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Old 11-28-2012, 02:05 PM   #1
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Furnace issue turns out to be AS issue

A few weeks ago I found that the rear bedroom furnace was short cycling by way of the thermal over temperature switch. It was the first time I tried to use the furnace since I purchased the unit a year ago. The furnace would turn on normally, but 5 minutes after the furnace lit it would shut off the gas valve until it cooled down some. After a minute or so it would fire back up and the cycle would continue until the room reached the proper temp.

After some diagnosis I found that the thermal limit switch was opening up causing the furnace to short cycle. I checked everything that could cause the problem without buying parts. I checked the voltage level, propane pressure, cleaned burner and burner chamber and checked the blower chamber for some infestation, but still the same. Both bedroom ducts were putting out a reasonable amount of airflow so they were not clogged. I replaced the thermal limit switch and the blower motor thinking that the blower speed was too slow. After all this it still was doing the same thing.

I then calculated the air volume that was available through the 2 4" ducts that AS installed and found that AS installed a marginal amount of outlet ductwork! Atwood’s specifications said that the 16K BTU furnace needed a minimum of 24 square inches of outlet duct. With the two that were installed I only had 25 square inches and that was with two long duct runs and several 90 degree bends to get around the shower. Each 90-degree bend was equal to 10 additional feet of duct according to Atwood's installation guide. I ended up added a third 4" duct and BINGO... everything started to work correctly.

Moral of the story... Don't assume that AS did a good job at specing out and constructing your coach's systems. I wasted a lot of time thinking that the installed duct work must have been adequate so assumed that the problem must have been with the furnace it self. Could have saved me around $120 in parts if I had just calculated the airflow to start with. I can only assume that the furnace has always done this and the PO’s just thought it was normal.
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Old 11-28-2012, 02:24 PM   #2
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In general, RV furnaces are installed using the absolute minimum required ductwork, regardless of the RV manufacturer.

I've added ductwork and made extensive changes to the furnace installation to improve performance, evenness of heat, and noise.
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Old 11-29-2012, 04:20 AM   #3
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Where did you add the extra 4" duct?
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Old 11-29-2012, 06:31 AM   #4
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I added the duct below my hallway closet just before the shower. Same place I added the CO/LP alarm.
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Old 11-29-2012, 09:51 AM   #5
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Would it not have been easier to just put a connector in the rear furnace duct system and join it to the existing front furnace ducting? They would seldom run at the same time unless you started them both together. And that would spread the heat throughout.

I have not experienced the same problem you have explained, but adding a new duct I envision as a major pain in my unit.

Dave
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Old 11-29-2012, 02:52 PM   #6
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Actually it was a lot easier to add the new duct then you might think. All it took was to cut a 4" hole at the base of the cabinet and run a short duct (~2 feet) to that opening. No big deal since all the area between the hole and the back of the furnace was open. I just knocked out one of the rear-prepunched cutouts on the furnace and inserted a duct adapter. I then ran a couple of feet of duct to the new grill. I used 4" hose clamps to secure the ductwork to the inlet/outlets.

The main system runs all along the roadside wall and the rear furnace only ran on the curbside so tying the two together would have been quite difficult. Tying them together would have also defeated the purpose of extracting additional air output from the rear furnace when the main furnace was on at the same time. And yes, both may be on at the same time on cold nights.
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Old 11-29-2012, 08:41 PM   #7
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Sounds easy enough.

On your unit, are the two furnaces not in the same location?

On the Clipper, the two furnaces are mounted together and are located below the fridge and HWT. The HWT would block and make it difficult to run a new duct to the same location under the wardrobe as you did and the pocket door would not allow snaking it around the front of the HWTank.

However, on the Clipper, it would be easy to run a new duct into the kitchen behind the drawer bank below the fridge if it was needed. I have actually eyed that cavity with the thought of putting in an electric 120 or 240 heater for when we are parked with power. I just have never had that need yet to justify the cost of installation.

Dave
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Old 11-30-2012, 06:20 AM   #8
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On my LY the furnaces are on opposite sides of the coach. Once located on the curbside under the furnace and one located aft of the kitchen.
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Old 11-30-2012, 05:33 PM   #9
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I missed the point about it being the rear furnace in your first post, I only have one furnace, but it does cycle on and off quite frequently and is very noisy. Can anything be done about the recycling and noise level?
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Old 12-01-2012, 06:52 AM   #10
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First you need to find out why it's recycling. Is the fan always running but the flame comes and goes until it reaches temp? Then it may be a similar issue as mine, which could be related to your noisy issue as well. With those symptoms it can be related to the following:
· Slow blower motor (Bad motor, damaged blower fan, debris on blower fan)
· Over temp switch (Weak)
· Insufficient outlet volume (Clogged output duct or marginal design)
· Insufficient inlet air (clogged air inlet or poor design)
· Intermittent sail switch (less likely)
· Too much LP pressure causing excessive flame

If the fan cuts off as well, then it's more likely related to your thermostat.

What kind of noise are you hearing? Could be a bad blower motor bearing causing the motor to be turning slower then required.
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Old 12-02-2012, 08:21 PM   #11
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I previously had a '94 LY. At one point, the furnace fan got progressively noisier. We determined that it was the bearings in the fan motor. That motor drives 2 fans, one on each side of it. Because the PO had had the moho in Florida, the salt air rusted that assembly so that it took about 1 week of daily dousing with penetrating oil to free it up. Once freed up, swapping out the old motor for the new was quick. The shop that worked on it told me that if they could not get the fan assembly out, it was going to be megabucks to replace the furnace. Thankfully, I was lucky.
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Old 12-03-2012, 03:57 AM   #12
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I was thinking it might be the thermostat, I'll try that first. When I said noisy I should have said loud. I don't hear any bearings or anything, just loud fan noise. Thanks for your replies.
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Old 12-03-2012, 06:20 AM   #13
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When it cycles on/off, does the fan keep running? If so, then it's not the thermostat.
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Old 12-03-2012, 07:17 PM   #14
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No, the fan turns off, thats why I think its the thermostat.
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