Heat pump control issues

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Wainert

Member
Location
Williston, vt
Hello,

last week I was sent to job to finish up the punch list which included 4 single zone heat pumps listed as not working. After much debate the Hvac team said the issue was with the wirring, with my meter in hand I rang all my lines found all units to be correctly terminated. I then went back to the hvac team said our wirring is correct and left for the day.

The next day my PM called me back to the site, the hvac team now stated it was an issue with the control wire. Just so everyone knows these heat-pumps wirring runs from the apt's panel (2-pole 20amp on 3wire romex) to a disco the roof (N/white wire capped in disco) then the outdoor unit; now another 3wire line returns from the outdoor unit passing back threw the disco to the indoor unit. When I meter for voltage I get 120v L-G, 208 L-L for the supply, now my 3rd "white" wire is the "control line" between my indoor and outdoor unit. When I meter this wire to ground I read right around 120v ac with a fluctuating DC voltage of 4vdc to 68vdc. With this info in hand I called tech support to find what I should be reading; the tech tells me the dc voltage should be fluctuating only to spike of around 27vdc, he tells me I have an induced voltage issue on the control wire.

Now as I stated earlier; I was there for punch list, so the building is complete. I don't want punch holes and pull another wire up three floors to get the same results. Has anyone run into an issue similar to this or have a way to reduce the induced voltage?

This building has 28 apt's total on 3 floors and all other heat pumps are working correctly. The 4 units that are not working are spaced out on different levels. I thank you all for reading and any input you may have!
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Firstly I would compare my readings with the readings from a unit that is working. Are these units mini split units?
 

kwired

Electron manager
Location
NE Nebraska
If you used proper wiring methods and followed manufacturer instructions with the installation of it - then it sounds like the HVAC installer's problem to me.
 

bob

Senior Member
Location
Alabama
you stated that you have a 3 wire romax cable running to the heat pump. Two of those
conductors are providing 208 volts to the heat pump. The third wire is the question. Are
you using the third wire as a neutral along with one of the other conductors to get the 120
volts you measured? Why would you expect to read DC voltage except maybe as a phantom
voltage Is the control voltage 120 volts?
 

kwired

Electron manager
Location
NE Nebraska
you stated that you have a 3 wire romax cable running to the heat pump. Two of those
conductors are providing 208 volts to the heat pump. The third wire is the question. Are
you using the third wire as a neutral along with one of the other conductors to get the 120
volts you measured? Why would you expect to read DC voltage except maybe as a phantom
voltage Is the control voltage 120 volts?

Third wire between units in these mini-split systems is a signal wire to communicate between units. It often is more then just a on/off signal and is used to transmit a digital signal that can be for multiple functions over one conductor, possibly using one of the other power conductors as a second conductor of the control circuit.
 

Wainert

Member
Location
Williston, vt
Kwired's got it. The panel to disco is a three wire; black/red for my 208, white is capped in the disco. From disco to outdoor unit there is no neutral. When I contacted tech support they said it's the induced dc voltage that is messing with the "white/control conductor" of the 3wire romex/nm-b. Tech support said I will have 120vac and 27vdc on the control conductor. Remember I'm reading 4vdc to 68vdc

The manual doesn't say anything about this issue. The hvac tech ran a 18/3 t-stat wire off the roof from the outdoor unit to the indoor unit and bam it worked. Switch it back and nothing.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
160110-1541 EST

Define an induced DC voltage.

DC implies a long time steady voltage that is unidirectional.

A varying or modulated DC voltage would be one of always the same polarity, but where the voltage fluctuated.

From one wire to another how can you induce a DC voltage from a magnetic field change without the magnetic field continuously changing in the same direction. Note: a magnetically induced voltage in a wire is described by v = K*dPhi/dt. Where dPhi/dt is the rate of change of the magnetic field.

Some might want to call a capacitively coupled voltage induced. Here again you would need to have a continuously varying monotonic input voltage to produce an apparent DC output.

Neither inductive or capacitive coupling can produce a continuous DC output for very long. Something will saturate and produce a reversal.

Why is there an AC voltage? That is probably capacitive coupling of an unwanted signal. Should you get an AC reading? Possibly. Disconnect both ends of the signal wire. Read both AC and DC voltage. Use a Fluke 27 or 87 or equivalent, meaning there is a series input capacitor in the meter in AC position. If you use a Simpson 260 use the Output and AC position to remove any DC (steady) voltage component. For a Simpson 260 on the AC position and using COM/- and + terminals the meter is a resistive divider to a bridge rectifier to the 50 micro ampere DC meter movement. Thus, either a DC of either polarity or AC input will produce a positive deflection of the meter movement. Use of the Output input terminal introduces an input series capacitor stripping DC from the signal being measured.

For a Simpson 260 on the 10 V range a 5 V DC input reads 5 V on the DC scale, and when AC is selected reads about 5.6 V on the AC scale. Change to the Output terminal and the voltage reads zero.

Disconnect the signal wire at both ends. This leaves the signal wire floating. Measure as you did before the AC and the DC voltage to neutral. Also see if there is a difference in the readings performed at each end. There should be no DC voltage. I expect AC voltage from capacitive coupling. When at each end of the communication wire while it is still disconnected measure the AC and DC voltages to neutral of the terminals from which the comm wire was disconnected.

From your last post I suspect the AC voltage is unwanted and is interferring with the modulated DC signal. You might solve the problem with shunt capacitance from the signal wire to neutral. The larger the shunt capacitance the lower will be the AC signal based on my conjecture. However, too much shunt capacitance will interfere with the modulated DC comm signal. There may be no solution other than a separate comm wire, and possibly shielded. I would expect the manufacturer of the HVAC equipment should know of this problem, and would provide proper installation instructions. But they apparently didn't or don't.

AC voltage read on a working system would be useful information.

.
 
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kwired

Electron manager
Location
NE Nebraska
I suggest not running back through same raceway/disconnect to go to inside unit - and would bet instructions tell you not to do that anyway, and your problems maybe go away. You must also use size/type cables they recommend between units or you may be asking for performance issues.
 
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