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  #1   IP: 71.236.46.55
Old February 15th, 2009, 10:33 AM
adcase adcase is offline
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Default Heat/AC works - blower never turns off

Starting yesterday my blower fan wouldn't shut off. even if I move the tsat to the off position it blows constantly. The heat turns on and off when it should so the house stays the set temp, but the blower is always on. From researching what I could it seems like my issue could be
wiring related
bad thermostat
fan relay
circuit board in furnace

I read on this board that if you disconnect the "g" wire on the thermostat and the blower turns off that the thermostat is bad? Is this the case? I had gotten the blower to turn off by unhooking the "g" wire on the tstat, but I can't seem to get it to shut off again, so it's looking like that was a fluke.

I have had the side of the air handler off and there aren't any circuit boards, but there are some relays, where do I start in testing relays? Help please!

Thanks,

Aaron
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  #2   IP: 98.30.163.230
Old February 15th, 2009, 11:24 AM
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Mr T Mr T is online now
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Most likely its your limit/fan switch thats bad.. They are usually together. If you can find it try tapping on it (carefully as there may be 120V on the terminals). Ive had a furnace where it would stick the first few heating cycles of the season.. Once I tapped on it, it ran fine. If you cant find it, post a pic of your furnace with the front cover off.
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  #3   IP: 71.236.46.55
Old February 15th, 2009, 11:59 AM
adcase adcase is offline
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OK, I'll put a pic on here. I found what I thought was the right thing, and lightly tapped it one time. It turned off instantly. The only down side is it hasn't come back on since either... I tapped lightly on the black part of the item in the right hand side of the pic.

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  #4   IP: 71.236.46.55
Old February 15th, 2009, 12:09 PM
adcase adcase is offline
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it has come back on and it looks like it's working normally again. THANKS A TON!

btw is fan/limit switch the "official name" of that part? I found a part number on it of 990185a02 (it's an Armstrong Air Handler)

Thanks again,

Aaron
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  #5   IP: 98.30.163.230
Old February 15th, 2009, 03:29 PM
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For the most part it is. Some manufacturers may have other names. If it only happens once and not again I am not sure if i'd run off and replace it right away. If it keeps acting up I would replace it for sure. If you decide to replace it make sure you have the exact one as specified. The limit switch part is a critical safety device.
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  #6   IP: 71.236.49.130
Old February 16th, 2009, 08:37 AM
adcase adcase is offline
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OK, I think that my limit/fan switch is OK now, if I put my thermostat in the off position the fan does turn off, but my fan still seems to run a LOT more than it normally does. could it be that my tstat is having issues? Under normal circumstances when the blower ran the air coming out was always "warm" and now it seems to just blow room temp air a LOT, even when the house is at the correct temp.

Thanks again for all your help. I am a HVAC n00b, although I am proficient with electronics and mechanical stuff, I haven't had opportunity (until now) to delve into the world of HVAC.

Aaron
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  #7   IP: 76.22.81.239
Old February 16th, 2009, 04:38 PM
suemarkp suemarkp is offline
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It could be the low limit is still stuck. This has to open to turn the fan off. It is there to extract every last ounce of heat from the heat exchanger once the gas turns off. When it is cool enough, that switch will open.

Not all furnaces care about the green FAN wire from the thermostat. A heating only system may not even have a separate fan control wire. I believe most furnaces operate the fan as they see fit when there is a call for heat. The fan wire can override this forcing the fan on continually.

You can see if its a thermostat problem by seeing if there is 24VAC from the green wire to the common on the transformer. All the thermostat is, is a giant switch. Power from the transformer comes in on the red wire. A call for heat closes a switch between red and white, and a contactor in the furnace is pulled in via the white wire. Similar for cooling and the yellow wire. The "FAN ON" switch connects the red to green. A thermostat may connect both green and white to the red on a call for heat.

So measure between the green wire to the furnace and the LV transformer side that the red is not connected to. This will tell you whether the thermostat is commanding the fan on or not.
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  #8   IP: 71.236.49.130
Old February 17th, 2009, 06:55 AM
adcase adcase is offline
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OK I'll give that a go when I get home from work, so if I DO have voltage than my tstat is bad and if I DON'T have the right voltage than my low limit switch is probably stuck?

Thanks for all the help guys.
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  #9   IP: 71.236.46.55
Old February 17th, 2009, 06:42 PM
adcase adcase is offline
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OK whenever I put my meter on the green wire that runs to the tstat, the box on the side of the air mover (some kind of accessory, I'll get the name posted her in a few -- it's a silver box that says "Skuttle Quality Indoor Air Products" on it, and it has a green light on top that lights up when the fan's running) started clicking, it has a green light on it, and it went on and off with the clicking, I am not sure what exactly I was supposed to be testing, but I did find a few things that showed up at 27v ish.

here is a pic of what I am seeing, it is an armstrong air air handler btw

http://flickr.com/photos/35414940@N0...85221/sizes/o/


I think I am back to a point where the blower fan is pretty much unresponsive to anything that the thermostat says. if I tap on the switch, it turns off, but otherwise it runs constantly. I pick up a thermostat today in hopes that it would be a cheaper fix, but at least to me it looks like things are pointing more towards the fan/limiter switch?

Last edited by adcase : February 17th, 2009 at 07:06 PM.
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