Sizing a retrofit ASHP against a heat-loss figure and existing radiator output
A 1930s semi-detached house has a calculated design heat loss of 8.4 kW at an external design temperature of -3 °C, with a flow temperature of 45 °C assumed. The existing radiators were sized years ago for a 75 °C flow / 65 °C return system and total 11.2 kW of nominal output at those conditions. The customer wants a monobloc air-source heat pump and is reluctant to change any radiators. Domestic hot water is a 180 litre unvented cylinder with a 3 kW immersion and no dedicated coil data available. The proposed unit is rated 9 kW at A7/W35 and 8.2 kW at A2/W45. The house has loft and cavity insulation but single-glazed windows in two rooms.
1. Assess whether the proposed 9 kW unit and the existing radiators are suitable, showing your reasoning.
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Start from the design heat loss: 8.4 kW at -3 °C. The unit delivers 8.2 kW at A2/W45, which is close to but below the design condition output needed once you account for defrost derating and the fact that A2 is warmer than -3 °C. At -3 °C the capacity will be lower still, so the unit is marginal-to-undersized and the design margin is inadequate.
Now derate the radiators. Radiator output scales roughly with the mean water-to-room temperature difference to the power ~1.3. At 75/65 the mean water temp is 70 °C; at 45 °C flow with a 5 K drop the mean is 42.5 °C. Room at 21 °C gives ΔT of 49 K versus 21.5 K. Ratio (21.5/49)^1.3 ≈ 0.34. So 11.2 kW becomes roughly 3.8 kW - far short of 8.4 kW. The radiators must be upsized or the flow temperature raised, which then penalises COP and may exceed the unit's practical range.
Conclusion: neither the emitter system nor the unit as proposed is adequate; recommend a full room-by-room heat loss, radiator resizing, and a unit selected with margin at the actual design external temperature.
2. What would you tell the customer about the single-glazed windows and the DHW cylinder before finalising the design?
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The two single-glazed rooms are likely to dominate the fabric loss and cause comfort complaints (cold radiation, draughts) even if the heat loss figure is met. Recommend glazing upgrade or at minimum thermal curtains, and re-run the heat loss - this may reduce the required unit size and flow temperature, improving COP.
For DHW: a 180 litre cylinder with unknown coil is a risk. Without coil surface area and reheat performance data you cannot confirm the heat pump can reheat the cylinder in a reasonable time at 45–50 °C flow. Ask for the cylinder's coil rating or plan to replace with a heat-pump-specific cylinder with a large coil. Also flag legionella cycle requirements and that the 3 kW immersion is a backup, not the primary heat source. Do not sign off the design until both are resolved.