Google Nest Learning Thermostat (4th gen) with Nest Temperature Sensor (2nd gen) vs Sensi Smart Thermostat ST55
These are side-by-side facts taken from each product guide: published specifications, strengths and caveats as their sources state them. This page is not a test, a ranking or a verdict about which model is better.
Google Nest Learning Thermostat (4th gen) with Nest Temperature Sensor (2nd gen)
Multi-stage 24V systems run through the Google Home app, in homes with or without a C wire
Google's top thermostat, bundled with one room sensor, a trim plate and a steel plate. The maker lists up to three stages of heat and cool, three-stage heat pumps with aux and emergency heat, plus humidifier, dehumidifier and ventilation terminals. Power Sharing means no C wire in most homes, and control runs through Google Home and Matter.
Published specifications
- Maker: most 24V systems; heating and cooling 1-3 stages; heat pump to 3 stages with AUX, AUX2 and E
- Maker: star connector takes only one of W3, Y3, G3, E, HUM or DEHUM; AQ+/AQ- limited to Y3 or VENT/HUM/DEHUM
- Maker: Power Sharing works with or without a C wire; under 1% of homes may need a C wire or Nest Power Connector
- Maker: Wi-Fi 802.11n on 2.4 GHz and 5 GHz; works with Google Home and Matter; Google Home app required to set up
- Maker: 3.9 in diameter, 1.1 in deep, 5.7 oz; 2.7-inch 600x600 circular display; Soli, temperature, humidity, light sensors
- Warranty: Google Store lists 2 years retail and 5 years for HVAC pros; the listing's warranty PDF says one year
Strengths
- Maker publishes the exact stages and terminals, including three-stage heat, cool and heat pump support
- Trim plate, steel plate and a Nest Temperature Sensor are all in the box
- Works without a C wire in over 99% of homes by Google's own figure, and takes 2.4 GHz or 5 GHz Wi-Fi
Caveats
- Setup requires the Google Home app and account; Alexa or Apple control goes through Matter and may need a third-party hub
- Warranty length differs between Google's spec page and the warranty PDF on the listing, and covers authorised resellers only
- The extra terminals are shared: you cannot use W3, Y3, G3, E, HUM and DEHUM all at once
Read the full Google Nest Learning Thermostat (4th gen) with Nest Temperature Sensor (2nd gen) guide
Sensi Smart Thermostat ST55
Conventional or heat pump systems up to two stages, on a 2.4GHz network, in the old thermostat's footprint
A button-and-LED smart thermostat whose manual configures one or two stages of conventional cooling, gas or electric heat, and one or two stage heat pumps, with a built-in RC/RH jumper you clip for two-transformer systems. Batteries are included, but the manual says a common wire is required on heat-only, cool-only and heat pump systems.
Published specifications
- Manual: terminals RH, RC, C, Y, Y2, W/E, W2, G, O/B and a non-functional L
- Manual: configures AC1/AC2 cooling, GA1/GA2 gas, EL1/EL2 electric, HP1/HP2 heat pump, fan-only and no-cooling
- Manual: common connection required on heat-only, cool-only or heat pump systems; listing says C wire not needed in most applications
- Manual: two-transformer systems supported by clipping the internal RC/RH jumper above the battery compartment
- Manual: Sensi app and Sensi account required to install; router must be a 2.4GHz network
- Warranty: three-year retail limited warranty; five years only for PRO models installed by a PRO contractor
Strengths
- Handles two-stage heat and two-stage cool, heat pumps and two-transformer systems at an entry price
- Fits the old thermostat's footprint with buttons, a built-in level, wire labels and AA batteries in the box
- Both the installation guide and the full warranty text are published on the listing itself
Caveats
- The manual requires a common wire on heat-only, cool-only and heat pump systems, and no adapter is included
- 2.4GHz Wi-Fi only, and the Sensi app plus a Sensi account are required to install it at all
- Warranty remedy is replacement only, is void if you attempt a repair, and excludes commercial installations
How to choose between them
Choose the Google Nest Learning Thermostat (4th gen) with Nest Temperature Sensor (2nd gen) if your situation matches: Multi-stage 24V systems run through the Google Home app, in homes with or without a C wire. Its main caveat: Setup requires the Google Home app and account; Alexa or Apple control goes through Matter and may need a third-party hub.
Choose the Sensi Smart Thermostat ST55 if your situation matches: Conventional or heat pump systems up to two stages, on a 2.4GHz network, in the old thermostat's footprint. Its main caveat: The manual requires a common wire on heat-only, cool-only and heat pump systems, and no adapter is included.
Both entries repeat what each guide records. Confirm the model, included parts and availability on the current listing before ordering.
Primary sources
- Google Store tech specs and wiring compatibility, Nest Learning Thermostat (4th gen) Source for Google Nest Learning Thermostat (4th gen) with Nest Temperature Sensor (2nd gen)
- Google Consumer Hardware Limited Warranty PDF supplied with the Amazon listing Source for Google Nest Learning Thermostat (4th gen) with Nest Temperature Sensor (2nd gen)
- Google Nest compatibility checker named on the Google Store specs page and in the listing Source for Google Nest Learning Thermostat (4th gen) with Nest Temperature Sensor (2nd gen)
- Sensi Wi-Fi Programmable Thermostat installation guide supplied with the Amazon listing (Emerson) Source for Sensi Smart Thermostat ST55
- Emerson Thermostat Limited Warranty supplied with the Amazon listing Source for Sensi Smart Thermostat ST55
- Sensi (Copeland) manuals index for the Sensi smart thermostat Source for Sensi Smart Thermostat ST55
Primary sources checked .