How do I wire a thermostat for a wood burning furnace?

Wow. If I understand your post correctly you are asking for something that is a bit beyond simple thermostat wiring. If you don't know how to wire a basic thermostat to your wood stove, I think it may be a bit beyond you to do a thermostat, relay and probably transformer just with help from here. A decent HVAC tech won't have any trouble, but I've even seen them do some convoluted solutions to non-standard setups like this.

Actually, as I was typing that last sentence I realized that it could be done really neatly with one 2-stage thermostat and (if required) relay isolation for the wood stove. Still won't be exactly simple. I don't have the time to do a wiring diagram drawing for you, and I would need some information that is missing in your post to really be certain I would be hooking things up correctly. Maybe one of the list professionals could help farther.

This is all I have for this

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Did you get an answer on this? I am doing the exact same set up. I have learned several things. I tried wiring a second thormostat across the pwr and fan terminals. From red to green thinking the original thermostat would remain un changed but this caused a problem. When the auto/on switch is in Auto and you power up the fan the AC will also come on. I moved the switch to ON, removed the green wire from main thermostat and ran it to a second thermostat. Powering the second thermostat also with a wire from the are of the first thermo. This works fine for now but is not the best set up. I believe I may be able to makes things better by adding a swich at the furnace and break the wire to the AC and having a summer / winter selector I would move twice a year. Let me know if you solved this problem already.

Piece of cake.

To make it simple, set up the wood burning thermostat for conventional heat. You can also connect C to C if present.

Jumper Rc to Rh if present and we'll call these terminals are.

Connect are of the wood t-stat to are of the furnace t-stat.
Connect W of the wood tstat to G of the of the furnace t-stat.

This would say if the wood stove called for heat, the fan would come on. It also says that the fan will run continuously when the wood stove was out.

The way to fix that is to put a NO thermal switch on the flue of the wood stove and interrupt the connection to the other thermostat.

This way, the flue temp, would determine if the fan stays on.

For the initial lighting of the wood stove, until the stove got hot enough and if you wanted to have the blower running during that time just use the G switch on the thermostat.

Another way, would be to use a time delay relay and a push button wired at the stove. This would engage the fan for x time and you'd push it when lighting.

Sorry, don't know enough about wood stoves to know if the fan nees to be running during lighting or not.

As I said earlier, use a two stage thermostat. You would hook the gas furnace up as the second stage of heat (W2) and then set the second stage differential to 10 deg or thereabouts. Nothing would be hooked up to the first stage terminals (W1). Set the thermostat to call for heat at 80 degrees (10 degrees or 2nd stage differential above where you want the gas to kick in) and the fan will be on as long as the temperature is below the first stage set temp. The gas furnace will kick in at 70 degrees and you will have it all done in one thermostat.

If you need W1 to call your wood stove somehow, you would have to isolate it with a relay. A 2 stage heat pump thermostat would work fine, as they often don't have a W1 terminal to worry about. Just make certain the one you use has an adjustable 2nd stage differential and shuts the second stage off when it is satisfied, not when both stages are satisfied.

If you need wiring help I will need to know more specifics on your existing thermostat and the exact requirements of both heat systems.

Some tstat manufacturers (Honeywell, for example) won't tell you how there tstats work.

EP:
Can you suggest a possible tstat? Link to a manual?

I actually think the higher units in the FocusPro line will do what I suggest. Honeywell's website is lousy for getting help quickly, so I am not going to look that up now. ClimateMaster has a unit that would do it, models in the ATP32U series. I think some of the ICM multi-stage tstats might. The 2210 would work and it's cheap. If you need more than 6 deg for the differential setting you would have to use W3 with that one. I use the ICMs because we need max temperature lockouts. They are not a high end unit, but as I said, they're not pricey. White-Rogers and RobertShaw have units but I can't come up with numbers quickly.

Just some ideas. Google them, the mfgr links come up early.

Furnace Filters, Air Conditioner Filters, Air Quality - iaqsource makes it easy to get info. It's organized very well for what they sell.

It's hard getting info as to whether a thermostat say drops W1 when a W2 call happens. I suppose it does. But try to find that in writing.

I looked here: http://www.ntsupply.com/files/products/9915i.pdf

For specs on the Robert Shaw 9915i stat and all I have is questions. Like what is the max differential? The table says O & B terminals. Does it mean it has both? There's probably another manual somewhere.

I only use manufacturers websites for information. Most mfgrs have installation manuals for their tstats and those are where you will find how to set them up correctly. Honeywell has very good information, but their search function is absolutely pathetic. Don't search for a complete part number! And keep everything you find on your computer so you don't have to go through the agony again.

I don't believe that I've ever seen a fossil fuel tstat that drops W1 when a higher number call comes in, they are called in addition (see next paragraph for exception). The functionality that I suggested for the OP required that when W2 (or W3) is satisfied, they will drop out and leave the earlier calls in. Of course if there is any stage not satisfied, the fan will be on, which I believe is what he wants.

A heat pump tstat may disable the compressor (1st stage heat) on a call for emergency heat. Depends on how it is configured, but even that will still have the fan called.

That RobertShaw that you mentioned is a NICE tstat. Expensive too. And yes, they don't give you the max differential in the manual. I think you got the sell sheet. Here is the manual from the manufacturer. http://www.icca.invensys.com/manuals...w/110-1069.pdf I have one on my parts shelf if you really need to know that information.

I wish the OP would come back and give more detail on his requirements. If he wants the fan to be on all the time the woodstove is hot, and when it is cold, then the fan will run only when the fossil fuel furnace is called, then my idea won't work well. He will need two tstats for that the scenario, the one to call the fan for the woodstove would have to be set for cooling, and then he probably would need relay isolation for the two. The question was vague enough that I almost didn't jump in. Now I have spent a lot of bandwidth just about thermostats.

EPM