Gas furnace issues


  #1  
Old 02-01-10, 11:09 PM
C
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Gas furnace issues

This problem driving me nuts.

Furnace - Snyder General model # GUG047A008N built 9/89
Fan limit control settings 90 / 140 saftey limit 220
manufacture specs say not to exceed these settings

Thermostat - Honeywell RTH7600D
Prior thermostat - Comfortmaker Q6747

Fan limit control - Honeywell L4064a-2782 w/ 11 1/2" probe

Problem - Fan / burner sequencing issues - erratic / sometimes varying behavior.

I recorded following behavior of blower & burner to illustrate problem.
*Thermostat set at 72 throughout test period

10:02
Thermostat temp reading 71 / burner off / blower off
10:05
Thermostat temp reading 72 / burner off / blower on
*why did blower turn on..no call for heat?
10:08
Thermostat temp reading 72 / burner off / blower on
*blowing cool air.
10:11
Thermostat temp reading 72 / burner off / blower on
*still blowing cool air
10:13
Thermostat temp reading 72 / burner off / blower on
*still blowing cool air
10:17
Thermostat temp reading 72 / burner on / blower on
*why does burner kick on when already reached desired temp
10:19
Thermostat temp reading 72 / burner off / blower on
*2 minutes later relay clicks and burner now turned off
10:22
Thermostat temp reading 72 / burner on / blower on
*why does burner kick on again when already reached desired temp?
10:25
Thermostat temp reading 72 / burner off / blower on
10:30
Thermostat temp reading 72 / burner on / blower on
*why does burner kick on again when already reached desired temp?
10:33
Thermostat temp reading 72 / burner off / blower on
*starts blowing cool air after a few minutes
10:39
Thermostat temp reading 72 / burner on / blower on
10:41
Thermostat temp reading 72 / burner off / blower on
*starts blowing cool air after a few minutes
10:48
Thermostat temp reading 72 / burner on / blower on
10:50
Thermostat temp reading 72 / burner off / blower on

Pretty much goes on and on like this with burner kicking on for 2-3 minutes then turning off while the blower constantly runs.

So I am thinking bad Fan limit control. So I do a test shutting off gas & pilot. Let cool for a good 10 or 15 minutes. Fan still blowing even though I visually watched the dial rotate on the fan Limit control. So then I push the thermostat setting down to 71...still blowing. Push thermostat setting down to 69....blower goes off!!! Hmmm??? Room temp now reading 71. Push temp setting back up to 71...wait a few minutes blower stays off. Push it to 72...wait a few minutes blower stays off. Push it to 73...wait a few minutes blower stays off. Push it to 74 and blower kicks on...30 seconds later burners fire.

I record the behavior for an hour with thermostat set at 74 and get same behavior as above.

By the way it was exhibiting this same behavior with old thermostat as it does with new thermostat. Just to make sure new thermostat wasn't bad...I took it back and got an identical replacement and it behaves same way. Safe to say we can rule out thermostat!!

Note: If I turn thermostat 10 -15 degrees above current room temp the burners will remain fired while trying to get to that temp so I know burners can run for extended period of time and are not being tripped off because hitting safety limits. I've watched the burners fire continuously for at least 15-20 minutes without shutting off to make sure not some safety limit issues associated with burners cycling on for 2-3 minutes before turning off.

The cool / cold air coming out of vents drives me crazy (and the wife even more so)....sometimes it will blow cool air for a good 10-15 minutes before burners will fire and will often fire even when room temp has reached the temp set on thermostat.

Infrequently the unit will behave as expected....room temp reaches thermostat setting...burner goes off....blower blows for another minute or two clearing warm air out of ducts and then the fan limit control turns and shuts off the blower. Behavior is inconsistent / erratic. A few times I've seen it sit 3 degrees below set temp cycling the burner on / off briefly while the blower runs continuously.

I should mention at anytime if I manually set the thermostat temp 2-3 degrees below the room temp everthing turns off as you would expect it to so components seem to be working...conversely if you push the thermostat several degrees above room temp everything fires up and runs as you would expect it to...it's just during normal operation it gets whacked out.

Like I said at the beginning the behavior is driving me nuts. The system is in a fight with itself. Heating things up then cooling things down by not shutting off blower and blowing cool air.

PS. As noted behaving like this prior to changing out thermostat. Also performed a thorough cleaning of unit for good measure.

PSS. Honeywell Tech Support told me how to wire new thermostat to match old thermostat.

Bad blower relay? Bad fan limit control? Thermostat old / new not wired properly? Voltage issue? All of the above or some combination of above or something else I am overlooking.

All thoughts and suggestions welcome!!!!


By the way how this should work is....call for heat...blower kicks on for 30 seconds (a safety feature to clear any gas)...burners ignite....stay lit until room temp hits temp set on thermostat....burners shut off....set limit controller turns off blower when chamber cools to below 90 degrees to clear warm air from duct work (efficiency thing).
 

Last edited by cpanddp; 02-01-10 at 11:26 PM.
  #2  
Old 02-02-10, 07:34 AM
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problem

1st thing,did you set the heat anticipator on the thermostat? if not it could cause short cycling.I feel that the fan limit is bad,as they get older the spring steel in them distorts,and causes problems.you also have to be carefull when installing the new one that you do not rotate the dial if you are changing the trip setting tabs
 
  #3  
Old 02-03-10, 05:13 PM
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I'm thinking you might have a failing flame sensor. The flame senseor senses flame to ensure unit is on. If no flame is sensed then unit shuts off. The sensor can fail to sense the flame even if it is on. Though what is happening to you doesn't usually happen but its my guess. either that or you need a new fan/limit control. Also some fan/limit controls have 24 volts going to the center to warm it up. if you lose 24 volts to this it is common for the unit to turn off and on repeatedly.
 
 

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