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 โhigh sideโ 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 โhigh sideโ 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
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 boardโno 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
