A building owner in Surrey recently replaced two ageing gas boilers with a pair of commercial air source heat pumps. Twelve months later the energy bills were barely better than before. The heat pumps were working — flow temperatures hitting target, no fault alarms — but the COP was hovering around 2.3 when the equipment was rated for 3.8. The cause, identified during a controls audit, was straightforward: the BMS was sending a fixed 60°C flow temperature setpoint regardless of outside conditions. The heat pumps were being driven exactly like the boilers they replaced, and punished for it in efficiency terms every single day.
This is the most common heat pump commissioning failure we see. A heat pump is not a boiler with a heat exchanger on the outside — it is a fundamentally different machine that requires fundamentally different controls to deliver the efficiency it promises. The good news is that the controls fix is not expensive. The bad news is that without it, the investment in the heat pump itself is largely wasted.
A heat pump's Coefficient of Performance (COP) is the ratio of heat output to electrical input. A COP of 3.5 means the unit produces 3.5 kW of heat for every 1 kW of electricity consumed. But this figure is not fixed — it varies continuously with the temperature of the heat source and the flow temperature the unit is asked to deliver to the heating system.
The physics are straightforward: the larger the temperature lift between source and flow, the harder the compressor works and the worse the efficiency. CIBSE AM17:2022 — the technical guidance for heat pumps in large non-domestic buildings — quantifies this precisely: each 1K reduction in compressor lift raises COP by 2–3%. An ASHP asked to deliver 55°C flow at 7°C outside achieves a COP of around 2.5–3.0. The same unit delivering 40°C flow at the same outside temperature achieves COP 4.0–4.5. That 15°C difference in flow temperature, which the building barely notices, costs or saves roughly 30–40% of the operating electricity bill. Rated COP figures quoted by manufacturers are measured at the test conditions defined in BS EN 14511:2018 — typically 7°C outside air and 35°C or 45°C flow temperature; real-world performance diverges from rated conditions the moment the system is commissioned to a different setpoint. BS EN 14825:2022 sets the methodology for calculating Seasonal Coefficient of Performance (SCOP) — measuring performance across a full heating season rather than at a single rated condition, which is why a unit rated at COP 3.8 may deliver a seasonal average of 2.8 or 4.2 depending entirely on how the controls are set up. Over a year on a commercial site, that difference is typically tens of thousands of pounds — and a substantial gap in carbon emissions.
The BMS is the mechanism that exploits this. By continuously adjusting flow temperature setpoints based on outside conditions, it keeps the heat pump running in its high-efficiency range rather than always targeting the worst-case design condition.
Weather compensation automatically modulates flow temperature as outside air temperature changes — lower flow when it's mild, higher flow when it's cold. For a gas boiler, weather compensation is a useful efficiency improvement. For a heat pump, it is the primary control mechanism. Without it you are not operating a heat pump efficiently; you are running an expensive electric boiler.
A typical commercial ASHP weather compensation curve runs from around 50°C flow at -5°C outside air down to 35°C at 15°C outside. The slope and end-points must be set during commissioning to match the building's actual heat loss — a poorly insulated 1980s office block needs a steeper curve than a well-insulated modern development. Getting this wrong means either under-heating on the coldest days or driving unnecessarily high flow temperatures on mild ones. The BMS reads the external temperature sensor continuously and sends the calculated setpoint to the heat pump via its Modbus or BACnet/IP interface. The curve itself needs monitoring and fine-tuning after the first heating season — the design assumption rarely matches real-world building performance exactly.
Most commercial heat pumps from manufacturers including Mitsubishi, Daikin, Stiebel Eltron, Kensa, and Dimplex communicate via Modbus RTU or BACnet/IP. A well-configured BMS integration covers: a weather-compensated flow temperature setpoint written to the heat pump continuously; enable/disable control tied to occupancy schedules; operating mode selection where the unit is reversible (heating, cooling, DHW priority); fault status monitoring with alarm codes translated into readable descriptions; and energy metering via heat meters on the flow/return pipework alongside an electricity meter on the heat pump supply to calculate real-time COP. For a guide to best practices when integrating heat pumps and other HVAC plant into a BMS, see our article on HVAC controls integration best practices.
That last point matters more than most commissioning engineers acknowledge. You cannot verify actual system performance without both thermal output and electrical input measurements. Real-time COP trending in the BMS is one of the most useful early indicators of developing faults — a gradual reduction in measured COP alongside increasing compressor current is typically the first sign of refrigerant loss, long before a low-pressure alarm triggers.
Commercial heat pumps need hydraulic separation from the distribution system to run efficiently. A buffer vessel — typically sized at 15–20 litres per kW of heat pump output — acts as a thermal flywheel, allowing the heat pump to run for extended periods at optimal conditions rather than cycling on and off as the small volume of the primary circuit saturates.
The BMS control strategy for the buffer vessel should allow it to float within a temperature band — typically 38–48°C for a low-temperature underfloor or fan coil system — rather than controlling to a fixed target. The heat pump runs until the upper limit is reached, stops, and is locked out until the lower limit is hit. The band width determines run duration: too narrow and you get short cycling with compressor wear; too wide and you lose comfort control. A minimum run time of 20–30 minutes should be enforced by the BMS regardless of temperature as an additional anti-cycling safeguard. Where multiple heat pumps feed a common buffer in parallel, the BMS sequences units using lead/lag rotation to equalise compressor run hours, staging the second unit on only when the first is approaching full capacity.
Air source heat pumps in cold, damp conditions — typical UK winter weather — periodically reverse their refrigerant cycle to defrost the outdoor coil. A defrost cycle typically lasts 2–5 minutes and occurs every 30–90 minutes during sustained cold periods. During defrost, the unit briefly delivers less heat and may draw higher current than normal. If the BMS is not programmed to recognise this, it generates nuisance alarms — "heat pump not meeting setpoint" or "low flow temperature" — that erode trust in the alarm system and lead operators to ignore genuine faults.
Defrost frequency monitoring is also a useful diagnostic. A unit defrosting every 20 minutes in mild weather, when ice formation should be minimal, indicates low refrigerant, a blocked coil, or a failing defrost sensor. The BMS should log defrost events with timestamps and generate a maintenance alert when frequency exceeds a configurable threshold.
Where a heat pump provides domestic hot water, Legionella control is a legal obligation under HSE Approved Code of Practice L8 (Legionnaires' Disease: The Control of Legionella Bacteria in Water Systems). L8 requires that regular thermal disinfection cycles raise the entire water volume to 60°C or above for at least one minute. Standard heat pump flow temperatures of 45–55°C do not achieve this without supplementary heating — typically an electric immersion element. The BMS schedules weekly disinfection cycles outside occupied hours, logs each cycle with the achieved temperature and duration to provide the compliance audit trail that L8 requires, and times the cycle to align with off-peak electricity tariff periods to minimise cost.
Approved Document Part L requires that heating systems in new and substantially refurbished commercial buildings are provided with controls that minimise energy consumption. For heat pump systems, this specifically includes weather compensation and optimum start — both delivered through a properly configured BMS. Buildings seeking MCS certification must also demonstrate adequate controls provision. CIBSE TM65 and BREEAM-referenced net zero pathways increasingly require verified operational COP data over a full heating season — something only achievable through BMS-integrated performance monitoring. For a broader view of how BMS technology supports decarbonisation strategies, see our article on the path to net zero with BMS technology. For buildings where the heat pump installation is part of a wider MEES compliance programme, our guide to MEES compliance and BMS EPC deadlines covers how heat pump and controls upgrades affect SBEM scoring.
The failures we most frequently find when auditing heat pump installations commissioned by others are: fixed high flow temperature setpoints used instead of weather compensation; buffer vessel control targeting a single-point temperature rather than a band; defrost cycles absent from alarm logic, generating hundreds of nuisance alarms; Legionella disinfection runs either missing entirely or scheduled during peak occupancy hours; and no COP metering — the site has no idea whether the system is performing to specification. Any one of these individually costs money. Together they can make an otherwise viable heat pump installation look like a failed experiment.
If your building has heat pumps installed in the past two to three years and there has been no controls audit since commissioning, the chances are one or more of these issues exist in your system. The audit is straightforward and typically identifies the configuration changes needed in a single site visit. For a new heat pump installation, the BMS controls specification should be agreed before equipment is ordered — not added as an afterthought at commissioning.
Alpha Controls commissions heat pump BMS controls across London, Kent, Essex, and the South East, integrating systems from Mitsubishi, Daikin, Stiebel Eltron, Kensa, and other commercial platforms. We carry out controls audits on existing installations and design BMS integration for new projects from pre-order through to handover and performance verification. Contact us to discuss your heat pump controls project, or see our BMS services and commissioning pages.
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