Atwood 8500 series furnace bench test on high limit switch

[JBarca]

Furnace: Atwood Hydroflame 8520-IV DC Installed in late 2003.

I am doing a total restore on a wet 16 year old camper. I am at the point of servicing the furnace and have a question concerning testing the high side (over temp), of the high limit switch. The furnace is out of the camper, on a bench test setup, been cleaned up, gas valve replaced due to failing a pressure leak test, gas burner and heat exchanged inspected and are in good shape. By the looks of inside the heat exchanger and the burner screen being in such good shape, this furnace has not had a lot of run hours. The control board works as it should and the system now is in good running condition.

The question is, what method have you used, or know of, to confirm the “high side” of the high temp limit switch opens when it should? The Atwood service manuals do not talk to testing the high side, only if the switch fails to make continuity and is stuck open.

I have run two 40 minute continuous burn tests, taking various temperature checks around the furnace. With the setup I am using, the system will stabilize and come to equilibrium, running it longer does not gain much more internal temp.

My only question I have now before installing it back in the camper, is there a test used on how to force the system to confirm the high limit switch will open on an over temp? I have tried blocking the duct ports to raise the cabinet temperature, and while the system does increase some, it will not trip the over temp limit switch to shut down the gas burner. The duct ports where close to only approx. 20% open and it still would not trip the high temp limit. That said, the air temp at the limit switch was not hot enough most likely to trip the switch as I had a remote thermometer measuring the air temp at the high temp switch throughout the testing. The way I am doing the bench test may be partly why I cannot get the temperature hot enough to trip the limit.

At this point, the only idea I can come up with, is to pull the switch out of the unit and test it against a known temperature hot plate and see it opens up in the range of 190 deg F. That is doable with what I have to work with, but that is a lot of parts to take out to do this. Figured I would ask here first if someone has been through this before.

I have all kinds of pics and data sheets I can share if that will help, but it still comes down to, how is the high limit switch tested for hi limit, or is it not tested as they have not failed in that manner? It may be, I am trying to go above and beyond what normal furnace testing does.

Here is the bench setup, doing the testing.


The temp probe measuring the air temp at the hi limit switch.


The 6 temp. check points during testing.


1= Exhaust port, outside surface temp.
2= Heat exchanger surface temp at discharge.
3= Heat exchanger surface temp at end of gas burner.
4= Outside cabinet temp, above gas burner.
5= Furnace exit duct temp.
6= Hi limit switch senor area air temp.

Any help, greatly appreciated.

Thanks

John

[JBarca]

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

[afidel]

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.

[smthbros]

Dusty R wrote:
That switch should never need to open. If it does there is not enough air moving through/over the furnace.

Absolutely true.

[dougrainer]

Dusty R wrote:
That switch should never need to open. If it does there is not enough air moving through/over the furnace.

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

[Dusty_R]

That switch should never need to open. If it does there is not enough air moving through/over the furnace.

[dougrainer]

JBarca wrote:
Thank you all for responding.

I agree, if you take the blower apart enough to get to the high temp. switch, then replace it. I do have a spare high limit switch, and I am in the process of replacing it. As FYI, cost for a Dometic PN 37022, the current day replacement, was $21.04 with tax and shipping. The prices are all over the map, just make sure it is an OEM supplied one.

From Doug's comments, it appears he has seen these switches fail before as they get older. Not sure if they fail open, stay stuck closed, how many years it takes, or run hours of the furnace to get to that point. It would be good to know.

On taking the time to test the furnace, if this was a charged job, time is money and the time spent with extra testing is hard to absorb or pass on to a customer. In my case, time is something under my control and cost is secondary, there is no waste. The knowledge learned from the testing, was worth it for me. I'm retired and restore water infected campers. I now know more now then I did before, and I know better for the next time.

Thanks again, grateful for all the responses/help.

John

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

[JBarca]

Thank you all for responding.

I agree, if you take the blower apart enough to get to the high temp. switch, then replace it. I do have a spare high limit switch, and I am in the process of replacing it. As FYI, cost for a Dometic PN 37022, the current day replacement, was $21.04 with tax and shipping. The prices are all over the map, just make sure it is an OEM supplied one.

From Doug's comments, it appears he has seen these switches fail before as they get older. Not sure if they fail open, stay stuck closed, how many years it takes, or run hours of the furnace to get to that point. It would be good to know.

On taking the time to test the furnace, if this was a charged job, time is money and the time spent with extra testing is hard to absorb or pass on to a customer. In my case, time is something under my control and cost is secondary, there is no waste. The knowledge learned from the testing, was worth it for me. I'm retired and restore water infected campers. I now know more now then I did before, and I know better for the next time.

Thanks again, grateful for all the responses/help.

John

[dougrainer]

Dutch_12078 wrote:
Why not just replace it with a new one as long as you're already in there? For the $15 or so they sell for, I wouldn't waste my time on the old one. Save it for a spare...

Bingo. You are wasting a lot of your time on a 16 year furnace testing a Limit switch that usually fails after years of service. Just install a new one. To get it to trip, block off the hot air ductwork holes. It should take less than 10 minutes for the Heat Chamber to reach over 200 degrees. Doug

[enblethen]

Could use a hair dryer, need to be able to measure with temperature probe to verify about what temperature it opens

[Dutch_12078]

Why not just replace it with a new one as long as you're already in there? For the $15 or so they sell for, I wouldn't waste my time on the old one. Save it for a spare...

[larry_cad]

If you remove the switch, and put it into an oven set to 200, it seems to me an ohmeter would indicate if it is open. Yes, this requires removing the switch, but seems to simple and effective or am I missing something?