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Old 10-29-2012, 08:12 PM   #1
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RV furnace


Hi all. I have an 89 Southwind with an Atwood 35,000 btu furnace. We just bought the motorhome during the summer, so havent really been able to use the heat yet, so yesterday I fired it up for the first time.
At a 75 degree indoor temp, the furnace is blowing at around 205 degrees. I checked with an rv repair shop today and they said that was too high but as I was looking at the spec sheet on the furnace tonight, it says air rise 136 degrees.
I checked the heat exchanger for cracks or holes and also made sure the blower is running at full speed.
Does this temp seem normal to you guys? I really dont think the blower in these things move much more than a couple hundred cfm, so I can see why at 35,000 btu, they would be so hot. Thanks for the help.

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Old 10-30-2012, 05:06 AM   #2
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At 35,000 BTUs output, a 135į temp rise would only be about 258CFM.

Don't work on RV heaters, but that does sound high.

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Old 10-30-2012, 08:56 AM   #3
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Thanks beenthere. This thing is only ducted with four 4-inch ducts and two of those have a pretty tight bend so as to make the unit fit where they had to put it.
I forgot to mention that the unit also goes off on high limit after it has run for a while but does restart after a couple minutes.
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Old 10-30-2012, 02:57 PM   #4
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Air flow is too low. Find a way to add more supplies, or increase the size of the existing ones.
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Old 10-30-2012, 03:55 PM   #5
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Quote:
Originally Posted by beenthere View Post
Air flow is too low. Find a way to add more supplies, or increase the size of the existing ones.
The problem with most rv's is that once they build them, your pretty much outta luck for adding more duct but I think I may be able to pull one more off of it.
I called Atwood this morning and the guy there says when he test runs them on a bench, he gets 140 degrees out of them. Most duct systems in rv's are a joke for airflow and I cant see having an air temp that low when installed in the coach.
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Old 10-30-2012, 06:11 PM   #6
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With Rv's you get what's designed no more no less can't add on or do anything but make sure the system is working as designed.
In your case your RV is approx 23 years old I would look at the fan motor . Can it be oiled, cleaned or replaced? Maybe a new heater is what you need. You can buy a replacement heater for that unit probably for a few hundred bucks which probably is the way to go with the age of the equipment.
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Old 10-30-2012, 08:35 PM   #7
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Quote:
Originally Posted by COLDIRON View Post
With Rv's you get what's designed no more no less can't add on or do anything but make sure the system is working as designed.
In your case your RV is approx 23 years old I would look at the fan motor . Can it be oiled, cleaned or replaced? Maybe a new heater is what you need. You can buy a replacement heater for that unit probably for a few hundred bucks which probably is the way to go with the age of the equipment.
I found a furnace on an rv site that sells for 450, same brand and design as mine, they havent changed in all these years. If I do buy a new one, I will go with a smaller btu output, in our general climate, 35,000 is too big for our coach.

I was looking into buying a couple heat pump units to replace our rooftop units but discovered that they dont have a defrost system on them, they just simply shut down and switch over to strip heat. I've been told that the Carrier models had defrost but Carrier pulled out of the rv game a few years ago.
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Old 10-31-2012, 11:36 AM   #8
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Keep in mind that the blower motor in these units is a DC motor. The voltage and available amperage to it is critical as to the RPM's that it runs at. Make sure the battery if fully charged and that all wire connections are good.

Adding additional heat runs usually requires a magician. I had a similar issue with mine and solved at least part of it by using a chassis punch and putting six 1 1/8" holes in the front cover of the furnace. I got away with this because the furnace front cover was not behind a cabinet style solid or metal mesh door.

Other thing to check is the gas pressure to the furnace. Make sure it's not overfiring.
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Old 11-01-2012, 09:29 AM   #9
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Before you buy another furnace, pull that one out the RV, and run it sitting on a bench outside, and measure the temperatures that it's putting out and it's performance. If it performs okay, then the fault is in the installation, not the furnace itself. If you can, I recommend replacing the flex-duct with regular metal ductwork
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Old 11-02-2012, 12:35 AM   #10
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I'd like to see some pictures of this unit. I replaced an rv unit not too long ago but there was no duct work involved, just drop in and it blew into the open space.
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Old 11-02-2012, 09:21 AM   #11
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I had the same type troubles years ago with an rv heater in a popup. very hot air and shut down on overtemp.

What I found was that the heat exchanger was filled up with a mud hornets next. They had built it in the fresh air intake and exhaust system. When the pop up got washed, the nest went into the heat exchanger and partially plugged it up, as well as plugging up the air intake for the burners. The whole thing overheated.

The fix was to take the system out, clean out the exchanger (and check for cracks and rust etc), then clean the dual (inside and outside air) blower cages and finally clean and oil the motor. Worked fine for the next 10 years till I sold the trailer. kept that leaky canvas monster nice and warm even in the coldest settings!!!

As suggested, pull the system out and bench test it!!!

I do have a question. Temp rise??? how is that calculated? is it the delta t between indoor (at return) and output (at supply)???? or is it the actual output temp of the supply ????? I am guessing the delta t....
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Old 11-15-2012, 05:53 PM   #12
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So I pulled the furnace out and ran it in my driveway, still got hot and that was with a proven good LP regulator running at 11" and free unrestricted air flow. Checked the HX and it looked fine but didnt really get too far into it.
I went ahead and bought a smaller btu output unit from a large rv place in Texas and had it overnighted to Vegas, installed it and it works great. Highest output temp is 160 degrees and the peace of mind of having a new safe furnace.

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