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 > Atwood 8500 series furnace bench test on high limit switch

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JBarca

Radnor, Ohio, USA

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Posted: 02/27/21 10:03am Link  |  Quote  |  Print  |  Notify Moderator

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.
[image]

The temp probe measuring the air temp at the hi limit switch.
[image]

The 6 temp. check points during testing.
[image]

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


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larry cad

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Posted: 02/27/21 10:35am Link  |  Quote  |  Print  |  Notify Moderator

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?


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Dutch_12078

Winters south, summers north

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Posted: 02/27/21 10:57am Link  |  Quote  |  Print  |  Notify Moderator

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...


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enblethen

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Posted: 02/27/21 11:06am Link  |  Quote  |  Print  |  Notify Moderator

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


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dougrainer

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Posted: 02/27/21 02:24pm Link  |  Quote  |  Print  |  Notify Moderator

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

JBarca

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Posted: 02/27/21 09:59pm Link  |  Quote  |  Print  |  Notify Moderator

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

Carrolton, Texas

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Posted: 02/28/21 06:33pm Link  |  Quote  |  Print  |  Notify Moderator

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

Dusty R

Charlotte Michigan 48813

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Posted: 02/28/21 07:19pm Link  |  Quote  |  Print  |  Notify Moderator

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

dougrainer

Carrolton, Texas

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Posted: 03/01/21 09:20am Link  |  Quote  |  Print  |  Notify Moderator

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

smthbros

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Posted: 03/01/21 11:46am Link  |  Quote  |  Print  |  Notify Moderator

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.

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