Adding Thermostat Controller For HVAC Blow Fan Relay.

Thread Starter

norzmen

Joined Nov 15, 2022
13
I'm looking to purchase a new thermostat or add a temp. controller to my current thermostat that will call my furnace blower on at a set temp range, before my furnace kicks on. The application is to distribute heat throughout the ducting in my house through the single return vent that is located in the same room as my wood stove. Let me know if anyone has ran a similar setup like this. My current thermostat has auto and on settings for the fan. Thanks
 

Reloadron

Joined Jan 15, 2015
7,084
My current thermostat has auto and on settings for the fan. Thanks
You should be able to work from that feature. Normally in a color coded system the green wire is the fan. When you set your thermostat to the fan setting The system normally sends 24 VAC to the thermostat. When you have the fan on the thermostat is just switching 24 VAC to the fan line (normally a green wire). You should be able to remove the fan line from your existing thermostat and get 24 VAC from your existing thermostat. Add another On/Off thermostat and let the added thermostat switch the fan line On/Off based on your setpoint. Most of this depends on how your existing system is wired.

Ron
 

MisterBill2

Joined Jan 23, 2018
13,805
What is not mentioned at all is if your furnace is also part of a cooling system,where at one time the furnace blower of heating only systems was switched on directly by a mains rated thermostat located inside the furnace, sensing the air temperature on the air side of the heat exchanger. That worked fairly well for heating-only systems, and for some heating-cooling systems where a higher fan speed was selected for cooling. BUT MOST heating/cooling systems now use a relay to control the fan motor. The lower speed is enabled through the normally closed contact and the same "bonnet" thermostat in the heat exchanger. For cooling, the relay is operated to engage a higher blower speed through the normally open contact. This avoids the possible engaging of two speeds at once, which leads toward blower motor failure.
So operating that relay to run the blower motor during the heating time will work, but the motor would be running at the higher speed selected for cooling.
So we need to know if he furnace is also part of the air cooling system, if there is one. Not allparts of the world need air conditioning.
 

Thread Starter

norzmen

Joined Nov 15, 2022
13
The furnace and heat exchanger are seperate systems. There is no auto change over, just manual switching at the thermostat from heating or cooling. The blower inside of the furnace moves all the air in the system and what is what im wanting to control. I was thinking about getting a more modern thermostat that has a setting to cycle the thermostat based on a time interval per hour, but also thought it would be nice if i could control the blower based on temp. instead of time. Awesome to have a resource like this for discussion and thank you so much for your response. Any more info. needed let me know.
 

Thread Starter

norzmen

Joined Nov 15, 2022
13
You should be able to work from that feature. Normally in a color coded system the green wire is the fan. When you set your thermostat to the fan setting The system normally sends 24 VAC to the thermostat. When you have the fan on the thermostat is just switching 24 VAC to the fan line (normally a green wire). You should be able to remove the fan line from your existing thermostat and get 24 VAC from your existing thermostat. Add another On/Off thermostat and let the added thermostat switch the fan line On/Off based on your setpoint. Most of this depends on how your existing system is wired.

Ron
I was thinking the same thing but what some concern there would be issues if furnace tries to kick on if the wood stove heat can't keep up. I guess i would conider that the second stage of the system.
 

Reloadron

Joined Jan 15, 2015
7,084
I was thinking the same thing but what some concern there would be issues if furnace tries to kick on if the wood stove heat can't keep up. I guess i would consider that the second stage of the system.
Less seeing a whole system schematic it's hard to make calls or guess, Today's furnace systems are a long way from a few decades ago. Just for heating the fan comes on first and a pressure sensor in the flu makes dure there is adequate airflow. Makes dure there is no obstructions. Then on a gas combustion system gas comes on and igniter. A Flame is sensed and things work, no flame and the system should shut down. Once the system has reached temperature set point the gas supply shuts off and the blower continues to run for a preset time. You want the fan to come on before the original thermostat calls for heat, So my thermostat id set for 70 F. I set my added thermostat for 67 or 68 to just operate the fan a few degrees before the original thermostat would start by calling for heat, That's what you are after right? Placing the Fan from auto to on will just result in the fan dunning 24/7. You just wans the fan to come on at a lower temp than the old main thermostat calls for heat.

Eon
 

Thread Starter

norzmen

Joined Nov 15, 2022
13
Less seeing a whole system schematic it's hard to make calls or guess, Today's furnace systems are a long way from a few decades ago. Just for heating the fan comes on first and a pressure sensor in the flu makes dure there is adequate airflow. Makes dure there is no obstructions. Then on a gas combustion system gas comes on and igniter. A Flame is sensed and things work, no flame and the system should shut down. Once the system has reached temperature set point the gas supply shuts off and the blower continues to run for a preset time. You want the fan to come on before the original thermostat calls for heat, So my thermostat id set for 70 F. I set my added thermostat for 67 or 68 to just operate the fan a few degrees before the original thermostat would start by calling for heat, That's what you are after right? Placing the Fan from auto to on will just result in the fan dunning 24/7. You just wans the fan to come on at a lower temp than the old main thermostat calls for heat.

Eon
That's right, wanting the fan to turn on to distribute heat from the wood burning stove throughout the house. If the wood stove can't keep up then the furnace kicks on and gas heat kicks in.
 

MisterBill2

Joined Jan 23, 2018
13,805
OK, now we have some of the information required. BUT there is the concern that the green wire that operates the relay to run the blower runs the motor at a higher speed connection than the speed used for heating. That is usually how it works, that the cooling function runs the blower quite a bit faster than the heating function. In those applications the green wire for fan operation is only active for the cooling mode.
For heating the motor is often operated by the thermostat mounted on the heat exchanger itself. So the very first step will be to determine if that is the case. That can be done by examining the blower motor connections tag to see if it is a multiple speed motor or not, and examining the thermostat on the heat exchanger to see if it has mains rated blower control contacts.
There is usually a second set of contacts to switch off the gas at some higher temperature, often tagged as "Limit".That is used to prevent overheating if the blower fails to operate. Leave that one alone.
A simpler method of implementing controlled blower operation will be to connect another relay with the normally open contacts across (in parallel with) the contacts of the motor control thermostat on the heat exchanger. THat relay can be controlled by your added thermostat heating contacts. So the added thermostat can be a a heating only type with only two terminals, usually red and white. Actually, this will be a simpler installation with less risk of problems, because of not changing any of the existing connections.
 
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Reloadron

Joined Jan 15, 2015
7,084
I was thinking the same thing but what some concern there would be issues if furnace tries to kick on if the wood stove heat can't keep up. I guess i would consider that the second stage of the system.
I doubt the latter would matter. The fan would already be running and if your system uses a pressure sensor to know the fan is running it should just start and ignite the natural gas supply making more heat. With a little luck a Google of your existing thermostat and furnace will yield a schematic or two to work from and make sure of things. I am basing what I suggest on how my system works.

Ron
 

Thread Starter

norzmen

Joined Nov 15, 2022
13
All suggestions make sense. My next step is to look into the purchase of the heat only thermostat. It would be really nice if a thermostat was available in which I could set temp. differential or a timer option. My concern is the blower fan will be cycle on/off quite a bit due to high cfm of the blower and limited heat available in the room with the wood stove / return vent. I checked into a few thermostats on grainger and all had the standard 2 degree temp. diff. Any suggestions?
 

MisterBill2

Joined Jan 23, 2018
13,805
OK, a valid concern about the fan cycling. But we have no clue yet as to the generation of the furnace.
Is it one of those complex super high efficiency types? Or a much older plain gas-fired forced air types of any vintage? Or a computer controlled from not recent production??
 

Thread Starter

norzmen

Joined Nov 15, 2022
13
OK, a valid concern about the fan cycling. But we have no clue yet as to the generation of the furnace.
Is it one of those complex super high efficiency types? Or a much older plain gas-fired forced air types of any vintage? Or a computer controlled from not recent production??
Older forced air type furnace with single speed blower and propane as fuel type.
 

shortbus

Joined Sep 30, 2009
9,762
The application is to distribute heat throughout the ducting in my house through the single return vent that is located in the same room as my wood stove.
Why not have a thermostat in the room with the wood stove, and an extra blower on the return duct from that room? The thermostat would need to be a cooling/heating type using the cooling setting. When the temp is above the cooling set point it would then cause the extra blower to turn on, sending the wood stoves heat through the normal furnace ducts.

Doing this would only bring heat from the wood stove room when heat was available.
 

Reloadron

Joined Jan 15, 2015
7,084
I checked into a few thermostats on grainger and all had the standard 2 degree temp. diff. Any suggestions?
That is how most come short of getting more expensive. One potential issue with the 2 degree hysteresis is knowing how accurate each is. A few years back I was curious so compared my system to a standard thermometer and found it was within a degree. Mine is a pretty basic Robert Shaw, heating/cooling and programmable for 7 day week. Standard 24 VAC system. As long as you have the fan running at a lower setpoint below the standard house furnace thermostat things should work fine. Actually you could just parallel the added thermostat with your existing for the fan only. I also like what shortbus mentions:

Why not have a thermostat in the room with the wood stove, and an extra blower on the return duct from that room? The thermostat would need to be a cooling/heating type using the cooling setting. When the temp is above the cooling set point it would then cause the extra blower to turn on, sending the wood stoves heat through the normal furnace ducts.

Doing this would only bring heat from the wood stove room when heat was available.
You can have pretty much whatever you want depending on how you want to go about things and budget. :)


Ron
 

Thread Starter

norzmen

Joined Nov 15, 2022
13
The extra blower would need to be large to push heated air from the stove through the ducting of the entire house. We currently have no room for the added blower in the current duct system so would need to cut another large opening in the wall for blower and duct into the existing duct. Large dollar cost for the project and the wife does not want another large vent in the wall. She already is on me for repainting or at least dressing up the current return vent.
 

shortbus

Joined Sep 30, 2009
9,762
I was just telling how I did it for my exwifes house. The blower and stove was in the basement with the furnace. The blower was actually an old furnace that I cut the top/heat exchanger part off and then added a sheet metal cover on it and ran a pipe from it into the existing furnace.
 

Thread Starter

norzmen

Joined Nov 15, 2022
13
I think another option would to use just a standard programmable thermostat with the feature to control the temp. differential. I could pull power from the existing thermostat and parallel the fan relay terminal on the new thermostat with the old thermostat.
 

Thread Starter

norzmen

Joined Nov 15, 2022
13
I was just telling how I did it for my exwifes house. The blower and stove was in the basement with the furnace. The blower was actually an old furnace that I cut the top/heat exchanger part off and then added a sheet metal cover on it and ran a pipe from it into the existing furnace.
I definitely like the idea and If i had the luxury of a basement I would be all in on it. Adding the extra hole in the wall could put me in an exwife situation, lol.
 

Reloadron

Joined Jan 15, 2015
7,084
I think another option would to use just a standard programmable thermostat with the feature to control the temp. differential. I could pull power from the existing thermostat and parallel the fan relay terminal on the new thermostat with the old thermostat.
Yep, that should get you where you want to be.

Ron
 

MisterBill2

Joined Jan 23, 2018
13,805
One option would be to get one of the much older mechanical thermostats that were used with coal furnaces. Most of those had adjustable differential points.. Even a modern mercury one should be able to increase the differential by using a slightly stronger magnet. OR use two thermostats in parallel
 
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