Question about phasing in Bard wall unit

Chapstick · July 5

Hey guys, I find myself in an odd situation. I landed a new job and I’m the sort of the lead guy in hvac troubleshooting even though I have zero experience in central air systems. We have 100’s of units so I’m learning rapidly but this has me stumped.

I came across a Bard wall unit yesterday that wasn’t running. Somebody was in there messing with it at some point and apparently blew up the original transformer and replaced it. They also removed the jumper on the low voltage board that went from W1 to W2.

First thing I did was replace the jumper. The phase monitor was also showing it was out of phase so I tracked the wire feeding it through the control module and all the way back to the transformer.

I removed the 120v feeding the transformer from the contacter and somehow the line side wire of the transformer was still showing 120v and that tells me the transformer was being backfed from somewhere.

It actually worked out perfectly because I was able to check the phases across to the contacter with a fluke and quickly determined which phase was the right one.

I de-energized the circuit then swapped the two wires on the line side of the breaker and it worked immediately.

So my question is, since the 24v transformer was apparently being backfed, where was the 24v that was backfeeding it coming from ? I scoured the schematic and the unit and I’m certain there’s no other transformers in the unit.

There was an identical unit that was running in the same building and each had its own thermostat. The wiring went up into a giant cable chase so I couldn’t tell if the two thermostats were connected without taking them apart but if they were connected and the phases were off wouldn’t the other unit show it was out of phase too ?

These units cool a bunch of electrical distribution equipment and we were in a heat wave so I didn’t want to risk opening up anything else.

pecmsg · July 5

They also removed the jumper on the low voltage board that went from L1 to L2.

You lost me with this. A jumper between L1 & L2 creates a dead short.

Chapstick · July 5

I’m sorry. I meant W1 to W2. I corrected my post.

pecmsg · July 5

The phase monitor checks high voltage not control voltage.

If it says, "Out of Phase" What were your voltage readings between legs and to ground?

Is there a Neutral in there? It's possible but usual.

109A_5 · July 5

I have replaced the transformer in a Bard wall unit, it simply just failed, the primary went open.

Sounds like it may be a 3 Phase unit, if so the 120 VAC may have been from the other phase. With 208 VAC Wye 3 phase any phase to neutral (or ground) typically is 120 VAC. However then your control transformer may have a 208 VAC primary.

If it is a 3 Phase unit, Phase rotation or Phase sequence may be important so the compressor and/or fans run in the correct direction (any 3 Phase motor load). I kind of wonder why someone would change that ? Was a circuit breaker changed or replaced and they wired it wrong ???

The W1, W2 wire thing, it may be mostly irrelevant. If the Bard wall unit was at a facility (like a cell site) they possibly are always removing heat, even if it is zero (0) degrees Fahrenheit outside. The only time you would need heat is if the majority of the electrical / electronic equipment is shut down in the winter.

EBEBRATT-Ed · July 6

We need to know if the power input is 3 phase or single phase.

And what type of unit is this?

Model #

Cooling and electric heat? or gas heat?

109A_5 · July 6

Looking into Bard Wall Mount units a bit more, I suspect it has a Phase Monitor similar to this (some have a Bard part number on them). In the wiring diagram below, note the Control Transformer's primary Voltage, it is not 120 VAC. Again, I suspect the primary of the control transformer is 208 VAC.

The Red wires are an example of the control transformer's primary connection.

I suspect when they changed the original failed transformer they either messed up the Phase Monitor sensing wires or reversed two of the Phases, which you may have corrected. And I hope you verified proper compressor operation. If the other unit is the same model, presumably wired the same, you may want to use it to verify the wiring is correct.

Text from a Bard Manual.

EdTheHeaterMan · July 6

This does not phase me at all.

Ironman · July 6

Is the unit supplied from a 3 phase Y connected circuit or a 3 phase delta circuit?

On a delta circuit, one of the phases is 208 volts to ground. If a 120 volt transformer was connected to that phase, then you have your answer why it failed.

EBEBRATT-Ed · July 6

That's why I asked if it was 3 phase. Phase monitors are fairly common on 3 phase on single phase usually not so much.

Kaos · July 7

Isn't reversing the phasing and running the compressor backwards how you convert these to a heat pump?

@Chapstick "

"I removed the 120v feeding the transformer from the contacter and somehow the line side wire of the transformer was still showing 120v and that tells me the transformer was being backfed from somewhere."

Most likely the thermostat has its own transformer that is wired improperly, only the C wire should be shared and the thermostat needs to have a relay output or connected through relays to the unit.

109A_5 · July 7

With the information given by @Chapstick, I believe it is a 208 VAC Wye 3-phase system. The Phase Monitor appears to work and measuring 120 VAC is common with a 208 VAC Wye 3-phase system when one of the meter leads is connected to the Neutral or Ground. There is probably no Neutral in this situation.

I believe the assumption that the control transformer is being back-fed somewhere is erroneous. Since even with one side of the transformer disconnected you would still measure 120 VAC to ground on the disconnected transformer wire. The meter simply 'sees' the voltage from the other phase through the partially disconnected transformer.

Here is a partial wiring diagram that is probably similar to what @Chapstick was working on.

109A_5 · July 7

With one wire going to the control transformer's primary open or disconnected (either wire, but just one) there will be 120 VAC on that wire from the transformer (measured from Neutral or Ground). Actually there will be 120 VAC on either side of the disconnection (measured from Neutral or Ground). There will be 208 VAC across the open circuit. When the open circuit is connected (closed) there will be 208 VAC across the transformer's primary.

Question about phasing in Bard wall unit

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