afidel
Cleveland
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One thing to check for is any domeing on the capacitors on the control board. 2003 was during the height of the capacitor plague so there's a significant chance the board will fail sooner or later.
2019 Dutchman Kodiak 293RLSL
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JBarca
Radnor, Ohio, USA
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I am giving an update and comments back.
dougrainer wrote: They are normally CLOSED. They usually fail OPEN. The Bi Metal of the switch gets "weak" over years of tripping and resetting. That is why if rebuilding an older furnace you replace the switches. Doug
Thank you, Doug, I know the technology of these thermal disk switches; we use to call them Klixon relays years ago at work. Klixon was one of the more extensive manufactures making them back then, and they may have even invented them, I'm not sure. Nowadays, there are lots of manufactures.
I can see your point about the metal fatigue on the switch, and if you are rebuilding the furnace, change it. Got it, good point.
Since you have seen many of these fail so much, it begs the question as to why? Then Dusty added this, which my mind already went to; this is supposed to be a hi "limit" switch, not a T stat switch to modulate the high-end temp of the heat exchanger.
Dusty R wrote:That switch should never need to open. If it does there is not enough air moving through/over the furnace.
dougrainer wrote: They open and close all the time. OEM furnace ductwork installs NEVER match the correct volume of air required to run the furnace without the Limit occasionally tripping. Also the RV'er will block off some floor ducts to get more heat or air to some areas of the RV. That will also cause the Limit system to trip. YES, it would be nice if the Ductwork was installed to meet the Minimum requirements to prevent Limit switch tripping. I can ALWAYS make both Furnace and Roof AC ductwork many times more efficient, but unless under warranty, the customer will not want to pay for a check out and making it to best operation. Rarely do I get a complaint about Ductwork under warranty. They just figure that is the way it is. Doug
Then your thoughts lined up with the reality of many RV manufacturers saved me from typing it. While the RV manufacture engineering department may have done some CFM calculations, not sure if they redid them on every floor plan on the duct routing. Then there is the main shop floor. Did they route the ducts without as many turns or hose kinks? Did they do CFM/pressure tests at the vents as part of a QC requirement? While they do this in HVAC in commercial buildings, I'm not sure the RV assembly line does on every floor plan and camper off the line.
This furnace I am working on now is a 20,000 BTU unit. Atwood states it must have 2, 4" ducts minimum. I looked up the part numbers on the air wheel, the motor, and the gas jet, it seems the blower wheel is the same across the range of the 8500 series. The motor changes in groups when the output gets larger, and the gas jet changes per BTU output of the furnace.
On my bench test, I had to work to get the hi Limit to trip on this 20K BTU unit and still it would not get hot enough to trip. I was at least 80% blocked, and it still did not trip. But, the bench test does not have all the duct turns and restrictions in it like the camper. Plus, my shop is at 55F, so the camper inside fresh air intake is constant and not rising like it usually would. The bench test is the best case, not the worst case to "not" trip the hi-temp Limit. Since the blower wheel is the same, it would have heated up faster if this was a 31,000 BTU unit and may have tripped the hi Limit. I have a larger camper in restoration camper in que that I will test out when I get to it.
All my restoration work so far has been on the Sunline brand of campers. They were a smaller east coast builder in PA; they went under in Nov. 2006, one of the first to go under back then. They had hourly waged line employees and not piece work. Their assembly quality and the camper itself, I would say was better than most that I have seen. But they are still an RV.
I have 6 of these campers in my shop now, 4 in restoration. Sunline upped the numbers of ducts per size. The 20K BTU units have 3 air ducts where the min is 2. The larger 31K units have 4 ducts where the min is 3. They may have sorted this out to have more ducts due to the floor plans and pressure drop losses. I have only been associated with this brand since 2003, and the hi-limit switch failing is not common; even on the 1900's campers. I have not heard of any failing yet. I'm sure there may be some, though, just not common.
Doug, has worked on many brands, and more years, you have seen more. Have no idea if all the other brands kept to the minimum duct requirements or upsized. Your comments are right about the customer not knowing that hearing the gas valve cycle on and off when the furnace is running is a concern. There is no fault of the system that comes up, it just happens, and they think it is normal.
When I install this furnace back into the camper and get it fired up, I will check if the gas burner cycles during a call for heat. If it does, I'll go hunting for the restrictions to see if they can be corrected.
Thanks again, everyone, for your help. I'll post some pics in the next reply of the new switch and bench test that followed.
John
* This post was
edited 03/04/21 12:53pm by JBarca *
John & Cindy
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dougrainer
Carrolton, Texas
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The OEM's have engineered designs for ductwork. BUT! The actual install is up to production people that are NOT told the specs and why the install has to be to the engineering prints. That is why over the years I have seen horrible installs on Roof AC ducting and furnace floor ducting. But, until the customer complains I do not dive into the ducting to see the problems. One exception was over 10 years ago when we sold a certain brand if inexpensive Trailers. ALL the roof AC ducting was not installed and sealed especially at the AC control box plenum. HUGE gaps. It was so bad(customer complaints under warranty), that we in service told our PDI dept to repair every AC ducting of that brand during PDI, which the OEM paid for under warranty. AS I stated, most customers never realize that the furnace is LIMITING during operation and if they hear the burner kick off and on, they assume that is normal. Doug
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JBarca
Radnor, Ohio, USA
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Here are some pictures of the new hi limit switch install and the tests to follow it.
With the furnace out of the camper, this is easier but the hi limit switch is buried pretty deep.
![[image]](https://live.staticflickr.com/65535/50996879912_aceb5a6411_b.jpg)
![[image]](https://live.staticflickr.com/65535/50996757596_36d88a4c4e_b.jpg)
The old discontinued hi limit switch. It has #4 screws with a #0 square bit drive head. Odd, on this whole furnace this is the only square drive head screw.
![[image]](https://live.staticflickr.com/65535/50996879872_826b4752e2_o.jpg)
The new hi limit replacement. PN 37022
![[image]](https://live.staticflickr.com/65535/50996879932_16b6976364_o.jpg)
The terminals are rotated some and smaller.
![[image]](https://live.staticflickr.com/65535/50996066208_54c218532f_o.jpg)
The bench test after installing the switch and the outcome.
![[image]](https://live.staticflickr.com/65535/50996066008_9d2e627a76_b.jpg)
![[image]](https://live.staticflickr.com/65535/50996066028_7f3cc99d29_b.jpg)
I used steel plates to block off all 3, 4" duct holes. Two of the plates has some holes drilled in them that allowed limited air flow. I would estimate I was 95 to 98% blocked on the three duct ports.
The test outcome. This is a 20,000 BTU furnace. Time zero was when the gas burner opened the first time. I used the same remote thermometer to measure the inside cabinet air temperature at the hi limit switch area. The old switch had ratings stamped on it, set point 190F, with a -20F differential.
Start
Time" 0'0" Gas burner on
Hi limit area temp: 57F
Time: 3' 59" Gas burner off
Hi limit area temp: 204F
Time: 5' 41" Gas burner on
Hi limit area temp: 154F
Time: 8' 20" Gas burner off
Hi limit area temp: 206F
Time: 10' 10" Gas burner on
Hi limit area temp: 154F
Time: 13' 20" Gas burner off
Hi limit area temp: 204F
Test over.
With blocking the ports more completely, the system will cycle the burner on and off as it should.
The furnace is now ready to install back in the camper.
Thanks again everyone.
John
* This post was
edited 03/04/21 12:53pm by JBarca *
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BFL13
Victoria, BC
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Good job on the furnace!
Never heard of those called "square drive" before. Weird! ![[emoticon]](http://www.coastresorts.com/sharedcontent/cfb/images/smile.gif "smile")
Looking it up, found this explanation;
https://www.popularwoodworking.com/quest........-a-square-drive-vs-phillips-head-screws/
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dougrainer
Carrolton, Texas
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Are you serious? Square Drives/screws have been utilized in the RV industry for over 50 years. The actual Square drive bit I use for that limit switch is No 1. Doug
PS, in a pinch you can use a small needlenose Vise Grip pliers. Hard to do, but they usually grip the outer round part of the screw.
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BFL13
Victoria, BC
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I only ever heard them called Robertson "up here". I suppose that is changing with younger people, don't know. Square Drive new to me until today.
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JBarca
Radnor, Ohio, USA
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Thanks for the good words on the furnace.
The square drive, yeh, it is the same as the Robertson. The Robertson drive originated in Canada and most likely why it is common language there still today.
Here in the US, square drive is common. Here is one of the fastener supply places I buy all my screws from when restoring campers. They sell the bits too, https://www.albanycountyfasteners.com/square-drive-power-bits/1050-600.htm
This is sort of like the term "Vise Grips". They were the first and as time went on and patents ran out, the name Vise Grips is still used by the trades even if they are some other brand of locking pliers... Old names, die hard. Sort of like a Crescent wrench, or other form of adjustable wrench.
John
* This post was
edited 03/02/21 07:02pm by JBarca *
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JBarca
Radnor, Ohio, USA
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dougrainer wrote: The actual Square drive bit I use for that limit switch is No 1. Doug
PS, in a pinch you can use a small needlenose Vise Grip pliers. Hard to do, but they usually grip the outer round part of the screw.
H'mm, that is odd. I tried my no. 1 bits and it would not fit. I used the mini Vise Grip method on the OD like you said and I'm ordering a few no. 0 bits for the next time.
No. 2 square head screws are all over the inside of the camper. Seems outside, 1/4" hex head is more prevalent on the moldings, door flanges, etc. Anyone owning a camper, needs to have a no. 2 driver of sorts in the camper tool box or else.
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dougrainer
Carrolton, Texas
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BFL13 wrote:I only ever heard them called Robertson "up here". I suppose that is changing with younger people, don't know. Square Drive new to me until today.
I am 66 years old and have been in the RV industry for 50 years. Never heard them called Robertson until about 15 years ago on an RV forum. Doug
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