How to wire multiple thermostats

Question

I would like to have two thermostats controlling a single furnace. However I'm not 100% sure how I would need to wire everything to achieve the correct effect.

Assuming a thermostat simply acts like an on/off switch, I think that I could run another set of wires in parallel to the existing ones. I believe the current thermostat simply uses two wires. Would this work? Are there considerations as to the type of furnace or thermostat?

I'd add the second thermostat in the basement and normally keep it at a low temperature unless I'm in the room. As it is now with the thermostat upstairs, it's very difficult to keep a consistent temperature below.

I don't want to move the upstairs thermostat because if I'm not in the basement I don't care how cold it gets and the upstairs would become inconsistent.

Answer 1

I'm going to go ahead and say that YES, you can do as you propose, wiring the two thermostats in parallel.

While, as others have correctly pointed out, this will not let you achieve temperature control in both the upstairs and downstairs simultaneously, I'm going to take you at your word that you simply do not want it to be uncomfortably cold in the basement when occupying the basement, and infer that you don't care what happens upstairs during those times.

In this case, go for it--install the second thermostat.

The upstairs area of the house will in fact be over-heated when you use this basement thermostat, but you may still consider the result to be an improvement over the current situation.

The "correct" way to solve the problem is to split your system into two zones, which requires installing baffles/valves on your heating ducts/pipes, which would allow independent control of upstairs and downstairs.

Answer 2

You want multiple zones on a single furnace. To achieve this you need more than an additional thermostat - you need electronically controlled baffles in your heating system.

The way such systems work is - you have each "zone" blocked off by a baffle, and a thermostat in each zone. Both thermostats can call for heat, triggering the furnace to fire, and each thermostat controls one baffle. When heat is needed, it opens the baffle (while calling for heat) - when heat isn't needed it closes the baffle.

Generally this is not a DIY project - not that you couldn't do it but it's a pretty specialized skill set.

Answer 3

A thermostat is basically a relay that energizes one or more circuits (ie: Fan AND heat), usually with 24VAC. The R/RW wire is the incoming power source, and then it sends power back through the other wire like Y, G, W, etc.

There are some other thermostats that are line voltage and simply switch 120/240VAC instead of 24VAC.

You could probably wire two up, but as tester101 mentioned, it won't really help with your problem.

What you probably want is one thermostat with multiple thermometers (one built in to the thermostat, and one remote). Usually the temperature is averaged across both thermometers, so while not perfect, it does help keep one level from being really hot/cold and the other level the other extreme (both will sit somewhere in between).

Look around for thermostats that support a remote wired thermometer.