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Category: Home Improvement
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Experience:  Handyman 40+ years - Gardener for 50+ years - Homeowner 40+ years - Internet Researcher
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gas furnace rewire for fan only operation

Customer Question

how do i rewire my gas furnace so i can manually turn the fan on in the summer for air circulation?
Submitted: 10 years ago.
Category: Home Improvement
Expert:  Glenn replied 10 years ago.


Can you supply the make and model number ?

Customer: replied 10 years ago.
Reply to glenn's Post: Glenn,

it is a Rheem Criterion with the jets on the bottom and squirrel cage fan on top.

Expert:  Glenn replied 10 years ago.

Hey Bud

Do you wanna just make this blow air >??

Customer: replied 10 years ago.
Reply to glenn's Post: Yup. Here in NW Washington, we have a normal, unairconditioned home. For the 2.5 months of warm weather, it would be nice to just get the fan to blow to circulate air. Lots more secure than leaving windows open and unlocked when we are not home.

Expert:  Glenn replied 10 years ago.


Not really sure but have you tried disconnecting the gas valve

Some of these units require a certain temp before turning the fan on

Customer: replied 10 years ago.
Reply to glenn's Post: Hmmmm. Not sure how disconnecting the gas valve will help. What I wan to do is to wire the fan motor into the thermostat to allow the "fan on" to turn on the fan.
Expert:  #1 Expert replied 10 years ago.

HiCustomer Thank you for asking your question on Just Answer. The other experts and I are working on your answer. By the way, it would help us to know:

-Is your furnace a Rheem Criterion or the Rheem Criterion II ?

-Do you normally shut off you pilot light in the summer?

Thank you again for trusting us with your problem. Please reply as soon as possible so that we can finish answering your question.

Expert:  Glenn replied 10 years ago.

Hello again Bud

I'm shure the fan will not come on before it knows there is heat

Do you want to make a connection on the blower and dissconnect it from the unit and put a plug on it so you can plug it in ?

Customer: replied 10 years ago.
it is the criterion...not the criterion II (at least there is nothing listing it as a II).

we do not shut off the pilot...there is no pilot. the ignition is via a heater coil...i just changed it out this past year.

Expert:  #1 Expert replied 10 years ago.

Due to the age of the furnace, I do not have the manual available, but I an tell you that the fire box has a thermostat controlled switch, that turns the fan on when the fire box temperature gets to a certain temperature.

This plenum thermostat, does the same function as an on and off toggle switch except it is temperature controlled.

Warning: Do not attempt this unless you have some basic electric knowledge. The voltages inside your furnace can cause sever shock and or death. Only open the furnace with the power turned off.

This plenum thermostat is normally enclosed in a sheet metal box. When you remove the box's cover to gain access, you will see a calibration knob.

Do not touch this control. It is factory set and could cause your furnace to get to hot if adjusted incorrectly.

Turning it could also cause your furnace not to work unless the thermostat switch was replaced.

If you look closely you will see 2 power wires going to the plenum thermostat and another brass looking tube going to the fire box, or the box it's self can be in front of the box with a large tube going into the fire box.

One of these wires will show hot and one will show no voltage. Put a toggle switch to allow it to connect these two wires and your fan will run.

Turn the toggle switch off and the plenum thermostat will control the fan.

These instructions are generic in nature and may or may not describe your furnace. I believe I have supplied enough information for you to do this modification to your furnace

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