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airheatman
airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 7971
Experience:  Furnace, A/C and heat pump specialist.
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Have Trane XB80. No Heat. At turn on, main fan seems to run

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Have Trane XB80. No Heat. At turn on, main fan seems to run normal and then turns off after EXACTLY one minute. Then it stays off for EXACTLY 30 seconds and cycle repeats. No ignition. LED error = 3 flashes = pressure sensor error, is it the pressure sensor, control board or inducer? Note: I do have a VAC meter here.

airheatman :

First of lets turn power off to the furnace and allow board to reset then turn power back on and thermostat to call for heat.At that point, tell me if the small inducer vent motor runs?

Customer:

OK one moment thanks

Customer:

small inducer fan runs (fan near pressure sensor right?), saw glow igniter on temporarily then gone, still 3 flashes for pressure sensor error

airheatman :

Look with a flashlight real close at glow bar and see if it might have a burned spot like this.

Full Size Image

Customer:

Be right back

Customer:

Has white powder color like discolorations

airheatman :

Can you set your meter to voltage, Unplug the hoy surface glow bar,Stick the probes of the meter into the plug where you disconnected glow bar.Turn thermostat to call for heat, inducer will start, you will get voltage to the plug,See if voltage continues or is dropping right back out,This will tell us if the ignitor is losing voltage or if it is bad

Customer:

OK but I will need 5 minutes

airheatman :

ok

Customer:

After Heater turned on 120VAC at glow bar for 20 Seconds, then sounds like valve opening for gas input, at that moment the 120VAC disappears.

airheatman :

I am thinking you may have a bad ignitor

Customer:

OK, if thats not it, could it be the pressure sensor or relay on control board? Just wondering for Plan B :)

airheatman :

According to manual, the inducer should come on, pressure switch closes and ignitor should warm up for 17 seconds at which time gas valve will open.I don't think your flame sensor is getting hot enough to send signal to board to open gas valve

airheatman :

Lets try one other thing

Customer:

OK

airheatman :

Is the little rubber hose in a convienient enough place for you to disconnect it from inducer and leave attached to pressure switch,Turn furnace to heat ,allow inducer to start and then suck on the hose keeping vacuum on the pressure switch so we know it is staying closed?

Customer:

I think so, One moment.

airheatman :

ok

Customer:

I did what you said and the furnace fired up! I could here the pressure switch clicking as I inhaled.

airheatman :

OK perhaps the pressure switch or inducer is the culprit here. Take a straightened paper clip and clean the fittings on both the pressure switch and inducer motor and blow through the hose to make sure it is clean and lets see what happens

Customer:

Ok

Customer:

It's working now (after I cleaned the fittings and blew through the hose and reconnected to inducer and press sw.

airheatman :

Very good

airheatman :

Must have been a bit of trash in the switch or fitting

Customer:

I think we are good to go. Thanks so much, if it fails later I will know what parts to start replacing.

Customer:

Bye.

Customer:

Accepted.

airheatman :

Thanks for allowing me to help

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