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Furnace Issues Baffling Me

PAToyota

PAToyota

Red Skull Member
Joined
May 27, 2020
Member Number
1545
Messages
1,233
Loc
South Central Pennsylvania, USA
A bit of background first. The natural gas furnace was here when I moved in thirty years ago as of this past July. I added a programmable thermostat to it the first year I was in the house and about a decade ago I swapped the pilot light out for an electronic ignition unit. It gets regular maintenance - basically cleaning and lubrication. This year I did start looking into replacing it - and the air conditioner - next summer for better efficiency. So, of course, after no issues for decades I now have issues... :homer:

For the parts replacement below, everything was replaced with units of the same specs - capacitor, relay, transformer. Double checked. Triple checked. Thermostat was a cheap unit just to test control.

So, it started with the fan relay chattering before engaging or not engaging at all. Straightforward enough to replace it. I also replaced the fan capacitor because some reading online said that a failing capacitor can add stress to the relay and they both go out. Replaced both - they're thirty plus years old. Then nothing worked...

Some testing... The burner side of things all works. Thermostat calls for heat, furnace ignites, everything works properly but the fan doesn't come on, so it shuts off again when it starts to overheat. I can jump the power terminal of the relay to the output terminals and the fan comes on. I can jump the terminals from the thermostat and hear the relay click - however, doing so does not send power to the fan... :confused:

So I thought I got a bad relay and replaced it again. Same thing. Got out the multi-meter and was concerned that the relay wasn't getting enough power, so replaced the transformer. Just to eliminate possibilities, I also picked up a cheap thermostat in case the fan control got damaged somehow. Nope. Still won't run the fan. I should note that switching the thermostat control for the fan from "auto" to "on" also doesn't do anything - not just that the heat cycle doesn't engage the fan.

If I bypass the fan relay and apply power directly to the fan, everything works as it should except the fan runs constantly instead of turning on and off with the thermostat. So it would seem it is narrowed down to the relay or transformer - right? The two replacement relays were the same make and model from the same supplier, so there is a chance they had a bad run - or that I somehow got the wrong one (not the same make as original, but same exact specifications).

I have to admit at this point that I'm baffled. It's not like it is rocket surgery (wiring diagram attached - arrows to transformer, relay, and capacitor). About the only other options for things being bad are the various limit switches (yellow box), but that should just mean the fan always runs on high and not medium high - right? I do find it odd that wires from the motor are hooked up to the medium low and low terminals on the terminal block, but there is no way to power them. I guess that's just a way of terminating the wires - or possibly set up for another model?

I'm probably missing something obvious, so I'm throwing this out here before making a service call and paying the big bucks to have my error pointed out.

IMG_6537edit.JPG
 
When the thermostat is calling for heat, and the burner is on, what is the voltage at the fan relay coil?

My guess is you have a wiring issue with the thermostat or the cable going to it, and aren't getting voltage at the G terminal.
 
Ok, laying everything out above and going back over everything to do so got me one step ahead. Looking over the wiring diagram, the component diagrams, and the original and replacement components I realized a difference between the original and replacement relays that I was missing.

Here is the diagram for the replacement relay:
Screen Shot 2023-11-14 at 11.32.01 AM.png


From the wiring diagram in the first post, I had things set up for "1 Form C" - low voltage white and black to #1 and #3, power (black) to #6, normally closed (brown) to #5, and normally open (violet) to #2. These were swapping the wires directly over from the original relay to the exact same posts on the new relay.

Going back over things - and pulling out the relay diagram for the new relay and looking at the physical relays - made me realize that the old relay had a jumper tab between #6 and #4 which you cut if you wanted to separate them (like cutting the tab on an outlet to separate the top and bottom receptacle). The new relay didn't have that tab... :homer: So I was applying power to the fan only through the "brown wire" side.

So I rewired things to create a jumper from #6 to #4 to apply power to both sides. Now the fan cycles with the thermostat, but it seems to only be using the "high" speed (violet wire to #2). That makes sense. Before, power was only going through to "medium high" (brown wire to #5) - which apparently isn't working.

I have an online meeting I'm sitting in now, but the next step is to bypass the fan switch to make sure that is the issue on the "brown wire" side.

As I said, just the process of writing everything out here forced me to analyze every step instead of just making assumptions (everything was the same as before). Always a valuable step. As such, I'll leave this up here in case it helps someone else out in the future.
 
Looking at the diagram I see it as the low speed is not used on this furnace. I added the blue squares to the diagram that show the lack of connections.
diagram.png
 
Yes. The Low and Medium Low are not used. The asterisk at the terminal board connections and the note at the bottom of the page state to look at the installation manual for proper speed connections for your unit. So the setup and motor serves multiple configurations.

My configuration still uses the High and Medium High speeds, though. Currently only the High setting works. The Medium High through the Fan & Limit Control is not working - so either the Vent Switch, Rollout Switch, or Limit Switch has a potential issue.
 
This evening I was able to put in a jumper between the fan relay terminal and the Medium High terminal bypassing all the switches and fan & limit control in the yellow box above. This is the red line below. Like that, the fan runs when you turn fan switch on the thermostat from "auto" to "on" and when (set to auto) the thermostat calls for heat the fan starts running immediately and then switches over to a higher speed when the fan relay clicks.

IMG_6537edit2.JPG


Looking at the connection diagram and the ladder diagram, I'm not quite sure I'm clear on how to test the switches and fan & limit control (red box and yellow box above).

Looking at the switches (red box above) - rollout switch, limit switch, and vent switch, they are all normally closed temperature switches. I take the arrow across them in the ladder diagram means they're adjustable? I haven't seen (and was unable to find) that specific symbol before, but the arrow across a symbol like that typically means variable or adjustable - like with a resister, capacitor, or rheostat.

So it looks like if any of them open, it is an overheat condition and it shuts off the gas valve? Since the heat side of things works as it should, I'm guessing that all of those are functioning properly. If I wanted to I could test them by pulling the wire off the terminal for each of them (opening the switch) while the furnace is running to make sure that the flame shuts off - right?

So I'm left with the fan and limit control (yellow box above). It shows as the normally closed limit switch at the top and the normally open fan switch at the bottom - but it isn't a relay where one circuit controls the other. Something needs to close the fan switch for the fan to run on medium high. The thermostat controls the switch between medium high and high - the green wire activates the coil on the fan relay, switching power from medium high to high.

I need to disassemble some stuff to get to my fan and limit control, but it basically looks like this internet image from what I can see:
Control.jpg


The brown wire comes from the fan relay, the yellow wire goes to the high volt terminal board medium high terminal, and the two red wires go out to the vent switch and rollout switch. Looking at the image above, it looks like there are three settings - off and on for the fan and off for the limit switch. So the low temperature where the fan goes off once the furnace is cool enough, the middle temperature where the fan goes on once the furnace heats up enough, and a high temperature which shuts everything down if there is an issue and the furnace gets too hot. The settings in the image above of (roughly) 80, 130, and 200 would seem to be applicable for those cases. From what I can see, mine is set about the same.

That makes sense as before the thermostat would click on, the furnace would ignite, it would be a moment or two before the fan came on, and then a bit later the fan would increase.

I'll have to read up on diagnosing the fan and limit control to see what my options are there. Everything looks normal and all the wires are solidly connected to it, so that is as far as I can go currently.
 
Looks like that fan and limit control just turns the fan on /off and shuts the furnace down if it overheats.
What controls the fan speed? How is it wired now "in the field?
 
Eric642 said:
Looks like that fan and limit control just turns the fan on /off and shuts the furnace down if it overheats.
What controls the fan speed? How is it wired now "in the field?
Click to expand...

Yep. That's exactly what the fan/limit control does.

The fan speed is controlled by the fan relay. Normally it is on medium high (the NC side). When the green wire (from the circuit diagrams) engages the fan relay coil, the relay switches over to the high speed circuit (the NO side). The thermostat controls the two speeds by sending power to the green wire.

The fan part of the fan/limit control is just basically a thermostat that sets the temperatures that the medium high circuit turns on (high) and off (low). As mentioned above, the on is at about 130℉. This allows the furnace to heat up a bit before the fan comes on so that it isn't just blowing cold air at you. The off is about 80℉ - which allows the furnace to cool down a bit after the flame goes off.

The limit part of the fan/limit control is set at 200℉. That cuts power to the gas valve if the fan didn't come on and the furnace is in danger of overheating - along with the temperature switches (rollout and vent). If any of those three switches open, power to the gas valve is shut off.

For whatever reason, it looks like the fan control isn't turning the fan on for the medium high circuit. With the fan relay on NC, it is sending the power through the fan/limit control but no power is coming out and going to the high volt terminal board. So the fan doesn't turn on until the thermostat engages the high circuit (NO).

I did verify that the temperature dial on the fan/limit control is rotating with heat. As I said, I'll have to disassemble things to be able to actually test the control. It's up under the draft hood, so I can see it but really can't get to it well enough to play with it.

Of course, this week I have a bunch of other stuff going on too so I can't just sit down and spend an uninterrupted day working on the furnace. :shaking:

I read a couple pages and watched a couple videos. This one was a better video description of the fan/limit control than the others I went through.

 
Last edited:
PAToyota said:
This evening I was able to put in a jumper between the fan relay terminal and the Medium High terminal bypassing all the switches and fan & limit control in the yellow box above. This is the red line below. Like that, the fan runs when you turn fan switch on the thermostat from "auto" to "on" and when (set to auto) the thermostat calls for heat the fan starts running immediately and then switches over to a higher speed when the fan relay clicks.

IMG_6537edit2.JPG
Click to expand...

Didn't your red jumper wire JUST bypass the "Fan Switch" in the fan/limit control unit? That would suggest that the unit is defaulty and the cause of your problem.

Jumper.png
 
Eric642 said:
Didn't your red jumper wire JUST bypass the "Fan Switch" in the fan/limit control unit? That would suggest that the unit is defaulty and the cause of your problem.
Click to expand...
Yeah. I realized that laying in bed last night. The other switches just turn off the gas valve if there is an overheat problem.

I still want to pull the fan/limit control unit to see if there is anything that can be fixed. The control dial is rotating as it heats up. It just isn't turning the fan on. Also, the control unit is at least $100, so more than the other money I've thrown at things so far.
 
I'm looking at the picture of the fan and limit switch and it looks like the wire for the fan terminal is broken off. It looks like the part of the stakon terminal is there but no wire Is that your picture?
 
never mind, I see your pic back a few posts Sorry
 
Just to follow up on everything, it's all working without replacing the fan/limit control. :bounce::bounce2::bounce:

Chances are that the original transformer is good (still have to check that), but after thirty-plus years I'll say that a new capacitor, fan relay, and transformer were just maintenance items.

Other than that, I don't know if I had jostled something with the other work that then shifted back with subsequent mucking around with the fan/limit control or what, but it is all fully operational now and has been for the past week.

Switching the fan from auto to on at the thermostat engages the fan relay and the fan runs.

When the thermostat calls for heat, it ignites, the burner warms up, and you can see the dial on the fan/limit control spin up. When it hits the "on" limit the fan comes on. If I remove power from the fan and allow it to spin up further, it shuts the burner off at the high limit. When the burner goes off, the dial settles back down and the fan goes off when it hits the "off" limit.

Checking everything with a multi-meter shows that everything is getting the proper voltages and such too - just to double check. Checked operation of the vent and rollout switch too just to cover everything.

And I now understand the inner workings of my furnace control to a much higher level than I ever did before. :laughing:

I have to say that although I didn't appreciate the wondering if I was going to have to spend a lot of money to have heat this season, being on the other side of the fix I appreciate the lesson on how it all works. I'm afraid that if/when I replace the furnace it will all be computer controlled, but the analog system here is pretty ingenious. Instead of sensors and computer control, the analog system is basically just mechanical "if-then" statements based on various temperature switches. One temperature switch to turn the fan on, one to turn the fan off, and then a bunch of them to shut down the whole system if it is going to overheat.
 
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