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Old 02-26-2008, 07:59 PM   #1
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HVAC Short Cycling


In the heat mode, unit is short cycling. If I take out the filter or not, it is the same. When I open the service door for the blower, and give it as much air as possible, short cycling stops!

I have made sure nothing has blocked the incoming and outgoing vents, even took off all the vent covers. Not sure if the unit has always done this.

Inside unit thermostadt is set at off=60, on=120, off=160 with a govener set at 160.

Unit shuts off at 160 in about 2-4 min. Room temp has not been met.
It seems there is not enough return air.

What can I do to resolve this??

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Old 02-26-2008, 08:26 PM   #2
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HVAC Short Cycling


with all the covers on whats going on with the return grills from the house any major sucking at the grill.if not something fell arcoss the duct going back down into the return off the unit in the basement.check the rounds on the squirrel cage make sure they are not rounded off with dirt or have a coating that wiould limit the air into the heat-exchanger.that 60/120/160 cycle is your normal settings but when your tripping on the HI-LIMIT as you have found only by opening the acess door.the unit was working OK and this limiting started tripping might want to raise it to 180F-200F...120F might be the operating range not the hi-limit and raising it to upper 190F the furnace will run.seen those pins move and that limiting knocking the burner off at the lower setting .one thing that 120/160 is too close.....go with 200F GAS OFF and 140F FAN ON it looks like the fan isn't moving the air but the lower limit setting is knocking your gas off when it should be on.

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Old 02-26-2008, 10:21 PM   #3
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HVAC Short Cycling


Return grills have suction. (No basement). Squirrel cage I did check before, not dusty. 160 hi-limit cannot be set any higher, a bar or governor stops it from going higher. Movede the 120 to 100 but still the same. Thinking of opening up the system and looking at the AC coils for dust clogging, but don't evpect to see any since the cage is not dusty.

Anything else???
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