Smart thermostats with a heat interface unit

Why it usually works

From the thermostat's point of view, a heat interface unit is just something that provides heat when asked. The thermostat's job is to close a contact when the room is cold and open it when the room is warm, and the unit's controls respond to that.

That is why most smart thermostats that can replace a conventional room thermostat will work with an HIU. The complication is not usually the unit; it is the wiring and the building.

What to check before buying one

What is on the wall now, and how is it wired. Some systems use a simple switched pair, some use additional wiring for power, and the thermostat you want may expect one and not the other. Photograph the existing control and the back of it if you can see the terminals without removing anything live.

Whether anything in your lease or your heat supply agreement restricts changes to the heating controls. A few developments do, particularly where the unit was installed and is maintained by the network operator.

Whether your unit's own controls are integrated with the existing thermostat in a way that makes swapping it less simple. Some units have their own controller on the front and the room thermostat talks to it; some do not.

The thing that matters more than the model

Where the thermostat is.

A thermostat sited in a hallway next to a front door that opens onto a cold communal corridor will keep calling for heat long after every room in the flat is warm, because the hallway never gets warm. The result is a flat that overheats and a bill that reflects it, and no amount of scheduling from an app will fix it.

In a flat, the other common version is a thermostat on an internal wall next to a hot pipe run, which does the opposite: it thinks the flat is warm when it is not, and the heating cuts out early.

This is why moving a thermostat is often a better investment than upgrading it, and why we look at where it is before talking about what it is.

Getting the useful part out of it

The part of smart heating that actually changes a bill is the scheduling and, where it is available, room by room control. The app is how you reach those things, not the benefit in itself.

Which means setting it up properly on the day matters. A smart thermostat left on its factory schedule is a more expensive version of the thermostat it replaced. We set the schedule up with you before leaving rather than handing over a box and a login.

Scheduling a flat on communal heating

Flats behave differently from houses, and the schedule that suits a house is frequently wrong for a flat, which is the practical part of getting this right.

A flat has fewer external walls and is surrounded on several sides by other heated homes, so it loses heat more slowly and recovers faster. The deep setback that makes sense in a draughty house, letting the temperature fall a long way overnight, often achieves less in a flat because the flat was never going to get that cold.

That means shorter, better targeted heating periods usually beat a long low background, and the saving comes from not heating an empty flat in the afternoon rather than from dropping the night temperature by four degrees.

Room by room control, where you have it, is worth more in a flat than scheduling is. A bedroom that nobody is in until ten at night, and a living room nobody uses before six, are the two rooms where the money actually is.

One warning worth giving: do not schedule the hot water off on a heat interface unit as though it were a cylinder. In most installations the hot water is made on demand and there is nothing to store, so there is nothing to schedule and switching it off simply means no hot water when you open the tap.

We set the schedule up with you on the day, on your actual week, rather than leaving the factory default and an app to work out.