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Old 06-08-2011, 12:52 PM   #1
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atwood 7900-11 operation

Hi, just installed a new atwood 7900-11 furnace, could any one tell me how it should run, when I turn up the thermostate the blower comes on and the furnace hasnt even lit yet, is this normal? It fails to light most of the time. I was wondering if i have the thermostate wired right? there are 3 wires coming out of the furnace red, black and white, i took the white wire and a positive 12v to the thermostate. I also installed a new regulator, gas refrig, stove and water heater all work fine, but the furnace is killing me, thanks if any one has any sugestions on whats wrong.

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Old 06-08-2011, 12:57 PM   #2
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Yes. The blower should run for a period of time to exhaust any propane in the combustion chamber to prevent an explosion upon ignition.

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Old 07-08-2011, 07:20 AM   #3
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Hopefully your furnace is working by now.

If not, here is some information for you concerning your 7900-II: (this information came from an original Hydroflame 7900 manual - Atwood bought Hydroflame some time ago.)

What the manual doesn't tell you:

The fan runs first to "purge" the combustion chamber. When the gas valve opens, you should hear a single "snap" sound. Next, you should hear the rapid "ticking" of the ignitor. These sounds occur in immediate sequence: "snap" then "ticking" (and they are different). The rapid "ticking" will last for about six seconds then stop. The fan will contine to run to purge the unlit propane from the chamber, then the board will try to cause ignition again. After three unsuccessful tries to ignite, all the furnace functions stop completely. It's a safety item referred to as board "lock-out". You must turn the t-stat "OFF" completely, then "ON" again to reboot the board and restart the sequence.

Have you heard the two sounds described? (You may have to get close to the furnace to hear them.) If you have NOT, the board is not getting the necessary 12 volts to function - check the t-stat and 12 vdc power supply. If you HAVE, you now have two other possibilities - gas flow and ignitor function. Check both.

Hope this helps.
Dale Lee Sumner
RVIA/RVDA Master Certified RV Service Technician
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Old 07-09-2011, 04:33 AM   #4
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thanks for the the help, good info, It is working great now.
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