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Old 12-23-2009, 12:47 PM   #1
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Carrier Cobra Gas Furnace Problem


We moved into a house this summer. The house was built in 1995. Not until this winter I notice the furnace that heats the house is not functioning properly. I was told the furnace is the original (installed when the house was built).

The furnace is a Cobra gas furnace by Carrier and is located in the attic. There are 2 thermostats, one upstairs & one downstairs.

The problem is the Flame-On duration varies between 3 and 7 minutes and is not long enough to warm up the house. For your information, I have recorded the sequence of the operation.

1. Surface Burner (Ignitor) On for 30 sec
2. Gas Valve Open & Flame ON for 50 sec
3. Blower ON for 3-7 minutes
4. Gas Valve Close & Flame OFF for 4 minutes (Flame shuts OFF even the temperature setting on the thermostat is not reached)
5. Blower OFF
6. Restart the cycle

Could the problem cause by the Flame Sensor?

The 2nd issue is the Blower-On-Time (4 minutes) after the Flame shuts OFF is too long. It blows cold air in the last 2 minutes period which makes house even colder. I adjusted "Blower Delay Time", but it didn't work.

Please help

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Old 12-23-2009, 01:13 PM   #2
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Carrier Cobra Gas Furnace Problem


could be a number of things. I'd start with verifying the tstat operation.

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Old 12-23-2009, 01:32 PM   #3
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Hi 3 legged dog,

Thank you for your assistance. Can you please be more specific about how to verify the state operation.
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Old 12-23-2009, 01:44 PM   #4
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Carrier Cobra Gas Furnace Problem


You said there are two thermostats. How is this controling 1 heater?
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Old 12-23-2009, 01:47 PM   #5
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Carrier Cobra Gas Furnace Problem


Quote:
Originally Posted by carriercobra View Post
Hi 3 legged dog,

Thank you for your assistance. Can you please be more specific about how to verify the state operation.

Sorry, I should have said thermostat.
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Old 12-23-2009, 02:13 PM   #6
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Carrier Cobra Gas Furnace Problem


Have you changed or cleaned your air filter recently.
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Old 12-23-2009, 02:33 PM   #7
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Carrier Cobra Gas Furnace Problem


Hi beenthere,

The filter is new and is 3M Filtrete Ultra Allergen Air Filter. Could it be due to too much constrain of air flow?

Thanks
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Old 12-23-2009, 04:54 PM   #8
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Carrier Cobra Gas Furnace Problem


Yes. Those are very restrictive air filters. And unless your duct was designed to use it. Don't use it.

Good chance either your furnaces high limit. Or your zone panels DATS are sensing too much heat and shutting down the burner to protect the furnace.
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Old 12-23-2009, 05:35 PM   #9
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Carrier Cobra Gas Furnace Problem


I figured he have one of those overpriced allergen type filters.

FYI for all DIYs: If you want allergy protection buy an electronic air cleaner or a media filter.

Those expensive filters that claim to to be good for allergies are just going to kill your furnace by causing it to over heat.
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Old 12-24-2009, 08:54 AM   #10
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Carrier Cobra Gas Furnace Problem


Note You need to put the delay back on 4 min. becasue you need to get all the heat off the heat exchanger much as you can after burner shut off . It gaves it extra life time on exchanger.
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Old 12-24-2009, 08:56 AM   #11
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Carrier Cobra Gas Furnace Problem


Plus if you have it turn off way to early. It will make the fan limit hot and short cycle the fan motor. That will save life on the run cap.
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Old 12-29-2009, 06:30 PM   #12
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Carrier Cobra Gas Furnace Problem


Thank you all for the tips. It's the overpriced filter that causing poor performance. I replaced it with low MERV filter and the furnace is working properly.
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Old 12-30-2009, 03:38 AM   #13
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Quote:
Originally Posted by carriercobra View Post
Thank you all for the tips. It's the overpriced filter that causing poor performance. I replaced it with low MERV filter and the furnace is working properly.
What number was the MERV?
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Old 12-30-2009, 11:47 AM   #14
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It was MERV 12. The new one is MERV 7.

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