OT Furnace Blower Motor Wiring

Re: OT Furnace Blower Motor Wiring

Phil Joines Krum, Tx

>Has someone been playing with the switches on the thermostat? Ours has a "Fan On" position.

Re: OT Furnace Blower Motor Wiring

Hal Smith

>Phiul may be right about a switch in the vicinity of the thermostat so make a check for that. The "device" you mentioned is probably a limit switch which keeps the fan from turning on until there is a certain temp in the furnace bonnet and keeps it running after the flame shuts down until the temp in the bonnet goes below a preset value. Don't know much to offer except play with the wiring and see if you can fix the problem.

A couple of things....

Mark Goodall - ATL - tooljunkie

>As already mentioned, there are usually two controls for the fan. You probably already know this, but in just case.....

One is at the thermostat, and it is either OFF - ON - AUTO and sets it so the fan is never on, always on, or on/off automatically based on what the furnace/AC is doing.

The second control is in the furnace itself, and it's what controls the offset delay for the fan. It will keep the fan off until the burners are hot enough, and will continue supplying power to the fan after the burners have turned off.

I can't think how replacing the motor and it's wires could mess you up like this, since there's nothing on the motor itself that decides when to go on or off, and there are only a couple of wires from the controller to the motor. Hang on, the motor should have at least 2 hots if not 3 for the difference speeds (slow speed fan when heating, faster fan speed when cooling).... Oh, I'm not helping much, I'm afraid....

I'll have to look for some of the diagrams I have from my thermostat and furnace beacuse I certainly can't recall what colors go where. Hopefully someone else will have them handy, cause I know where the diagrams SHOULD be, but that's not the same as know where they ARE.

Happy Woodworking!

Re: OT Furnace Blower Motor Wiring

Kneale Brownson in Northern MI

>No, this is a simple two-wire thermostat with a mercury switch.

Re: A couple of things....

Kneale Brownson in Northern MI

>The old motor and the new have only three wires, one green ground and two other leads, one, I presume, is neutral and the other is hot. Motor direction is controlled by which posts the motor's internal red and black wires go on. That's all the wiring from the motor. Power source wiring has two lines, one, I presume, the neutral and the other the hot, plus the green ground. Only other wires are the white and black from the device to which the thermostat's two wires are connected. That's all that was there when I took the old motor out. I think the way the system worked in the past was if the thermostat called for heat, the motor started up and the gas valve opened simultaneously. The motor ran until the thermostat was satisfied and the gas valve opened and closed as needed until the heat level reached the thermostat's setting.

Hmmmm....

Mark Goodall - ATL - tooljunkie

>I don't think I'm going to be much help to you. All the furnace motors I've worked with have more multi-colored wires for 2 or 3 speeds. Usually red/blue/black/white/green IIRC. And the thermostats have 3 or 4 wires, red, yellow, green, etc. Is your furnace for heat only rather than cooling as well?

Happy Woodworking!

Re: A couple of things....

Gary Bindel

>If as you say, the burner and the blower motor came on simultaneously and went off simultaneously, the box with the two wires that the thermostat wires go to could be a relay that is controlled by the thermostat. If you have an Ohm meter, you could check for continuity across the two wires. You would check it first with the tstat turned down at which you should not have continuity. Then, check it it with the tstat turned up at which you should have continuity. If this is the case, I would connect the incoming hot to one of the wires, and then connect the motor hot to the other.

Re: Hmmmm....

Kneale Brownson in Northern MI

>Heat only.

Re: A couple of things....

Kneale Brownson in Northern MI

>Thanks, Gary. I'll dig out the ohm meter and have a check.

Re: A couple of things....

Bill Turpin in WNC mountains

>Some of the combo fan/limit controls that are stuck into the plenum have a push/pull switch for constant fan. They are the diameter of a pencile eraser and about an inch long. You might have bumped this during motor replacement.

Bill in WNC mountains

Re: OT Furnace Blower Motor Wiring (long)

Larry Clinton At Frankfort (Central)., Indiana

>I installed and worked on Furnaces for about 30 years so am familiar with most circuits. I believe yours is an older unit since I don't think any of the models made in the last few years have belt drives.

First all furnaces have the wiring diagram fixed to one of the doors when they come from the factory. This should show you all of the internal wiring in the furnace.

Second, almost all of the older furnaces have only a couple switches in the blower motor circuit. These were not connected to a "control board & relay system" like the new high efficiency furnaces made today. The blower circuit was not connected in any way to the thermostat unless a fan relay was added for central air conditioning. The normal mode of operation is 1) Thermostat calls for heat - opening the gas valve (providing the pilot thermocouple is hot)turning on the burners. B) When the heat exchanger becomes hot enough, the "fan limit" switch makes the "on" limit switch. This turns the blower motor on. (the only other switch in the motor circuit is the safety switch that opens when the blower door is removed). C. When the temperature rises enough to satisfy the thermostat setting, it opens the circuit to the gas valve - the gas valve closes and the burners go out. D) The blower continues to run until the heat exchanger cools off enough to activate the fan limit "off" limit switch. Note there are two positions that control the On / off of the motor. The high or On position of the Fan limit may be set to 110 - 130 Degrees and the "Off" to 90 - 100 degrees. (I have seen furnace blowers turn on when the house gets very hot in the summer and the On limit is set too low). The reason for the two setting is so that the air is hot enough to keep from having "cold drafts" from the heat vents when the blower starts. The new high efficiency units do operate differently and do have some of the cold draft problems if the heat runs are under the floor in an unheated crawl space. I would check that the fan limit switch has not had the manual "on" button pushed in as mentioned above. Chances are you did not wire anything wrong - unless you changed some wiring you didn't mention.

Re: OT Furnace Blower Motor Wiring (long)

Kneale Brownson in Northern MI

>Thanks, Larry, this isn't an older furnace, it's a really old furnace that's had the blower replaced by someone else in the past because the fan bearings broke down. There is no wiring from the heat exchanger plenum, only the wiring I described before. I'm familiar with (have even replaced in the past) limit switches for blower motors. This setup has no such wiring or switch. So I cannot have disturbed something in the motor exchange.

Re: OT Furnace Blower Motor Wiring (long)

Larry Clinton At Frankfort (Central)., Indiana

>Kneale, the only way I can think of (without seeing the unit) that the fan can be turned on and off with the thermostat is that a relay is used. If so the relay may have welded the contacts when the old fan motor went out. I would be very surprised if there isn't a Fan limit controller somewhere in the hot air plenum however. The Fan limit is also used to interrupt the thermostat line to the gas valve when the temperature reaches the setting of the limit switch. Without the limit switch you could overheat (melt or crack the heat exchanger) Some of the very old furnaces did use a klixon switch for the high limit however. If the thermostat is running the fan through a relay, and the fan only runs when it calls for heat, the system is using a whole lot of extra fuel. Once the heat exchanger is hot and thermostat shuts off, you need to keep the blower running until the heat exchanger cools. The thermostat has a wire inside that heats up the thermostat slightly (heat anticipator setting) to compensate for the added heat. If the fan shuts off while the heat exchanger is still hot that heat will go out the chimney. I would add a fan limit to the system if it were mine. Anyone knowable about furnaces should be able to do that.