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Trane gas pack problem - It won't blow warm air.

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14 yr old Trane gas...
14 yr old Trane gas pack: Problem - early at the start of each heating season for last 4-5 years it craps out and won't blow warm air. Each time a technician been called they diagnose a faulty pressure switch. Remedy has often been to simply power-off then on the furnace a couple times and it fires right up and is good for the rest of the heating season...but not this year.
Submitted: 7 years ago.Category: HVAC
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Answered in 19 minutes by:
11/3/2010
HVAC Technician: M Oglesby, HVAC Technician replied 7 years ago
M Oglesby
M Oglesby, HVAC Technician
Category: HVAC
Satisfied Customers: 2,038
Experience: Universally Certified HVACR Technician
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You will need to go to the furnace. Remove the panel door. Locate the inducer fan motor. The inducer Fan is connected to the flue pipes and it has a rubber hose attached to it that connects to the pressure switch. The pressure switch is a semi round device with two wires attached to it. On a call for heat, does the inducer Fan Motor come on?
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Customer reply replied 7 years ago
yes fan comes on
HVAC Technician: M Oglesby, HVAC Technician replied 7 years ago
After the inducer fan has run about 30 sec. do the igniter attempt to ignite?
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Customer reply replied 7 years ago

Yes I think so as I can smell combustion gas.

HVAC Technician: M Oglesby, HVAC Technician replied 7 years ago
Do you see a spark attempting to igniter or a glow coil? If so do the burners ignite?
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Customer reply replied 7 years ago

to this question I can't answer as I am not at my furnace - I am at work. Will I have to open the top cabinet of the gas pack to see this or can it be seen with the front panel off?

HVAC Technician: M Oglesby, HVAC Technician replied 7 years ago

It can be seen through the front panel, but I would prefer for you to remove the front panel off because we may need to perform troubleshooting test. When troubleshooting a Furnace, the first step is to know the operation process. Once you learn the operation process, then you watch the furnace go through this process and wherever it fails, that is your problem. Then you have to find out why it is failing at this particular point. Here is the Operation process of a gas furnace:

1)Thermostat is set to call for heat

2)Inducer Fan Motor starts (prepurging system of flue gases)

3)pressure switch closes

4)Igniter attempts ignition (spark or glow coil)

5)gas valve opens to light burners

6)Flame Sensor proves flame is present

7)control board allows gas valve to remain open since flame is present

8)Heat Exchanger heats up & blower motor starts

I will need you to observe the Ignition process of your furnace and compare it to this process and let me know where your furnace fails.

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Customer reply replied 7 years ago

I will try to observe the ignition process later today. Just to get a little more insight on this problem I have another couple questions.

What creates the air pressure that trips the pressure switch? Could there be insufficient air pressure or a leak or a bad switch?

I live in NC and often this problem has arisen in October where we get a few cold nights middle of the month and the furnace comes on at night and works normally for many days. Then we get warm weather again for a couple days and switch back to using A/C for a day or two. It is then the reversion back to heat (yesterday) that we run into this problem...but after "fixing" this by "rebooting- power on/off" we don't have the problem again for rest of fall/winter. Puzzling...

HVAC Technician: M Oglesby, HVAC Technician replied 7 years ago
The inducer Fan Motor is what causes the pressure switch to close. It produces a negative pressure in the flue pipes that indicates that all flue gases are clear from the system and it is safe for the igniter to attempt to ignite, which causes the pressure switch to close. It is possible for the rubber hose that is between the Inducer Fan Motor and the Pressure switch to have a small break in it preventing the pressure switch from closing. It is also possible for the Inducer Fan Motor connection that the rubber hose attaches to to be clogged. Also the pressure switch could be malfunctioning and sticking, not opening and closing when it should. It could be many problems but we will have to take one step at a time.
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Customer reply replied 7 years ago
Ok I ran the furnace sequence again and watched for ignition. Did not occur.
HVAC Technician: M Oglesby, HVAC Technician replied 7 years ago
What step did it fail? If the inducer Fan motor came on, I want you to remove the rubber hose that is attached to the Inducer Fan. Blow through it to make sure it is clear. Use a paper clip to clean the Inducer Fan Connection to make sure it is clear. Apply force if neccessary. Also inspect the rubber hose to make sure there are no tares. Reconnect the hose then attempt ignition. Let me know what happens.
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Customer reply replied 7 years ago

Followed your advice. No change. Could it be inducer motor not drawing enough vacuum. Is it normal that the inducer fan shuts down after about 15-20 seconds after the heat is called for?

HVAC Technician: M Oglesby, HVAC Technician replied 7 years ago

Remove the two wires attached to the pressure switch. Attach them together metal to mtetal and attempt ignition. See if the igniter attempts ignition while you are holding the two wires together metal to metal.

Also it is not normal for the inducer fan to shut off before the pressure switch is closed.

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Customer reply replied 7 years ago

did that. no ignition.

HVAC Technician: M Oglesby, HVAC Technician replied 7 years ago

Did the ignitor attempt to ignite (spark or glow coil)? If not, that is an indication that your igniter is bad and needs to be replaced. But before you condemn it, use a voltmeter to see if the igniter is getting voltage at the point of ignition.

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Customer reply replied 7 years ago

Had a tech come out this morning. He positively diagnosed as the "flame roll-out sensor". He replaced it and now have heat.

HVAC Technician: M Oglesby, HVAC Technician replied 7 years ago

I apologize that we could not solve your issue. We are happy that your issue is resolved.

Thanks for the info.

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