Achieving the highest efficiency from your heat pump requires attention to your radiators. When set correctly, the pump will run more smoothly, with less noise and reduced 'short cycling,' leading to greater comfort due to more even temperature control in your rooms.

I would also recommend setting your air source heat pump to 'Weather Compensation' and adjusting your 'Leaving Water Temperature' (LWT) to a moderate setting, starting with 35 degrees Celsius for the central area of the 'slope.' More guidance on setting 'LWT' can be found elsewhere; this article focuses exclusively on the 'Lockshield Valve' setting.

The lockshield valve acts as a 'flow regulator,' controlling the amount of heat supplied from the flow pipe into the radiator. It should not be confused with the Thermostatic Radiator Valve (TRV) fitted at the other end of the radiator. The TRV serves as a temperature limiter, reducing the flow of heat only in extreme circumstances, such as solar gain heating up a room. Otherwise, the TRV should not restrict the flow.

To begin setting up the valves, ensure all valves (both lockshield and TRVs) are fully open. Ideally, this task should be performed in colder weather to simplify the procedure. At this point, the heat pump can be turned on. Then, feel each radiator to determine which is warming up the most—make a note if necessary.

The radiators, especially if newly installed, should have been sized according to room size and calculated heat losses, along with the desired room temperature. Starting with the radiator that warmed up the most, gradually close the lockshield valves from room to room. Initially, you might close the valve by two full turns, with final adjustments usually being half a turn or less from the fully closed position. Continue adjusting each lockshield on the next warmest radiator.

Allow time for the radiator temperatures to settle after each round of adjustment. If some radiators remain warmer than others, they should be the first to be closed further. This process, requiring multiple steps of adjustment, aims to restrict flow minimally until each radiator sufficiently heats the room, allowing the rest of the heat to flow to other radiators. For instance, we prefer a warm bathroom and keep the lockshield valve fully open on this radiator, which is the first fed from the secondary pump circuit, keeping the room very cosy.

Once the radiators provide proportionate amounts of heat for the rooms, allow the system to settle for about a day. Then, you might find some lockshield valves need a bit more tweaking. Proper adjustment takes time; although installers may make initial adjustments, fine-tuning to your comfort level is crucial.

Consider using thermometers in rooms and a temperature probe on radiators to gauge progress. An infrared handheld thermometer can also assist in assessing radiator temperatures and the efficacy of your adjustments.

If using a room thermostat, ensure it's correctly set so the radiator in that area warms the room to, but not above, the set temperature. Setting the thermostat slightly higher than desired can initially help avoid frustration. Once radiators are balanced, return to the heat pump controls for any final adjustments.

TRVs should now be set to start shutting at approximately two degrees above the required room temperature. Under normal circumstances, TRVs will not disrupt the heating system but will activate if the room experiences solar gain or another heat source.

An efficient heat pump system prefers a constant load and resistance to water flow; while a lockshield valve provides consistency, a frequently adjusting TRV can reduce the system's efficiency and increase operational costs.

*Note: Installers might set a low temperature heat pump to a leaving water temperature of 45 or even 50 degrees Celsius by default, even though a lower temperature is more efficient and still provides adequate heat.

Please note: While I am not an expert, plumber, or heat pump installer, I had an ASHP installed in 2023 and have carefully adjusted it after the initial commissioning. This required waiting for colder weather in December 2023 to fine-tune the lockshield valves and pump speed, achieving a COP well above 4 most of the time.

At installation, the smart controller was not available, so a basic thermostat was installed. Setting this thermostat a few degrees above the required temperature prevented frequent on/off cycling. After installing the smart controller, the temperature is maintained within 0.6 degrees of the desired level. I also repurposed a programmable TRV from a previous gas boiler system for the bedroom radiator, allowing for temperature adjustments at night, while the other nine radiators remain with TRVs fully open.