Trane XB80 runs intermittently

[Smokindog]

I've got a Trane XB80 that was installed in 2002 when the home was built.  In the past week or so it's begun to operate intermittently meaning it doesn't always provide heat when called for.  Last night I can see in the data from the thermostat that there was a 2.5 hour period where the thermostat called for heat and the room temperature didn't change which is indicative of the failure.

When this situation occurs, the code on the main board is 3 flashes indicating "Pressure Switch Error" per the sticker on the inside of the main panel.  Sometimes it just starts working and other times a power cycle (completely remove power) of the furnace may get it working.

When it's not working the behavior is

• Inducer motor starts and immediately cuts out
• Multiple relay clicks from the main board (I can take a video with sound if helpful)
• LED indicates call for heat before switching to 3 blinks

Here is what I've done so far

• Replaced the filter and used a "cheaper" one allowing more airflow
• Removed the pressure switch and, with a multi-meter, tested it was switching between open and closed by gently sucking on the input port to supply vacuum
• Used a small test tube brush to clean the port on the inducer motor/vent stack
• Inspected the connector tube for cracks
• Checked the vent stack to be open and clean
• Replaced the thermostat with new

I'm about to condemn the main board but was hoping for some suggestions.

I'm looking for further steps on isolating the problem.  I like to know what needs to be done before I call an HVAC company.

[Admin]

It's an older install so I suspect the venting size, slope and termination are correct.

I would just make sure there's nothing outside blocking the vents and check the furnace drainage system inside.  There could be a blockage in the drain line.

I would replace the pressure switch before the board.  It's more likely to be bad and cause the lockout than a faulty board.  You need a manometer to measure the venting airflow to see how close to the pressure switch rating it might be.  There could be a legitimate problem causing the pressure switch to open.

[Smokindog]

I found a board and a switch online for the cost of the board locally ($130).  Can't even get a tech out here for that!!!!  I'll try them in order of cost when they arrive.  This is the third unit and only serves the master suite.  I put an oil filled space heater back there for now.  It's Texas, we'll be AOK!

I'm betting on the main board though given it emits a hum when the error exists and this afternoon the error code survived a power cycle.  I'm guessing failing and/or stuck relay.

It's an older install so I suspect the venting size, slope and termination are correct.

I would just make sure there's nothing outside blocking the vents and check the furnace drainage system inside.  There could be a blockage in the drain line.

I would replace the pressure switch before the board.  It's more likely to be bad and cause the lockout than a faulty board.  You need a manometer to measure the venting airflow to see how close to the pressure switch rating it might be.  There could be a legitimate problem causing the pressure switch to open.

[Smokindog]

Got home from a night out and the pressure switch had arrived. The main board isn't due until tomorrow.

Decided it was simple enough that even with a full belly and a couple bourbons I could tackle it

Boom, everything fired right up. I won't declare victory just yet as it's run intermittently all along and it has been powered down now for some time.

I'll review the run time data versus the room temp from the thermostat in the morning and see what's up.

The switch was only about $40 shipped and the board was about $80 (give or take). I didn't even look at the return policy as even if the repair cost me the $130 (tax) I'm still light years ahead of calling the HVAC tech!

Thanks to all who helped. I promise I'll provide a final read out because I hate when people ask for help and then never tell me what if anything fixed the problem!!!

[Smokindog]

Well it was too soon to declare victory.  Behavior is back to the same.  Tomorrow I'll swap the main board that arrived late this afternoon.

PS - I don't mind having a reasonably inexpensive tool in the chest for occasional use.  This may have saved me the cost of the pressure switch.  Will this device work for testing an HVAC system pressure switch and flue pressure?
https://www.amazon.com/Manometer-Differential-Backlight-Ventilation-Measurement/dp/B07K7HT3XJ/ref=pd_sbs_328_4/145-3565811-3354046?_encoding=UTF8&pd_rd_i=B07K7HT3XJ&pd_rd_r=2587d815-b158-476d-8dcf-3da33e077190&pd_rd_w=PAyM5&pd_rd_wg=StUKz&pf_rd_p=7c0dad87-8a25-4c4f-9349-026039ea6cb3&pf_rd_r=PKG5ASS4DTAQGVZ21KSK&psc=1&refRID=PKG5ASS4DTAQGVZ21KSK#detail-bullets

OR

https://www.amazon.com/Noblik-High-Precision-Manometer-Barometers-Differential/dp/B07Z4LXN6T/ref=sr_1_30?keywords=manometer&qid=1578286021&s=industrial&sr=1-30

[Sergroum]

Could you confirm that you've checked and cleared the drainage of the furnace? I would definitely confirm that before ordering a new board.

[Smokindog]

There is no moisture that I can see anywhere.  The unit is mounted horizontally and I see nothing in the main cabinet nor in the pan beneath the evap coil.  Where else would you like me to confirm is dry?  Are you thinking the drain float switch as the problem?

I already have the new board.  Ordered it at the same time I ordered the pressure switch and it arrived today.

Could you confirm that you've checked and cleared the drainage of the furnace? I would definitely confirm that before ordering a new board.

[Porcupinepuffer]

I'm assuming this is a mid-efficiency/non-condensing furnace? Maybe that's why there's confusion about cleaning a drain. If the furnace doesn't have a secondary heat exchanger, no drain will be found.

[Admin]

My bad, given the model number this is likely an 80% furnace with no drain.

[Smokindog]

OK, I follow you now.  I've had similar issues on my high efficiency tankless when we had 3-4 days in the teens a few years back and the condensate drain line froze   Baffled me for awhile until the sun came out, hit that wall on the house, and I had hot water again   Learned a good lesson!!!!

Just installed the new main board.  Before doing so I called White Rodgers tech support to review what I've already done and planned to do.  They confirm that the low level hum from the main board is most likely a relay problem on the board and to just call back if there are still issues.  Much more helpful than I anticipated to tell you the truth.  Pleasantly surprised.

Everything fired right up.  As some comments on the vendor websites noted for the updated replacement board, there are extra features and care had to be taken when transferring wires given the extra posts on the newer model.

We'll let it go a few days and see what transpires.  Going to be warm during the day this week so it wont get much of a work out.

All's well for now!

[Smokindog]

I am pleased to report that I am declaring the problem to be fixed with the replacement of the main board.  I now have a spare vacuum switch but I'm happy with the resulting cost of about $130.   A small price to pay for this repair IMO!!!!

THANKS ALL FOR THE HELP AND WISDOM!!!!