Post some pics of the furnace with both doors off and the fan relay.

Your ohm meter tests continuity and does not short per se.

The fan control should send either 120 volts or 24 volts to the fan relay depending on what that relay coil is rated for.

Try check for voltage at one side of the fan control when it is warming up and go to ground or C on the terminal strip where the thermostat wires attach.. One side should be live.

When it hits the fan on setting then you should get voltage on the other side of the switch to ground.

If none then the fan switch is no good.

If the relay is getting voltage to the coil then it should close a circuit and turn on the blower.

If there is a wiring diagram on one of the doors take a pic of it and post it here.

Use photobucket if the site does not let you attach pics until after 5 posts. Put the links here.

 

The blower works in AC mode but not heat mode. Sounds like an issue with the thermostat wiring or settings.

 

As I mentioned, a new thermostat was installed over the summer - technician was troubleshooting a difficult starting AC unit - after some frustration, he finally changed out the thermostat and the CA has been working great. But this is the first time we fired up the furnace and we're experiencing this problem.


Is there something in particular I should check?

 

G turns on the fan relay with AC and the old school dial type fan control turns it on with the heat.

There may be 2 relays also.

AC fan has nothing to do with the heat.

 

Here are some pictures of the furnace. I noticed last night there was a little water (very little) inside furnace and I see it turned to rust as shown in the pic (must have been dripping in for the summer?). We've had an extremely rainy summer with some pretty good winds - so the water must have come through the stack (hopefully not dripping from the condenser above!).
Wondering if this might have caused my fan issue?


As far as further testing of the Fan Limit Switch (beyond the continuity test I already performed), I'm not sure where I should put my probes for the voltage. Again, the burner lights up so I thought that side of the Fan Limit Switch is good (white wire), but the fan side is what needs to be tested (yellow side), correct? I believe I need to connect one probe to the yellow wire contact and the other probe to the "C" terminal?


One last thing, based on some research I checked the voltage between terminals R and C for about 27 volts.

 

You may have damaged the fan/limit switch be moving it by hand. After figuring out the problem, you may have to get a new one.

Were you testing resistance across the fan side?

If yes, the problem is downstream of the fan/limit. Unless the dial isn't rotating when the burners come on.

You'll need to check all the wiring downstream of the fan/limit, compare to diagram. If the wiring isn't messed up, either the normally closed contacts on the fan relay are bad or it's the heating speed tap of the blower motor. Both are pretty unlikely.

 

Thanks for your advice.


The Fan Limit Switch still works and moves normally when the burners heat up - the dial rotates to the activate the fan - so I don't think its damaged. I've seen a few videos showing that checking resistance is one way to check the Fan Limit Switch operation as shown in this video https://www.youtube.com/watch?time_continue=190&v=Pyz9MxgHn5k.

 

I am concerned about the water and rust. Looking over the furnace, I noticed the clear drain hose for the AC evaporator pan has a lot of rust in the hose. Same color rust that is inside the furnace. I'm starting to think the evaporator pan is rusted through and leaked all over the furnace this summer. Wonder if it damaged something related to the fan.


Could the water have shorted out the low speed winding on the blower motor? Is there an easy way to test the blower motor with my multimeter?


My next issue will be to repair that AC pan before next summer...

 

It may not be accurate any more.

Assuming you checked the resistance of the fan side, the problem is most likely downstream.


Are you sure the guy who worked on it didn't mess up the wiring? trace everything downstream of the fan/limit before bothering with more testing.

 

OK, I can't believe I fixed it. In my original post I mentioned that an AC tech was messing around with the wiring inside the furnace - I could tell he was getting frustrated, he was at my home a long time. Well he was the cause of the issue.

I opened this box (in the pic) below the fan relay to trace the yellow wire from the Fan Limit Control switch, and I noticed the yellow wire went into the relay, and another yellow wire came out of the relay but it didn't look like it was fully pushed it. So I grabbed the wire and it just fell out of the hole. I reseated the yellow wire, and tested it. Sure enough the fan works now.
:vs_clap: :vs_cocktail:

Thanks for helping me out guys! Time for a beer!

 

good.

something to keep in mind is this furnace is from the 1980s or early 90s and should be thoroughly inspected for cracks.

 

Anyone happen to know what this part is called? New to this website with new problems on what seems to be a really old unit

 

It is a combination transformer and fan relay ( fan center )

https://www.supplyhouse.com/Resideo-R8285A1048-40-VA-Fan-Center-w-SPDT-Switch-Includes-R8222B

 

It is a combination transformer and fan relay ( fan center )

https://www.supplyhouse.com/Resideo-R8285A1048-40-VA-Fan-Center-w-SPDT-Switch-Includes-R8222B

Thank you! Got it and installed successfully

 

Yur Welcome