How to hook up a 110V thermostat to furnace fan

I need to know a little more about the outdoor boiler system like how it connects and is controlled. You might benefit by a multi-stage thermoststat or you may be able to use your existing one, but for know whithout that additional info:

I assume that you want to turn on the fan of the furnace when the line thermostat goes on. To do that, take a relay with a 240 or 120 coil (Voltage of line tstat) and connect to the line tstat and then take the normally open contacts and connect to terminals are and G of the furnace. Leave existing wires there.

You may be able to put a switch that says propane or boiler and use the existing stat, but I need a little more info about the boiler controls.

-or- use a multi-stage stat.

From PM. Your post should have posted right away. I'm posting it here.

Don't use the 12/2 line.

What it's basically saying is that you will have two stats, one for propane and one for the boiler and a two stage stat would probably be the best way to go. In this case the boiler can supplement propane, because both can be on at the same time.

But based on what the boiler Co wants you to do is to is to have two stats. One for the boiler and one for the propane. So, lets' explore this a bit.

The limit switch has Nothing to do with what goes on. What the limit switch does is turn on the fan when the gas is hot enough and turn it off a bit later to increase efficiency. Some furnaces use timers and sometimes it's done in the stat.

R to W says Call for heat
R to G turn on the fan

You do have to MAKE SURE that your furnace will operate the fan by itself in the heat mode. Some tstats and furnaces will not do this.

What it's saying is, if there is a call for heat on the boiler stat, turn on the fan in the furnace. This also assumes that the propane stat is off.

A better way would be a 2 stage stat, but it may involve extra wires that you may not have available. It could involve at least 2 (C) or common and W2 ( second stage heat).

One of the Honeywell 8000 series stats will work. I'm not sure exactly which model, but you need a 2H/1C at least. Prices range from $100 to $200 depending on model. If this is a vacation home, then you could consider stats that can be controlled via the telephone or internet.

The Vision Pro stat has way too many options, so you might get lost configuring.

Furnace Filters, Air Conditioner Filters, Air Quality - iaqsource has manuals and the stats in an easy format to look at.

One stat (the most expensive), uses only 3 wires to connect to the furnace.

It does look like the $121 model TH8320U1008 would work, with an additional relay or solid state device.

Stage 1 would be your boiler, stage 2 would be propane.
Problem is in your case stage 1, the tstat controls the fan and in stage 2 the furnace controls the fan. OOPS

But that's solveable. Just take a 24 V relay connected to C and W and have the Normally open contacts in parallel with are and G. W2 would go to where W of the old stat goes.

You'll need 2 more wires at the stat.

The highest price model only needs 3 wires.