Can I control the fan with my old furnace

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Old 01-21-19, 11:17 AM
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Can I control the fan with my old furnace

I have a 1954 (65 yr old) Sears, Roebuck and Co. gas furnace for just heating my finished basement. Model 867 w/H-Q blower.

When I bought my house the previous owner installed a Carrier high efficiency furnace for my main living up stair area ( ranch) and kept the original Sears furnace ( with new duct work) for the basement. I'm upgrading the downstairs (removing the paneling and installing sheet rock and insulation) and noticed it's a two wire system (red and white) going to the thermostat. So I can control the heat but not the fan which runs all the time. The 1954 basement furnace heat works great. I know I would have to run other wires for fan control.
Attached are some pictures I took of the main components of the furnace.
Does anyone know if I can control the fan with a thermostat with this older furnace based on the model # and attached pictures?

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Last edited by PJmax; 01-21-19 at 11:56 AM. Reason: cropped/resized pics
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Old 01-21-19, 11:26 AM
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Yes.... you could have your thermostat control the fan.
You'd need a 24vAC NO (normally open) relay and one extra wire from the relay to the thermostat.
 
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Old 01-21-19, 11:31 AM
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Where would I make the connection to the thermostat and the furnace?

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Last edited by PJmax; 01-21-19 at 11:42 AM. Reason: add pic
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Old 01-21-19, 11:33 AM
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When you say the fan runs all the time do you mean it runs 24x7 without a heat call?
 
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Old 01-21-19, 11:44 AM
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Good catch A. Sounds like a stuck fan control.
 
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Old 01-21-19, 11:45 AM
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I had an old mercury Honeywell thermostat. I believe the red and white connection controlled the gas going on and off based on the thermostat feedback to the furnace But the fan kept running. I have an electrical engineering background but am not that knowledgeable with heating systems and thermostats. I'm assuming with my 2 wire system (red and white) controlled the temperature (gas on and off) but not the fan. Unless my old thermostat was bad. I've read some and thought that 2 wire systems will not allow you to control the fan.
 
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Old 01-21-19, 11:58 AM
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Correct. The thermostat only controls the gas valve. The left side of your fan & limit switch turns the fan on when the furnace warms up and turns it off when it cools down.
Article on fan & limit switches: https://inspectapedia.com/heat/Fan_L...ion_Wiring.php
 

Last edited by Astuff; 01-21-19 at 12:17 PM.
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Old 01-21-19, 12:00 PM
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In your fourth picture is the fan/limit control. That controls the blower. That metal disc turns as the temperature changes. If you look on it you'll see ON and OFF. Looks to be set fan on at 140 and off at 100.

Allow the furnace to fire and see if that disc turns. If not.... that control is bad.
 
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Old 01-21-19, 12:19 PM
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Astuff and Pete,
Thank you. My basement is currently in demolition stage so my contractor removed my thermostat so I currently cannot start up my furnace till I get another digital one. I will go get one at Home Depot and try what you are telling me to do.

Question (to make sure I understand):
1) So with my 2 wire system (which controls the gas on and off based on temperature feedback from my thermostat), my limit switch, if working correctly, should turn the fan on and off based on the settings (100 and 140 currently or what ever I set it to.)?
Other questions:
2) Can I run another connection wire with my old furnace to control the fan (based on my pictures)?
3) Should I not do that with a 64 year old furnace (let the limit switch do its job)?
 
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Old 01-22-19, 01:22 PM
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So I just picked up a new thermostat and set the temperature to 50 deg. F. Turned my furnace on and the fan goes on right away without any heat. My basement is 59 deg. F and the heat is not firing yet. Does the same even if there is no thermostat connection. Should I just assume I have a defective fan and limit switch? Should I take it out and test it using an ohm meter?
 
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Old 01-22-19, 02:49 PM
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I haven't used my basement heater for about 9+ years so I manually moved the dial of the fan and heater limit switch.
I started the furnace and everything is working as it should. Maybe the limit switch needed to move for it to work. I regulated the temperature with my thermostat 2 deg above ambient and I watched the limit switch turn as the heat continued. My fan turned on at the correct setting. The gas shut off at the limit. The gas came back on as the dial went down below the limit and proceeded to go to the limit again and turned the gas off again. I turned the thermostat 5 deg. below the new ambient and I watched the gas go off. As the limit switch turned to a cooler furnace to the fan off setting, the fan shut off. Makes sense to me now. Good education. I will keep my eye on it but it looks like everything is working. Thank you both for your help!!!
 
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Old 01-22-19, 03:25 PM
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With years of non use..... there's a very good chance the bi metal element got rusted inside preventing the hub from turning. Be very careful forcing the silver disc as the bi metal spring can become damaged.

The good news is that you've freed it up.
The bad news is if it doesn't turn freely and completely..... it may not shut the furnace down in case of overheat.
 
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Old 01-22-19, 03:41 PM
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Just another point to mention is that those pointers are manually adjustable if you want the fan to come on sooner or later or move your shut off temp higher, or you want to lower your high limit. Some people feel the air is a bit cool at 100 deg. since it's so close to body temp so it doesn't feel as warm.
 
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Old 01-22-19, 04:37 PM
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With most gas furnaces the limit switch is for safety, not for normal cycling as sounds like what it is doing. It appears to be set for default of 200บ. Running hot increases the likelihood that you crack the heat exchanger. Also 200บ air coming out of the vents can cause issues.

Possible fan limit switch it is out of whack since it was stuck earlier. Get something to measure the real temp. More likely not enough airflow for this furnace since no longer feeding the entire house.
 
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Old 01-23-19, 03:01 PM
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In my area, I would find furnaces installed basements with a "summer fan" arrangement. It would be wired from the furnace to the head of the stairs with a switch located there.
 
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Old 01-23-19, 03:58 PM
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Honeywell makes a version of that same fan/limit switch with a white manual override button on the control itself which can be controlled by a service switch at the top of the stairs.

This is just a sample of your options. They have different insertion lengths.

https://www.supplyhouse.com/Honeywel...1b10f138a86cf0
 
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Old 01-24-19, 06:03 AM
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Can this go to the thermostat fan control switch too?
 
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Old 01-24-19, 08:31 AM
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There is no short cycling problem here. The problem is the blower won't shut off.

The fan/limit switch should be replaced to keep the furnace operating safely.
You would still need to add a relay to have the thermostat control the blower.
You don't want the fan/limit with the pull out switch.
 
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Old 01-24-19, 11:55 AM
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PJ,
I'm just curious. Why did you advise against the F/L with the manual button.

They were on all the older furnaces before electronics and on oil fired units where you had to have a service switch upstairs it was convenient in the summer to put on manual and control with the service switch. Never had any issues.
 
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Old 01-24-19, 03:24 PM
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There is nothing wrong with that type of switch but it doesn't do what he wants. He wants blower control from the thermostat.
 
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Old 01-24-19, 03:48 PM
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Thanks Pete. Didn't know if they found a problem with that control I wasn't aware of. That control will not solve his problem of the stat controlling the fan but just another option if he didn't wan to rewire.
 
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Old 01-25-19, 04:16 AM
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Construction is proceeding in my basement so i haven't posted any updates until now. I took out my fan control and heat limiter switch and inspected it and it was in good shape (no rust). I lowered the heater shutoff from 200 to 185 degree F. from feedback/ concerns I've gotten in this thread then put it back in. I ran a new 5 wire line to my thermostat thinking of the future (still only using the red and white connections) since the line will be behind new sheet rock. Ran the furnace and it works fine. The heat and fan turn on and off according to the settings on the fan control and heat limiter.

The heat turns off at 185 degree F. then comes back on after dropping below 185. So the limiter fan control is regulating the heat and the fan during operation. Is that not what it's suppose to do?

Also, I'm no longer concerned about manually controlling the fan with my old system since it does shutoff after ambient temperature is reached.

Thanks for everyone's feedback and expertise!
 
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Old 01-25-19, 02:55 PM
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It is working as it is suppose to. Thank you for the update.
 
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Old 01-26-19, 05:55 AM
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Spott,
Thank you for responding. I watched the limit switch run through many heat cycles and with my new/limited knowledge, thanks to this doityourself thread, it makes perfect sense that it was working correct. Amazing that before we had electronic control boards designers would be able to use physics and use the metals of the limit switch to turn accurately based on temperature. Pretty cool. I'm thankful for everyone who responded to my question and concerns.
 
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