# Furnace transformer 24V hot?



## Artfuldodger (Feb 17, 2017)

My brother in law changed his Rheem furnace transformer.
It got it going but then his fan ran continuous even in the "auto" position.
I took a look at it and noticed he had reversed the low voltage wires with the neutral going to the 24v hot connection. I reversed the secondary transformer wires and the fan started working properly.
The new transformer was a Packard and had the "blue" marked as hot and the "yellow" marked as neutral. I think this is backwards from most furnace transformers.

My question is why did changing the wires have any bearing on his fan running continuously as I didn't think AC current has polarity?
I guess it has something to do with the way the circuit board is wired to work.


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## Artfuldodger (Feb 17, 2017)

I did read this about finding the 24 volt output hot vs the neutral. I guess it matters otherwise the new transformer would not distinguish between the two secondary wires.

On the input side, black is hot (sometimes marked with brass), white is neutral (sometimes marked with silver). On the output side, if the terminals are unmarked, power up the transformer and test AC voltage from the input neutral to each output terminal. One should read about zero -- this is the output neutral; one should read about 24VAC this is the output hot. 

Maybe it has something to do with the 24 volt neutral being grounded. Also since the 24 volts go to a control board instead of just a thermostat, it is affected by phasing or the grounded secondary. I'm not sure if the 24 volt secondary neutral was grounded.

Maybe the wire swap did something to the way the fan relay on the board works with the thermostat.


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## Deerslayer83 (Feb 17, 2017)

He crossed the common and hot 24 volt wire going back to board. Luckily it did not burn transformer or blow fuse.  If furnace doesn't have a fuse wired into transformer it would be good idea to add 5 or 3 amp fuse by breaking hot 24 volt wire with spade terminals. Gas furnaces have a lot of limit switches that will operate fan when temperatures rise, you were getting some kind of feedback from some where. When ever your wiring is wrong they can do some crazy things.  But the big question is why did the transformer fail there is a reason for this that needs to addressed or your gonna have another bad transformer.


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## Artfuldodger (Feb 17, 2017)

Deerslayer83 said:


> He crossed the common and hot 24 volt wire going back to board. Luckily it did not burn transformer or blow fuse.  If furnace doesn't have a fuse wired into transformer it would be good idea to add 5 or 3 amp fuse by breaking hot 24 volt wire with spade terminals. Gas furnaces have a lot of limit switches that will operate fan when temperatures rise, you were getting some kind of feedback from some where. When ever your wiring is wrong they can do some crazy things.  But the big question is why did the transformer fail there is a reason for this that needs to addressed or your gonna have another bad transformer.



You mean transformers don't ever just go bad? This furnace is probably from the nineties. His new transformer has a circuit breaker. His old one didn't nor was it fused.

I agree that they usually don't just fail. He replaced it with a 50va. The old one was probably a 30 or 40.

If it starts tripping the transformer breaker, what would be the first thing to check? If there is no common to the thermostat, I don't think it would be that. I'll check to see if it has a common if it trips the transformer breaker.
Maybe start with a new thermostat and check the low voltage wires. Especially out by the condensing unit. 

It's not something instant like a short, I guess maybe a relay coil  on the board or a contactor coil going bad.


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## Deerslayer83 (Feb 17, 2017)

Normally there is reason for them to fail. Most common reason would be shorted tstat wire. Check all wires for nicks or ohm wires out for shorts. I would definitely install a inline fuse on 24 volt side. I would say number one reason I have seen for blowed transformers would people hitting wires outside at condensing unit with a weedeater. Or rats and squirrels chewing up wires.


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## Artfuldodger (Feb 17, 2017)

It didn't blow the 3 amp fuse on the control board. Does the fuse on the control board offer protection for the transformer? If so and that didn't blow, maybe it was age related or an amperage underrated transformer.


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## Artfuldodger (Feb 17, 2017)

Deerslayer83 said:


> Normally there is reason for them to fail. Most common reason would be shorted tstat wire. Check all wires for nicks or ohm wires out for shorts. I would definitely install a inline fuse on 24 volt side. I would say number one reason I have seen for blowed transformers would people hitting wires outside at condensing unit with a weedeater. Or rats and squirrels chewing up wires.



OK, thanks. Do I still need a fuse even with the built in circuit breaker on the transformer?


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## JohnK (Feb 28, 2017)

No...


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