Although heat pumps are now standard across much of the UK newbuild market, there is sometimes still some confusion around how room controls interact with a heat pump system, particularly when a buffer tank is involved, as Fausto Grilo, Technical Director, ESi Controls, explains.
The main point is that in a typical buffer tank setup, the room controls are not controlling the heat pump; they are controlling the distribution of heat within the property. That distinction matters, because it clears up a lot of misunderstandings around compatibility, run times and system operation.
System layout
In most modern new-build heat pump installations, the system is split into two separate but linked parts: the heat pump and its own manufacturer controls; and the controls for central heating in the different zones within the property.
The heat pump retains its own dedicated controller (the human to machine interface or HMI), supplied by the heat pump manufacturer. That controller manages compressor operation, flow temperatures, defrost cycles, domestic hot water production and all of the internal logic required for the heat pump to operate efficiently.
Alongside this, the home may have one, two or multiple zones, controlled by room thermostats and motorised valves. These are usually supplied separately to provide the homeowner with simple room-by-room temperature control.
The important point is that in a buffered system, these two control layers operate independently.
So, what’s the buffer tank for?
A buffer tank acts as a thermal store for the central heating circuit. The heat pump keeps this store of water at the required temperature, while the heating system draws from it as needed.
Think of it as a reservoir of heated water sitting between the heat pump and the house.
When a room thermostat calls for heat, the thermostat opens the relevant zone valve(s), the wiring centre enables the circulating pump and heated water is circulated from the buffer tank through the emitters (radiators or underfloor heating). Then, cooler return water comes back to the buffer tank, where it can be warmed up again to the required temperature.
At no point in this sequence does the room thermostat directly instruct the heat pump to start or stop. The heat pump simply monitors the temperature in the buffer tank using its own sensor. If that stored water temperature drops below its set target, the heat pump automatically tops it back up. The room controls and the heat pump are performing different jobs.
Why this matters to installers
Once you understand the hydraulic separation, wiring becomes much easier to interpret.
In a standard new-build installation with a buffer tank:
- The heat pump talks to the buffer tank sensor
- The heat pump controller decides when the unit runs
- The room controls switch valves and circulation only
In practical terms, the installer is usually wiring the thermostat system to a wiring centre, which in turn controls the zone valves and circulation pump. The heat pump remains connected to its own controls exactly as designed by the manufacturer.
This is why buffered systems have become so widely adopted in housebuilding: they simplify zoning while allowing the heat pump to operate as intended.
Why it’s important not to treat a heat pump like a boiler
A large proportion of heat pump performance issues come from applying boiler habits to heat pump systems.
With a traditional boiler, end users often switch heating on and off in short bursts, raise the setpoint sharply and expect a quick response.
However, heat pumps are different. They work most efficiently when operating continuously at lower flow temperatures, maintaining the building temperature steadily rather than repeatedly reheating it from cold. Frequent stop/start operation reduces efficiency.
This is one reason buffer tank systems work so well in central heating applications. They separate room demand from heat pump operation. Occupants can adjust room temperatures in the normal way, while the heat pump continues to operate according to its own internal algorithm.
What if there isn’t a buffer tank?
In some single-zone systems without a buffer tank, the room control may provide a simple external demand signal to the heat pump. Even in these cases, the external control is typically only a demand input, asking for heat. It is not controlling the heat pump’s compressor speed, run profile or internal operating logic.
The heat pump manufacturer’s controller still determines how long the unit runs, how it modulates output, how it protects itself from short cycling and how domestic hot water is prioritised.
So, whether the system uses switched live, volt-free contacts or low-voltage signalling, the principle remains the same: the external thermostat indicates demand, while the heat pump controls itself.
Easier and more comfortable for the end user
The best setup is often the simplest one. The heat pump’s own controller can stay where it belongs, typically by the cylinder or plant cupboard, available for commissioning, servicing and advanced settings.
On the wall, the user sees only what they need day to day: a straightforward room thermostat with simple temperature adjustment and zoned scheduling, which may also be controlled using an app on a smart device. This provides a familiar user experience while preserving the manufacturer’s intended heat pump operation.
And finally…
For installers specifying or fitting controls on a buffered heat pump system, the main point to take away is that the room thermostat system is not managing the heat pump. It is managing the heating circuit downstream of the buffer tank.
That means when you add zoning controls, you are generally not replacing the heat pump controls or interfering with its operating strategy. You are simply adding comfort control for the occupied spaces.
Once that separation is understood, heat pump controls become far less intimidating. In many respects, the installation is simpler than it first appears – just a different hydraulic arrangement delivering the same end goal: efficient, comfortable heating.