Smart Thermostats With Geothermal – What To Be Careful Of

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?

Rolls of conduit/UPVC pipework on the edge of a building site.
Pipework on the edge of a building site.

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 35-50°C, 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?

Most 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

Two Google Nest thermostats in store, the Learning Thermostat 3rd Gen and Thermostat E.
Two Google Nest thermostats (the Learning Thermostat 3rd Gen and Thermostat E)

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:

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.

About Tristan Perry

Tristan Perry is a software developer who is passionate about tech gadgets, DIY and housing. He has therefore loved seeing smart homes hit the mainstream. Tristan also has an academic background (in Math & Computer Science), and so he enjoys digging into the technical ways that smart home devices work.

Tristan owns close to a dozen Amazon Echo devices, way too many Philips Hue bulbs and lightstrips, a boat-load of Ring Cameras and Doorbells... and a bunch of other smart home devices too (from Reolink, Google Nest, GLEDOPTO and others).

If you have any questions, feedback or suggestions about this article, please leave a comment below. Please note that all comments go into a moderation queue (to prevent blog spam). Your comment will be manually reviewed and approved by Tristan in less than a week. Thanks!

14 thoughts on “Smart Thermostats With Geothermal – What To Be Careful Of”

  1. Hi
    I have a carrier Geothermal HVAC system and I wanted to upgrade the older looking carrier thermostats.
    I purchased a Honeywell T6(because that supposedly supports geothermal) and I have followed all the instructs and tutorials, but I can’t get the heat to come on…just air blows…….I haven’t tried the cooling becasue I have been hung up on the heating…..I can send photos of how I have it wired and also describe the ISU settings…….
    Let me know if you can help me

    • Hi George,

      Unfortunately I haven’t used the T6 myself so I can’t be too much help sorry – but this comment is now live, so if anyone else knows the answer, please do reply back to help George – thanks 🙂

  2. The main benefit to using a smart thermostat with a geothermal system is not to program temperatures going up and down. But there are at least two other great reasons to use one: the ability to place sensors anywhere in the house, instead of being tied to the lone sensor on the wall unit itself, and the ability to control other variables, such as humidity. When I first moved into my current home in 2003, two thermostats were installed on walls which communicated directly with the ATTIC upstairs. So during the winter I would set my thermostat at 59 and the house would be in the high 80s, and I eventually resorted to manually running those thermostats until an insulation company fixed the ridiculous problem. I still have other issues with poor thermostat placement which we are finally fixing. So no matter how awesome your system is, if the thermostat is placed in a terrible location, the system will never work properly. We fixed this with ecobee4 sensors placed in the middle of rooms (away from hot or cold walls) we actually use.

  3. Thermostat placement has been our biggest problem too. Our geothermal unit uses tubing circulating through two holes drilled ~120m into bedrock (rather than the shallow buried tubing). It provides heat and AC (plus hot water) via forced air ducts from the original oil furnace for our heavily-renovated home (~200m²).

    The thermostat was installed with the unit in 2006, but was placed in out kitchen. During the winter, warm air from a woodstove used in an adjacent room circulates through that area, so the thermostat “thinks” the heating (and therefore fan circulation) isn’t needed, so it results in other rooms getting cooler than we like.

    We’ve found that the best strategy is to keep the fan on Circ (instead of Auto) so the wood stove heat is moved around the house. I’ll look into the ecobee4 sensors mentioned above by Ed, as this looks like it might work better for our situation.

    • Thanks for the comment, interesting that keeping fans on Circ have worked well for you (but yes, the ecobee sensors are pretty sweet too – it’s all extra data for the smart heating setup, after all).

  4. There is another reason why I want a smart thermostat for my geothermal unit other than changing temperatures during the day. I travel frequently and there is no reason to cool my house to a comfortable temp when I’m gone for days or weeks. I also do AirBnB at times. In either case it’s nice to be able to change the temperature from anywhere. I do agree with your basic premise as my Waterfurnace has been basically within a few degrees of the last 20+ years (except when I’m away several days).

  5. We have a geothermal system sized correctly for our 4200 sq/ft home, have manual zone controls in HVAC room for Basement, 1st and 2nd floors.
    I agree with Set it an Forget It, but.
    It’s wintertime here in Indiana and I like the temperature lower (66) when we sleep. During the day my wife likes the temperature higher (74+). This is an 8-degree swing.
    We have agreed to 68 at night and 70 during the day, for now. I do this by setting our programmable thermostat. I have the breaker for the Emergency Electric Heat (backup) off. I know not a good idea, but it has worked for the past few years. I turn it back on when we leave on any trips.
    What I would like to do is set it to 66 at night and 74 during the day.
    What is the electric cost to change the heat from 66 to 74 in the morning?
    What is the electric cost difference to keep it at 74 vs 70 during the day?
    What is the savings to lower the heat from 74 or 70 to 66 at night, 8 hours (11pm 66 to 7am 74, no recovery time added in, which is fine.)
    I have not found a calculator that can tell me this.
    Cost is only part of this. I cannot sleep when it’s 74, its too hot. I have been looking at programmable air vents (FLAIR SMART VENT) for the bedroom to help sleeping if there is no cost savings and the house needs to be left at 74.
    Any advice? Thank you.

    • Thanks for the comment Thomas, and your question/issue makes sense. Unfortunately I’m currently not best placed to answer your questions (I’m a Brit and haven’t had access to hands-on geothermal testing for a while), but your comment is approved – and hopefully someone else will answer your questions soon.

    • If you raise the temp at 1 degree increments, that should allow the geothermal time to raise the temp without triggering the auxiliary heat. This is what I do manually. It takes time though, so usually I’m only adjusting the temperature a couple degrees between nighttime and daytime settings.
      With a/c I don’t think it matters as there is just one cooling setting and no auxiliary cooling? Am I correct on this?

  6. Good article. I’ll chime in a little late but the reason I want a smart thermostat is to notify me to when my auxiliary kicks in. We have an 1850s house and the largest geothermal system we could put in is a 6 ton. With no exterior wall insulation and leaky windows the auxiliary kicks in at about 18deg F or lower, depending on the wind and the time of winter. We have a supplementary pellet stove that I run to keep the auxiliary heat from kicking in, but I have to turn it on manually. I want to be notified so that I can choose to kick on my pellet stove, reduce my indoor temperature or let the auxiliary heat run. I wish I had a setting where I could manually turn off my aux heat. Maybe I’ll just turn off the breaker for my Aux heat when I’m not away from home.

    • Thanks, and that’s definitely a good use case for smart thermostats. I hope you can find a good solution to the aux power at some point (although as you say, manually cutting the power could work for now).

  7. My brand new waterfurnace unit is wired off of 2 breakers. A 60amp for the main unit and a 100amp for the auxiliary heat. Oddly, it turns out that my unit operates off both breakers so I can’t simply turn off my 100amp breaker so my auxiliary never kicks in when I’m home. I will have to get a smart thermostat and hopefully it will alert me when my aux heat kicks in, and if I’m lucky, allow me to prevent aux heat from turning on when I’m home. I’ll try to remember to post my findings.


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