Never saw this before

hbiss

hbiss

EC, New York NEC: 2017
Location
Little Falls, New York NEC: 2017
Occupation
EC
I just watched a You Tube video by a plumber who was troubleshooting some thermostat and zone valve problems. He found a dead short on the wiring off the 24vac control transformer which blew the internal fuse. So, he replaced it and in doing so checked for 24v on the secondary screws to see that it was working. But what I never saw anybody do is measure from each side of the secondary to ground, noting that one side is higher than the other and call the higher side the "hot" and the other "common". He actually marked H and C on the transformer with a Sharpie.

I submit that any voltage to ground is a phantom of his DVM since the secondary is floating. Secondly, what does it matter?? There is no reason for a H and C.

-Hal
 
It's been a long time since I seen a control xfmr secondary that one side wasn't grounded.
In hvac units anyway.....
 
On most furnaces the ignition and flame sensing the current is returned to the ignition module via the metal on the furnace so that is one reason one side of the transformer is grounded
 
Grounding the secondary was started a number of years ago for hvac manufactures.
It does make TS easier.
 
DooWop said:
Grounding the secondary was started a number of years ago for hvac manufactures.
It does make TS easier.
Click to expand...
Eddie702 said:
On most furnaces the ignition and flame sensing the current is returned to the ignition module via the metal on the furnace so that is one reason one side of the transformer is grounded
Click to expand...
LarryFine said:
I believe it's more because of electronic thermostats that the 'common' leaves the AHU.
Click to expand...
You guys miss my point. Yes, you are all correct EXCEPT that is after the transformer is installed. This guy takes the new transformer out of the cardboard box, connects the primary to 120V then proceeds to measure from each UNCONNECTED secondary screw to the transformer case. When he sees that whatever the random voltage on one screw is higher than the other, he labels the higher one "HOT" and the lower side "COMMON". WE all should know that all he is reading is phantom voltages since the secondary is floating at this point.

-Hal
 
hbiss said:
You guys miss my point. Yes, you are all correct EXCEPT that is after the transformer is installed. This guy takes the new transformer out of the cardboard box, connects the primary to 120V then proceeds to measure from each UNCONNECTED secondary screw to the transformer case. When he sees that whatever the random voltage on one screw is higher than the other, he labels the higher one "HOT" and the lower side "COMMON". WE all should know that all he is reading is phantom voltages since the secondary is floating at this point.

-Hal
Click to expand...
Yes, we should, but don't always think about it.

There is something else we should also know but don't always think about. When grounding one secondary leg of the transformer, the other secondary leg may be additive or subtractive to the voltage on the hot leg of the primary. I'd prefer to ground the secondary leg that results in the two hot leg voltages are subtractive.
 
qcroanoke said:
It's been a long time since I seen a control xfmr secondary that one side wasn't grounded.
In hvac units anyway.....
Click to expand...
Had similar issue with a HVAC tech that came to troubleshoot our HVAC. has a 230V/24V control trx. He said we had power issues since he was getting only 40V/gnd one leg of trx and 60V/gnd on the other. HVAC company sent me report we need to fix our power issue. I told them send a different tech as the control trx was a ungrounded trx(floating neutral) but there is 24v on the trx seconday. Ended up one of the control relay contacts were fried, lol.
 
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