Heat pumps

Size against heat loss, not floor area

Floor area rules of thumb ignore the two things that matter most: insulation level and air tightness. A room-by-room heat loss calculation to EN 12831 gives the design heat load in kW at your local design outdoor temperature, and that number, plus an allowance for hot water reheat, is what the heat pump must cover.

• Use the design point, not the nominal rating. A unit rated at 12 kW at 7 °C outdoor and 35 °C flow may deliver considerably less at minus 10 °C outdoor and 50 °C flow. Read the capacity tables, not the model name.
• Do not add a safety margin on top of a margin. Heat loss calculations already assume steady-state worst case. Adding 30 % on top produces a unit that spends most of the season below its minimum modulation.
• Check the minimum modulated output. An inverter unit that turns down to 30 % of rating behaves far better in a mild autumn than one that only turns down to 60 %.
• Count the hot water load separately. A 190 L or 240 L cylinder reheat is a short high-demand event, usually handled by scheduling rather than by extra rated output.

Flow temperature is the main efficiency lever

Every degree of flow temperature costs efficiency. As a working rule, each 1 °C reduction in flow temperature improves COP by roughly 2 to 2.5 %, so the same machine on the same house can swing between poor and excellent depending on how the emitters are set up.

• Underfloor heating runs at 30 to 40 °C flow because the emitting surface is enormous. This is the natural pairing and gives the highest seasonal efficiency.
• Oversized or replaced radiators can run at 40 to 45 °C. Doubling radiator surface area in the two or three worst rooms is usually cheaper than accepting a 55 °C system for twenty years.
• Existing radiators sized for a gas boiler at 65 to 70 °C will work, but the COP penalty is severe and the unit may need its high-temperature envelope for only a handful of days a year.
• Weather compensation should always be enabled. A fixed flow setpoint wastes the whole benefit of a modulating compressor.

COP, SCOP and what the numbers mean

COP (coefficient of performance) is heat output divided by electrical input at one stated operating point, for example A7/W35, meaning 7 °C outdoor air and 35 °C flow. SCOP is the seasonal average over a defined climate profile and a defined flow temperature, calculated to EN 14825. SCOP is the honest figure for comparing running cost because it includes part-load operation, defrost cycles and standby.

• Always compare like for like. A SCOP quoted at 35 °C flow and one quoted at 55 °C flow are not comparable numbers.
• Defrost matters. Between roughly minus 5 °C and plus 5 °C with high humidity, the outdoor coil ices up and the unit periodically reverses to melt it. Frequent defrosts pull the seasonal figure down.
• SPF is what you actually get. The seasonal performance factor measured on site includes pumps, controls and any backup heater, so it sits below the brochure SCOP.

Monobloc or split, R32 or R290

In a monobloc, the entire refrigerant circuit is sealed inside the outdoor unit and only water pipes enter the building, so no F-gas handling is required on site but the external pipework must be protected against freezing. In a split, the compressor sits outdoors and the condenser indoors, connected by refrigerant lines, which removes the freezing risk and generally lowers outdoor noise, but requires a certified refrigerant handling engineer to commission.

• R32 is the current mainstream refrigerant, with a GWP of 675, mildly flammable in class A2L and well supported across the supply chain.
• R290 (propane) has a GWP of 3 and reaches higher flow temperatures efficiently, but it is class A3 flammable, which imposes stricter siting distances from windows, doors and drains.
• Charge size and siting rules follow EN 378 and the F-gas regulation. Confirm minimum room volume and clearances before the outdoor unit position is fixed.

Bivalent point and backup heater

The bivalent point is the outdoor temperature at which the heat pump alone can no longer cover the building's heat loss. Above it the compressor does everything, below it a second heat source contributes. In a monoenergetic system that second source is an electric backup heater, typically 3 to 9 kW in the indoor unit. Choosing a bivalent point around minus 7 °C to minus 10 °C lets a correctly sized unit cover the vast majority of the annual heat demand, with the element supplying only a small percentage of it.

• Watch how often the element runs. A backup heater that engages regularly in November means the sizing or the flow temperature is wrong, not that the unit is faulty.
• Legionella cycles need it. Most systems use the immersion element to lift the cylinder above 60 °C periodically, which is normal and short.
• Check the electrical connection. Units up to about 10 kW are often single phase at 230 V, larger units and most backup elements need a three-phase 400 V supply and an appropriately rated breaker and RCD.

Combining a heat pump with photovoltaics

A heat pump is the single best self-consumption load a household has, because it converts each kWh of surplus solar into three or four kWh of heat. The practical route is a photovoltaic array with an export-aware controller: a smart meter at the connection point detects surplus, and the system raises the hot water setpoint or lifts the heating curve slightly while the sun is on the roof.

• Store heat, not just electricity. Lifting a DHW cylinder from 45 to 55 °C absorbs several kWh of surplus at a fraction of the cost of battery capacity.
• Seasonal mismatch is real. Peak heat demand is in January, peak PV yield is in June, so a heat pump improves self-consumption strongly in spring and autumn and much less in midwinter.
• Watch the starting current. Inverter compressors have a soft start, but the backup element is a blunt resistive load and should be interlocked so it does not fight the PV control logic.

Noise and outdoor unit siting

The outdoor unit is the only part neighbours notice. Sound power level in dB(A) appears on the datasheet, but what matters legally is sound pressure at the boundary, which falls with distance and rises with reflective surfaces. Keep the unit off a corner between two walls, avoid pointing the fan at a neighbouring window, leave the manufacturer's clearance for airflow, and mount it on anti-vibration feet on a rigid base rather than directly on a house wall. A night quiet mode reduces fan speed and compressor frequency at the cost of a little capacity.

Key parameters compared

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