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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.

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You won't be able to heat your basement and upstairs separately simply by adding a thermostat. If the basement thermostat calls for heat, the heat will be delivered to the whole house. So your basement will be comfortable, but the upstairs will be extra toasty. –  Tester101 Nov 14 '12 at 19:46
    
There are wifi-enabled thermostats that allow remote control from a computer/phone/tablet (I have a CT30 that does this). There are some that allow multiple thermostats to be linked together (eg, NEST, which also has motion sensors to automatically recognize when you're there), although this still doesn't get a multi-zone system, just a way to heat or cool the entire house from multiple locations. –  gregmac Nov 14 '12 at 21:38
    
I have a similar dilemma with a 3-storey house. A thermostat on each floor will enable the boiler/ furnace to be fired up whenever that floor is not at the desired temperature. Rooms coming off each floor can be controlled by mechanical thermostatic valves on each radiator. We cannot beat natural science and heat will rise so any space that is left open to heat coming from downstairs will be heated beyond the thermostat setting. Therefore keep doors shut. –  user15356 Sep 30 '13 at 20:26
    
Two risks that I see are: you can't turn off the heat from one floor if it's turned on from the other and if one thermostat calls for heat and another calls for AC, it could have unexpected results. –  BMitch Sep 30 '13 at 21:53
    
If you do this be aware each thermostat will be oblivious to actions of the other. Any cycling restrictions enforced by thermostat to prevent damage to heating or cooling systems will be ineffective. –  draftking Dec 16 '13 at 0:14

3 Answers 3

up vote 4 down vote accepted

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.

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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.

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This is a good answer too but mac specifically addressed my concerns, I'm going to try the second thermostat and if it doesn't improve the situation, I'll think about adding zones and baffles. –  case nelson Nov 15 '12 at 8:08

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.

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What they really want is a multi zone heating system, which cannot be achieved by simply adding thermostats. –  Tester101 Nov 14 '12 at 20:03
    
Thanks for describing the wiring. Although a multi zone system would be ideal, I'm working on the assumption that I can properly regulate the zone I'm in and ignore the other zone. That should put me in a (slightly) better situation than I am now. –  case nelson Nov 15 '12 at 8:14

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