[zz28zz]

Have a Trane XV80 in the attic. Also in the attic laying next to air handler is a Relay Panel (Trane BAY24VRPAC52DCA). In the hallway is the Trane thermostat (model# TCONT850AC52UBA)

Noticed A/C wasn't working last night. Thermostat display is blank (appears un-powered). Checked drain pan water sensor and additional float sensor on the condensation drain pipe. Both dry.

Opened air handler and after bypassing the panel safety switch, I see the Status LED on main board showing a singe green blink (which per the diagnostic codes listed on schematic attached to unit, means "Standby Mode of Call for Cooling".

Pulled thermostat off the wall and measured 15Vac at red/blue wires. Went into attic and found the same red/blue wires from thermostat connecting to the Relay Panel connections R (hot) and B (Com). Also connected to the R and B terminals are wires that go to main board inside the air handler. (The R terminal wire actually goes thru a float switch on the way to the main board and float switch tests OK)

If I disconnect the R and B wires from the main board at the Relay Panel, the voltage on those 2 wires is 28Vac. When I connect them to the Relay Panel, voltage drops to ~15Vac. It appears the Relay Panel is pulling the 28Vac down to ~15Vac OR the main panel can't produce enough current and voltage is dropping. None of the LEDs on the Relay Panel are illuminated.

(With everything hooked-up, the 15Vac I saw was the first measurement at red/blue wires at thermostat. Subsequent measurements have been anywhere from 7Vac up to 12Vac.)

It appears to me the Relay Panel is bad but would like to find a way to confirm that. Thought of installing a precision resistor in series with the red wire and measure V/D across it to calculate if current is excessive, but don't know how much current it's supposed to draw.

Any ideas to verify if Relay Panel is bad or not??

 

[aa7483]

This isn't a DIY site.

 

[rundawg]

This isn't a DIY site.

:ditto:

The forum rules state that we can not assist with DIY repairs or troubleshooting.

To ensure your system gets fixed in a timely manner, I would suggest you reach out to a qualified contractor.

Sorry we could not be of assistance.

 

[zz28zz]

I saw that after I posted while awaiting a reply. Should have read the first posts. My bad. Got an appointment but they can't get out here for a few more days.

 

[rundawg]

I saw that after I posted while awaiting a reply. Should have read the first posts. My bad. Got an appointment but they can't get out here for a few more days.

Thanks for your understanding. We would still like to know how it was resolved, should you choose to share it with us.

We are always here should you have "Non Technical" questions about your system.

 

[zz28zz]

Well, it just keeps getting better and better. Got the AC tech onsite and he confirmed the Relay Panel was bad. He spoke to Trane tech support and they recommended he take a look at the condenser unit. He popped off the cover and discovered evidence of fire! Melted wires and soot everywhere. He assumed the contractor fried and put that on the estimate along with the Relay Panel. I was sent a quote and I approved it. Still waiting to hear back from them.

In the mean time I cleaned up the soot and discovered the contactor looks fine. Appears the main issue was one of the incoming power wires overheated at a splice. Everything around that splice was charred including the splices for the other hot and the ground wire. 24V wire from airhandler to contactor melted and we assume high voltage got back into the airhandler 24V wire and took out the Relay Panel. He climbed inside, ohm'ed-out the compressor and said it looked OK. No shorts to ground either.

I had initially experience sticker shock on the labor rate and got another company to provide a quote. The second tech said that the only way to keep my warranty intact was to use the same outfit that installed the units. I had assumed any Trane approved outfit could do warranty repair (I got their name/number from the Trane website). Not sure if this is really true.

Second tech said he thought it was a power surge. The first tech mentioned something abt power surge too.
Got a heavy duty Type II (140,000A/ <1 nanosecond) surge protector installed at main panel so if it really was a surge, hopefully it won't happen again.

I personally think the splice wasn't proper and ~90 consecutive days of 100+ degs temps was more than it could take. It's been running 23-24 hrs a day for abt 3 months.
Curious if anyone has seen this particular situation before and if anyone else thinks the power surge theory make sense.

The only good part of this situation is that a cold front arrived and we're in the 70's again. Yeah!!

 

[dougfamous]

What kind of engineering do you do?

Some thoughts:
I have seen a Trane unit where the wire melted at the splice. In that case the wire melted through the low voltage wires and fried everything in the control circuit.

If this unit is still under warranty then you really shouldn't be in there messing with it. You could inadvertently make your problem worse or get injured.

Any HVAC contractor (Trane dealer or not) can get parts under warranty from Trane. It can be a massive hassle which is why a lot of companies won't do it for units they didn't install.

Your relay panel might be OK. If you powered it and it showed a cooling call the voltage drop you measured might have been from the contactor outside. The contactor might have been pulled in and holding but the unit didn't run because of the shorted wire.

I doubt it was a power surge. I think we all know why it melted.

Good luck. Update us on what happens.

 

[zz28zz]

What kind of engineering do you do? >>>>>>> HA. Lots of different stuff over many years.

Some thoughts:
I have seen a Trane unit where the wire melted at the splice. In that case the wire melted through the low voltage wires and fried everything in the control circuit.>>>>>> I'm surprised the air-handler board is giving an "awaiting cooling signal" message. Awaiting wiring repair completion before we know for sure it survived. Seems to me there should be some effort made to keep the HV and low-voltage wiring separated, but that's just me.

If this unit is still under warranty then you really shouldn't be in there messing with it. You could inadvertently make your problem worse or get injured.>>>>>>>>> I've done many post incident inspections/investigations. Think I'll be Ok.

Any HVAC contractor (Trane dealer or not) can get parts under warranty from Trane. It can be a massive hassle which is why a lot of companies won't do it for units they didn't install.>>>>> Sadly, that makes sense.

Your relay panel might be OK. If you powered it and it showed a cooling call the voltage drop you measured might have been from the contactor outside. The contactor might have been pulled in and holding but the unit didn't run because of the shorted wire.>>>>>>>>>> Relay panel is dark, no LEDs illuminated. Air-handler board LED continuously indicates it's awaiting a cooling input signal. The contractor 24V signal wire was found completely severed (melted).

I doubt it was a power surge. I think we all know why it melted.>>>>>>>> ;-)

Good luck. Update us on what happens. Will do..

.

 

[VTP99]

..

 

[zz28zz]

Got a 3rd tech out to the house and he figured out the control circuit issue after a bit. Found the airhandler board was limiting the amount of current available at the 24V output to Relay Panel and thermostat. Had 28V at transformer but only 23V leaving the airhandler board (no load). He connected the 24V transformer directly to the Relay Panel and it came to life, same for thermostat. He replaced the airhandler board, the contactor, start/run cap and repaired the burnt wiring. All good now.

In regards to the first outfit, after I approved the quote a few days went by with no contact other than the auto-generated email "we'll get back with you soon". Called them after 5 days and they told me their guys weren't allowed to touch HV wiring and I need to hire an electrician. HUH?!?! They installed the unit! At least I know who not to call from now on.