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My Furnance has stopped working

1747 Views 7 Replies 2 Participants Last post by  jagans
We were waken up by the cold and found out that the furnance is not running. The furnance is a TempStar. So I did some testing

1. On the thermostat I put the fan switch from Auto to On. The fan comes on.
2. I set the thermostat to heat and hold the temperature to way above room temperature, the system on indicator is on on the thermostat.
3. I open the furnance, override the door open safety switch.
4. There is 24V ac across C and R and C and W

But nothing is running. If my memory is right, I think the exhaust motor should start running when the thermostat calls for heat.

I did a very quick test with the exhaust motor. I switch the power off to the furnance, disconnect the wire going to the motor and give it 110V. the motor turns. I donlt want to run into any risk because I donlt know what happen with just the motor running. So I disconnect the 100V as soon as I saaw the motor runs and reconnect the motor back to the furnance network.

I read some forum entry that the exhaust motor is also controlled by the pressure switch is is open at this time. But I am not sure if the pressure switch actually controls the motor when the thermostat first call for heat or it controls it after it is started first to create a pressure. Can some one confirm ? Is there a good way to test the switch if it can be the cause of the problem?

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Please give the exact model number of your furnace so the HVAC pros can help you. I know a little but some guys like Yuri and Doc know a lot.
I saw this on a sticker inside the furnance NTG9075FGA3. Not sure if it is the model number
Go to this link and download your manual. The first thing you need to do is make sure the trap and condensate lines are clean, and your intake and exhaust lines, usually 3 inch PVC are both clear. If that inducer blower cannot create enough draft through your intake and exhaust, your pressure switch will not close and you will not get ignition. It is a safety. check in the ends of your pipes outside for birds nest etc. I cut my lines inside about 5 foot from the furnace, and had my son put my gas leaf blower on them outside. Blew some debris through but mine weren't blocked. I put them back together with Rubber 3x3 fernco connectors for easy future checks. My condensate line from the lower heat exchanger was blocked. Do these first then come back.
Thanks jagans, What is "the trap and condensate lines" I have checked all the PVC pips (yes about 3" in diameter). They are good. This unit also has A/C. It has 3 small hoses (may be 1/2 to 3/4 " dia) coming out of it leading to the basement drain. Are those trap and condensate lines ? one of these is connected to the exhaut motor module. That may be the condensation line. If it is, that one is clean also. The pressure switch is now open (I get more than 10 V across its terminals (my meter goes from 10V AC to 250 VAC, so I know that the voltage is definitely more than 10)
Just one thing to confirm. Are you saying that the pressure switch should be closed at this time (i.e a normally closed switch)
I just traces the three drain hoses that I mentioned. One comes out of the drain pan of the A/C. One actually is coming from the Hunidifier . One only is coming out of the Exhaust motor unit. I have unplugged them and check. they all look good.
This is a condensing furnace. It has a two stage heat exchanger. The first stage are those big S tubes in the top of the furnace. The second stage is below and there should be a drain from the secondary heat exchanger that runs into a trap. Clean out the trap and the drain line.

You have the power off at the breaker box right? I just looked at the schematic and it looks like your pressure switch is supposed to be normally open. Take a look for yourself.

Anyway what should happen is that the inducer (Exhaust) motor should go on when the stat calls for heat. Air flow should close the pressure switch and initiate the ignition sequence. The igniter should glow or spark or whatever yours does, and the gas valve should open pilot, and then main gas.

Check to see if all your vacuum lines are good, not cracked etc, and if they are not, you should call a good HVAC tech to go over your entire unit, because you have gone about as far as you want to go. Pressure switch, Igniter and Flame sensor is about my limit, but maybe somebody good will happen along, at least you got the obvious things out of the way.

You do change your filter regularly right?
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