C-Wire Check product 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.
Listing checked . Confirm the model, included parts and availability before ordering.
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
Fit and limits
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
A button thermostat that fits the old footprint
The Sensi ST55 is the plain one. It has physical buttons and an LED display rather than a touchscreen, and the listing's pitch is that it looks and feels like a thermostat and fits the same wall space as a traditional one, so you do not have to patch and paint. Amazon's table gives the body as 1 inch deep by 5.88 wide by 3.75 high and 10.5 ounces. The documents we read do not give dimensions, so treat those as listing figures.
Amazon's included-components list is unusually complete for a thermostat: the thermostat itself, the mobile app, two AA batteries, mounting hardware, wire stickers, an installation guide, a welcome guide with the Sensi code, and a warranty booklet. The installation guide confirms the wire labels and screws are in the box, and that you remove a battery tag on the back of the thermostat to switch it on. A built-in level is mentioned in the listing bullets.
One identity note. Amazon lists the manufacturer as Copeland, the product description says the thermostat is manufactured by Emerson, and the warranty document supplied with the listing is issued by White-Rodgers, a business unit of Emerson Electric Co. The Sensi thermostat business moved from Emerson to Copeland, so all three names attach to this product line. Neither the maker's current website nor the supplied manual uses the string ST55 on the pages we read; that number comes from Amazon's fields, so check the model printed on the box if you need it for a rebate.
Ten terminals, and the systems the maker's manual configures
The installation guide's terminal table is the best fit-check available for this thermostat, and it is more generous than the price suggests. It lists RH for 24V heating power, RC for 24V cooling power, C for the common wire, Y for first-stage cooling on a conventional system or first-stage heat and cool on a heat pump, Y2 for the second stage of either, W/E for first-stage indoor heat or first-stage auxiliary and emergency heat, W2 for the second stage of those, G for the indoor blower, O/B for the changeover or reversing valve on a heat pump or zone panel system, and an L terminal that the manual says has no function.
The configuration menu in the same guide shows what that supports: conventional cooling in one or two stages (AC1 and AC2), no cooling at all (AC0), gas heat in one or two stages (GA1 and GA2), electric heat in one or two stages (EL1 and EL2), heat pumps in one or two stages (HP1 and HP2), and a fan-only, no-heat setting. The reversing valve position is selectable between O and B for heat pumps, and the wiring notes mention a dual-fuel option that de-energises the first heat-mode stage. Two-stage heat with two-stage cooling is inside what this thermostat configures, which several dearer smart thermostats will not do.
Two-transformer systems are also handled. The manual says that for two-transformer systems with separate RC and RH wires you clip an internal jumper located on the back of the Sensi above the battery compartment, and that the thermostat otherwise has that jumper built in, so you should throw away any jumper wire from the old thermostat. We are not reproducing the wiring diagrams here. Follow the printed guide, and confirm your own system on Sensi's HVAC compatibility tool on the Sensi website before you buy.
The C-wire claim, and the three cases where the manual wants one
The listing bullet says a common wire is not required in most applications. That is the headline reason a lot of people choose this thermostat, and it is backed by the AA batteries in the box. But the manual's own wiring-diagram footnotes are more specific, and they are what you should read before ordering.
On the single-transformer diagram, the footnote says a common connection is required for heat-only or cool-only systems. On the heat-pump diagram, the footnote says a common connection is required on heat-only, cool-only or heat pump systems. So if you have a heat pump, or a boiler with no cooling, or cooling with no heat, the maker's own document points at needing a C wire. Those are exactly the cases where a buyer expecting a battery-powered install can be caught out.
There is no power adapter or extender kit in this box, and none is mentioned in the documents we read. If your system falls into one of those categories and you have no common wire at the thermostat, that is a wiring job at the equipment, not something the box solves. Sensi publishes a dedicated C wire page on its site, and it is worth reading before you order.
The app, the account and the 2.4GHz requirement
This thermostat cannot be set up without the Sensi app. The installation guide states it plainly: the Sensi thermostat is a Wi-Fi enabled device, and to install and configure it correctly you must use the Sensi app, which walks you through wire identification, labelling and connection as well as the network setup. You are prompted to create a Sensi account with an email address and password, and the thermostat is linked to that email address once registered.
The network requirement is specific. Under the information you will need before starting, the guide lists your router name and password and adds, in a note of its own, to make sure this is a 2.4GHz network. If your router hides a 2.4GHz band behind a single combined network name, sort that out before you start the install rather than halfway through it.
There is a sharing behaviour worth knowing about. The guide says that anyone you give your Sensi account email and password to can log in and control any thermostat registered to that account, and that there is no limit to the number of devices that can access the thermostat. Sensi's privacy claim in the listing is that it will not sell your personal information to third parties. Amazon's table lists Alexa, Apple HomeKit, Google Assistant, SmartThings and Vera as controller types; none of the documents we read confirms that list, so check Sensi's own smart home compatibility page for the platform you care about.
Three years, unless it is a PRO model in a pro's hands
The warranty document supplied with the listing sets out a three-year limited warranty from White-Rodgers, a business unit of Emerson Electric Co., covering retail thermostat models including the ST series against defects in material and workmanship under normal use, running from the later of the purchase date or the manufacturing date code. Amazon's Item details give the manufacturer warranty as three years, which matches.
The document's first page explains the other version. A five-year limited warranty is available to professional contractors installing PRO Sensi thermostat models, and if you buy a PRO model but do not use a PRO contractor to install it, the warranty converts to the three-year retail warranty. So the five-year figure you may see elsewhere is not available on a retail unit you fit yourself.
The limits are worth knowing. The sole and exclusive remedy is replacement at the manufacturer's discretion, replacement units may be refurbished or a different but functionally equivalent model, and repaired or replaced product is covered for the remainder of the original period or ninety days, whichever is longer. The warranty does not apply to products installed in a commercial or industrial application, and it is void if you attempt to repair the product yourself. There is a 30-day satisfaction guarantee that requires the product back in new condition with all original packaging, accessories and the receipt. Keep the box for a month, keep the receipt, and check the seller and condition of the offer you select.
What this model does not try to be
There is no learning algorithm here and no occupancy sensing in the documents we read. The features Sensi advertises are flexible scheduling, remote access, usage reports, equipment alerts and maintenance reminders such as filter replacement. The listing's energy claim, about 23 percent savings on HVAC energy, is the maker's own figure and is marked with a disclaimer pointing at the product documentation section.
Equally, the equipment alerts are a prompt, not a diagnosis. The listing describes them as helping monitor the performance and efficiency of your HVAC system through usage reports, alerts and reminders. Nothing in the documents we read claims they detect faults reliably, and none of them is a substitute for a technician.
The multi-thermostat story is simple: the guide says that if you are installing more than one, you repeat the same process in the app under Account and Install thermostat, and all thermostats on one account are controlled from that account. Technical support numbers and hours are printed in the guide, which is more than most listings supply.
Check the current listing
Confirm the model, included parts and availability before ordering.
Primary sources
- Sensi Wi-Fi Programmable Thermostat installation guide supplied with the Amazon listing (Emerson)
- Emerson Thermostat Limited Warranty supplied with the Amazon listing
- Sensi (Copeland) manuals index for the Sensi smart thermostat
Primary sources checked .
Compare with
- Google Nest Thermostat (Charcoal)
Single-stage 24V systems that need a schedule and app control, not multi-stage equipment
- Mysa Smart Thermostat V2 for Electric Baseboard Heaters (BB.2.0.01.NA-US)
120V or 240V electric baseboard, fan-forced convector and radiant ceiling heat in a standard gang box
- ecobee Smart Thermostat Premium with SmartSensor
24 VAC forced-air, heat pump, boiler or PTAC systems, checked first with ecobee's compatibility tool
- 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
- ecobee Smart Thermostat Essential (EB-STATE7-01)
Simple single-transformer systems with one stage of cooling, on a 2.4GHz network
Side-by-side pages, this model next to: Google Nest Thermostat (Charcoal), Mysa Smart Thermostat V2 for Electric Baseboard Heaters (BB.2.0.01.NA-US), ecobee Smart Thermostat Premium with SmartSensor, Google Nest Learning Thermostat (4th gen) with Nest Temperature Sensor (2nd gen), ecobee Smart Thermostat Essential (EB-STATE7-01).