I recently purchased the ecobee3 lite and am now stuck trying to install them. I have 3 Honeywell thermostats I am looking to replace and my heating system is oil/hot water. My home was built in 1970 and I believe the wiring involved here is pretty old. I am not an electrician but I believe I should be able to handle this project given some help.
For starters, each of my thermostats is hooked up with a red and a white wire, with one additional (blue) wire coming out of the jacket in the wall, but not hooked up to the thermostat:
As far as I can tell, there's no voltage coming from that wire. I checked with a multimeter and it came up 0 every which way I checked it. So then I made my way to the furnace. Here's what I see:
So this box is a Honeywell Aquastat Relay L8148A whose schematic can be found here.
Here are two angled views of the controller that give a better picture of the few different connection points:
Here is the inside of the cover plate for the controller that details the wiring a bit:
As you can see, the main controller has 1 pair of red/white wires labeled Thermostat in the top-left corner. Then there are two transformers affixed to the furnace that also have red/white wires. I'm at a loss because most "add a c-wire" videos show a controller that has 5 different connection points, one of which is clearly labeled C, and they run a wire from there and everything works. I can't find a spot that I'm confident would be a "C" wire spot. The ecobee3 lite offers a PEK but that looks like it also requires more than just two wires to be run into it to operate properly. Many of these smart thermostats assume you are also controlling a cooling system which I am not; I only have a furnace.
So my question is: Can I install an ecobee3 lite myself in this system without a ton of work, or should I just buy something like the Nest E that can power-steal from my furnace?
Best Answer
You have a bigger problem than chasing down a blue wire here
Here's the rub -- the Honeywell L8148A doesn't even provide a C terminal for the blue wire, PEK, or whatever to connect to, as it was designed for dumb mechanical 'stats. Considering it not only contains a transformer and relay, but a high limit function as well, this leaves you with limited options -- simply replacing it with a separate transformer and relay won't do. One could add another transformer and relay as an isolation relay, but that's probably more complexity than is called for here, considering that Honeywell makes a more versatile version of what you're looking at, called an L8148J.
Replacing your L8148A with an L8148J configured for a 24VAC gas valve (valve from B1 to B3) lets you connect the C wire to the TV terminal, jumper T to TV, and then pull the jumper from Z to W, connecting R to Z and W to W. Of course, this all presumes that the cable doesn't transition from 3-wire to 2-wire somewhere in the middle; if it does, then you need to use a PEK or add-a-wire device to handle the situation atop the aforementioned aquastat changeout.