Why Is Furnace Unit Blowing Cold Air?


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Old 01-07-18, 09:22 AM
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Why Is Furnace Unit Blowing Cold Air?

Hello, I recently turned on my upstairs furnace unit in my attic for the first time this winter season (my downstairs furnace unit is working fine) and for some reason it is blowing cold air. This same attic furnace unit was working last winter season. Here is some history about this unit:

- It is a Carrier WeatherMaker 8000 (Product / Model: 58ZAV050-13112 / 58ZAV050-12)
- Manufactured September 1995
- During the summer, the blower motor was replaced under my home warranty program as it stopped working.
- Upon entering my attic the other day to begin troubleshooting, I could see a bright orange glow coming from inside the furnace unit when I turned up the thermostat. This was a good sign for me since I meant that I didn't have to light a pilot light. Upon further inspection, it looked like the gas line was turned off by the technician in the summer when he replaced the blower motor.
- I then turned down the thermostat so that the furnace, blower motor, etc stopped. I then turned on the gas. Once I turned up the thermostat, I could hear the furnace kick on and I could now see the flame inside the furnace unit (good sign)
- For some reason with furnace running (and a flame inside the furnace), there is cold air coming out of my 3 vents upstairs even after running for 5 or 10 minutes.
- Not sure if it is relavant or not, but I thought at some point 'after' turning on, or up, or off (I can't remember), I think I felt heat coming out of the vent for about 5 seconds, but that was the only time.

Does anyone have any suggestions on what to look for? Please let me know if you need any additional information or pictures, etc.

Thanks in advance!
 
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Old 01-07-18, 09:32 AM
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Do you have good air flow on that furnace ?
 
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Old 01-07-18, 09:38 AM
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I 'guess' I have good air flow... the reason I say guess is that I have gone to at least 3 other ducts fed off this furnace I can feel all of them blowing 'cool' air.
 
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Old 01-07-18, 10:08 AM
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Make sure the return air duct is not damaged or disconnected.
 
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Old 01-07-18, 11:31 AM
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It sounds like the burner is only staying lit for a short time.
Once the burner is lit..... and you have airflow.... you'll get heat.

That furnace shows a control board with a fault LED. You should be able to the fault LED flashing thru a sight glass in the blower door.
 
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Old 01-07-18, 03:07 PM
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skaggsje - the return air duct seems to be ok.... in fact it looks like it is only about 5' feet length.

Pjmax - it sounds like you might be familiar with this sort of thing. Originally, when the gas line was off, etc I could see the LED flashing through the small plastic window. Unfortunately I didn't bother troubleshooting the LED behavior because I knew the gas was off. At any rate, since the burner lights, and is on the 'entire' time when I turn the thermostat up to turn on the heat, and the LED is on solid, I'm not sure what to look at next.
 
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Old 01-07-18, 03:51 PM
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If the burner is lit and you have air coming out of the registers...... you must get heat. The heat can't go anywhere else. If air was not flowing thru the furnace..... the burner would shut off in a minute or two.
 
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Old 01-07-18, 06:51 PM
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PJmax, I agree 100%... it makes no sense at all. The burner is staying lit, the blower motor is running and it is blowing cold air. I've tried resetting the relay switch as well as removing the batteries from my smart thermostat - still no difference.
 
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Old 01-07-18, 07:01 PM
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Sorry..... I've got nothing else to add. Your problem appears to defy logic.
 
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Old 01-07-18, 07:12 PM
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Looks like I unfortunately need to call the professionals :-(
 
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Old 01-07-18, 07:44 PM
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Have you gone back up after the furnace has run for a while to verify that you still have the burners lit?
 
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Old 01-08-18, 01:36 PM
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Well... after calling out the professionals this morning, I have an update on this issue. The reason it was blowing cold air when the furnace was engaged was that for some strange reason when I would turn up the thermostat for the heat to kick in, the AC would start up at the same time!

During my troubleshooting, I didn't notice that the outside condensor unit would start when I turned up the thermostat to turn on the heat. The HVAC technician ended up removing one of the jumpers from my smart thermostat which fixed this problem. I told them that nobody has touched the jumper and it was working just fine previously last season. Even more interesting is that my HVAC unit for the downstairs has the same thermostat (everything is working fine) and it still has the jumper in position.

I wonder if there is some type of defect or bug with the model of smart thermostat because I recall around Thanksgiving having a similar issue in another home I own with the same thermostat? It was a little different - -the heat wouldn't come on either (the tech thought it was wiring, etc and I told him nobody has touched the wiring or jumpers on it either).

Thank you everyone for all of your help and suggestions!
 
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Old 01-08-18, 10:22 PM
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Hmmmm....... the mythical jumper.

Can you tells us what terminals the jumper is/was on ?
What stats do you have ?
 
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Old 01-09-18, 07:49 AM
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It was a jumper between Y1 and W1... however, looking at my other thermostat that is working, I see that the jumper between Y1 and W1 is not actually 'jumped' together - it is only connected to one pin. What I don't know is if the jumper for the attic unit was jumped across both pins, or just hanging on one of them. Either way, the HVAC technician just removed it and put it in one of my drawers near by.
 
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Old 01-09-18, 09:34 AM
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Y1 is A/C and W1 is heat so a jumper across there with a furnace would not be a good thing.
Different with a heat pump system.

With that jumper..... the heat would also come on when A/C was selected.
 
 

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