Heat Pump Controls and Commissioning: Weather Compensation and Flow Temps

Quick Answer: A heat pump must be commissioned to run at the lowest flow temperature that meets the design heat loss, using weather compensation so the flow temperature rises only as the weather gets colder. Commissioning under MCS MIS 3005 includes setting the heating curve (weather compensation), balancing the system, configuring DHW and anti-legionella cycles, verifying flow rates and the temperature differential (typically ΔT 5°C), and documenting the results. Correct commissioning is the difference between the 3.5-4.5 SCOP the design promised and a real-world 2.5 from a unit left running like a boiler.

Summary

A heat pump can be sized and installed perfectly and still deliver poor efficiency and comfort if it is commissioned badly. The most common failure is leaving the heat pump set to a fixed high flow temperature — running it like a gas boiler — instead of using weather compensation to run at the lowest flow temperature the conditions require. Every degree of unnecessary flow temperature costs efficiency: the rule of thumb is roughly 2-3% loss of efficiency per degree Celsius of flow temperature. Commissioning is where the design's promised SCOP is either realised or thrown away.

The defining control strategy for heat pumps is weather compensation (sometimes called load compensation or the heating curve). Instead of a fixed flow temperature, the controller varies the flow temperature according to the outside temperature: mild day, low flow (high efficiency); cold day, higher flow (more output when needed). This keeps the heat pump running steadily at the minimum effective temperature, maximising SCOP while maintaining comfort. Setting and fine-tuning this curve is the central commissioning task.

This article explains the commissioning process, weather compensation and the heating curve, flow temperature and ΔT, system balancing, DHW and anti-legionella, controls philosophy (run it low and slow, not boiler-style on/off), and the MCS documentation requirements. It follows on from heat pump sizing heat loss and mcs heat pump standards; see also heat pump servicing, radiator balancing and low temperature design.

Key Facts

Quick Reference Table

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Parameter Heat Pump Target Boiler Comparison
Flow temperature 35-50°C (design-set) 70-80°C
ΔT (flow − return) ~5°C ~11-20°C
Control strategy Weather compensation, continuous On/off thermostat
Operation Low and slow, long run times Short high-output bursts
DHW temperature 50-55°C + periodic 60°C 60°C+
Night setback Minimal / none Deep setback common
Efficiency sensitivity ~2-3% per °C flow temp Low

Detailed Guidance

Weather compensation and the heating curve

Weather compensation is the heart of efficient heat-pump control. An outdoor sensor feeds the controller, which uses a heating curve to set the flow temperature: as the outside temperature falls, the flow temperature rises, and vice versa. The curve is configured so that at the design external temperature the flow reaches the design flow temperature (the value the emitters need to meet the heat loss), and on milder days the flow drops well below it.

Setting the curve is a balance: too low and the house under-heats in cold weather; too high and efficiency is wasted in mild weather. Commissioning sets the curve from the design figures, then fine-tunes it over the first weeks based on comfort feedback — typically nudging the curve down as far as comfort allows, because lower is always more efficient. This iterative tuning is part of good practice and should be explained to the customer.

Flow temperature — commission low, not boiler-high

The single biggest commissioning error is leaving the heat pump at a fixed, high flow temperature (e.g. 55-60°C) "to be safe." This runs the unit like a boiler and destroys the SCOP. The heat pump should be commissioned to the design flow temperature from the heat loss/emitter calculation — often 35-45°C with adequately sized emitters or underfloor heating, up to ~50-55°C only where small existing radiators are retained. Because efficiency falls ~2-3% per degree, the discipline of commissioning to the lowest viable flow temperature directly determines the customer's running costs.

ΔT and flow rate

Heat pumps are designed around a smaller temperature differential than boilers — typically ΔT 5°C (flow minus return) versus a boiler's ~11°C. This means heat pumps move more water for the same heat output, so pipework and flow rates must be adequate, and the circulating pump set correctly. Commissioning verifies the flow rate meets the manufacturer's minimum (too low triggers faults and poor defrost behaviour) and that the actual ΔT is close to design. An incorrect ΔT signals a flow problem, a balancing problem, or insufficient emitter surface.

System balancing

With weather compensation, all emitters receive the same flow temperature, so each must be balanced to draw the right flow for its output — otherwise near rooms overheat and far rooms stay cold. Balancing adjusts the lockshield valves (and UFH manifold flow setters) so every emitter reaches temperature together at the design ΔT. Heat-pump balancing is more critical than boiler balancing because there is no high flow temperature to mask poor distribution. See radiator balancing.

Buffers, volumisers and defrost

Air source heat pumps periodically run a defrost cycle — briefly reversing to melt frost off the outdoor evaporator. This draws heat from the system, so there must be enough water volume in the circuit to supply the defrost without large temperature swings or the unit faulting. Where the system pipework volume is below the manufacturer's minimum, a buffer tank, volumiser, or low-loss header is added to provide volume and hydraulic separation. Commissioning confirms defrosts complete cleanly without nuisance cold blows or lockouts.

Domestic hot water and anti-legionella

The heat pump reheats the hot water cylinder, typically to 50-55°C (higher than this becomes inefficient for a heat pump). Because heat pumps run the cylinder cooler than a boiler, anti-legionella protection is essential: a periodic pasteurisation cycle raising the cylinder to ≥60°C (often weekly, sometimes using the immersion as a boost), in line with HSE L8/HSG274 guidance on Legionella control. Commissioning sets the DHW setpoint, the reheat schedule (ideally timed to off-peak or warmer parts of the day), and the legionella cycle. See heat pump cylinders.

Controls philosophy and setback

Heat pumps reward continuous low-output running, not the boiler habit of deep night/away setbacks followed by a hard reheat. Recovering a deeply cooled house forces the heat pump to its least efficient high-output, high-flow-temperature mode. Modest or zero setback, with weather compensation maintaining a steady low flow, is usually more efficient and more comfortable. Part of commissioning and handover is re-educating the customer away from "turn it off when out / blast it when cold" boiler behaviour.

Documentation and handover (MCS)

MIS 3005 requires documented commissioning: the flow temperature and heating curve set, flow rates and ΔT verified, balancing completed, DHW and legionella configured, and the results recorded in a commissioning sheet. The handover explains the controls, the low-and-slow philosophy, and maintenance. This documentation supports the MCS certificate, the BUS grant, and any future fault diagnosis or service.

Frequently Asked Questions

What flow temperature should a heat pump run at?

The lowest flow temperature that meets the design heat loss — usually 35-45°C with properly sized emitters or underfloor heating, rising to perhaps 50-55°C only where small existing radiators are kept. It should never be left at a fixed high temperature like a boiler. Weather compensation should vary it: low on mild days, higher only as the weather turns cold. Every unnecessary degree costs roughly 2-3% efficiency.

What is weather compensation and do I need it?

Weather compensation automatically varies the heat pump's flow temperature according to the outside temperature, using a "heating curve" — low flow (high efficiency) on mild days, higher flow only when it's cold. It is essential for heat-pump efficiency and is the central control strategy; running a fixed high flow temperature instead can drop the SCOP from 4 to under 3. Commissioning sets and tunes the curve.

Should I turn my heat pump off when I'm out or at night?

Generally no — heat pumps are most efficient running continuously at low output. Deep setbacks or switching off mean the unit has to reheat the house hard later at its least efficient high-flow mode, often using more energy than steady running would have. Modest or no setback with weather compensation usually gives lower running costs and better comfort than boiler-style on/off habits.

Why does my heat pump need an anti-legionella cycle?

Because heat pumps run the hot water cylinder cooler than a boiler (typically 50-55°C) for efficiency, and Legionella bacteria can grow in water below 60°C. A periodic pasteurisation cycle raises the cylinder to at least 60°C (often weekly), killing the bacteria, in line with HSE Legionella guidance (L8/HSG274). It's set up during commissioning and is a safety requirement, not optional.

Regulations & Standards