underfloor heating clearly the better choice? A worked example for one room would help.

[MEP Center Engineering]

A heat pump works best at the lowest possible flow temperature, so underfloor heating (flow about 30 to 35 °C) is the natural partner. Radiators also work if they are sized for low temperature, typically 45 to 55 °C flow, which usually means two to three times the nominal output of a boiler-era radiator.

1. Why flow temperature matters

The ideal (Carnot) heating COP is T_flow / (T_flow − T_source) in kelvin. With outdoor air at 2 °C: at 35 °C flow, 308 / 33 ≈ 9.3; at 55 °C, 328 / 53 ≈ 6.2. Real machines reach roughly 40 to 50 % of this, so typically COP 3.5 to 4.5 at A2/W35 and 2.5 to 3.0 at A2/W55. A common rule: each 1 K lower flow temperature improves seasonal performance by about 2 to 3 %.

2. Worked example: one living room

Room: 20 m², design heat loss 1400 W (70 W/m²), 20 °C.

Underfloor heating. EN 1264 basic characteristic: q ≈ 8.92 × (θ_F,m − θ_i)^1.1. For 70 W/m²: θ_F,m − θ_i = (70 / 8.92)^1/1.1 ≈ 6.5 K, so a mean floor surface of about 26.5 °C, well under the 29 °C limit for occupied areas (that limit gives about 100 W/m²). With a tiled floor this is typically reached at about 35 °C flow. A thick carpet (R about 0.15 m²K/W) raises the required flow temperature noticeably.

Radiators. Radiator output is rated at ΔT 50 K (75/65/20 °C, EN 442). At 45/40/20 °C the mean water-to-air difference is 22.5 K (log mean 22.4 K), and with exponent n = 1.3:

factor = (22.5 / 50)^1.3 ≈ 0.35

Required nominal output = 1400 / 0.35 ≈ 4000 W at ΔT 50, against about 1400 to 1700 W for a boiler-era radiator. Keep each radiator at 180 cm or less and split larger outputs into two units, for example two radiators of about 2000 W under two windows, connected diagonally (flow top on one end, return bottom on the other) so the whole panel is warm at low ΔT.

3. How to decide

• New build, floors not yet laid: underfloor heating, designed for 30 to 35 °C flow.
• Existing house: calculate the room heat losses again (after any insulation upgrade), check each radiator at 45 to 55 °C flow and replace or add only those that fall short. Fan-assisted low-temperature radiators help in small rooms.
• Mixed systems: avoid a design where one undersized radiator forces the whole house onto a high flow temperature.
• Design ΔT for heat pumps is usually 5 K (sometimes up to 8 K), not the 20 K of a boiler: flow rates are about four times higher, so check pipe sizes and pump head.

Common mistakes

• Keeping 70/50 °C radiators and setting the heat pump to 60 °C.
• Using catalogue ΔT 50 outputs without the correction factor.
• Ignoring floor covering resistance in underfloor design.
• Sizing the heat pump without a bivalent point and backup strategy.

Related: Heat Pump Sizing and the Bivalent Point, Underfloor Heating: Quick Design Rules, Radiator Sizing: Correction Factors for Low Temperatures.

Check the edition of the code that applies to your project.

[Author]

Posts