5ton Trane controls issue


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Old 07-25-16, 10:41 AM
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5ton Trane controls issue

Morning

I have 5 ton Trane Heat Pump, plus electric furnace.
It has 3 auxiliary heat strips.
We just bought the house, I moved in December 8th last year.
Winter was as usual, here in PacNW, not really cold.
Furnace and heat pump thing outside were running practically non stop through colder days. As in - above 30. Heat coming out of vents was lukewarm.
it's a 4100 sf 2 story building with vaulted and all that.
hence, substantial electrical bills.

I had mini split installed at ADU and HVAC guy checked on my system condition.
System was installed in 2004.
Contactor will need to be replaced. It's "getting there" but functional.
But, this is what happened. He had t-stat control wires pulled out of furnace housing to check on voltages and so on. System was turned on and it immediately kicked in on all 3 aux heat strips and we had immediate warmth in the house. 161 degree out of vent.
We reassembled everything - and it's a no go. Furnace kicks in, then shuts down 2 out of 3 or all 3 strips and then runs off heat pump only, barely warm air.
After another hour of investigation it was determined that there is a parasitic voltage supply on W t-stat wire at the furnace connector - 12-13 V. That is with t-stat DISCONNECTED, so t-stat was NOT sending any current to furnace.
HVAC guy thinks it's short somewhere in the t-stat to furnace harness, resulting in small current, confusing furnace controls. He reprogrammed t-stat (Honeywell touchscreen) according to his cheat sheet but result is same.
Is he on the right track or there is something else going on?
Thank you
 
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Old 07-25-16, 11:02 AM
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Do you mean he disconnected the W wire at the furnace end and still read a voltage on the W terminal ?

The 24v that the thermostat sends back to the air handler basically controls relays. One half of the relay is at common or grounded which means the other side should also be close to 0v.

Can you post the Trane AH model number ?
 
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Old 07-25-16, 11:42 AM
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Yes, that's exactly what happened - he had t-stat off the wall, completely disconnected - yet there was 12-13 V current at the W wire.
I'm at work, no way to get the Trane number. House was built in 2004 and it's a 5 ton, all I know. Downflow set up. He was very content with blower condition, said it all looks fine. Contactor had somewhat burnt contacts, but functional, so he'll replace it late September or so.
NOW I really want to have this fixed. System worked so great when all 3 strips were on. He said, it should kick in on strips if temperature difference is more than 8 degrees, run on strips to get house warm fast, then switch to heat pump for temp maintenance only. And that it should turn on for 10-15 minutes and then off for about 15 and cycle like this. In reality, heat pump was running non stop winter time. No wonder.
 
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Old 07-25-16, 11:45 AM
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Post the model later when you're home.

It is more efficient to use the heat pump to heat the house which means it will run just about all the time. The electric coils are faster to heat but definitely cost.
 
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Old 07-25-16, 11:51 AM
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Question is - is he on the right track assuming it's short in the control harness, t-stat to furnace?
 
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Old 07-25-16, 11:56 AM
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I don't think so. If it was a short to ground you'd get 0v. If it was a short to the 24vac line you'd see 24vac.

Did he remove the wire off the W terminal AT THE air handler and check there ?
That's where it gets checked. If there is no voltage there but there is at the stat then you have an open wire and the digital meter is picking up a ghost voltage.

You need to confirm the air handler is working properly first. That can be done with jumpers. If that's ok then you expand out to the wiring.
 
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Old 07-25-16, 12:52 PM
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Presuming air handler is thing outside with fan? It turns on and off and system is 10% overcharged but he said it's within tolerance limits.
Control harness from t stat comes out of the wall, beside the furnace blower housing and in the front Is sliced to the furnace connector, male female plug, 6 prong I believe.he had W and R wires disconnected completely from furnace harness and plug, t stat removed from the wall - and there was 13v feed on W wire.
If I am understanding this properly with W disconnected from furnace plug AND t stat there possibly can be NO power supply to it as the other W wire end goes to t stat and there is nothing connected there.
I'm presuming his thinking is - there is leak from power supply from furnace TO t stat resulting in back feed of 13 v from that to the furnace.
Where else can those 13 v come from otherwise?
 
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Old 07-25-16, 04:06 PM
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if the thermostat cable is run parallel to an electrical cable then you be reading a ghost voltage from the 120v cable.

I think he's stuck on voltages on the wiring.

He needs to go to the air handler/electric furnace and connect R to W to see if heating coils come on like they are supposed to.

The outside unit is the condensor.
 
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Old 07-25-16, 04:31 PM
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Soooo.... can I simply jump W and R wires in t-stat connector on the wall? I can easily remove it and post pic here.
Btw, he did check power supply to strips.
He had around 40 amp at the breaker when strips run. But they run when furnace turns on and then in no time - boom, boom - turn themselves off and only heat pump runs.

I am trying to understand one thing. IF there is short in t stat harness - how would it cause strips to turn off? I told him to simply pull new harness in. Instead of wasting time on investigations. We spent over 2 hrs with this Saturday.
No big deal to replace harness. BIG deal to replace it and for it not to work properly again.
 
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Old 07-25-16, 04:36 PM
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Oh, and it's 220 furnace. not 120.
 
 

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