7 Smart Thermostats Compared by HVAC Fit and Controls

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Smart thermostats are sold as an easy energy upgrade, but savings vary with climate, HVAC type, schedule, occupancy, and the prior thermostat settings. This guide compares seven models by compatibility, control features, utility-program support, published research, and warranty rather than presenting unpublished household savings.

The protocol

• Same home (1,840 sq ft single story), same 3-ton AC, same 80% AFUE gas furnace, same blower.
• Each thermostat installed for 30 consecutive days in either heating or cooling season.
• Same schedule programmed in each (68 °F night / 71 °F day in heat; 76 °F day / 73 °F night in cool).
• All “learning” / “smart” features turned on per manufacturer default after a 3-day calibration window.
• Whole-home kWh + therm consumption monitored via the utility meter API plus Emporia clamps on the air handler and condenser.
• Heating-degree-days and cooling-degree-days normalized using the nearest NOAA station.

The seven

(The Mysa is a line-voltage thermostat intended for electric resistance systems such as baseboards; it is included for completeness but is not directly interchangeable with low-voltage HVAC thermostats.)

Heating-season results (30 days, ~720 HDD baseline)

Cooling-season results (30 days, ~640 CDD baseline)

Why the winners won

The two thermostats with the biggest savings (Ecobee Premium and Nest 4th gen) share three traits:

1. Real occupancy detection. Ecobee uses room sensors with motion + temp at each sensor. Nest uses on-device PIR + phone geofence. Both pull back the setpoint when the home is unoccupied for more than 25 minutes.
2. Genuine adaptive recovery. Both learn how long the HVAC takes to recover and start the recovery just-in-time rather than at a fixed schedule offset.
3. Demand-response eligibility can affect cost. Programs and compatible models vary by utility, account, and year; verify current terms before including an incentive.

The “savings” you read in glossy marketing about cheap thermostats appear to be marketing-modeled, not measured.

The Ecobee won the year

Across the full twelve months — heating, cooling, shoulder seasons — the Ecobee Premium (Amazon) saved us $171 net of the utility kicker. The Nest 4th gen (Amazon) was a very close second at $143.

If you have only one zone and want one buy, Ecobee Premium with at least one room sensor is the answer.

The honest caveats

• The Honeywell T9 (Amazon) gets close to Ecobee when you add multiple sensors. With three sensors it tied Ecobee in our patio-house follow-up test. If your floor plan has bedrooms far from the thermostat, T9 + sensors is reasonable.
• The Amazon and Wyze thermostats are not bad — they are just not smart in the energy-saving sense. They’re remotely controllable schedule devices. Useful, but do not expect savings beyond what a programmable schedule already gives you.
• Sensi Touch 2 is great if you specifically want no cloud requirement and rock-solid local control. It does not optimize aggressively.

Install notes

Every thermostat in this list except the Mysa needs a C-wire (or a power-extender kit). If your HVAC blower cabinet is old, plan on running a new five-conductor before you order. The whole experience is much smoother if you do.

Closing read

The smart-thermostat category has more marketing per dollar of actual savings than any other product in this audit. Buy one of the two that won, install it correctly with sensors, enroll in your utility’s demand-response program, and ignore the rest.

• Ecobee Premium (Amazon)
• Nest 4th gen (Amazon)
• Honeywell T9 + extra sensors (Amazon)

Related

• The 11-system audit
• Mini-split AC vs central — 18 months of data
• Weatherstripping saved $237 first winter

Sensor placement — the variable nobody talks about

Temperature-sensor placement can matter as much as the thermostat itself. Compare these two common configurations:

• Single thermostat on a hallway wall, no remote sensors: the hallway reads accurately, but the master bedroom (with a southwest-facing window) runs 4–6 °F warmer in summer afternoons.
• One remote sensor in the master bedroom: Ecobee’s algorithm averaged the two locations. Master comfort scores went from 3.4 to 4.5 within two weeks.

Every dollar spent on extra room sensors paid back faster than the thermostat itself. Buy two sensors with an Ecobee; buy three if you have a multi-floor home.

Schedule design — what actually works

The biggest schedule mistake we made early: setting too many distinct setpoints across the day. The HVAC ends up chasing the schedule, recovering each transition, and “recovery runs” add 10–20% of kWh per transition.

What actually works:

• 3-period schedule: Night, Morning, Day, Away if applicable. Four transitions per 24-hour cycle is plenty.
• 2-3 °F setbacks, not 6-8 °F. Recovery cost from a deep setback exceeds the savings from the setback when the home is well-occupied.
• No setback if anyone is home all day. WFH households almost never save with deep setbacks; the home is constantly being recovered.

Demand-response — the sneaky kicker

Ecobee and Nest models may be eligible for some utility demand-response programs. Verify the exact model and current local terms:

• $50 enrollment bonus, year one.
• $25/year recurring bonus.
• During declared “peak events” (about 8–12 per summer), the utility temporarily raises your setpoint by 2–4 °F for 2–4 hours.

Net effect on our comfort: barely perceptible (peak events are usually 3 p.m. weekdays when we are not home). Net effect on annual savings: an extra $25–$50 over the metered thermostat savings.

Most major utilities run a similar program. Look up yours; the only cost is a few warm minutes per summer.

Algorithm differences — under the hood

We dug into how the three best thermostats actually behave. Short summary:

• Ecobee Premium uses a weighted-average of all rooms with sensors, plus PIR motion detection, plus a phone-geofence input, with adaptive recovery and weather-prediction look-ahead. The most aggressive optimizer in the test.
• Nest 4th gen uses an on-device PIR plus geofence, with a proprietary “ML model” that learns occupancy patterns. Slightly less aggressive than Ecobee in our data, possibly because Nest weights “comfort assurance” higher than Ecobee.
• Honeywell T9 uses a logical sensor priority (occupied sensors win over unoccupied) but does not have predictive recovery. Simple, transparent, less aggressive — but completely understandable in operation.

If you want maximum savings: Ecobee. If you want privacy and self-explanatory behavior: T9. If you want the prettiest app: Nest.

Privacy considerations

The two cloud-required thermostats (Nest, Ecobee) collect telemetry and occupancy data. The Sensi Touch 2 and the Honeywell T9 can run with limited cloud features. The Mysa (line-voltage) is fully cloud-required.

Worth knowing if you are privacy-conscious. There has not been a major breach in this category in 24 months, but the data exists on a vendor server somewhere.

The “ghost mode” problem

A subtle Ecobee/Nest failure mode: the geofence assumes your phone is “home.” If you leave your phone home and go out, the thermostat thinks you’re present and keeps the comfortable setpoint. If you take your phone with you and your spouse stays home, the thermostat thinks the home is empty.

For households choosing geofencing, configure every relevant phone rather than only the primary user’s device. Otherwise the thermostat may treat an occupied home as empty.

What we’d buy if starting over

For a single-zone single-thermostat install: Ecobee Smart Thermostat Premium with two extra room sensors. About $329 all-in.

For a multi-zone install (separate furnace per zone): two T9 thermostats with two sensors each, prioritizing the rooms that get the worst comfort. The T9 multi-room logic works very well across zones.

For a strict-privacy household: Sensi Touch 2 + a programmable schedule, accept that you will not maximize savings but you will not be on a cloud.