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HVAC blows cool air in the morning but warm in the afternoon


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9 replies to this topic

#1 Koffeehound62

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Posted 16 December 2013 - 12:03 PM

Here is the description of what my in-laws heater does in the mornings recently but works fine in the afternoon. The morning temps recently have been in the 20's - 30's.

Thermostat calls for heat. 2. Draft inducer motor starts. 3. Pressure switch attached by a small plastic or rubber tube senses the negative pressure produced by the draft inducer and closes. 4. Draft inducer runs for 30 seconds to a minute before you hear a gas hissing sound. The ignitor did not glow, the flame sensor (a small metal probe about 1/8" in diameter, with a white porcelain base) does not sense the flame, so after 8 to 10 seconds the hissing sounds stops with no ignition of gas to heat your home. Your furnace shuts down and goes into a lock out condition until you turn your power switch back off and on again. Then the sequence starts all over again with no ignition of the gas.

The first day I did find the draft inducer was binding up and not spinning. After freeing this up with wd40 it has been working since. I have ordered a new one and will replace it when it arrives. Not sure what could be causing the heat not to work in the mornings. I suspect the colder temps lately have something to do with it but I don't know what component could be " cold sensitive" . Any help would be great!

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#2 Bobice

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Posted 16 December 2013 - 12:31 PM

A drawback to gas heat is that in colder weather is that the gas flows much slower sometimes not at all
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#3 Koffeehound62

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Posted 16 December 2013 - 12:36 PM

Is there any troubleshooting I can do to rule out the gas? The system is a central unit on the roof, I think a Carrier or Bryant. Not sure because I can't seem to find a manufacturers label.

#4 Dan Webster

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Posted 16 December 2013 - 12:42 PM

I wonder is it could be the hose from the pressure switch to Draft inducer?


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#5 Koffeehound62

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Posted 16 December 2013 - 01:12 PM

I just went over and checked it since it is cold out and the draft inducer motor wasn't spinning. The new one should fix the problem but I'll let you know what happens after it arrives and I install it.

#6 BryanS

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Posted 16 December 2013 - 10:11 PM

I'm confused. In your post you said the ignitor did NOT glow but you heard a hissing for 8-10 seconds. Shouldn't the ignitor be glowing if gas is flowing. I didn't think it would even get to that point if the pressure switch isn't making, which would mean the inducer fan would need to be running. You said it was binding up. It sounded like it was cycling properly once you got the fan running except the ignitor didn't glow. Shouldn't we be figuring out why the ignitor didn't glow even though gas was being put out? I feel like in a typical sequence the ignitor kicks on then gas flows to ignite. Maybe I'm wrong or misunderstanding. I haven't ran many gas furnaces.

#7 Daveb1972

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Posted 17 December 2013 - 07:38 AM

Guys, you are missing the gold here, igniter should be glowing. Disconnect it and measure its resistance. It should be 60-80 ohms I'd SiC and 10-14 if SiN ( you can tell the difference by the look of the element, SiC or Silicon Carbide is black with shiny flects in it. It is VERY BRITTLE and often is flat with a large flat white ceramic base. The other type is SiN or Silicon Nitride, and is way more durable, and has a matte finish and often a solid bar for the heated part, and also a white ceramic, thought this ceramic will most likely be glazed, the wires are likely to be silicone or Teflon; whereas SiC are usually fibreglass braid coated wires).
The sequence you describe is exactly what should happen when the igniter has failed.
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#8 Dan Webster

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Posted 17 December 2013 - 05:17 PM

You could unplug the ignitor and fire up the system with ohmmeter connected to the harness to see if power is getting tothe ignitor. If no voltage hunt down an open lmit and look for blocked stack or something else that would cause a limit to trip.


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#9 Koffeehound62

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Posted 19 December 2013 - 12:13 PM

The new draft inducer fan fixed the problem.

#10 Daveb1972

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Posted 19 December 2013 - 12:39 PM

If the inducer fixed it then the pressure switch wasn't closing, and that keeps the igniter from lighting. This also means the gas valve should NOT have been opening. This could be a miss hearing problem, or a problem with the control. If the igniter isn't beeing activated the gas shouldn't be flowing even for ignition trial.
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