Sensi Smart Thermostat, 100 Years Of Expertise, Wi-Fi, Data Privacy, Programmable, Easy DIY Install, Works With Alexa, Energy Star Certified, Mobile App, ST55

Works with boilers and does NOT require a C-wire
I moved to the East coast last December. We have a six-zone boiler heating system with baseboard heaters throughout 4,200 sq. ft., on three floors (not bragging, just trying to give an idea of the size and complexity of the system). With six old-school manual thermostats spread throughout the house, it was nearly impossible to evenly regulate the heat during the cold winter, so some rooms were very cold and others too warm. I found myself constantly going up and down stairs to monitor temperatures and make sure I wasn’t wasting heat or keeping areas too cold that would affect other zones.My boiler was installed in 2007, so it isn’t very old and it’s very efficient. However, it’s a two-wire system with no C-wire, so the fact that 99.9% of “smart” thermostats on the market require a C-wire to operate was very frustrating.I stumbled across the Emerson ST55 and saw in the description that it could work either with or without a C-wire, so I called Emerson tech support to make sure before I plunked down hundreds of dollars on six thermostats that might not work. Tech support confirmed my fears that it would not work without a C-wire despite the Amazon description, so I kept hunting.Fortunately I found a YouTube review of the ST55 and the owner said he had been successfully using it with the same kind of two-wire system that I have for over two years. So I figured I’d buy one and see if it worked. Worst case, I’d send it back for a refund.The instructions inside the box clearly state that it will not work without a C-wire, but I ignored them and kept going. With only two wires it was a breeze to install. I put in the batteries, snapped it into the wall plate, downloaded the app, created an account and was ready for testing in about 15 minutes.I’m not sure if it’s just my system or all boilers, but I didn’t realize it doesn’t turn on instantly when the thermostat temperature is raised above the existing temperature. So when I first tested it, I was bummed that it didn’t work. But as I walked downstairs to the boiler, it was running! So I did some more testing, and found that there’s about a 20-25 second delay between increasing the temp and the boiler kicking in.Granted it’s only been a couple months since I got it, but I’ve played with it plenty and it’s working great. Instead of a C-wire, the thermostat is powered by two AA batteries. The guy in the video said he’s been using the same regular alkaline batteries for two years and they’re still at 90% charge capacity, which is awesome. But I figure that even if I have to replace batteries every year, 12 AA batteries is a helluva lot cheaper than paying an electrician to rewire all six zones!On top of the six heat zones, I also have two central AC units. Thankfully those have C-wires, but I’m going to replace those thermostats with the Emerson ST55 also so that I’ll have full control of all heating and cooling from one simple-to-use app. With any luck it’ll go on sale on Prime Day so that I can save some money! :-)I can’t speak to the Alexa integration as I don’t use that functionality. But plenty of other reviewers have said it works fine.I appreciate Emerson’s thoughtfulness in adding a built-in level to the back plate. I’m a stickler about things being mounted straight, so it was nice to not have to break out a level to make sure I didn’t mount it crooked.As for aesthetics, the ST55 is nice. It’s not too large to look obnoxious, but it has a good-sized display. And I’m not sure if it’s just because I’m running it on battery, but the back light doesn’t stay on constantly, so it doesn’t light up a dark room at night (not too important in common areas, but important if you’ve got a thermostat in a bedroom).So, to make a long story short, don’t believe Emerson. This thing really does work on a two-wire boiler heat system with no need for a C-wire!

Step by step app is AWESOME!!!!! Install app BEFORE removing old thermostat.
We’ve had this thermostat a month and I really like it so far. It makes me think our old thermostat wasn’t actually working for a long time before it started setting itself to 85°. Since it wasn’t even programmable, I knew it had to go and if I was going to upgrade I may as well go with something just a bit more…modern than a set-temp-and-it-stays-unless-you-change-it thermostat. I’m going to do pros and cons below. I do want to say, I took my old thermostat off the wall first, wrote down all the wire colors and the letter each was connected to on a piece of paper. The app removes the need for pen and paper. I just used my pen and paper to place the flags mentioned below on the correct wires.Pros:1. This thing is super easy to install. Open the box and there are instructions for you to download the Sensi app. Once the app is downloaded it has you choose which letter slots you have wires connected to (R, RH, C, E, OB, etc.). The app then tells you how to use the flags they provide to label the wires. For example I had a wire that went into a slot labeled “R”, but Sensi only had “RH” and “RC” (I think it was RC), so the app told me which R my wire needed to go in. Not having any electrical wiring experience, the app was SUPER helpful. It even had you take a picture of your wiring to save to your phone, just in case.2. The piece that attaches to the wall has a built in level. I am super annoyed when things aren’t straight on walls, so this right here, was awesome.3. The app is really easy to set up. I just have a daytime and nighttime cycle, but I could set up all kinds of things super easily. Vacation modes, turning the AC up when I leave the house… It was great. It even walked me through how to sync the app with my Google assistant and the Samsung Smart Things app, which I didn’t end up continuing to use. (See cons #1)4. The report that tells you how long your AC is running is really helpful as well, especially since we had issues last year with it not being able to keep up. We have better insulation and updated windows and it’s nice to see they are working, even when it’s 99° with a heat index at 112°. (See cons #2)5. I like that you can do things like temp and humidity offsets, have the AC turn on to help maintain proper humidity levels (we live in Florida, so this is a big thing for me), change the cycle rate (ended up choosing this one over another brand because people had issues with not being able to adjust the cycle rate on that one), and easily turn heat boost on or off. (See cons #3)Cons:1. It doesn’t work with Bixby. This is something I don’t really understand, but I’m guessing they assume if you’re gonna use Bixby you use Smart Things? But I can just use Google for that instead.2. The recent usage section only shows how long the AC ran that day, not which times it ran or how long at a time it ran. It would be nice to see how long the AC was running at a time, especially in the afternoon when it gets really hot. I have not yet gotten a monthly report since I got the thermostat mid-June and it’s now mid-July. Interested to see what the monthly report looks like at the end of the month.3. Sometimes I had to change settings a couple times before they “stuck”. I found after changing a setting I needed to wait a few seconds before returning to the settings menu and that helped.4. Shortly after I got the thermostat the AC drain line clogged. This resulted in my app/thermostat constantly saying cooling was on, but the AC wasn’t running. My usage for that day was like 21hrs, but I know it wasn’t actually on for the majority of that. Not really a huge issue, but you won’t have an accurate reading for that day.Summary:Overall, this is a really amazing unit and I have been really glad to have it. It was seriously a breeze to instakl and set up and functions well. I do wish I could connect it to Bixby without using Smart Things and I do wish I had a more detailed daily usage report, but those are very small inconveniences. I normally keep our house at 78° during the day and 77° at night and sometimes that feels cold. I would often feel hot before and wondered if our AC was not working correctly, but it would seem it was actually the thermostat. Very happy with this purchase.