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Friday, October 23, 2009

Faulty Furnace Feared

I have an older Hydro-Flame 19,000 BTU furnace. The thermostat is a Robertshaw CM60. My problem; I can’t get ignition. I have had the DSI board checked and the technician said that it was okay. What I need to know is how I can eliminate each component one at a time, such as the thermostat. How can I wire across it to call for heat to see if it is okay. How can I eliminate the DSI board, the sail switch, the fan limit and the solenoids on the gas valve? The instructions that came with the furnace are very vague. I have a full bottle of propane and it’s hooked up with a very short hose so I have fuel. Any help will be greatly appreciated. Charlie, (Wheeling, WV)

Charlie, there are actually two versions of your furnace, though the difference involves only the flame sense circuit and does not apply directly to your question. Here's how to test the components you mentioned. 

The thermostat can be tested by eliminating it from the circuit. Remove the two wires and connect them together. If the furnace fires up, the thermostat is faulty. The circuit board can only be tested by using an approved electronic board tester, or by swapping the board with a known, good one. Most RV service facilities will have the correct board tester. The sail switch is a "normally open" switch. With the voltage disconnected, attach a VOM to the two terminals on the sail switch. There should be continuity measured only when the paddle of the switch is manipulated. When released, the meter should show infinity. 

The limit switch is just the opposite; it’s "normally closed." Being a thermal switch, the limit switch will open at a pre-set temperature. Attach the VOM to both contacts and carefully heat the thermal disc portion of the switch with a match. The switch should open when the disc is heated. 

The board itself performs basically three different functions. It sends voltage to the gas valves, it creates the spark for ignition and monitors the micro-amp remote sense circuit, and it will perform a lockout if ignition did not take place. The voltage to the board first passes through a breaker, the relay, the sail switch and the limit switch. 

If the motor turns too slowly, the sail switch will not close and voltage will never reach the board. This is common when the squirrel cage becomes heavily laden with dust and lint. The extra weight slows the revolutions to the point the sail switch won't even close. If you have voltage present at the gas valve while the fan is turning, then the relay, the sail switch and the limit switch are all okay. 

On some early Hydro-Flame units, there is an additional on/off switch located between the board and the gas valve. If this switch is turned off, the gas valve will never open. If the fan does not operate at all, it could be the relay or the fan motor. If the fan runs, the board activated and no spark occurs, but there is voltage at the gas valve, chances are the board is faulty. Be sure to check the ground connection carefully and be sure you have a fully charged battery in place. Low voltage can cause a plethora of additional problems as well.

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