Old furnace fan blowintg 24/7. Wiring question.
 

I have a Gaffers and Sattler U80-1 gas furnace circa 1974 that blows all the time. I think the termostat portion is OK. The only way I can get it to stop is to unplug it (the furnace) from 110vac. :wink:

I looked at the wiring and the transformer is outputting 28VAC and it's got 24v written on it so I assume that's OK. I checked the thermostat with a meter and the relay is working OK. It's a 2-wire thermostat, powered by AA batteries (which are now brand new). So I think something on the other side of the transformer is closed when it should be open.

I'm just guessing here but I think the thermostat controls only the flame and some other sensor turns the blower on/off (see diagram).

http://www.musictechnologiesgroup.co...es/furnace.jpg

There is a big two-wire switch of some kind or other that is closed. It has a screw adjustment for voltage (I assume) marked 80-90-100 ... 120-FAN OFF. What is this? There are also two wires going into the guts that I have not had a chance to measure, but imagine they might be the culprit? Any recommendations? The gas is still on (pilot only), so should I kill that before proceeding? I have a DMM. Thanks!!

Does the stat have rc and rh terminals?

Check your thermostat :thumbsup:

Switch in question..

http://www.musictechnologiesgroup.com/images/switch.jpg

can you post a picture with an overview

I think the thermostat is OK. It doesn't control the fan anyway and the flame is not going other than pilot (I think) ... though maybe I should double check that ;-)

Sounds like the fan switch is close all the time.

Agreed. Something that vintage wouldn't have a battery stat, the g circuit (or even full on) can happen by not splitting r circuits. I've also seen g tied into an r at the unit to allow not running a 3 wire to the stat, which produces that same result.

Is the fan switch the blue thing? Where do I get a replacement? What are the other two wires that disappear into the guts (you can see them in the 2nd photo in front of the blue thing).

Another question, the blower motor is quite noisy. Are replacements easy to come by?

That looks like some odd style of fan/limit I think?

It still should be verified first that there is a separation of heat to fan modes. With two wires to the stat the fan only will come on for a call for heat. Make sure that only wires are where they should be and nothing is bypassed.

Next, you will need to reset your limit fan thing there to open only when the appropriate temp is reached (I'm pretty sure that's what "the blue thing" is)

Lastly, check the capacitor and confirm the motor doesn't need to be lubricated

You have only 2 wires on Fan SW....one goes to motor and the another wire to L1.

Maybe this part?

http://www.americanhvacparts.com/Mer...Code=FAN680201

Yes, To be sure disconnect the two wires and do a test ..... if it shows 0 ohms between the 2 wires you have to replace it.:thumbsup:

Nice find Grant,The part number is right there 40T3.
Just make sure you install it with the instructions because that line also feeds the transformer.