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My heating system only blows out cold air, not hot. I've left it on for a good 15 minutes to see if it will warm up, and I get nothing but cold air. I've even put it on Auto and waited some more, but still all I get is cold air.

What could be the problem?

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4  
Can you tell us what kind of heating system you have. It's impossible to even guess without knowing that. –  Niall C. Sep 24 '11 at 18:56
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-1 Without more details, this question can't be answered and should likely be closed. –  BMitch Dec 14 '13 at 15:53

2 Answers 2

Several possible issues:

  1. Bad thermostat
  2. Pilot light out (if a gas furnace with a pilot light)
  3. Heating element out (if electrical furnace)
  4. It's cold outside (if heat pump - coupled w/ #3)
  5. Electronic ignition shot (gas furnace w/ no pilot light)

Without more information, that's the best guess I can make.

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1  
6. You have mistaken the heating system for the air conditioner. –  Alex Feinman Sep 30 '11 at 19:12
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Technically that would be the other way around, but - LOL –  The Evil Greebo Sep 30 '11 at 19:26
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A bad high limit switch would also cause this. however, if the limit switch was bad the blower would never shut off. –  Tester101 Dec 14 '11 at 15:46

Turn off the furnace, remove the thermostat, touch the R and W wires together (as well as R and G), and then turn it back on and see what you get. If this works, then your thermostat isn't calling for heat correctly, but the fan is operating when it attempts to.

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