Summary: 3rd party thermostats or smart TRVs are very likely to result in high energy bills and can lead to early failure of your heat pump system.
What are 3rd party thermostats or smart TRVs?
3rd-party thermostats are thermostats that are not from the same manufacturer as your heat pump. Typical brands include Heatmiser, Hive, Salus, Nest, Netatmo, Honeywell, and more.
Smart TRVs are radiator valves that you can program to a certain temperature via your phone.
They were designed to control boilers, but are not good for heat pumps.
3rd party thermostats and smart TRVs are very likely to result in poor performance of your heat pump
Here are the core issues with these devices:
They can only provide on/off signals to the heat pump. That means your heat pump doesn’t know how warm it is in your house. If it gets an on signal, it will turn on. But it doesn’t know how much it needs to be on to keep you comfortable. As a result, the heat pump will run too hard (which is inefficient) until the house overshoots it target. It will then get an off signal, turn off, and the house cools down. A bit later, the room stat or TRV notices the temperature has dropped too much and will fire the heat pump. The cycle repeats… This is not how heat pumps are supposed to work. It’ like driving a car at full throttle, but also having the break on.
In many cases, they don’t even provide on/off signals and don’t talk to the heat pump at all. So the heat pump will run even if none of the stats or TRVs are calling for heat. Sounds crazy, but we often see this.
Room stats and TRVs are often used to create “zones” in the home. When you have multiple zones, parts of the heating system get isolated from the heat pump. This results in a reduction in the water volume in your heat pump system, which creates “cycling” (the heat pump turning on/off rapidly). Zoning also means that your heat pump needs to run at higher flow temperatures, which is inefficient (and therefore costly).
It sounds counterintuitive, but having many “zones” in the house results in higher energy bills with heat pumps, not lower bills!
This results in:
Inefficient heat pump performance, meaning higher bills. The heat pump is operating harder than it should, in an inefficient operating mode.
Shorter lifetime of the heat pump. The heat pump turns on and off too often, which can lead to early failure.
Comfort issues. Because of the on/off behaviour, your indoor temperature will zigzag up and down around your target temperature, instead of just staying at the target.
Read [ here ] how Havenwise works if I have 3rd party thermostats or smart TRVs.