Common Commissioning Mistakes With Air Source Heat Pumps (UK Guide)

Common Commissioning Mistakes With Air Source Heat Pumps

A heat pump can be designed correctly and installed neatly, and still perform badly. The reason is usually commissioning the final stage where the system gets configured and tuned for the specific property it is going into.

In a significant number of systems reviewed by independent technical advisers, commissioning is where things have gone wrong. Sometimes it is an oversight. Sometimes it is done quickly without proper verification. Either way, the impact on comfort and running costs can be substantial.

What Commissioning Actually Covers

Commissioning is more than switching the system on and making sure it produces heat. It involves setting flow temperatures, adjusting flow rates, configuring weather compensation curves, balancing the radiator circuit, and verifying how the system performs under real conditions. Each of those steps affects the others.

Done properly, commissioning tailors the system to the property's specific heat loss, radiator output, and usage patterns. Done poorly or skipped the system runs on default settings that may not suit the building at all.

Mistake 1: Flow Temperature Set Too High

Setting the flow temperature higher than necessary is probably the most widespread commissioning error. It usually happens because an engineer wants to make sure the house heats up quickly during the handover visit so they push the temperature up, the client is satisfied, and the system is left running that way permanently.

The problem is that every extra degree of flow temperature reduces the heat pump's efficiency. A system running at 55°C when 45°C would be sufficient is using meaningfully more electricity than it needs to, every single day.

Mistake 2: Weather Compensation Not Configured Properly

Weather compensation is one of the most effective tools for reducing heat pump running costs. It adjusts the flow temperature automatically based on how cold it is outside so the system runs warmer on the coldest days and cooler when the weather is mild.

When the weather compensation curve is not set up correctly, or is disabled entirely, the system runs at the same temperature regardless of outdoor conditions. On mild days that means the heat pump is working far harder than the property requires.

In many of the systems we review, this setting has either been left on default or switched off entirely at handover. If you want to check your own system, our step-by-step guide on how to set weather compensation on a heat pump explains what to look for and how to adjust the curve based on how your home is responding.

Mistake 3: Incorrect Flow Rates

Heat pumps need water to circulate at a specific rate. Too little flow and the heat pump cannot transfer heat effectively the system short-cycles, efficiency drops, and the unit can fault. Too much flow can also cause problems, including a reduced temperature differential across the heat exchanger and increased pump energy use.

Flow rate should be set and verified during commissioning, not estimated or left at a default value.

Mistake 4: Radiators Not Balanced

If the radiator circuit has not been balanced, some rooms will overheat while others struggle to reach temperature. This problem is particularly common when a heat pump replaces a boiler, because the original radiator circuit was balanced for much higher flow temperatures.

At heat pump temperatures, the distribution of flow across the circuit matters more the system has less margin to compensate for imbalance through sheer temperature.

Mistake 5: Controls Set Up Incorrectly

Control configuration goes beyond just setting a target room temperature. It includes deciding how the heat pump responds to demand, whether it heats continuously or in cycles, how it interacts with any smart thermostat, and how the hot water cylinder is scheduled relative to space heating.

Incorrectly configured controls can cause short cycling where the heat pump turns on and off in rapid succession which both reduces efficiency and increases wear on the compressor. They can also lead to situations where the hot water cylinder and space heating compete for heat pump capacity at the same time.

Mistake 6: No Proper Testing Under Real Conditions

Some systems are commissioned quickly a brief check on a warm day, or during a period of low demand and signed off before the system has been tested under the conditions it will actually face in winter.

Issues that only appear at colder outdoor temperatures, or under heavy demand, will not be caught this way. The homeowner then experiences problems in the first cold snap and has to go back to the installer to investigate.

Why Commissioning Errors Look Like Other Problems

This is the part that causes most frustration. When a heat pump has not been commissioned correctly, the symptoms high running costs, rooms not reaching temperature, constant running look identical to the symptoms of a genuinely undersized system. Many homeowners spend months being told their system is too small, when the actual issue is that it was never configured to run properly.

A structured technical review can separate the two. If the system is correctly sized but poorly commissioned, fixing the configuration resolves the problem without any hardware changes.

If you are an installer who has identified a potential issue during commissioning and wants a second opinion before handing over to the homeowner, our Installer Technical Support Call is designed for exactly that situation — a direct conversation with an independent technical adviser to work through the specifics before they become a complaint.