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I'm about to look for a programmable thermostat to replace a manual one. There are already a couple of good questions on thermostat replacement that make the task seem pretty straightforward for anyone who's done a bit of wiring (which I have done). The problem for me is this nagging doubt that, from a wiring perspective, all the thermostats I look at are not equivalent and only some would be able to work with the existing wiring.

So, my question is, as I'm looking for a replacement thermostat, is there anything I need to take into consideration? Do I need to read the back of the box for any specific statistic? Do I need to check the existing wiring first (number of wires, voltages, etc.)?

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nice question - i am also going to be considering this soon- hope some good feedback is give – ppumkin Aug 8 '11 at 8:24
up vote 5 down vote accepted

You have to check that the thermostat is compatible with your HVAC system. Heat pumps need the emergency heat switch, multi-stage high efficiency HVAC's have multiple fan settings, and electric baseboards may have 120/240v lines instead of the 24v wires. For the lower end models, or for those seen in the far north or south, you may have a thermostat for only heating or cooling, but not both.

So before buying a thermostat, check the type and features on your existing HVAC system and limit your selection to the thermostats that support your system. Then, see what features you want and how easy the system is to use. Some systems have wifi, some have programs for all 7 days instead of 5/2 split, but if you have all 7 days, how easy is it to change the setting for every day?

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Honeywell even has a 5+1+1 programmable which is roughly halfway in between the costs for 5+2 and a full 7. – user7116 Aug 8 '11 at 16:09

99% of thermostat installs won't have an issue. If you have a strange heat pump configuration or a hybrid energy system, i would look into having a pro do it... Otherwise, just make sure you look at he back of the existing thermostat and identify the letter that the wires are connected to before you remove them. The colors of the wires don't always match up with what they are connected to.

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Make sure the number of zones in the thermostat matches the number of zones in your home.

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If your HVAC is relatively new, you shouldn't have a wiring issue, but you should double check to be safe. Different systems having different wiring requirements, but the thermostats should be able to handle them. I just installed a Wifi thermostat over the weekend and it could handle several wiring configurations. For the Wifi, the only requirement I had to worry about was a C-wire to provide power to the thermostat. Most systems don't have a C-wire hooked up. Luckily my system had an extra wire that I was able to hook up to the C-terminal.

I've had 7 day and 2 day (weekday & weekend) programmable. I like the flexibility of the 7 day, but in reality, I use the same program for every day. If the house was empty during the day all week, I would probably have 2 separate programs, one for weekday and one for weekend.

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