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Building Science

Does the Nest Thermostat Save Energy?

Three independent studies plus a white paper by Nest give the answer

Image 1 of 3
The Nest Learning Thermostat was introduced in 2011 and has been installed in hundreds of thousands of homes. But has it saved energy?
Image Credit: Energy Vanguard
The Nest Learning Thermostat was introduced in 2011 and has been installed in hundreds of thousands of homes. But has it saved energy?
Image Credit: Energy Vanguard
Natural gas savings for control group in the Nest white paper study. Notice that the graph is centered around zero.
Image Credit: Nest
Natural gas savings for Nest users in the Nest white paper study. Notice the shift to greater savings.
Image Credit: Nest

The Nest Learning Thermostat has been on the market for nearly four years now. One of the biggest things the Nest folks use as a selling point is energy savings. “Programs itself. Then pays for itself.” That’s the first thing you see when you go to the Nest homepage. But what do the data say? Three independent studies plus a white paper from Nest provide some answers. (GBA first reported on these three studies in a February 2015 news story.)

The first study was done on 185 Oregon homes that used heat pumps, and the study was conducted over one heating season (2013-14), with the report released in October 2014. Two nearly identical studies run by two different gas and electric utilities in Indiana looked at both heating and cooling in a total of 700 homes. The results were nearly identical, too. Finally, Nest published a white paper in which they analyzed the energy use of 735 Nest owners with gas furnaces and 624 Nest owners with electric cooling. (See links to all studies at end of article.)

Let’s take a look.

A study in Oregon

This one was focused on heating with heat pumps. The goal was to find a way to reduce the amount of electric resistance heat that typically is installed as a supplemental heat source in heat pumps. The Energy Trust of Oregon, which launched the research project, was looking for an alternative to their “advanced heat pump controls measure.” As I showed in my recent article on Michael Blasnik’s presentation on big data from Nest, there’s a lot of opportunity for savings in heat pump supplemental heating.

The Energy Trust installed the Nest thermostats in 185 homes, and 174 of those made it all the way through the study with the thermostat installed and working. The researchers looked at energy use in the heating season before and the heating season after installation of the Nest. They also interviewed the participants at the end of the study period.

Here what I think are two of the biggest results they found when they looked at the final data:

  • Homes averaged 12% savings of electricity used for heating after installing the Nest. (These numbers have been adjusted for weather.)
  • More than 60% of the participants found their homes “somewhat more comfortable” or “much more comfortable” while using the Nest thermostat.

Then there were some interesting things that fell out when they analyzed the data in other ways. They looked at all kinds of demographics — age, education level, housing type, geographic location, and much more — and here are a few that struck me as noteworthy.

  • Manufactured homes had the highest savings, almost twice as much as the overall average.
  • In homes where the Nest replaced a programmable thermostat, the savings were about twice as high as in homes where it replaced a non-programmable thermostat. The uncertainties were high enough, though, that the report says those numbers only “suggest” that the Nest does better when replacing a programmable thermostat.
  • The homes that used the most energy saved the most energy (1,785 kWh per year), whereas the group with the lowest incomes had the highest percent savings: 11.1% (1,654 kWh).

Although the results are encouraging, the Oregon report urges some caution:

We have some reservations about the reliability of the results from the subgroup analyses. Each subgroup comparison began with a relatively small sample of pilot homes and cut it into even smaller pieces to analyze. With such small samples to work with and so many comparisons that the team was interested in, there may have been random fluctuations in the data that resulted in observing spurious differences.

In other words, they feel more confident about the numbers based on all the participants, less so about the ones based on things like those who live in manufactured housing, those who used to have a different programmable thermostat, and those who floss religiously on Sunday, Tuesday, Thursday, and Saturday every single week. Also, the data cover only one heating season, so running it for multiple years would provide a better picture of what’s happening.

Still, one of the biggest takeaways here is that this study found good savings with the Nest thermostat connected to heat pumps. In their white paper, Nest discussed the difficulties of using setbacks with heat pumps and included a U.S. Department of Energy warning: “Programmable thermostats are generally not recommended for heat pumps.” That’s because many of them tend to cause the electric resistance heat to run more as the heat pump attempts to recover from setbacks.

Two studies in Indiana

These two projects were done by two different gas and electric utilities in Indiana. The Cadmus Group conducted both studies and wrote the reports. Both studies looked at gas use during heating season and electricity use during cooling season. One of the studies had 400 participating homes, the other 300.

The Oregon project was a pre- and post-installation study with the same group of homes. The Indiana studies were different. They compared the energy use in homes with a Nest thermostat to homes with a standard programmable thermostat (the Honeywell TH211) by looking at each relative to homes with non-programmable thermostats as the baseline.

Here are the main results from both studies:

  • For heating, the Nest showed ~13% energy savings compared to the baseline homes.
  • For heating, the programmable thermostat showed 8% and 5% savings compared to the baseline.
  • For cooling, the Nest showed 16% and 14% energy savings compared to the baseline homes.
  • For cooling, the programmable thermostat showed 15% and 13% savings compared to the baseline.

The Nest beat the baseline homes (non-programmable thermostat) by double digits for both heating and cooling. It also beat the standard programmable thermostat for heating. For cooling, the Nest and the programmable thermostat came out about the same.

A white paper from Nest

This white paper is a good read even without the energy savings information because the paper discusses a lot of the issues around trying to determine whether or not a given measure saves energy and how you would go about determining the amount of savings. (Read the Background and Methodology sections.)

The Nest energy savings analysis is different from both the Oregon and the Indiana studies. The researchers used “pooled” data from a control group of non-Nest users and compared that data to data from the Nest users. The first chart below shows percent savings for the control group. Notice that the center of the data is at 0% savings. In contrast to the control group, the data from the Nest users are shifted to the right of the 0% savings line, as you can see in the second chart.

The actual numbers they found for savings of natural gas for heating and electricity for cooling are:

  • 9.6% for heating (as a percentage of heating, not total energy use)
  • 17.5% for cooling (as a percentage of cooling, not total energy use)

The heating number is lower than the other studies, but the cooling number is about the same.

The Nest white paper discusses potential biases and the issues surrounding using data from the MyEnergy customers. In the end, though, they conclude that there wasn’t any large bias in the results.

Yes, the Nest does save energy

The results here provide a lot of evidence that the Nest Learning Thermostat does indeed save energy. It saves on heating energy with both gas furnaces and heat pumps, and it saves on cooling with air conditioners and heat pumps. All four of the reports go into the details of their methodology, telling how they chose the participants, how they collected and analyzed the data, and more. (Use the links below to download the papers.)

One important thing to point out here is that the Oregon and Indiana studies used carefully chosen participants, and they installed the Nest thermostats for them. One question that comes up with Nest data sometimes is, what is the effect of having self-selected gadget-geeks or energy savers on the savings data? In those three studies, that’s a moot point. It seems that the average Nest owner will save energy even when they’re not the kind of people who would go out and buy one on their own.

Of course, not everyone buys a Nest because it saves energy. Some people get them just so they can adjust the thermostat remotely from the app on their phone. Maybe it’s even because it’s flossing day, and they have to get up early.

The Four Reports

Oregon Study (pdf)

Indiana Study – NIPSCo (pdf)

Indiana Study – Vectren

Nest white paper (pdf)

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. Check out his in-depth course, Mastering Building Science at Heatspring Learning Institute, and follow him on Twitter at @EnergyVanguard.

8 Comments

  1. Malcolm Taylor | | #1

    Allison
    For us less technologically minded readers would it be possible to describe what the Nest is doing that a programmable thermostat is not? How is it saving the energy? What is it learning and how is it reacting to what it learns?

  2. User avater
    Dana Dorsett | | #2

    A signficant difference with the Nest
    Unlike most programmable thermostats, a Nest has: "... the ability to adaptively lockout a heat pump ’s backup electric resistance heat based on weather conditions. ", per the preamble to the Oregon study.

    That's huge for managing heat pumps, since it allows the use of setbacks without dramatically reducing average efficiency, saving money by the lower heat loss from the cooler room-temperature averages.

    The Nest monitors the occupant behavior to determine when and how much the occupants want the temperatures to be higher. It automatically steps back the temperature (in heating mode) periodically to "test" the occupant tolerances as it is learning, and tracks the time of day/week and the outdoor temps at which occupant adjustments are made, then ramps the temperatures appropriately in anticipation of the manual adjustment to the setpoints by the occupants. It's constantly testing and probing this way, but after the first few days direct occupant interactions with the thermostat drop considerably, sometimes to fewer than once per day.

    While programmable T-stats can do about as well (for non heat-pump applications) if one has a very rigid schedule, they don't automatically adapt to revisions in schedule, nor do most of them measure the ramp-rates of the recovery from setback at different outdoor temps to deliver the desired room temp at a precise time the way a Nest (and a few other ) thermostats can, maximizing comfort while minimizing on-time, since there is less pre-heating/pre-cooling necessary to be comfortable with thermostats that are agnostic regarding outdoor conditions.

    Nest and some other Wi-Fi enabled thermostats can also be aggregated & pooled into demand-response programs in some areas, where the utility pays you for the ability to bump the thermostat a few degrees during peak load times.

  3. Tim C | | #3

    Occupancy Detection
    The Nest also has occupancy detection. My programmable thermostat is configured with the assumption that I'm home during the weekend (and less fancy thermostats have that assumption built in). Sometimes I am, sometimes I'm not; the Nest can detect the times I'm not home and apply a setback.

  4. Malcolm Taylor | | #4

    Thanks Dana and Tim
    That 's very useful. So is it fair to assume that the benefits would be less significant in a very well insulated house, or one with radiant heat in a thermal mass where occupant comfort didn't rely on immediate inputs from the heating or cooling system?

  5. User avater GBA Editor
    Allison A. Bailes III, PhD | | #5

    Response to Malcolm Taylor
    Yes, that's a fair assumption. Setback isn't going to help much at all in a Passive House, for example, because you'd have to keep it set back for a long time to see a change in temperature. In homes with hydronic systems, it's also not going to help, nor is it recommended, because of the thermal lag. Setbacks can save energy in both of those cases, though, when the home is unoccupied for an extended time.

  6. User avater
    Dana Dorsett | | #6

    With high mass radiant setbacks don't work well anyway
    The ramp rates on high mass radiant are glacial, but that's not to say a Nest wouldn't be able to handle it and do well, if it was a bang/bang on/off type boiler rather than a modulating condensing boiler under outdoor reset control.

    Most high-mass radiation required a PID algorithmic approach to avoid temperature under & overshoots, and with outdoor reset on a mod-con the higher heat losses of a higher average room temp are usually more than offset by the boiler's higher combustion efficiency when burning at it's lowest possible modulation rate and temperature.

    But the average radiation temp of the room responsible for human comfort is more than the air temperature- the wall & window temps are a big factor. Radiant thermostats that work on room temp alone don't squeak the last 2% out of it, the way measuring the average radiant temp in the room does. (I'm told such controls exist, but they're not commonplace.)

    A Nest would still produce savings in a very well insulated house (even a Passivehouse), but a 10% savings on a nothing load is 10% of nothing- hardly worth thinking about. And with modulating equipment like mini-splits using setbacks typically INCREASES rather than decreases energy use. The changes in efficiency due to modulation rate with a mini-split is significantly higher than with a mod-con boiler. At 45F outdoor temps better mini-splits will cruise along at a COP well north of 4, but running full blast on a recovery ramp they'll only deliver 2.5 or so. Fujitsu's "economy mode" with occupancy sensor controlled setbacks doesn't have a good rationale, and I've yet to see data that it actually achieves any power savings (from either Fujitsu or third party testing.) I suspect based on bench testing data from other Fujitsu mini-splits that the auto-setback would burn more total power, not less, except in the least insulated air-leakiest homes.

  7. Kevin Dickson, MSME | | #7

    Setback can be VERY important with radiant
    In the winter, you'd like to have warm floors in the master bath when you get up. The best way to guarantee warm floors in the early morning is to set back the thermostat overnight, then set it up at, say, 4am.

    It's a comfort issue, not an energy saving issue. Also, buying Nest thermostats for all six of your zones can get too pricey.

    BTW, I never have any overshoot problems with my 3" mass floors and plenty of south facing windows. I set back to 55F at night, but it only drops to 63F, then it sets up to 65F in the morning. The sun can then drive it to 68F. All done with $30 thermostats.

  8. User avater
    Dana Dorsett | | #8

    Of COURSE you are getting overshoots!
    A 68F temp is a 3F overshoot from the 65F set point, it's just not a particularly uncomfortable overshoot.

    If you're heating with a modulating condensing boiler any energy saving from letting drop to 63F overnight are being chewed up by the lower combustion efficiency during the recovery ramp back to 65F, and if it takes until 4AM to drop that 2F, how much is that affecting sleeping comfort?

    It's not clear what would be considered " ...VERY important..." about the use of setback here, but maybe there are some nuances I'm not quite getting.

    A "set and forget" with the thermostats combined with finely tweaking the outdoor reset curves would use less energy. I know of a hydronic heating pro in MN runs his radiant slab ONLY via outdoor reset, without benefit of a thermostat of any kind, hard-wiring it to be always calling for heat, not that he recommends this to his clients (or he'd be getting a dozen callbacks per installation to adjust the outdoor reset curve here & there.)

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