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
