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]

Boundless2281 wrote:
Okay thank you. That's what I was afraid of. I did that and nothing happened. Verified 13+ volts to the positive and negative lead. No noise or anything from the furnace. I am going to check all the connections, but it seems like I may be buying a dinosaur board to replace mine.

Atwood had several control vintages of this furnace, the prior version used a thermal timing relay to run the fan and fan delay. The PC board, in this case only ran the gas valve, ignition, and flame sense circuit.

The next generation of controls did away with the timing relay and added the fan control to the PC board, which then controlled that fan and the gas valve, ignition, and flame sense.

Does your furnace have the thermal timing relay?

See the timing relay here; it is on the right end of the blower housing.



Here is the newer vintage with the fan control on the PC board—no thermal timing relay. A PC wire plug is on the front of the PC board; a blue wire, the T stat run signal, was added and is part of the fan control. The T stat run signal on the older thermal timing relay received the T stat signal.






This PC board has fan control on it. The black relay on the PC board in this pic is for the fan motor (the red wire is the fan motor hot wire, and the blue next to it is incoming 12-volt power for the fan motor). The round coil igniter also has a red wire but different insulation and goes to the electrode.

Some checks, and this assumes you have a 12.5 DC power or higher coming to the furnace.

If yours has the timing relay, they have been known to go bad over time. If you connect the two blue wires within a short period, the blower should run; if the blower will not run, either the motor is bad (less likely) or the thermal timing relay is bad (more likely). Check the fan power wires and see if it is getting power; if there is no power, then the relay is bad.

If you have no timing relay and twist the two blue wires, the fan should run after a few seconds as the PC board controls the fan motor. Check the fan motor power wire coming out of the PC board to see if the board is sending incoming DC voltage to the motor. If the board is sending power, then the motor has issues. If the board is not sending voltage, the board has issues.

Hope this helps

John

[Boundless2281]

Okay thank you. That's what I was afraid of. I did that and nothing happened. Verified 13+ volts to the positive and negative lead. No noise or anything from the furnace. I am going to check all the connections, but it seems like I may be buying a dinosaur board to replace mine.

[trailrider]

Connect the blue wires together to run the furnace.

[Boundless2281]

Hello,
I know this is an old post, but I have the exact same furnace and need to bench test it. I am unsure what to do with the blue thermostat wires. I applied 12V to the positive and negative wires, but I am not sure what to do with the 2 thermostat wires. Do I need to connect them together? Or do they get supplied with 12V? They are both blue, does the polarity matter if I am supposed to be connecting them to 12V??

Thank you.

[dougrainer]

Zero(0) IS the smallest square bit. I was thinking of the No 1 that is used on some Canadian built door cabinet latches. Doug

[larry_cad]

dougrainer wrote:

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

Great information. Thanks Doug

[dougrainer]

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

[JBarca]

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.

[JBarca]

BFL13 wrote:
Good job on the furnace!

Never heard of those called "square drive" before. Weird! 🙂
Looking it up, found this explanation;

https://www.popularwoodworking.com/questions-and-answers/q-a-square-drive-vs-phillips-head-screws/

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

[BFL13]

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.

[dougrainer]

BFL13 wrote:
Good job on the furnace!

Never heard of those called "square drive" before. Weird! 🙂
Looking it up, found this explanation;

https://www.popularwoodworking.com/questions-and-answers/q-a-square-drive-vs-phillips-head-screws/

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.

[BFL13]

Good job on the furnace!

Never heard of those called "square drive" before. Weird! 🙂
Looking it up, found this explanation;

https://www.popularwoodworking.com/questions-and-answers/q-a-square-drive-vs-phillips-head-screws/

[JBarca]

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.




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.


The new hi limit replacement. PN 37022


The terminals are rotated some and smaller.


The bench test after installing the switch and the outcome.




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

[dougrainer]

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