I've got an older home (plaster walls) and an older furnace, It's a Peerless. My current thermostat is located on my main floor and only has 2 wires to it, red and white. I should mention that my heating system is hot water with radiators. Anyway, I want to get a new wireless thermostat installed. I'm pretty sure I will need a new thermostat wire with a C wire in it, but can I just install that straight from the furnace? The "router" or base will be located in the furnace room so I'm not worried about having to snake it through the walls but I'm wondering if I have 24v coming from the furnace and if I can easily replace my current thermostat wire with a newer one with a dedicated C wire.

  • Do you have a link to any details on the specific thermostat you're considering? – TomG Aug 2 '14 at 2:42

Trace where the wires from the thermostat to your furnace. You'll find that one connects to a relay and the other connects to a terminal on a transformer. R is the wire from the transformer, W is the wire to the relay. (Your thermostat may be connected to the wire backwards, but that doesn't matter.)

Your new C wire will be the other terminal on the transformer. Or if the transformer has multiple taps/terminals, then it will likely be labeled COM or -.

I suggest instead of trying to monkey with the wire to your existing thermostat that you just investigate where it's connected to and make new connections. That way if you run in to problems you can simply reconnect the old thermostat easily.


The R wire (or wire wired into R at the thermostat) will be your 24v continuous power. If you want to use a thermostat with wireless capabilities you will need to run a new wire (at least 18/4 wire) to accommodate the new thermostat. Depending on the type of thermostat you get, you will either need to wire it to the furnace, or to an EIM (equipment interface module) and then to the furnace.

Another solution is to get a wireless communicating thermostat. These types of thermostats only require continuous 24v power, and a common to run. This may be taken from any source (such as a transformer in the furnace). Then you will have to install an EIM in the basement that communicates remotely with the thermostat. Then he EIM will be wired into the furnace and will activate certain operations from the unit.

This type of thermostat may be the easiest for your application, however there are very few of these on the market, and are quite costly thermostat packages.

  • Thanks Mnc123, and that's exactly what I want to try and do. Set up an EIM in the furnace room, near furnace and have it communicate wirelessly to my thermostat, which I can place wherever I want. So what you're saying is I can "add" a wire (18/4) to the "R" wire on my furnace and then put in on my new EIM, and then add the 2 wires that ran to my thermostat, with a total of 3 wires now to my EIM, that should work right? I'm only looking to control heat as I have hot water heat with radiators. Thanks. – Bryang Jan 3 '14 at 15:25
  • Yes im aware that this is a heating only system. The only reason you would be adding this type of thermostat is to get Wi-fi capabilities. This includes being able to monitor and control the temperature of your home via a smart phone app. If you do not need this function, and just would like a newer style thermostat, i would recommend getting any standard thermostat that you prefer and only hook it up for heating. Now wiring between the thermostat and equipment interface module is dependent on the thermostat manufacturer and the specific thermostat. – Mnc123 Jan 4 '14 at 0:04

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