Heat pump design — the essentials

An electric air-to-water heat pump moves heat from its source into the heating water. Under comparable source conditions, a lower required water temperature generally improves efficiency by reducing the temperature lift. Design the building, emitters, hot water and controls together.

Heat loss first. The system is sized to the property’s calculated heat loss, not to an old boiler’s output. Fabric improvements can reduce heat demand and the plant or emitter duty; their financial return depends on the work and the property.

Check emitters at the proposed temperature. A radiator gives less heat as its mean water temperature approaches room temperature. Compare its rated output at the design conditions with that room’s heat loss. Existing emitters may be adequate; enlarge or change them where the calculation shows a shortfall.

Hot water needs its own design. Where an indirect cylinder is selected, match its coil performance, usable volume and recovery to the heat pump and household demand. A larger effective exchanger area can help at a smaller temperature difference.

Controls such as weather compensation keep the flow temperature as low as conditions allow, protecting efficiency (the seasonal performance, often quoted as SCOP). Exact figures come from the heat-loss calculation and the manufacturer/MCS design.

Sizing in more detail

• Size from a room-by-room heat-loss calculation at the design outdoor temperature, the cold local condition the system must meet, and do it on the building as it will be after any fabric improvements. The room figures set each emitter, the flow temperature and the peak output.
• Check an inverter unit’s minimum output as well as its maximum. If the building demand falls below its available modulation range, it may cycle; excessive short cycling can harm efficiency and increase component wear. A unit that cannot meet its design duty leaves unmet demand or needs the supplementary heat allowed by the design. Direct resistance backup gives about one unit of heat per unit of electricity.
• Low-temperature domestic designs may use flow temperatures around 35 to 50 °C, but this is an illustration, not a universal operating range; select the actual temperature from the emitter and appliance data. Check existing emitters at the proposed mean water-to-room temperature difference. For a given heat duty, a smaller flow-to-return temperature drop needs a greater water flow; lower flow temperature alone does not determine that rate. Balance to the calculated duties.
• For the same heat transfer, reduced water flow increases the flow-to-return temperature difference. Flow below the appliance requirement can impair operation or cause a lockout. A buffer or volumiser earns its place only by solving a real problem, such as low system volume, cycling, defrost or a flow mismatch, and it cannot make an undersized emitter give more heat.
• A controller may use a bivalence temperature to permit supplementary heat, but the actual switching strategy can also depend on demand, tariffs and appliance limits. In a hybrid, agree how the heat pump and boiler operate together or separately; do not assume one fixed annual share. Under MIS 3005-D, all the heat sources together must meet 100% of the heat load, and the heat pump alone at least 55%, rated at 55 °C flow at the design outdoor temperature.
• Check the required heat-pump output at the design outdoor and flow temperatures, and its ability to turn down in mild weather. Under the standard MIS 3005-D sizing route the heat pump meets the full design heat load without resistive assistance; the hybrid route has the separate capacity requirements above.

Hot water and controls

• Choose storage volume and actual recovery performance together. Heat-pump output, primary temperature and flow, coil rating and peak draw-off all matter; recovery is not universally slow. A home changing from an instantaneous combi to this stored-water arrangement needs a suitable store and space for it. For the MIS 3005-D daily volume estimate, compare the known occupants with the bedroom count plus one. Use the larger count and allow 45 litres per day for each.
• Worked example (illustrative figures). Four bedrooms and three occupants give N = the greater of 5 and 3, which is 5, so the daily demand is 45 × 5 = 225 litres. This estimates daily use, not the cylinder’s required nominal capacity. Check the draw-off pattern and usable hot-water volume, then agree a reheat time that the selected equipment can meet.
• A higher hot-water target generally requires greater temperature lift and can reduce heat-pump efficiency. Preserve the specified hygiene regime and verify its temperature, exposure and frequency. Supplementary heat may legitimately support hygiene or recovery; its use alone does not prove poor operation. The equipment determines whether the heat pump can complete the cycle unaided.
• Weather compensation sets the flow temperature from the outdoor temperature. A heating curve set too high asks for hotter water than the building needs all season. Setback reduces room heat loss while the building is cooler, but recovery may take longer or call for hotter water or backup. Set schedules for the building, controller, comfort requirement and tariff, and check the result. Continuous operation is not a universal requirement.
• With PV generation, storage and a tariff whose price changes through the day, the heat pump can use more home-produced or cheaper electricity. This can reduce running costs if the controls and usage suit it.