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Old 01-16-2021, 12:52 PM   #21
chuckster57
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What concerns me (and I have no answer for) is why two furnaces are doing the same thing ???

Me too!!
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Old 01-16-2021, 03:12 PM   #22
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Originally Posted by chuckster57 View Post
What concerns me (and I have no answer for) is why two furnaces are doing the same thing ???

Me too!!
Are you sure it’s not a duct issue? It sure sounds like insufficient return air or restricted supply duct and I know you said you added two 4” ducts but what does the whole duct system look like?
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Old 01-16-2021, 03:16 PM   #23
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Originally Posted by jasin1 View Post
Are you sure it’s not a duct issue? It sure sounds like insufficient return air or restricted supply duct and I know you said you added two 4” ducts but what does the whole duct system look like?
He has said it does it on the bench with no ducts hooked up. That should rule out airflow.
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Old 01-16-2021, 03:18 PM   #24
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I would remove the flame sensor and sand it well using sand paper. If that doesn't work, run a small brush through the burner tube (or compressed air). All it takes is a spider Web to alter air flow to burner.
It’s a good idea to clean the flame sensor but it’s best not to use something too abrasive..yes it will get the furnace started if that’s the problem but the rough surface from sanding will attract more impurities and just cause the problem again in the near future.if you notice flame sensors are very smooth. I use a piece of rough canvas or anything I can find that isn’t too abrasive.
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Old 01-16-2021, 03:19 PM   #25
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He has said it does it on the bench with no ducts hooked up. That should rule out airflow.
Oh ok well that should rule out his regulator then also because he can’t bring all of that in his shop..
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Old 01-16-2021, 03:22 PM   #26
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AirExcel sent us a new furnace and it does the same thing. We replaced the gas regulator, thermostat, added two more 4" ducts and even bench tested the unit and it still does the same thing.
When you "bench tested" was that with a different LP source?
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Old 01-16-2021, 03:26 PM   #27
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And if you bench tested it did you just jump out the thermostat connections? Bench testing it is fine but if it’s doing that on a brand new furnace with a separate gas supply and a separate electric and thermostat setup it will be hard to diagnose the problem in the rv with all of those factors changed.if you bench tested how was it setup? Was it level? Tstat jumped,flue accounted for and not recirculating the exhaust gas. I mean it’s not really recreating the actual design parameters.
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Old 01-16-2021, 03:38 PM   #28
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Originally Posted by chuckster57 View Post
He has said it does it on the bench with no ducts hooked up. That should rule out airflow.
Do rv thermostats have the sliding heat anticipators? I’ve never worked on one before
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Old 01-16-2021, 03:52 PM   #29
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Originally Posted by jasin1 View Post
Do rv thermostats have the sliding heat anticipators? I’ve never worked on one before
There are analog thermostats with the “slider” to set temp and there are digital thermostats, he didn’t state which one.

When bench testing its normal to just twist the thermostat wires (blue) together to complete the “request” for heat.
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Old 01-16-2021, 03:53 PM   #30
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Hey Chuckster do rv furnace gas valves have adjustable regulators? On residential units when i convert a furnace over to propane I have to change the orifices and the regulator spring then adjust with a manometer. With natural gas I just clock the gas meter and adjust the regulator by that method. The chances of two separate furnaces with the same problems is pretty slim unless it was just a furnace design issue. But was wondering if there is any adjustment at the gas valve
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Old 01-16-2021, 03:55 PM   #31
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Originally Posted by chuckster57 View Post
There are analog thermostats with the “slider” to set temp and there are digital thermostats, he didn’t state which one.

When bench testing its normal to just twist the thermostat wires (blue) together to complete the “request” for heat.
Yes I realize that. I’m talking about the heat anticipator on older thermastats. ( like the round Honeywell t87 that was in millions of homes) they and a lot of other stats had heat anticipators under the cover. Anyway when I install newer furnaces on these style of stats, they can shortcycle wasn’t sure if rv thermostats(non digital) had heat anticipators
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Old 01-16-2021, 03:57 PM   #32
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You have to set the anticipator to the amp draw of the heat circuit to match the gas valve in milliamps
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Old 01-16-2021, 04:01 PM   #33
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I don’t think the RV thermostats have that.
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Old 01-16-2021, 04:01 PM   #34
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Anyway lol enough of this I don’t know why I’m tryin to diagnose a furnace over the internet. It’s hard enough when your looking at it in person and even harder when it’s second hand news relayed by a third person on a forum. I will leave it to you guys that actually work on them everyday.... me trying to compare residential furnaces to rvsis like apples and oranges
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Old 01-16-2021, 04:04 PM   #35
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And thanks for answering..I just want to be able to help because it’s my nature but I need to refrain from throwing too many things out there. It can confuse things
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Old 01-16-2021, 04:22 PM   #36
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Your welcome and thanks for posting. It’s always good to have another set of eyes when trying to figure things out.
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Old 01-18-2021, 09:36 AM   #37
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This thing is driving me crazy!*

I recorded another video bench testing the OLD furnace completely outside the trailer with a separate gas line and a new battery (and bypassing the thermostat) and it worked for 42 minutes straight without the flame going off. I turned it off otherwise it would probably continue indefinitely.*

Knowing that it's working as it should have while disconnected completely from the trailer, I went back to the new furnace (the one installed in the RV) and bypassed the electrical in my trailer by connecting the new battery and it failed: flame runs for about 3.5 minutes, off for 1 minute and turns on for another 3.5 and this cycle repeats (fan is on all the time unless system is turned off by the thermostat). I then connected the trailer electrical back and bypassed the propane AND it did the same thing! WHAT?!

For those of you who would think at this point that this is an airflow issue, I have over 400 sq.inch of return (minimum required by AirExcel is 56 sq.inch) and I have all six duct openings utilized so much better than the successful test I did with only two open (see video) AND the old and the new trailer failed the bench test while connected to the trailer electrical and propane.*

It is a*new RV we bought four months ago. We used it only once and my wife doesn't want to put the kids there until we know it's safe (after all we run this furnace 6 hour during cold nights) - I think she has a point knowing it could be a safety*issue. One tech told us that we are abusing the hi-limit switch allowing it to run this way. The hi-limit switch is a safety*mechanism not intended to be on*and off 12 times an hour and over 70 times over night.

For the past three weeks AirExcel and Grand Design worked diligently to help us find the problem but they are now pretty much telling us that this is normal. I disagree.

Here is the video:
https://www.dropbox.com/s/57kcg20lko...rnace.mov?dl=0

Help What do you think?
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Old 01-18-2021, 10:04 AM   #38
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Do you have insect screens on the furnace exhaust on the outside?
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Old 01-18-2021, 10:47 AM   #39
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@flybouy, yes insect screen is installed. We have two new furnaces so I don't think this is the problem. By the way, we did blow air into the combustion chamber of the old furnace to make sure it was clean and it was.

@jasin1 yes, I jumped the thermostat and the furnace is leveled. What do you mean: "flue accounted for and not recirculating the exhaust gas". No, I don't think it has a sliding heat anticipator but I might be wrong. Here is a picture of my thermostat: https://www.dropbox.com/s/l2krd2u1yr...tat.jpg?dl=0No,
I'm not sure it is not a duct issue mainly because I'm confused from the contradicting data I have so far. Sorry for the poor drawing but this is how my duct system looks like:*https://www.dropbox.com/s/105themuxb...awing.png?dl=0 *

@chuckster57 yes, that is exactly what I did. I twisted the two blue wires to bypass the thermostat.

@flybouy I bench tested it while connected to the trailer gas line and another time connected to a new gas line -- the one you see in the video I just posted (same propane tank and I have two of them running in parallel*while connected to the railer).
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Old 01-18-2021, 11:05 AM   #40
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We had a couple of folks that have posted similar issue after installing the insect screens. They removed the screen and the unit functioned fine. Might be worth a try.
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