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buckwheat1 · Jul 17, 2008, 04:22 AM

Hi,
I'm putting in a Outdoor Wood Boiler system. I need to connect a Line Voltage thermostat to my existing forced air furnace fan and need assistance. I will be keeping the low voltage thermostat operational to use my propane system when I'm away. Obviously I'll be running a 12/2 line from the thermostat down to the furnace area, but I'm not sure what to connect the wires to. Could someone give me a clear idea on this procedure?

Thanks,
Buckwheat1

KISS · Jul 17, 2008, 04:45 AM

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 R 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.

KISS · Jul 17, 2008, 08:19 AM

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

Originally Posted by buckwheat1

Hi, my apologies. I'm new to this forum and obviously don't know how to use it yet. I tried to reply to your answer but I don't think it went through so I'll restate my response.

The OWB has a Honeywell Aquastat that controls the temp. of the water surrounding the firebox. A circulation pump circulates the hot water in to the heat exchangers located in the existing plenum of the furnace. The hot water circulates constantly keeping the plenum warm and ready to use. I have gotten instructions from both the stove company and the installer. The installer is a plumber and admits that he doesn't like to work on the circuit boards of the furnace so he suggested running a 12/2 from a new 110v t-stat. On the other hand, the diagram I have from the stove company shows a new electronic t-stat, which I am assuming is a 24v, connected to the old t-stat. The wiring from the new to the old is; "R" and "W" from the new to ""R" and "G" on the old. I would prefer not to have to run a 12/2 line if I could avoid it, and the stove companies set up looks easier, but I don't know which would be better. The only thing I thought of that might cause an issue is using the "new to old" set up is concerning the limit switch. With 110v I'm just turning on the fan, with the other set up I assume that the fan is operated by limit switch to turn on the fan. Is that a correct assumption? Being that the heat exchangers are higher up in the plenum will it get enough heat to activate the fan?

Thanks in advance,
Buckwheat1

KISS · Jul 17, 2008, 08:38 AM

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.

KISS · Jul 17, 2008, 09:51 AM

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 to 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 R 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.