Furnace Trouble-Shooting - Fiberglass RV
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Old 03-20-2018, 09:32 AM   #1
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Furnace Trouble-Shooting

Time to get this thing running! If I can...

Last fall it would come on and the fan would run, but failed to ignite. Probably a problem with the circuit board. While I was poking around, having forgot to pull the fuse, I saw a spark. I was checking connections, pulling them with pliers. After that, nothing. Furnace wouldn't even turn on. No fan, nothing.

I replaced the board and sail switch, I tested the limit switch (it's fine), and I bypassed the thermostat. Still no power to the board. So at this point, so far as I can figure, it's either the time delay relay, or the motor itself.

So the questions...how to test the motor? If the motor is bad, would that cause the circuit board not to get power? Would a bad time delay relay cause the board not to get power?

My furnace is an Atwood Hydroflame, HF8012D. From a 91 camper.

Where I'm at is...IF it's the motor, everything I can find tells me it cannot be replaced. The 8012 II has a lot of compatible parts, but the motor ain't one. So if the motor is bad, I'm wasting my time and money on the furnace to trouble-shoot any further. So I'd like to test it. But I'm not sure if a bad motor would cause no power to the circuit board. ? The time delay relay also seems to be a no-longer-available part, but I'm still checking some places. The 8012 II is the same exact size, perfect-fit replacement for my furnace, but interior parts have changed.

If the motor is bad I'm looking at needing a replacement furnace. Suburban is affordable but means modifications to my cupboards and fiberglass shell. The 8012 II, direct-fit replacement from Atwood is over $700.

Any ideas?
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Old 03-20-2018, 09:57 AM   #2
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my thoughts

zack have you checked at your final output off your board if you have 12v to your fan using a vom or a test light? if you do and the fan wont run then its your motor.

you can also run a clip from 12v to your fan motor directly to see if it will run. I hate all those circuit boards and new things makes it hard to work on them!

if that furnace is shot I would consider a wave 3 for heat with warning you need to vent top and window slightly for makeup air.

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Old 03-20-2018, 10:18 AM   #3
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Maybe this will help if you don't have one.
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Old 03-20-2018, 10:23 AM   #4
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I would guess that you blew up control circuits. At this point the best thing to do is take the furnace to a Atwood repair center. Let them fix if for you. Yes it'll cost a few bucks but that's better than blowing up your trailer.
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Old 03-20-2018, 10:32 AM   #5
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Zach, Bob has a good point. You need to test for voltage to the furnace.
You started by saying it was running. Until you were poking around with pliers.
And then a spark. Spark means short, means blown fuse.
Until you can determine if you have power to your unit. We can’t help, throwing parts at it isn’t the answer.
Things that will help us help you.
Verify 12 volt going to furnace.
Then let’s go from there.
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Old 03-20-2018, 10:33 AM   #6
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How much time and money do you want to invest in a 27 year old furnace? If it was mine, I would consider a new $700 furnace.
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Old 03-20-2018, 10:55 AM   #7
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Did you check this breaker?
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Old 03-20-2018, 03:52 PM   #8
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seen many things

I sold expensive restaurant cooking gear, 10k steam cookers and pots thrown out because I got them and had my repair guy who was a sharp electrician friend check them out.

guess what some minor 100.00 part blew and the place didn't have the sense to contact me or find someone who knew circuits to come check the piece of equipment out.

I have had schools with 50 year old ranges in them they would keep squacking this didn't work throw it out. I would order a new shut off valve or a new thermocouple and maintance would get them going again.

unless that exchanger is burned out I bet that furnace is fixable! a hospital kitchen just had to have new ovens with a bunch of electronics in them expensive. I told them they were buy a service nightmare those electronics wouldn't cope with all that heat. sure enough repairs all the time!


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Old 03-20-2018, 06:02 PM   #9
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If all goes bad. I found a used suburban that came out of a pop-up and it was the exact size and shape of the hydroflame in my Boler, at a RV dealer shop who sells used parts.
Paid $200 and it's been working great for 4 years now. New furnace has electronic ignition and old one had a standing pilot.
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Old 03-20-2018, 06:20 PM   #10
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Thanks everyone. A couple thoughts...

I don't want to invest any time or money in a 27 year old furnace. But it's visibly in almost brand new condition, and I can't afford $700.

My thinking is replacing the circuit board, known to fail and known to cause my initial issue of the furnace kicking on but being unable to fire, and while I'm at it, put in a new sail switch since they also fail, is reasonable. So I've invested under $110. I think that was a safe bet. Except it didn't pay off But it was worth it to me with $100/hr repair rates.

It's a fairly simple appliance as far the electronics. Circuit board, sail switch, limit switch, time delay relay, motor, fuse, thermostat. I'm just testing and replacing electronics as whole units, a couple screws and a couple plugs. I don't figure that's doing anything dangerous.

Ok, so- I forgot to mention that the fuse is good. Assuming the fuse in the diagram is the one I think it is. There are three fuses: one on the circuit board (brand new and functional), one in the power center (functional) and the breaker in the furnace (functional). I tested all these with a multi-meter using a continuity test.

So now how to do I tell if the furnace is getting 12v?

What I know is:

good fuses
limit switch has continuity
new circuit board
new sail switch

The light on the circuit board isn't lighting up so it's not getting 12V to the board.

So I guess I look at the diagram to see where 12V first comes into the furnace...which I'm afraid means pulling it again but I'll check...First I'll check at the thermostat, though.

Thanks.
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Old 03-20-2018, 06:37 PM   #11
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Zach, you should have four wires running into the furnace.
Two are thermostat and the other two will be your 12v dc.
The heavier gauge wire should be the dc circuit.
Test at that point for 12v dc. If you have it there then itís in the furnace.
If no power trace back to power source.
If power, next test for 12v dc at relay.
If you have power to furnace, you may have trashed the thermostat. Put both wires together, see if it fires.
Youíll get it, take your time and use your meter or test light.
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Old 03-20-2018, 07:06 PM   #12
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This may help

http://bryantrv.com/docs2/docs/hflameservice.pdf
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Old 03-20-2018, 07:34 PM   #13
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Thank you.

I did hold the two wires at the thermostat together and still no light on the circuit board.

The wires going into the furnace are accessible by taking the furnace out...so Iíll look over the wiring diagram and see where they first connect in the furnace and test for 12v there. Looks like it may be at the breaker.

I have the original manual, which is slightly better than that link, since the link is for the newer model. But thank you!

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Old 03-20-2018, 07:42 PM   #14
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Youíll get it!
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Old 03-20-2018, 07:42 PM   #15
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Quote:
Originally Posted by mszabo View Post
If you have power to furnace, you may have trashed the thermostat. Put both wires together, see if it fires.
Hey just making sure this is correct. Iíve been assuming that if the furnace is getting power, the thermostat must be good. Is that wrong?

I thought all power went first through the thermostat then to the furnace for the first time.

Let me know. Iím used to being wrong but want to make sure that wasnít a typo!
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Old 03-20-2018, 07:46 PM   #16
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Keep thinking and studying you will get it bob
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Old 03-21-2018, 03:48 PM   #17
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I have 12.6 volts on both sides of the fuse in the power center. I have something that settles around 178 mV at the thermostat...

Seems like anywhere Iím checking for 12v where there isnít any (?), my meter shows me some mV number. Might be some fluke with how my meter works, I donít know.

So I still donít know if power goes to the thermostat before going inside the furnace.

I have continuity through the thermostat, but no 12v. So obviously no 12v coming through the thermostat to the breaker in the furnace.

What next? Whatís between the power center fuse and the thermostat?
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Old 03-21-2018, 04:32 PM   #18
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Zach, the milamp at thermastat is what you want.
Those wires wont carry the load to motor.
If you read through the manual, once the thermostat calls for heat it’s 18 seconds before relay makes contact. Which then will put 12v to the motor, and sail switch.
Make sure you have the 12v at the relay, see picture. If you do, you can jump or remove wire and touch other side. Bypassing relay and putting 12v to motor.
Also turn on thermostat and check for voltage at relay. If you have it, 15 seconds later it should pull in coil.
My money is on that relay.
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Old 03-22-2018, 09:03 AM   #19
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Well I'll admit I'm not quite sure what you're saying on a lot of this, but I'll keep checking stuff.

If the furnace calls for 12V for a few seconds then gives up, am I in a rush to test at the thermostat? Will there be 12V when I first turn it on, then it'll stop when the furnace fails to do anything?

Well, I'm taking another unadvisable step. It's fairly safe to assume that the wire from the power center to the thermostat didn't melt, and that the connections are still good, and that whatever shorted is in the furnace itself. I don't see a tiny spark causing a wire or connection that far away from the furnace to melt or break, though one last test I'll do is a resistance or continuity check on that wire.

Other than the electronics on the propane valve in the furnace, I've tested or replaced every switch, circuit board and fuse associated with the furnace, except the time delay relay. That's a $30 part. I'll throw $30 more at this thing, then if it doesn't run I'll decide whether to pay a technician or just buy a new furnace. I'm moving back in at the end of the month so it's time to get the camper functional.

Thanks for the help!
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Old 03-22-2018, 09:44 AM   #20
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I admit I have never messed around with those relay circuits or el. circuits in a furnace I really don't understand them. they involve very low voltages such as millivolts to make things work including opening up the gas valve.


in your case I would pay a furnace guy to troubleshoot it for you I took out a bunch of el. stuff on my Vermont castings heating stove in order to put in a simple operated gas valve. all the operating controls are on the gas valve no electronics after looking at it he may or may not suggest how to fix it!


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