Dual Stage Furnace and Nest
It’s important to do your homework, and pay attention to what your furnace installer is doing. That’s the takeaway from my recent furnace replacement and Nest installation. Here are some details that you may find helpful.
My furnace died on a very cold day, so I called my local furnace repair company. They’re great; they’ve done work here before and I’ve always been happy with them. After some quick diagnosis, turns out the draft inducer (fancy name for one of the fans in the furnace) is seized. Furnace is 22 years old, replacement part is $600, and it’s not a high-efficiency furnace, so we decide to replace it.
The new furnace we went with is a Trane 95 XV, which is a dual-stage, variable fan speed furnace.Â
These are nice features.  Dual stage means the furnace has two burners, so it can run in low mode or high mode. And the variable fan speed means that when it’s in stage 1, it can run the fan more slowly as well, so you get a slow, quiet heat that’s great for days that aren’t incredibly cold, where your furnace would be cycling (coming on to warm, turning off when the house reaches the right temperature) more often.
Stage 1 will run longer, but uses less power, and keeps the temperature more constant.
Back to my install, the thermostat in my house had 4 wires running to it from the furnace. One brings power to the thermostat, one sends power back to the fan, one to the furnace to call for heat, and one to call for the air conditioner.
The Nest needs power itself, so it uses this “power stealing†technology to run a very small amount of current from the power line to the furnace line, siphoning some of it for itself. This is clever, but causes all kinds of compatibility problems. I went through two Nest thermostats with my old furnace before their technicians finally declared it incompatible and said, in a nutshell, to buy a different thermostat. This time, I wanted to make sure my furnace was going to be compatible. This is a common problem; read here and here. Even if it works initially, over time as the Nest’s battery loses compatibility and it needs to steal more power, it may cross a threshold from “not a problem†to “problemâ€.
There’s another wire, the “C†or Common wire, that acts as a power supply for your thermostat. Nest’s Pro Installer Guide says “Common wire (C) is not required in most cases, but strongly recommendedâ€. So I wanted a common wire.
There are two ways of controlling a dual-stage furnace. One is to let the furnace do it. But the furnace itself doesn't know anything about the temperature in your house, or the temperature outside; all it knows is “heat on†or “heat offâ€. So it uses the run time to determine when to kick from stage 1 to stage 2. When calling for heat, the furnace will always run in stage 1 for some time period (I believe it’s 15 minutes) and then will automatically switch to stage 2.
With my existing 4 wires, I could have had the system installed and working with the Nest using power stealing (which it says is “Compatible†with the 4 wire config with no C wire), and the furnace managing the switch from stage 1 to stage 2 itself. This is what my installer would have done, had I not done the homework and found out the drawbacks to doing this. Â
So I had them fish a 6 wire conductor through. They did this at no additional charge once I showed that I had a reason for wanting it.
Now, the Nest itself will decide when to turn on Stage 2, and it makes a big difference. Stage 1 is barely noticeable (much quieter than the old furnace); Stage 2 pumps out a lot of heat.
The Nest even tells you when it’s using Stage 2 Heat, which is kind of cool.
So my advice in a nutshell:  Don’t install the Nest without a “C†wire supplying power to the Nest itself, and don’t run a dual-stage furnace without having a smart thermostat control the stages.