Upgraded my Thermostat

A few months ago,  I spent $50 on a new thermostat, and as you can see it looks much better than the old one.  More importantly, it works much better than the old one. Due to the new one being more horizontal, and the old one being more vertical, I purchased the back-plate as well at that time to make installation easier. the sizes nearly match, but if you look closely, you can see where I will need to touch up the paint a bit.  It’s an easy task, and there are more  than a few such places I will need to do that with.

The unit’s installation was easy enough, although I had some hurdles that I will describe below.  In short, this would have taken 15 minutes if the old setup had proper labeling, but it was not the end of the world. I’ve always been a big DIY fan, which I credit my parents with encouraging.  They were very big on that.  Themselves, having grown up in Brooklyn apartments, were able to raise us in a nice suburban home.  They did not know a lot, but learned to do almost everything themselves.  It’s a great way to save money, and that’s why I’m posting this here.  In effect, hey this is easy, and you don’t have to hire someone.

My old thermostat failed last winter.  Fortunately, it was stuck at a fair temperature, about 65F or 66F. It was an original one, and the house was built in the 80s.  It was time to replace it.  My research online showed me that it was common in the US to wire furnaces with just 2 wires for control with Red and White wires having special meaning.  That may be incorrect, but it’s what I found.  Certainly, it’s true enough because the manual of the new thermostat acknowledges this.  The new thermostat warns, correctly, that you can not trust the colors.

You can see this was one of the old mercury ones, and I am not sure why it stopped working.  The device is quite simple, which is a good thing to me.  Simple tools are usually easier to fix.  Part of the reason for the upgrade is to save me the effort of running around the house to turn up and down the heat before we leave each day, and after we get home. Mostly, though, it’s the not working thing.  I did spend some time trying to fix it last winter, but I can’t determine which part of it is broken.

So when I pulled it out, I saw a nice hole in my wall, and the back of the device that you see in the next image.

One of those two is the source, and the other is the drain.  That is to say that one comes from the power supply, and the other goes to the controller.  Allegedly you can blow some components up, either in the new thermostat or in the furnace if you wire it wrong.  I am sure that could be true with other setups, but not in this simple one.  The device just works as a short, where if the temperature is too low, the two wires are connected.

Oh wow, I just had an epiphany.  The thermometer wire on the thermostat was definitely not working.  I have a hand held digital thermometer and hygrometer (humidity) meter that I keep in the house for obvious reasons.  That showed it was off. Well, that metal coil which is the thermometer failed.  That was stuck at 65F or so, and would of course not tip the mercury to tip the device on or off.  If you have never come across this, metal gets longer and shorter based on temperature.  Some metals and alloys more than others, and if you put 2 together, you can get a great thermometer. Anyway, like I said, it’s clear to me that it was the failed part here, and I do not feel like replacing it.

So reading the instructions, they tell you to make sure there is some insulation behind the device to protect it’s temperature reading from any drafts.  Mine did not have that, so I grabbed a small piece of fiberglass insulation from an exposed place in my basement, and stuck it in the tiny hole.

The instructions go on to show much more complicated designs, but fortunately for me, mine was simpler than the simplest one they had. The problem was that nothing, and I mean nothing in my furnace was labeled. I have an Electrical Engineering background, and that made it easier, but it’s really not needed  The trick was to look for the wires, you can see the brown jacket that holds the red and white wires in the next image.

From there, I had to work out which was connected to which of my 3 zones. That was no easy task as you can see in the image above and below.

It took me a minute or two to find that the device you see in the image above, which is the transformer that supplies power for the thermostats.  It took me another minute or two to realize that the left side is the source for all 3 zones / thermostats. Understanding how the red wires are connected to the zone relays (green and silver things in the images 2 above) took a bit longer.  The wires look like spaghetti, and are partially hidden behind some fire safe product. I even took apart the controller for the furnace, which the relays connect to (no picture), to find the full path of the wires.  This should have taken less time than it did, and ultimately, it was obvious to me that the 3 white wires in the image above are the source.  Once I worked that out, then it was just plugging it all in.

The copper wires are old and a bit brittle, and I was a bit out of practice in moving and stripping the appropriate cladding away.  So, not the end of the world, but after 2 tries, I got it all on. My thermostat uses either power from the supply, via the C wire, which is not there, or a battery that needs an annual change.  That’s reasonable to me.

Programming it was easy enough.  You can program 4 time blocks 7 days a week. You can adjust what time of day the blocks start. You can then copy it from day to day, and of course there is a manual override.  This is all pretty simple, and I think not worth explaining unless anyone wants to know that part.

In case you’re curious…

Now you might be wondering about my furnace type.  My furnace burns oil as many homes have in the New England region of the US do.  Why, well for a few reasons.  When the home was built, oil prices had crashed to the $10 to $20 dollars per barrel range.  Much of the land in New England is rocky thanks to the Appalachian Mountain chain that runs through the area; including New York City.  That means, along with this region being one of the older ones, and one of the first suburb areas in the country, that the costs of laying natural gas are high.  Streets must be dug up.  The home I grew up in which is only about an hour or so away and within commuting range to NYC and was built in the 1970s used natural gas. The specific development was built on farm land that had been actively farmed by Europeans since the 1600s, and likely by the tribes in some fashion for much longer. The point being that my current home is rocky soil, and is a “newer” house in an older region.  There are plenty of places where the housing is dense enough, the land easy enough to dig (no roads yet) or other similar factors that make it cheap enough to put in natural gas.  I actually asked the natural gas company, and they said the closest mains were too far to make a connection to the street my street connects to, let alone my street.  There aren’t enough homes on it, and it’s too rocky and hilly to dig cost effectively. So no natural gas for me, and that’s why we added those air source heat pumps that I like so much

The air source heat pumps that have dropped our oil usage about 30%, and thus saved us more than $500 a year in oil usage.  The great thing about the heat pump is that we now have flexibility.  If oil prices rise compared to electricity, then we use the heat pumps at lower ambient air temperatures to warm our home.  If oil prices drop, we’ll use more oil.

What about a Nest or Wifi Thermostat

Well those things are really cool, and a lot easier to use.  They all need either that C wire power, which I knew my home did not have, or they needed a real power supply. They also cost a lot more, and have functionality that I don’t see the value in.  This $50 device is plenty good enough for my needs, and that of my family. In fact, had the other not failed, in spite of the aesthetic concerns, my wife and I both prefer the new white one to the old beige one, it would still be on the wall.


  1. I work at a public utility and regularly interact with our energy efficiency/demand-side management groups on smart thermostat programs. While reading your write up, it struck me as odd that you hadn’t considered a Nest or EcoBee smart thermostat but saw at the end why you chose not to go with one of those. I’m working with a digital thermostat at my house, which are shown to have better energy usage statistics compared to the legacy thermostat like you replaced. It isn’t programmable, however, and I’m strongly considering the upgrade to a Nest. I haven’t quite gotten around to making the investment yet because I don’t know how much longer we’ll be in this particular house. The $50 investment would be a no-brainer were I upgrading from an analog thermostat, but going to a Nest would carry a higher price tag I’m not entirely sold on shouldering at the moment. Regardless, very thorough write up with lots of detail for any engineers who want to read.


    1. Thanks for the kind words. In truth, I love the gadgets. Its a level of self control that I am working on to not go with the superior products. I could, after all, add a power wire myself to the system for even more money. That project would be fun for me. Still, in a sober appraisal of the situation, the value is not there. I can not connect my air source heat pumps to the system, nor could I make it work the way I want even then. I prefer to use them for heat on days above 30F to 40F, which saves money as well. So, I replace the broken, with the good enough. In the spring, I will likely replace the other 2 working old style thermostats in the house. My next task is mounting a cable tv line in the wall. That is a long story for another post. I hope to get time to do it in the next few weeks.

      Liked by 1 person

  2. Thanks. That’s a bit more motivation to share more of these projects I do. I am most comfortable with electricity, and have replaced many outlets myself. Home Depot sells the outlets with the baby protection built in for like $1-2 that the kids can’t pry off. Still, I hired a pro when it was time to install the sub panel for our generator. That was not a mess I wanted to deal with, nor to learn the specific building codes so that I did it up to spec.


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