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I recently had an air source heat pump installed and in two rooms the underfloor heating inexplicably stopped working. I changed the wall-stat on each and in one room the heating is now working while the other is not. The two room are close to the manifold. Can anyone suggest a reason for this fault?

In theory a heat pump can have a variable speed and so a variable output, but there is little gain doing this, where as the gas boiler with a variable output can gain latent heat from the flue gas, so you have likely moved from a modulating boiler to an on/off motor, and so the method of controlling the heater has moved from the return temperature of the water, to wall thermostats.

With gas the TRV is king, as it closes it pushes water first through those TRV's still open, and then the by pass valve opens and as the return water gets warmer the boiler first modulates (turns down) then start to cycle (turn off/on) and the wall thermostat was only there to turn off boiler on warm days.

The heat pump is very good with underfloor heating, as it only needs the water to reach 30ºC so it can work far more efficient than if it had to heat water to 70ºC for radiators, however clearly will not work if you have a mixture of under floor heating and radiators, or DHW, it can be configured only to heat DHW at set times, so normally runs at 30ºC and only runs at 70ºC for an hour or so, when it stops working the underfloor heating, as clearly don't want floor at 70ºC.

And there is no point heating water to 70ºC then blending it to reduce the temperature to 30ºC as the heat pump would not run as efficiently.

The problem is the same using multi-fuel heating, to install efficient controls costs too much, yes the wood burner works well, and the heat pump works well, but it takes a lot to claw back the £20k installation cost for an efficient system, so they cross their fingers and hope it is near enough.

Thanks for the response. Our underfloor heating system, prior to Heat Pump, was all electric/no wood burner (this is a conversion from an agricultural building) we have never had radiators. Taking this into account can you suggest an alternative reason for one room not getting heat?

In the main our liquid central heating system is piped in a parallel arrangement. to get the water through each section, there needs to be a differential in pressure, and we use valves to set this.

In the main with underfloor heating these valves are all together in a manifold with wired on/off controls, and often a sight glass of some sort to help adjust the zones.

But there are so many different manifolds used, but as with radiators often one area being cold is not because some thing wrong with that area, but other areas are allowing too much flow.

The big problem with UFH is it's so slow to react, but in theory be it UFH or radiators as each area reaches temperature the area should switch off so forcing more water though other areas, or the by-pass valve, however if some wall thermostat turns off the heating before all rooms are warm, then this will not work.

What seems odd is to swap from a conventional boiler to a heat pump without the heating being re-configured for the new system. You need to get installers back to sort it.