Fan centre

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Old 10-22-03, 03:00 PM
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Fan centre

What exactly is the purpose of a fan centre. Aside from the fact its there where the thermostat wires connect. Is it there only to control the fan for the AC unit, or does it also control the fan for heating too?

The reason i ask is because trying to hook up my fan centre I can't get the fan to come on with the heat. If i bypass the centre for the heat then the fan comes on when the plenum switch reaches its temp, and turns off when it gets to the proper cool setting. The Guy that originally installed this for me as a favour does mostly industrial AC and said he wasn't overly familiar with the furnace aspect of it, i assume it was just wired wrong. It worked OK with the AC, but the furnace wouldn't turn on. I got the furnace fireing now (thanks to the member here, simple switching wires) and if i bypass the fan centre the fan will come on. If i don't by-p[ass then the Hi limit switch eventually cuts the gas flow.

BTW its a natural gas, forced air with ac.

Thanks
 
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Old 10-22-03, 03:23 PM
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The fan center should not be necessary to energize the fan for heating mode. The fan control for heat usually parallels the fan relay. Sometimes on a multi-speed blower, the two energize different speeds of the blower. It sounds like there is a problem with the line voltage wiring of the fan control for heating. On some furnaces the fan & limit controls in the same control box. Were any othermodifications made when the fan center was installed?

Ken
 
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Old 10-22-03, 03:56 PM
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OK.
so if i understand you the fan centre should parallel the high low switch. If this is the case then where should i pick up the hot for the fan centre? I would assume the load side would go to the fan, same place the current wire from the high/low fan switch goes? Also I would assume that the black would be line and the red load? Is it normal to have 2 lines(hots) going into the fan centre. One for the thermostat wires area and the other for the other end (transformer)?
In my case the fan high low and the high limit for gas are in the same housing but are two separate units. My blower is a single speed. The fan centre does have a 3rd wire.. I just left it capped

Should the fan centre control the fan for both the ac and heat?

thanks

Scott
 
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Old 10-22-03, 04:42 PM
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The hot from the power switch should go to line side of fan center relay (not sure what color wire it should be) and one side of fan control for heat. The load side of the fan control for heat should go back to the load wire on the fan center and then down to the fan motor. Either one can power the fan motor. Hope that makes sense. Check out the wiring on that fan center. One wire is common side of relay, one is a normally open contact and one is normally closed contact. Your understandiong of the fan control operation is correct.
 
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Old 10-24-03, 06:26 PM
bigjohn
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If you are absolutely sure that the blower motor has only one speed then the fan relay contacts and the heating fan switch should be paralelled. On a furnace with a multispeed motor, usually high speed is used for cooling and medium or maybe medium hgh is used for heating. In this case, the heating fan switch should be wired in series with the normally closed contacts of the fan relay. This prevents 2 speeds on the motor from being energized simultaneously which would burn out the motor.
 
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