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#1
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HVAC Fan Runs 24/7, Please Add Opinions. Thanks
Hi gang,
A friend called. The HVAC fan runs all the time in her 1 story home built in 1979. She has a digital stat above her return duct in the hall, the only return, 1700 square feet or so. The problen started when she used the heater for the first time last season a few weeks ago, Houston, Texas is hot most of the time. :-) I have no specifics: model, make, electric or gas, original unit... She has a digital stat. It is set to Auto.. I am not a pro HVAC man, I am a locksmith, I make lost car keys. That said: I think the possible cause IS NOT the thermostat (she wants to buy this first), but a relay AND/OR a thermal limit switch located by the heater output on a PCB or on the side of the attic unit some place, (max limit switch with a double pole [winter/summer push/pull switch depending on the year of production and ajustable temp settings?). I think the heater switch is effecting the AC too because the heater around here isn't used much and once it shorts it shorts and you know.., (now she says the AC fan stays on too), it shorted last year. (Would this be correct?). She can cut the AC or heat off or on at the stat, the fan stays on hi BTW, not low which I understand happens up north on purpose to allow the basement air to go upstairs and vise/versa to lower the bill, not that complicated here. We don't have basements being Houston. That said, Possible causes; (1) Bad limit switch? (2) Bad fuse or relay on PCB controlling limit switch? (3) Bad relay from thromastat controling fan? (Note: style of heater unknown) (4)Rat bit the stat wires shorting the green control wire to the feed wire? Naww...) Am I missing something, Thanks. Tomorrow is Sunday. Thought I could help her out while not doing nothing else. Ed |
#2
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HVAC Fan Runs 24/7, Please Add Opinions. Thanks
put the thermostat to Auto not On
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#3
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HVAC Fan Runs 24/7, Please Add Opinions. Thanks
"Glen Cooper" wrote in message ... Hi gang, A friend called. The HVAC fan runs all the time in her 1 story home built in 1979. She has a digital stat above her return duct in the hall, the only return, 1700 square feet or so. The problen started when she used the heater for the first time last season a few weeks ago, Houston, Texas is hot most of the time. :-) I have no specifics: model, make, electric or gas, original unit... She has a digital stat. It is set to Auto.. I am not a pro HVAC man, I am a locksmith, I make lost car keys. That said: I think the possible cause IS NOT the thermostat (she wants to buy this first), but a relay AND/OR a thermal limit switch located by the heater output on a PCB or on the side of the attic unit some place, (max limit switch with a double pole [winter/summer push/pull switch depending on the year of production and ajustable temp settings?). I think the heater switch is effecting the AC too because the heater around here isn't used much and once it shorts it shorts and you know.., (now she says the AC fan stays on too), it shorted last year. (Would this be correct?). She can cut the AC or heat off or on at the stat, the fan stays on hi BTW, not low which I understand happens up north on purpose to allow the basement air to go upstairs and vise/versa to lower the bill, not that complicated here. We don't have basements being Houston. That said, Possible causes; (1) Bad limit switch? (2) Bad fuse or relay on PCB controlling limit switch? (3) Bad relay from thromastat controling fan? (Note: style of heater unknown) (4)Rat bit the stat wires shorting the green control wire to the feed wire? Naww...) Am I missing something, Thanks. Tomorrow is Sunday. Thought I could help her out while not doing nothing else. Ed We could guess for hours, the only real way of correcting the problem is to go there check the settings. I remember when I moved into my last house with a digital stat, took me a couple of weeks with the manual to get it working the way I wanted. Any of the above are POSSIBLE, I would WAG doubtful. Go on line get the manual for the stat, then set all of the variables for her. or call someone on Monday. Personally I would never attempt working with locks. |
#4
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HVAC Fan Runs 24/7, Please Add Opinions. Thanks
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#5
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HVAC Fan Runs 24/7, Please Add Opinions. Thanks
Go to the furnace, shut the power off and disconnect the green wire
from the thermostat (the green wire connects to the G terminal and runs the blower). Then turn the power back on. If the blower still runs, the problem is not the thermostat or thermostat cable. If you are not familiar with electrical testing, you are done there. If you can use a meter, check the fan relay. Use a clamp on amp meter to determine which wire is powering the blower motor and go from there. If the blower motor has multiple speed taps, usually the black wire is high speed, blue is medium speed and red is low speed. If the color code is differemt, it will usually be on the motor or the furnace schematic. If you can't test it, you are just guessing. Guessing with a gas furnace is not a good idea. In that case, the best bet is to hire a company to test it, but all techs are not created equal. Ask if their techs are NATE certified. For more information on this, go to www.natex.org Good luck Stretch |
#6
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HVAC Fan Runs 24/7, Please Add Opinions. Thanks
"Stretch" wrote Go to the furnace, shut the power off and disconnect the green wire from the thermostat (the green wire connects to the G terminal and runs the blower). Then turn the power back on. If the blower still runs, the problem is not the thermostat or thermostat cable. Not trying to question your knowledge there Stretch, but: What if (and I have seen it happen many times) the wires got crossed somewhere in the wall? Someone was hanging a picture and drove a nail through the wire. Wire is run in the attic someplace and they set something on or against it and pinched them together. Someone had the cover/door off the furnace and the pinched the wire putting the cover back on. What you describe could maybe eliminate the thermostat but not necessarily the thermostat wire.... |
#7
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HVAC Fan Runs 24/7, Please Add Opinions. Thanks
"Stretch" wrote Dr. What if? It is possible but not likely that they used a color other than Green for the blower control wire. Even the old Lennox systems used Green, and they had the weirdest color code out there. Ain't Lennox wiring great? I like the old V and VR terminals. or York's 66 and 60 ones. The nail through the wires is possiblity, but then disconnecting the green wire AT THE FURNACE would stop the blower, which would show the thermostat OR thermostat cable is at fault, as I said in my post. Same thing for pinched together. I said to disconnect it at the FURNACE so the thermostat cable would be included in the test. O.K. I guess the way I read it, it sounded like you said to disconnect it from the thermostat: "Stretch" wrote Go to the furnace, shut the power off and disconnect the green wire from the thermostat |
#8
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HVAC Fan Runs 24/7, Please Add Opinions. Thanks
Hello again gang. Great group I must say.
I disconnected the green fan wire at borh ends with the power off, checked it for a short to the other wires. The fan cut off with the power on and the safety switch jumped. I replaced the thremostat, and bingo! Thanks again Ed |
#9
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HVAC Fan Runs 24/7, Please Add Opinions. Thanks
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#10
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HVAC Fan Runs 24/7, Please Add Opinions. Thanks
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