Room Thermostats, Zone Valves and ASHP efficiency - what we did

 As part of our new build's design, the underfloor heating (UFH) was zoned and every room has a room thermostat.

Section 8.2 of the Scottish Government's 2022 "Domestic Building Services Compliance Guide for Scotland" states in "Table 25: Recommended minimum standards for control of wet and electric underfloor heating systems" that "Each room should have its own thermostat, sensor or programmable thermostat." and we have exactly that.



However, it is generally accepted that zoning is not a good setup to allow an air source heat pump (ASHP) to run efficiently. Zoning restricts how much of the UFH system is used to heat the house.

As part of our ASHP installation, we specified two key things:

  • the ASHP should have a controller in the main living area
  • the UFH room thermostats should not be used to control the ASHP
and that's what we have.

Our Mitsubishi ASHP has a wired Mitsubishi controller in our main living area and the system is configured to use Mitsubishi's "Auto Adaption" aka "Room Temperature" mode.

We have a Heatmiser neoStat v3 wired room thermostats in our rooms and they're linked back to a Heatmiser UH8 wiring centre. We've also got a neoHub Gen 3 which we can use to set all the room thermostats.

A key part of the installation is that the Heatmiser heat demand/boiler enable is NOT connected to the ASHP. We don't use the Heatmiser thermostats to control the heating. We do use them to stop any potential overheating but we don't expect that to ever kick in.

Using the Heatmiser NeoApp, we've set all the room thermostats to 24C which is warmer than we ever want the house to be.

Using the Mitsubishi MELCloud Home app, we've set the ASHP's target room temperature to be 20C during the day and 19C overnight.

This means that the Heatmiser system is always(*1) asking for heating and, this is the key point, all the zone valves are open allowing use of the entire UFH system.

As far as the ASHP is concerned, it uses its room controller to maintain a comfortable 20C and the entire UFH system is open and flowing. This allows the ASHP to operate at maximum efficiency.

What's the downside of all this? The main one is that we heat the entire house all the time. That's not a big issue though because it's very well insulated and so the heat loss is low. 

(*1) we do have quite a lot of solar gain so sometimes the room temperatures go over 24C and the Heatmiser closes some of the zone valves. However, as that's well above the ASHP's maximum target room temperature it's already turned off its heating.


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