my furnace occassionally blows cold air


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Old 11-13-08, 06:08 PM
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my furnace occassionally blows cold air

I have a natural gas furnace and it sometimes blows cold air. If I shut if off and turn it on a few times on the thermostat it will start to blow warm air. I have had different people come to look at it and they all said the furnace is fine, however it continues to do this. It has done this for at least 4 winters now that I can remember. Can anyone help me with what may be wrong or is it time to get a new furnace??
 
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Old 11-14-08, 04:16 PM
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Nothing worse than sporadic problems. Especially REAL sporadic ones. People in the business can sometimes make intelligent guesses, but it is still a guess. You can't diagnose electrical failures when at the time, it works.

How often does this occur? If frequent enough, it might pay for you to open the big panel to the furnace to take a look and see if you can see what it doesn't do, that normally it should do? Or, see if it lights, but goes out in just a few seconds.

Can you describe what kind of furnace you have and how it works? Like, do you know if it ignites by a direct spark to the burner? Or if sparking first lights a pilot? Or if there is this glow coil that gets real bright and lights the burner?

To give you a general idea- The furnace inducer fan exhaust gas blower should come on first when you turn on the stat.Then there should be an ignition process, that follows after the inducer motor running for maybe 30 seconds first. If you have a fan trimer, that house air circulation blower may come on in seconds after the furnace starts up, even if no fire comes on. If your blower though comes on by sensing heat, it might take up to about 1 1/2 minutes to come on AFTER the burner fire comes on. The ignition process would use one of those methods I mentioned in the paragraph just above this one. Then, the burner(s) should come on, and stay on, until the call for heat has ended, for which then the burner and inducer fan go out at the same time.

Tell us what part of this sequence fails. First observe things when it DOES work, so you know what to report when some phase no longer allows it to work.
 
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Old 11-22-08, 03:10 PM
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furnace blowing cold air too

I currently have the same problem with my gas furnace heating system; on occassion it blows warm air, but at least in the last few days, when it comes on, it is cold. I took the liberty of opening the panel on the furnace and turning it off and back on again to see if that would help -- no luck. There is a glow coil that gets real bright and lights the burner, but then the burner turns off very quickly thus leaving the cold air (I'm assuming). The thermostat is digital and seems to not be communicating with the furnace properly; even when I put the setting on 70 or higher it doesn't turn on the furnace -- only when I press it lower than the current room temp (it will click) and then I'll press it higher than the current room temp (it will click) and ONLY then does it turn on and usually to blow cold air for a few minutes and then it turns off. Any suggestions on how to fix this?
Thanks!
 
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Old 11-22-08, 04:21 PM
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Originally Posted by wolf6
There is a glow coil that gets real bright and lights the burner, but then the burner turns off very quickly thus leaving the cold air (I'm assuming).
This is the key part of your post. I'd say a good bet your flame sensor needs cleaning. A DIY job, if you have one of those.
 
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Old 11-22-08, 05:46 PM
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Thanks for the advice -- I think you're correct -- the coil (I'm assuming this is the flame censor) looked like it had white deposits all over it. The problem was that when I attempted cleaning it, it broke into pieces, so I'll need another. You think a hardware store should carry this (Norton model #201N)? Tomorrow I might be warm again thank you!
 
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Old 11-23-08, 03:34 AM
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Hb81

4 years????? at least it is consistanly doing the same thing.has that been the same stat on the wall since the problem started?if yes on that it could be the stat...quick check for the stat....(unit switch off) pull the stat off the subbase and with a thin piece of wire connect R to W with the screws that hold the exsisting stat wires(leaving them on just add the jump).this will give you a solid call for the furnace HEAT to see if you get that on/off situation.if it stays on a heats consistantly for say 10 minutes the stat might be short cycling it or keeping it off...you need to sit in front of the furnace and note the cycles.power switch on,should be PILOT......MAIN burner.....FAN and as long as that jump is on the subbase it should heat main burner and fan.anything shutting off there is then within the unit ..not the stat!NOTE that jump is only 24Vs and won't shock you at the stat..small paper clip will do the trick you can screw down on it if your short on wire.what type of STAT do you have digital,round or retangular with mercury bulbs.....
 
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Old 11-23-08, 08:33 AM
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A note of caution: the coil (the part that ignites) is not what you want to clean -- it will break; you want to clean the flame censor (gently, with sand paper) which is located on the opposite side near the bottom of the burner. I replaced the ignitor, cleaned the flame censor with sand paper and there is now warm air (and the burners stay on). So grateful for heat and Brett who sold me an ignitor from his home on a Sunday morning and gave me free advice on cleaning the flame censor and threw in a piece of sandpaper.
 
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Old 11-25-08, 10:23 AM
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Thank you everyone for your responses. I've had another heating company come and look at our furnace and they said that the gas valve needs to be replaced. so now i am wondering if they are just trying to make a buck, because they also said i dont have any room in the furnace for an air filter so all this time dust and such has been collecting on the 2nd air exchange so the temperature is super high inside the furnace and one of the safety features is for the furnace not to kick back on so it doesnt get any hotter. therefore, blowing the cold air. but this company also said there wouldnt be any need to clean it because the furnace is going to need to be replaced within the next 2 years, but instead build a place to put a filter. my husbands friend works with furnaces and is going to come look at it this weekend and hopefully figure it out so we're not waking up to a 53 degree house!! oh and the thermostat is a digital one. Beer 4U2
 
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Old 11-25-08, 03:44 PM
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don't have anything changed til, you or have the friend do a visual on the "A coil" of the top of the furnace.if you were missing a filter the entire summer that coil could be matted with dust causing the heat from the furnace t build up and trip the HI limit as it is designed to do.can you access the coil from the back and brush it down "go easy" on it if you do watch those aluminum fins.i hope it is just a matted evaporator coil to why the heat is short cycling...and do get a filter in there or put one in the return grill in the hallway
 
 

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