Atwood 8500 series furnace bench test on high limit switch
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
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
