Trying to Wire a Second Thermostat to My Furnace Blower...

I would like to add a second thermostat to my furnace that would turn on only the blower when a high temp is reached in the basement where the coal stove is located to help distribute the heat. After reading the Continuous Blower Mode section of the install manual I thought I would be able to do this by wiring the second thermostat to the R G terminals in the furnace. The manual states "When the R G circuit is made, blower motor operates on heating speed."

Well the blower does come on, but so does the outdoor air conditioner compressor. I looked for what would kick it on and the Cooling Mode paragraph states "...the R Y circuit starts the outdoor condensing unit" . The second thermostat is not wired to the Y terminal, but could it somehow be getting energized via the main thermostat upstairs since its wires are all still connected to the furnace? The upstairs thermostat is in HEAT mode and is set to activate the furnace at 60*.

I guess I could leave it this way and just turn off the air conditioner breaker for the winter so the compressor won't come on, but I thought getting just the blower to come on would be fairly simple. The furnace is a propane 350MAV unit. Any thoughts?

Does your thermostat have a fan ON and AUTO switch ? Mine does, when in the ON position the fan runs all the time without anything else(heat or A/C) running. I have an aquastat and a bypass switch at the squirl cage fan. I switch to fan ON and an aquastat to make on temperature rise in the air handler when the pipe is hot. When it's cold, the switch breaks and the fan stops. I have a bypass switch for summer A/C.

How many wires are hooked up to your thermostat? Do you have separate fan control on your existing thermostat? Like he said, can you switch the fan ON independently from the furnace. If you have only have 2 wires, then the fan operates automatically with heat/cool controls, unless you wire directly to the circuit board on the furnace, which is what I think you did?

I would make sure the breaker is off to the A/C. Not like you need it in the winter.

The second thermostat (a simple mercury switch style) has only two wires, connected to R G on the thermostat and the furnace circuit board. The upstairs thermostat (a programmable LCD) has R W Y G & Com connected to the circuit board.

I could leave the breaker off so the AC compressor doesn't run when the 2nd 'stat calls for the fan, but if it is somehow backfeeding through the upstairs 'stat I didn't want that to happen.

The upstairs 'stat does have a fan control that I can turn it on manually, but the idea was to have the fan come on automatically based on the temp in the basement. I thought from what I read in the install manual that I could control just the blower in this manner.

I just did this to my furnace. I only had a 2 wire thermostat so I couldn't wire it to automatically turn on. So, what I did instead was wired a 12 hour timer switch to the limit switch on the blower. Its 110v and I only use it once in a blue moon. I have enough trouble getting my house above 67* in this weather.

When I get home I can look at my wiring and let you know how I have it wired.

Titleist, make sure it isn't something simple like a short between G and Y terminal in the t'stat from striping too much insulation off the wires and causing the short circuit. Do you have an RC-RH terminal in the t'stat or just an R terminal?

Your right, energizing terminal G should not energize terminal Y.

There are RC and RH terminals in the upstairs Honeywell LCD programmable 'stat that controls the furnace/AC. There is a jumper between these terminals in that 'stat. There is an R terminal in the basement 2 wire 'stat that I want to control only the blower.

I double checked both 'stats and the G and Y terminals are separated and there is no short from the wiring between them. There is no short between the terminals from the wiring on the furnace circuit board either.

I guess I'll just continue to manually turn on the blower from the upstairs 'stat occasionally.

Possable you could run line voltage to a relay then to the fan motor, use the thermostat to close the relay. I would want the fan blowing air that is alot warmer than 70, I used a heat pump in the past and the air always felt cool.My ductwork is in the ceiling(good for A/C but not heat) Low fan speed would be good.

I agree if it was just that it was blowing the 70* air that would not be ideal, but by doing this (manually) I am able to balance the temps across the house a little better.

The temp can get up to 85 in the basement where the stove is and I have a return vent in the ceiling just in front of the stove (there is also a supply to balance out the pressure) so I pull nice warm air from there. At the same time I am pulling air from the returns in the back bedroom and hallway which can be 7 - 10* cooler than the rest of the main floor depending on the wind direction.

The result of manually using the blower is that I can get each of the rooms within 3 or 4 degrees of each other which greatly lessens the whine factor in the house. I was just hoping to do this automatically by using a second 'stat in the basement to kick the blower on when it hits 82* or so down there. I hadn't thought out the fan speed issue, but it would be better to run it on low speed.

Titleist, do you have a separate air handler in the attic for your A/C? Is there a wire on terminal RC and another wire on terminal RH? If this is your set up, try taking out that jumper between RC and RH. Some how you are back feeding some where that is making the Y terminal to become energized.

There is no separate air handler in the attic. It is a single unit in the basement. There is a single red wire on the RH terminal in the upstairs 'stat and that is terminal is jumpered to the RC terminal.

I played around with it a little more tonight and the Y at the furnace is definitely getting energized through the upstairs 'stat module (not just the terminal block) when the downstairs R G get energized. I was able to pull off the honeywell 'stat module and leave the terminal block behind with all the wires connected to their correct terminals. When I did this and triggered the basement 'stat to call for the blower it worked correctly and the AC compressor did not come on. As soon as I replaced the upstairs 'stat and the pins from the module touched the terminal block the AC compressor kicked on. I'll have to see if I can find any documentation for the upstairs stat and see if there is a setting that would explain this.

That's good, you have the problem isolated to the t'stat. Yup, time to break out the manual to that unit. One of the reasons I don't like those programable stat's, every modle is different.

I think I have it working as I want!!

I found the manual online

http://www.instructionsmanuals.com/u2/pdf/termost ... 30B-en.pdf

There is a J2 jumper that has an HE (electric heat) and HG (gas or oil heat) setting. It was set to HG when I looked at it which makes sense since I have a propane furnace. When the fans switch is in auto mode both settings act the same when the stat is set to COOL, the fan activates when cooling is called for. When in HEAT mode and HE setting, the fan activates when heat is called for. When in HEAT mode and HG setting the fan is controlled by the furnace fan control circuitry.

Apparently they need an HC setting for coal heat! Anyway, I switched the jumper to HE and tried everything out. The blower comes on when the upstairs stat calls for heat and goes off when the temp is satisfied. There was a delay before it came on as per usual until the heat exchanger came up to temp and the blower ran past the firing time the same 135 seconds that it had before.

When the downstairs stat calls for the blower, it alone comes on and the AC compressor does not kick on.

I don't see anything in the explanation of the HG setting that would cause the AC compressor to come on when in HEAT mode and R G is energized, but it does. Of course the stat could be defective.

Anybody know something I don't about this HE / HG setting that will get me in trouble with how I am using it?

titleist1 wrote:Anybody know something I don't about this HE / HG setting that will get me in trouble with how I am using it?

Nope, I think you are to go. A little research paid off, good going.

Glad to hear you solved the A/C with the fan problem. I've been toying with doing something similar for my gas furnace as well...more as a protection from coal-stove fan failure rather than too warm in the basement.

One suggestion...attach a note to the thermostat and to the coal stove as a reminder to put the jumper back to the original position when shutting down next spring. I have done just that on my furnace as some changes I implemented concurrent with my firing up a few weeks ago would damage my furnace if I simply turned it on without thinking.

This "getting old & forgetful" thing has forced me to resort to written notes rather than trying to trust my memory like I used to...