The Nest Learning Thermostat proudly talks of supporting 95% of heating systems, whilst Ecobee also says that it supports “most” systems including geothermal. But do they work properly with geothermal, the green heat pump-based alternative?
A properly sized geothermal heating system shouldn't need frequent changes to its thermostat. Smart thermostats make constant changes to the thermostat. Therefore you need to be careful to avoid costly geothermal setbacks, and unnecessarily high energy bills.
What is geothermal heating?
The ground under our feet holds a fairly steady temperature because of how much solar energy the ground absorbs, but also because the earth is a naturally good insulator.
This means that you can dig a series of deep trenches or boreholes into your yard, and run pipes containing anti-freeze mixture through them. This could be pumped through the series of pipes, and heat will be collected from the ground. This heat energy can then be collected/'transferred' to a heat pump via a heat exchange which sits within the pipework system.
This works fine in winter because whilst the top of the ground might feel cold, the earth a few feet below has a more consistent and steady temperature – giving consistent heat which can be harvested to heat your home. Equally in the summer, heat from your house can be diverted back to the ground, acting as a cooling system.
Heat pumps work at around 95-122°F, and they are more of a ‘slow and steady' heating system. Just like underfloor heating, the idea of geothermal heating is that the system stays on for longer and providers a more gentle heat. Geothermal is also a very efficient way of heating, compared to gas-powered furnaces which are relatively wasteful and provide ‘short and sharp' spurts of heating.
Geothermal heating systems also contain an auxiliary heating source which is electric powered. A common myth is that this provides a quick ‘boost' to temperatures if it's extremely cold. In reality, a properly sized geothermal unit won't need this aux heating – it's actual use is as a backup incase the geothermal heating system fails entirely.
Which smart thermostats claim to support geothermal?
The best smart thermostats support geothermal heating systems, or at least they claim to! This includes:
Nest geothermal support
Nest's compatibility checker will help you find out exactly, but in general:
- The main Nest Learning Thermostat (3rd generation) says that it works with 95% of heating systems, including geothermal.
- The previous generation Learning Thermostat (2nd generation) also supports geothermal.
- The cut-down Nest Thermostat E works with around 85% of heating systems, which also includes geothermal.
In other words, all current Nest thermostats should support your geothermal system.
ecobee support
ecobee offers good geothermal support, with ways of specifying that yours is a geothermal heat pump in the app (under installation settings -> equipment -> heat pump).
Even older versions have been described as having “very solid” geothermal support – better than early Nest versions.
Hive geothermal compatibility
The UK based Hive range of smart thermostats do not support geothermal, only mainstream HVAC systems.
Emerson Sensi geothermal compatibility
Sensi's compatibility page is quite clear: both their Sensi Smart and Sensi Smart Touch thermostats support “Heat pump/geothermal” systems, with the ‘small text' caveat that “a common wire is required”.
Honeywell geo support
The Honeywell thermostat range have long supported geothermal systems, even though this is not mentioned much as a specific feature on their website.
Like Sensi, they support any geothermal system which have a common wire. Their website also has a compatibility page if you want to learn more.
Problems with ‘smart' thermostats controlling geothermal
Geothermal heating systems revolve around the idea of being correctly sized for the space they are heating, and working (‘being on') for a longer period of time, at a lower temperature.
Many systems are then supplemented with backup heating, either a separate heat source (such as gas or electric powered underfloor heating) or the built-in aux electric heating.
The issue with many early smart thermostats – especially Nest – was that they would still try and reduce (set back) the target temperature that the geothermal system would operate at, like gas-based heating systems.
The problem with this is that when the required target temperature is then increased (for example 15 minutes before everyone arrives home from work/school), geothermal systems take a much longer time to ‘catch up' – meaning that the backup system kicks in instead to give the temperature a boost.
This is undesirable, however, because this backup heating is more expensive to run.
So these early smart thermostats would effectively manage the geothermal system in a pointless way: running it too-low (since it was already correctly sized for the house) when everyone was out, and then relying on expensive backup heating when everyone was due to be back at home.
The solution to this is to ensure that any smart thermostat you have does not start programming set-backs (target temperature reductions) in your geothermal system.
The best smart thermostat for geothermal heating
Whilst I am doubtful as to whether you need a smart thermostat for your geothermal system (see below), if you are dead-set on being able to see (and manage, if required) your heating system from your phone, then naturally a smart thermostat makes sense.
In this case, people in the HVAC community are very positive about ecobee's range of smart thermostats. They had proper geothermal support a few years before Nest did, and even now they are described as being “pretty sharp” and having “lots of runtime statistics that Honeywell doesn't offer“.
Recommended ecobee geothermal settings
There are a couple of useful official guides showing how to best configure your ecobee to best manage your geothermal system:
- Ecobee support which shows how to select that yours is a geothermal heat pump.
- Ecobee's supplementary guide (pdf) which covers a few more settings you want, including compressor and aux heat min/max temperatures.
A really useful alternative guide from u/ziebelje shows the recommended settings you should use (after lots of trial and error on ziebelje's part), including:
- Aux Heat Max Outdoor Temperature: set to your balance point, which you can find out where the outside ground temperature cannot heat your home sufficiently – this can be seen by looking at your heat profile diagram.
- Heat Differential Temperature: 1.0°F
- Cool Differential Temperature: 1.0°F
- Compressor Min Outdoor Temperature: 0°F
- Compressor Min On Time: 5 minutes
- Aux Min On Time: 5 minutes
- Compressor Min Cycle Off Time: 600 seconds
- Heat Dissipation Time: 30 seconds
- Cool Dissipation Time: 30 seconds
The guide goes into more detail about each recommended setting, but the list is ordered from most important to least important – meaning that setting just the top few will have a greater overall impact than setting all of them.
Controversial opinion: you don't need a smart thermostat with geothermal
There, I said it. Phew, it's good to get that off my chest.
The whole point of a geothermal heating system is that it is properly sized to your house and the local climate, and the heat pipes are dug sufficiently low in the ground to harvest the exact amount of heat required – even in winter.
Geothermal is therefore a ‘set it and forget it' type system, unlike a gas-powered system which requires you to frequently adjust it when someone complains that it's too hot or too cold.
A smart thermostat is designed to learn your routines, and then slowly reduce the call for heating – and hence slowly reduce your energy bills by reducing the target temperatures in your home.
Therefore geothermal and smart thermostats are basically contradictory: one should be setup and then ignored, whilst the other works by constantly tweaking things.
If you have a specialized heating system (like geothermal), speak to your installer about whether a smart thermostat will really benefit you.
You will almost certainly be better off using a geothermal specific thermostat, even if that means that you can't ‘turn the heating up' on a phone app.
