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Building Science

Does the Nest Thermostat Save Energy?

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

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 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…

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  1. Malcolm Taylor | | #1

    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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