BDP (Carrier) furnace (20+yr old) faulty?

[spinn]

I'm new here, so please bear with me.

I have BDP 394GAW000050 furnace (no AC) that was installed when the condo was built in 1987. The furnace worked a week ago before recently turning it on again.

Last week, we turn the thermostat on to try to get some heat. About 5 min goes by and we don't feel any heat, but we smell something like paper burning. We shut off the furnace's electrical switch and check around to make sure there's nothing burning. We don't find anything. We turn the furnace back on and the blower goes on. We check the thermostat, it's still registers a cold reading, and the furnace is blowing out air. The furnace was not lit. We checked the thermostat and forced it to turn on the furnace, but no response from the furnace. I could hear the click on the thermostat, but nothing coming from the furnace. To top it off, the blower is still blowing, even though the thermostat is set to off.

I replace the thermostat first. No change. Blower still blowing as soon as I flip on the shutoff switch. And still no ignition at the furnace. No click or nothing.

The furnace has a limit safety switch (HH12ZA251A) which I was thinking failed and caused the blower to blow continuously. Checking the line with a multimeater, it appears to be a closed circuit (0 ohms). The fusible link, located on the upside of the limit safety switch, also showed to be OK. Am I correct to think that the closed circuit should indicate that the limit safety switch is OK for now?

And if it's not a limit safety switch issue, what's causing the furnace not to respond to the thermostat signal to light up?

I've not had a chance to open up the box that houses the furnace control circuit board. Could the burning smell (which was different from the type of musty smell you get when you haven't used the furnace for months) been because of a fried circuit board? (I will not have a chance to look at this until NYE)

Should I call in and expert to look at this and face getting reamed? I know a circuit board replacement online runs for about $170. Is an expert going to actually test everything, or just tell me to replace the whole unit? Any recommendations for a reputable repair guy in the Fremont, CA area? Thanks!

[SeattlePioneer]

This furnace turns the fan on whenever the limit switch is open or disconnected.

If you have a counterflow furnace with the fan on top of the furnace and the burner compartment on the bottom, there is a second limit switch inside the fan compartment that has a manual reset --- you have to push a small red button on the switch to get it to reset.

I'm supposing you checked the limit switch and the fusible link just above the burners.

If you have the counterflow type furnace, check for that second limit switch.

[spinn]

Thanks for the reply.

Yes. The limit switch and fusible link is located right above the burners and both were checked. The circuit is closed, which the schematic on the back of the furnace cover indicates is normal.

I believe this is not a counterflow furnace, as I believe the blower is located below the burners and the schematic didn't indicate more than one limit switches. (but I could be wrong)

You can view some photos of the furnace at:
Carrier Furnace - a set on Flickr

(sorry for the crappy camera phone shots)

The schematic does indicate a relay is located on the circuit board, but I have not,and won't have time til NYE, checked that out.

[SeattlePioneer]

Yes, the furnace you have is an upflow model and does not have the second limit switch.

I don't suppose there is a "fan" toggle switch on the furnace or on the thermostat that might have been turned on?

I would check the limit switch circuit with an AC voltmeter connected to the furnace ground or chassis. You should get 24 VAC on both side of the fusible link and limit switch.

[ecman51`]

Does the inducer vent motor come on after you reset the furnace when trying to get furnace to run the right way?

[spinn]

There is no fan toggle switch that I'm aware of. Only and on/off switch for the furnace. I guess a quick test would be to disconnect the fusible link connection and see what happens. And also check the voltage going across the limit switch as you mentioned.

As for whether the inducer vent motor comes on after resetting, I'm not quite sure what that (inducer vent motor) is. All I know is that the blower is on as soon as I flip the power switch and it seems to be bypassing the thermostat control. I'm not quite sure what you mean by resetting, as there is no reset switch. I did remove the limit switch and tapped it to see if that loosened anything, but it didn't change anything.

[ecman51`]

You say that limit circuit is now closed, right? Hmmm.


It is the initial chain of events that triggered this whole thing, that is interesting. Could it be you got the paper burning smell initally because the burners came on? - but the fan switch did not have the blower fan come on, as it normally should have?

But then the blower only came on when say the furnace limited (due to no blower fan running?). But - if THAT happend, you would have got heat, at least in the beginning. Only after the blower continued to run and run and run, would the heat cooled and then yuou only would have had cold air blowing out the registers. So, did you ever get heat to come out the registers in the very beginning, when you smelled that paper smell?

The furnace can reset itself, as long as say the limit circuit is not employed, by turning the thermostat down, then back up, or better yet, to turn off the furnace and turn it back on after about 30 seconds.

The inducer is that smaller blower fan that is connected to the exhaust pipe.

[spinn]

Based on what the Ohm meter indicated, I believe the circuit to be closed.

I'm really not sure about the burning smell. My wife turned on the thermostat. We waited for the heat to start. Smelled something funny. Turned the thermostat heat setting to off. *don't recall if we actually got any heat, but the system worked a week ago* Waited. Fan/blower was still on after 10 minutes. Finally turned it off at the furnace switch. Waited 15-20 min. Turned the switch back on, and the blower turned on.

As you indicated, the system should have resetted by me shutting it off at the furnace. But it does not appear to have any effect.

[ecman51`]

I'm going to look at some of the pics you sent. Haven't been there yet.

It sounds like what would happen if you had a limit switch that had a reset button on it. And if no such thing, then it sounds like a control that does this, is stuck or toasted, say on the board. I'll be back.

I'm back. Is that a fan switch?, a limit switch?, or a combination fan and limit switch?, that is just above the gas valve: That plate with the 2? wires coming out of it. Or are there 4 connections and 4 wires? We need to know conclusively if voltage exists on the outgoing terminal(s) of that switch.

**IF** there is, then I'd say your problem lies with some relay inside that control box, that is in the blower compartment, that has that silver cover on it. Unplug and replug on wires in that box, to make sure they make good contact.

[Skip4661]

I don't think this furnace has an inducer. It is an older carrier with a 2 wire pilot assy.. I would recommend checking the heat relay on the circuit board. It may have burned. Also check for voltage across the limit terminals. 0 ohms could also indicate an open switch.

[SeattlePioneer]

This furnace has a circuit board in front of the fan motor. When you turn the power on, you should hear a "clunk" of a relay closing, and when you turn the 120 volt power to the furnace off, you should hear that relay open. Try turning the power on and off several times to see if you hear that relay opening and closing.

If you don't the relay on the cicuit board may have failed which would require a replacement. Good form would also suggest having a knowledgable repairman inspect for other possible cause of this kind of problem before replacing the circuit board.

If you can take a good close up picture of the circuit diagram on the cover of the fan compartment and display it some place, that would be helpful as well.

[spinn]

ecman51, the plate with the 2 wires above the gas valve is the limit switch. The red wire to it has the fusible link which connects to the limit switch. As I mentioned, I would guess that voltage was going through because the circuit does not appear to be open. But I will check on Wed.

The control circuit board is located inside the box above the blower (pic 4). I have not had a chance to open that up yet and will do so on Wed. The schematic does indicate one relay on the board. Replacement circuit boards show that they use 2 relays now...don't know why. While I've seen switches and boards available for my model, I have not seen any replacement relay for my unit. Is it something that normally can be replaced? Are they plug in or usually requires soldering?

Skip4661, I will check the heat relay on Wed. AFAIK, if a circuit is open, Ohm reading should read infinity; 0 ohms usually means there is no break in the circuit. But I will check the voltage just to be sure.

Seattlepioneer, I definitely do not recall hearing any clunk when turning off and on the furnace switch. So it may very well be a bad relay. I'll try to get better photos up on Wed.

Thank you all for your assistance!

[SeattlePioneer]

Originally Posted by Skip4661

I don't think this furnace has an inducer. It is an older carrier with a 2 wire pilot assy.. I would recommend checking the heat relay on the circuit board. It may have burned. Also check for voltage across the limit terminals. 0 ohms could also indicate an open switch.

Well, it's a three wire pilot, but who's counting?

Carrier did have this pilot assembly as a two wire pilot originally, but that was when this model first came out more than a third of a century ago, and only for a brief period.

I think I still have a two wire pilot, just waiting for an opportunity to move that old inventory out the door....

[mbk3]

There a four things that make the blower go into operation and no heat with this system:

1. bad transformer (loss of 24 volts)
2. open limit
3. open fuse link
4. bad board

[SeattlePioneer]

Originally Posted by mbk3

There a four things that make the blower go into operation and no heat with this system:

1. bad transformer (loss of 24 volts)
2. open limit
3. open fuse link
4. bad board

Good list. And of course a transformer failure would prevent the relay from closing when the power is turned on, too.

[mbk3]

Originally Posted by SeattlePioneer

Good list. And of course a transformer failure would prevent the relay from closing when the power is turned on, too.

Exactly.....the heat relay contacts are held open and when the 24 v is lost it closes in to power up the blower.

S/P..how's your weather.....I'm heading up to Portland within the next few days, hope the roads are clear


Spinnnn....the transformer you show in the pic with wall disconnect is not the transformer for the furnace. (doorbell)

[SeattlePioneer]

Originally Posted by mbk3

Exactly.....the heat relay contacts are held open and when the 24 v is lost it closes in to power up the blower.

S/P..how's your weather.....I'm heading up to Portland within the next few days, hope the roads are clear


Spinnnn....the transformer you show in the pic with wall disconnect is not the transformer for the furnace. (doorbell)

The weather here and in Portland is back to normal --- daytime temps in the 40s and rain. The snow is gone or rapidly melting.

[SeattlePioneer]

Originally Posted by mbk3

Exactly.....the heat relay contacts are held open and when the 24 v is lost it closes in to power up the blower.

Yes, and a bad transformer is more likely than a bad relay of the circuit board.

So checking the transformer secondaries for 24 VAC with the power on and the door switch closed would be the first thing to check.

[kaman]

Hi, I'm having a similar issue with a Carrier 58GSC080-DB furnace. My furnace works fine for lighting the burners and kicking in the fan about a minute later. However, as the furnace enters it's cool down period (after the burners shut off), it begins to make a surging sound, as if the fan motor is stoping/starting repeatedly several times. Then once the blower does shut off, it immediately starts back up again (or tries for about 2 seconds), and then shuts off again, only to try to start 1 more time. Once it finally gives up, its fine until the t-stat kicks the process off again.

I've tested the voltage on both sides of the limit switch, and I'm reading about 25.5 volts on both sides (it varies slightly while the system is running). I have two limit switches, and both appear to be closed and completing the circuit. Also, the burner never fails to light. I've opened the circuit board cover, and the relays visually appear to be OK (nothing burnt or blackened). They click as I apply power and remove it from the furnace.

Also, if I manually turn the fan on at the thermostat, it starts instantly, as well as shuts off immediately with the switch, just as it should. If the fan starts on its own in the 'auto' mode, the motor sounds like it's struggling to get started. I thought the motor was going bad. But if I turn it on manually, it just starts right up, no growning.

Any thoughts or ideas willl be greatly welcomed, as I'm baffled at the moment. The fact that I can manually start and stop the fan without issue tells me that my relay is OK, but then the limit switches seem OK, at least when the furnace is cool.

Please help!

Thanks!

[mbk3]

Originally Posted by kaman

Hi, I'm having a similar issue with a Carrier 58GSC080-DB furnace. My furnace works fine for lighting the burners and kicking in the fan about a minute later. However, as the furnace enters it's cool down period (after the burners shut off), it begins to make a surging sound, as if the fan motor is stoping/starting repeatedly several times. Then once the blower does shut off, it immediately starts back up again (or tries for about 2 seconds), and then shuts off again, only to try to start 1 more time. Once it finally gives up, its fine until the t-stat kicks the process off again.

I've tested the voltage on both sides of the limit switch, and I'm reading about 25.5 volts on both sides (it varies slightly while the system is running). I have two limit switches, and both appear to be closed and completing the circuit. Also, the burner never fails to light. I've opened the circuit board cover, and the relays visually appear to be OK (nothing burnt or blackened). They click as I apply power and remove it from the furnace.

Also, if I manually turn the fan on at the thermostat, it starts instantly, as well as shuts off immediately with the switch, just as it should. If the fan starts on its own in the 'auto' mode, the motor sounds like it's struggling to get started. I thought the motor was going bad. But if I turn it on manually, it just starts right up, no growning.

Any thoughts or ideas willl be greatly welcomed, as I'm baffled at the moment. The fact that I can manually start and stop the fan without issue tells me that my relay is OK, but then the limit switches seem OK, at least when the furnace is cool.

Please help!

Thanks!

Typically the motor speed for heat is low. When you use the "on" position on the thermostat you are using a different speed. Usually it is the "High" speed that is used for a/c. You must have a thermally activated fan switch, since when the furnace blower goes off and then starts again a few seconds later. It probably should be set about 5 degrees lower. Hopefully that will settle the restart problem

[kaman]

Agreed. Is that something separate from the fan limit switch located on the furnace wall just above the burners? Mine is non-adjustable. It has two blade connectors about an inch and a half apart on a plate that's roughly the size of a light switch wall plate. It seems to check out continuity wise at cooler temps, not certain about run temps, but then the fan only seems to sound funny in the cool down period, not while the burners are still burning.

I don't think I have a two speed motor, but I'll have to double check. I wasn't aware of it any way. The motor is a GE HC43SB116. I'll have to look it up to confirm the speeds.

Where else would I look for the fan control? Every place I've checked so far seems to be OK, although I can't prove it beyond all doubt. I should also mention that this problem started about two weeks ago, and seems to be getting gradually worse. At first it would only restart once. Now twice, and I've noticed the 'moaning' during cool down.

Thanks.

[SeattlePioneer]

Frankly Kaman --- your problem has nothing to do with that earlier in the thread, and that person may well want to continue posting.

Mixing two different problems rapidly becomes confusing.

My suggestion we be to start a new thread an re post your comments. It would be a lot easier to help you that way.

[kaman]

My apologies. I've read through so many posts today searching for an answer, I thought I was attaching to a different post when I entered mine. I will start a new thread.

Thanks.

[spinn]

I wish I had looked at MBK3's reply first before I started taking apart everything this morning. DOH!

Anyways, I added some higher resolution pics to my set...
Carrier Furnace - a set on Flickr

Looking at the transformer pic, it looks like there's some burnt areas to it. Before I put it back to test for sure, would others think, from the look of the photo, that this might be the culprit? Can I test the transformer hooked up to the board without connecting the other wires? Which wires (based on the schematic diagram) should be checked...red/blue (http://www.flickr.com/photos/3017801...11857684888/)?

Being that it's NYs, I won't be able to get a part til tomorrow anyways.

[SeattlePioneer]

From the appearance, it looks like the transformer is likely to be the problem. The correct way to test it would be to use a multimeter to test for electrical continuity across the primary wires and then the secondary wires. You're probably going to find an open circuit in one or the other which indicates that the transformer burned out.

The likely cause of this is a thermostat wire that is grounded, often by being cut by some sheet metal that functions as the ground.

You should use a multimeter to locate that ground and repair it before replacing the transformer, or it will just burn out again.

If you don't have the skill to find that short to ground, you may need to hire a repairman to find it for you.

[spinn]

That's basically what I did...took the transformer out and tested the input and the output wires. The input side looked like it had the open circuit as there was no continuity. The output side barely had continuity (2.5 ohms).

I'll hopefully get a replacement tomorrow.

As for testing the grounding issue, can you give me an idea of what/how I should figure that out? Are you saying it's a grounding issue on the thermostat? If so, would replacing the thermostat with a new one be one way to fix it? Or is it a grounding issue at the furnace end? I guess I'm not sure how it could've been shorted if there really hasn't been any other contact other than turning on the thermostat switch.

Thanks for your help SeattlePioneer and mbk3!

[SeattlePioneer]

The likeliest cause of the transformer failure is that the thermostat wire was cut by some sheet metal which grounded the 24 VAC thermostat wire, causing high current flow and the transformer failure.

The chassis of the furnace is grounded, so if the thermostat wire is shorted to ground anywhere, you get a circuit and a short circuit.

Of course, it could have been caused by something else.

But if you test for continuity between ground (the furnace chassis or "G" on the circuit board) and one or the other leg of the thermostat wire, you will likely find a zero or near zero resistance.

[spinn]

when you say the thermostat wire was cut by metal, you're saying that one of the thermostat wires to the board is somehow touching the furnace metal (ground)? or one of the wires is touching metal somewhere?

I guess that's hard to believe since there hasn't been any wire to metal contact that I can see taken place prior to the incident.

I'll look though...

[mbk3]

I might add to what S/P says about tstat wires touching metal, also this product transformers are about the only ones I know of that go out regularly with no explanation.

[spinn]

I checked the wiring in the thermostat, and for some reason, the red wire had a section of plastic stripped before that possibly could've been the culprit of wire touching metal. I wrapped it in electrical tape and popped in the new transformer. $32 later, it works again!

Thanks for all the solid advice and help!

[mbk3]

Yippppeeeeeeeeeeeeeeeeeeee