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Old 03-14-2009, 10:44 AM   #1
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Furnace won't shut off

One day I hope to have enough experience to actually GIVE advice on this forum instead of asking for it all the time...

Anyway, today's issue is just as described; our furnace comes on and won't shut off, regardless of the tstat setting, regardless of if the system is even switched on. The only way to stop the heat is to pull the furnace fuse. Any thoughts?
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Old 03-14-2009, 04:37 PM   #2
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A lot of us draw upon the bad things that have happened to us in order to answer posts. Just look at the experience you are gaining to be able to answer furnace problems in the future.

Knowing your make and model of furnace would help. Typically there are two makers - Atwood and Subruban. I have two Atwood 8525-IVs so my information is based on my experience with those. Now, a couple of points:

1. When you say the furnace continues to run, does that mean that you hear/feel the blower or that the ignition/burner continues too? If the latter, I'd guess you have a bad T-stat. You could prove that by looking at the input to the furnace after the temperature set has been reached (or perhaps slightly exceeded). If you still have 12 volts, it is the T-Stat.
2. If just the blower continues, something is interrupting the ignition before the T-stat temperature is reach, I'd wager. While it is difficult to suggest why the ignition is dropping out, the sail switch and limit switch would be a great place to start. If the temperature hasn't been reached and the blower is running, you can check a point on the ICB (Ignition control board) for voltage. If those two switches are closed, there will be voltage to board from them. If no voltage, one of them has interrupted the flow. In my case, it was a bad sail switch. If you have the voltage from those switches, the problem likely is in the board or the gas valve. That gets a bit more complicated to troubleshoot.
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Old 03-14-2009, 06:21 PM   #3
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Originally Posted by chasfm11 View Post
1. When you say the furnace continues to run, does that mean that you hear/feel the blower or that the ignition/burner continues too? If the latter, I'd guess you have a bad T-stat. You could prove that by looking at the input to the furnace after the temperature set has been reached (or perhaps slightly exceeded). If you still have 12 volts, it is the T-Stat.oard or the gas valve. That gets a bit more complicated to troubleshoot.
The former is definitely the case. Heat and blower continue well past the point where they should shut off (it ran the heat up into the 80s, even after turned down to 64). Switching the wall t-stat from HEAT to COOL or OFF also had no effect on the heat and blower--only pulling the fuse to the furnace will make it stop. When the fuse is plugged back in, it's quiet for a few minutes, then starts up again.

Is the tstat in question the wall thermostat, or one located on the furnace itself? I ask because the wall thermostat is a relatively new digital unit (less than 1 yr old).
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Old 03-14-2009, 07:10 PM   #4
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Chris....My first step would be to remove the thermostat from the wall, most can be unsnapped. This will also tell you if the thermostat is the issue or something is wrong with the heater. It may also cause the thermostat to reset and cure the problem.
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Old 03-14-2009, 09:23 PM   #5
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When you pull the thermostat off wall and it keeps running find the therm wire into the furnace and cut it. You may have a crushed wire some where in coach or the same wire maybe shorted to metal frame of furnace.
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Old 03-14-2009, 10:04 PM   #6
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I've no advice to offer. I hope you solve this problem, it is dangerous. If that happened while you were away from the MH for a full day it could start a fire that destroys your coach.
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Old 03-14-2009, 10:23 PM   #7
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Mother board was what fixed our furance with this problem.
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Old 03-15-2009, 09:21 AM   #8
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Quote:
Originally Posted by The Incorrigible Dirigible View Post
One day I hope to have enough experience to actually GIVE advice on this forum instead of asking for it all the time...

Anyway, today's issue is just as described; our furnace comes on and won't shut off, regardless of the tstat setting, regardless of if the system is even switched on. The only way to stop the heat is to pull the furnace fuse. Any thoughts?
What model furnace, T'stat and roof or basement air cond. do you have? Some furnace circuit configurations use the air conditioners to control 12 volt power between furnace and T'stat. Do you have separate Tstat for furnace and air conditioners? If not, have you tried turning the 120VAC circuit breakers off on the air conditioners?
Please give us as much info as possible to better help you on this matter.
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Old 03-15-2009, 10:26 AM   #9
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Replace the ignition control board

I can tell you that our furnace did the exact same thing and that it drain our batteries down....

It ended up being the control board....

Here is a link to the company that we bought our replacement board from

http://www.partsfortechs.com/asapcar...3_168_167.html

Since replacing it has worked fine.
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Old 03-15-2009, 03:48 PM   #10
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What model furnace, T'stat and roof or basement air cond. do you have? Some furnace circuit configurations use the air conditioners to control 12 volt power between furnace and T'stat. Do you have separate Tstat for furnace and air conditioners? If not, have you tried turning the 120VAC circuit breakers off on the air conditioners?
Please give us as much info as possible to better help you on this matter.
Furnace is a Hydro Flame 8535-III. Thermostat is a digital RV Comfort unit. Rooftop AC units are Carrier and Dometic, all controlled through a single wall thermostat. The Carrier has its own thermostat and can work independently of the furnace/t-stat, while the Dometic will not work without the wall thermostat. When the wall t-stat is on, the Carrier listens to it. (Hope that makes sense...) Have not tried turning off AC circuit breakers but the problem seems to persist with AC units turned off.
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Old 03-15-2009, 03:50 PM   #11
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I've no advice to offer. I hope you solve this problem, it is dangerous. If that happened while you were away from the MH for a full day it could start a fire that destroys your coach.
Agreed; that's why the furnace fuse has been pulled and we're only using the heat pump until this is figured out. At the very least, the silly thing's going to use up all our LP! :(
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Old 03-18-2009, 08:20 PM   #12
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Just pulled up the service manual from this site:

http://bryantrv.com/docs2/docs/hflameservice.pdf

It lists defective thermostat, shorted thermostat leads, or bad relay as the possible causes. 007 gave you info on the first two issues, I've had the bad relay on my old MH.

If I remember correctly, I could get at it by removing the access panel from the outside of the coach. BTW, http://www.marksrv.com/hydroflameparts.htm had the relays, though the price seems to have gone up a lot in the past 4 years!

Good luck
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Old 01-01-2014, 06:56 PM   #13
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Let me know what you find Im having the same problems
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Old 01-02-2014, 04:11 AM   #14
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doc, you may want to start a new thread as the last post here was almost 5 years ago.
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