Radiator Sizing for Heat Pumps: Why Existing Radiators Often Fail at 45°C Flow, Oversizing Rules and Emitter Types

Quick Answer: Radiators are rated at 75/65/20°C (75°C flow, 65°C return, 20°C room temperature) in standard manufacturer data sheets (Delta T 50°C). At 45/40°C flow/return (Delta T 22.5°C), the same radiator outputs approximately 42% of its rated output. A radiator rated 1,500W at standard conditions outputs only ~630W at 45°C — typically insufficient for the room's heat loss. To run a heat pump at 45°C flow, existing radiators must typically be upsized by a factor of ~2.4×, or low-temperature emitters (underfloor heating, fan coil units) must be used instead.

Summary

The single most common reason for poor heat pump performance in UK retrofit installations is undersized radiators left over from the previous gas system. Gas boilers run at 70–80°C flow; heat pumps run efficiently at 35–50°C. The mismatch means existing radiators don't have the output needed at heat pump temperatures, so either the heat pump runs at a higher (inefficient) flow temperature, or the building is cold.

For heat pump installers, the radiator sizing check is part of the MCS 007 design process. Where existing radiators are inadequate, the customer must be advised of the upgrade requirement — and the cost must be included in the installation quote.

Key Facts

Quick Reference Table: Radiator Output at Different ΔT

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Flow / Return / Room Mean Water Temp ΔT (mean - room) Fraction of ΔT50 Output
75°C / 65°C / 20°C 70°C ΔT50 100% (rated)
65°C / 55°C / 20°C 60°C ΔT40 ~75%
55°C / 45°C / 20°C 50°C ΔT30 ~52%
45°C / 40°C / 20°C 42.5°C ΔT22.5 ~39%
40°C / 35°C / 20°C 37.5°C ΔT17.5 ~29%
35°C / 30°C / 20°C 32.5°C ΔT12.5 ~20%

Detailed Guidance

The Radiator Sizing Calculation

Step 1: Calculate room heat load From the BS EN 12831 heat loss calculation (see heat pump sizing heat loss), determine the design heat load for each room in watts.

Example: Living room design heat load = 1,800W.

Step 2: Determine target design flow temperature Identify the heat pump's target DFT (design flow temperature). For most ASHP retrofits: 45°C or 50°C is typical.

At 45/40°C flow/return, 20°C room: ΔT = (42.5 - 20) = 22.5K. Correction factor = (22.5/50)^1.3 = 0.39.

Step 3: Calculate required rated output of radiator Required rated output (at ΔT50) = Room heat load / Correction factor = 1,800W / 0.39 = 4,615W rated output required at ΔT50

Step 4: Select radiator A radiator with a rated output ≥ 4,615W at ΔT50 is required. This is a large radiator — for context, a typical steel double-panel double-convector (Type 22) measuring 600mm × 1400mm has a rated output of approximately 2,400W at ΔT50. Two such radiators, or one much larger unit, would be needed.

This is why gas system radiators are typically undersized for heat pump operation at 45°C.

Practical Options for Radiator Sizing at Retrofit

Option 1: Replace with larger steel radiators The simplest approach: calculate the required rated output for each room; specify and install new radiators of the appropriate size; remove old radiators. Most straightforward rooms (living room, bedroom) can accommodate a longer or taller radiator in the same position. Cost: typically £150–£400 per radiator supplied and fitted.

Option 2: Add a second radiator in the same room Where wall space or radiator size restrictions prevent a single large enough radiator, adding a second radiator in the same room (on a different wall or below a window) distributes the output across two emitters.

Option 3: Fan convectors / fan-assisted radiators A fan coil unit or fan-assisted radiator can maintain heat pump-compatible output at low flow temperatures by forcing airflow over the heat exchanger. The fan significantly increases the effective output:

Option 4: Underfloor heating addition Where the flooring is being updated as part of a wider renovation, adding UFH to the ground floor (screed or low-profile retrofit UFH) allows those rooms to be heated at 35–40°C flow — maximising heat pump efficiency for the rooms that are retrofitted. UFH is more disruptive (floorboards up, screed laying) but delivers the best long-term efficiency.

Target Design Flow Temperature and Radiator Sizing Relationship

The design flow temperature has a significant impact on required radiator size:

DFT Correction Factor Radiator Oversizing Needed
55°C ~0.52 ~1.9×
50°C ~0.45 ~2.2×
45°C ~0.39 ~2.6×
40°C ~0.32 ~3.1×

Raising the design flow temperature from 45°C to 55°C reduces the radiator oversizing requirement from 2.6× to 1.9×. This is why some ASHP retrofit installations are designed to run at 55°C initially (to avoid replacing all radiators), accepting a lower COP, with a plan to reduce flow temperature as rooms are gradually upgraded.

This is a legitimate strategy for phased retrofits: fit the heat pump at a higher initial flow temperature (COP ~2.5); upgrade radiators room by room over 2–3 years; lower the DFT progressively as each room is upgraded; achieve COP ~3.5 long-term. Discuss this approach with customers as a way to reduce upfront cost.

Communicating the Radiator Requirement to Customers

Customers often do not understand why their existing radiators need replacing. A clear explanation:

Frequently Asked Questions

The customer says their neighbour has a heat pump and never replaced their radiators. Is that possible?

Possibly — if the neighbour's property is well-insulated, their room heat loads are low enough that the existing radiators (even at 42% of rated output) provide sufficient heat. Or the heat pump runs at a higher flow temperature (55–60°C) than is optimal. Check the neighbour's heat pump COP and electricity bills — they may not be getting the performance they expect.

What about cast iron radiators in a period property?

Cast iron radiators are actually excellent for heat pumps due to their large surface area and high thermal mass. A large Victorian cast iron radiator may have a ΔT50 rated output of 3,000–5,000W — and at ΔT22.5, still outputs 1,200–2,000W. Check the actual dimensions and estimate the output using the correction formula. Many cast iron radiators survive the transition to heat pump temperatures; some may still need supplementing.

Can I set the heat pump to 60°C flow to avoid replacing radiators?

You can set most heat pumps to 60°C, but the COP at 60°C is approximately 1.5–2.0 — barely better than direct electric heating. The running cost advantage of the heat pump largely disappears. The correct approach is to upgrade the radiators so the system runs at 45°C or lower, capturing the full COP benefit of the heat pump. In the short term, a 55°C compromise may be acceptable; 60°C is generally not an appropriate long-term setpoint for an ASHP.

Regulations & Standards