I am in the process of installing a new outside wood boiler and connecting it to my existing force air furnace. My question is regarding the installation of a second thermostat to control the blower on my existing furnace.
I have fan relay switch that was for adding air conditioning to my existing furnace. Could I just connect the fan relay switch to the existing furnace and then connect a second thermostat to the relay and control the blower that way? I want the blower to come on when the second thermostat calls for heat and also when the existing furnace comes on and trips the fan limit switch. Will this work?

Thanks

You probably already have a fan relay in your furnace, this relay will bring on the fan motor in high speed and the fan switch will bring it on in low speed, if you don't have air conditioning you could change the wire on the relay to low speed operation or you could leave it on high speed which will cause it to run in high speed when the furnace is in normal heating mode and the relay is energised from the additional thermostat.

It's hard to understand exactly what you are trying to say. Somehow I get the impression that you have an extra fan control center. I'm also assuming that the second thermostat is a "normal" furnace 1 stage t-stat.

Thermostats don't provide "dry" contacts so you need to add a relay with a 24 VAC coil and a transformer. I think the fan control center does do that, but it may become unreliable in the long term since it's not designed to switch low level signals. You will have to check if the contacts are "isolated" or "dry".

Although all you need to do is to supply 24 VAC to the tstat and connect a 24 VAC relay so it activates on heat, you may wish to add two relays. One for heat and one for fan. Put both contacts in parallel and then place them in parallel with the existing t-stat's heat contacts. That way fan auto and fan will both work.

Digging up the terminals and wiring diagram will be a bit more difficult.

You can run it off the existing transformer, but having it connected to the controls for the woodstove makes more sense. More of a safety issue than anything else.

One of the cleanest set ups would be to buy a 2 stage heat thermostat. Connect the wood stove controls as stage one, and the furnace as stage 2. That way, when you aren't around to keep throwing in more wood, the thermostat fires up the furnace.

Labman is on it with the 2-stage thermostat suggestion. Assuming this same system is not used for cooling, a heat pump thermostat would provide blower control for your first stage (wood boiler) independent of the fan/limit control on the furnace. An auxiliary relay could be added to the "W" or "W2" circuit and used to break the "G" circuit, giving blower control back to the fan/limit when heating with the furnace. Use a DPDT type relay if you want to also break the "Y" circuit and shut off first stage. You may have to "tinker" with the relay-controlled blower speed to get the ideal first stage temperature rise. If you encounter problems with the boiler, simply switch the t-stat to emergency heat and the furnace will take over.

I have mine on a 2 stage heating digtal thermostat. I put my wood heat on stage one ,that cuts on my indoor blower motor and my hot water punp on my wood burner. I put the second stage on my geo-thermal heat pump.