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Phil, Mechanical Engineer
Category: HVAC
Satisfied Customers: 8429
Experience:  Retired HVAC/ Electrical & Boiler contractor. Industrial
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Billy, I contacted about my second floor theromstat no

Customer Question

Hello Billy,I contacted about my second floor theromstat no functioning about 6 months ago. We believed it was a bad thransformer because the transformer to my 1st floor unit had a slight him to it with a 27.8 volt reading with a Greenlee tester. The second floor transformer was silent with power coming in to the transformer but it did not have any reading on the other leading out.I finally replaced the bad transformer with an identical transformer. I still do not have any power to the thermostat and the thermostat in question, functions on the first floor unit.I have reset the breakers to the ac/forced hot air system and I still do not have any power to the thermostat. The board the thermostat attached to is also dead. Thanks Steve
Submitted: 11 months ago.
Category: HVAC
Expert:  Phil replied 11 months ago.

Hello, the humm is common in transformers and is not a sign of failure. These cannot be diagnosed by humm,. only by measuring the primary side voltage and the secondary side voltage.

Have you replaced the transformer that you think was bad, be very explicit please.... even it it is obvious. (first floor or second floor and which thermostat on each floor)

what was the primary side voltage reading on that transformer, what is the primary and secondary side voltage on the new transformer...

and what reading to you get on the new transformer from each side of the secondary side to ground (bare threads on the gas line).

What reading do you get from R on the thermostat where the same wire connects at the thermostat terminal strip in the furnace to ground.

We can go from there.... be unnecessarily explicit please. Pointedly explicit, that will help a lot.

Customer: replied 11 months ago.
The unit is a Carrier AC that also uses forced hot air here in South Carolina. My thermostat on my second floor does not have any electric reading from R. It is completely dead and is a snap on electrical type that does not use batteries. I used a Greenlee DM-20 multi-meter to confirm this. It should be noted that I have two units in the attic. One for the 1st floor, the other for the 2nd. The 1st floor functions fine. Also, I tried the 1st floor functioning thermostat on the 2nd floor and it does not function on the 2nd floor, but functions perfect on the 1st floor. Because it has electricity to it.I was advised to install a new transformer (Carrier HT01CN241) in my second floor air handler because power, 27.8 V? was going into the old transformer and there was not any reading coming out. Therefore, I was told to change the faulty transformer.I changed the transformer and installed the new one. Again 27.8 going into the transformer and something like 24-25 volts? coming out.The is a 5amp fuse in the air hander which I observed was ok. I reset the garage circuit breaker for the unit, but this did not help. The a/c electric box outside at the outside unit on the side of the house is live. Any help you may give me at this point would be appreciated until I can get an HVAC Professional here by the weekend. Thank YouThank You.
Expert:  Phil replied 11 months ago.

27.8 vac from the second floor transformer is fine and quite common, (that is because the primary voltage varies by 5 to 10% daily, and also the specification might be 230 volt transformer and a 240v primary transformer is not available....)

Some of the counter men at the wholesale houses are quite new to the business without any hands on experience.. those get paid about a quarter of what an experienced high quality tech gets... taking their advice will often get a person in trouble.

Regarding your comment:

"I changed the transformer and installed the new one. Again 27.8 going into the transformer and something like 24-25 volts? coming out."

That is now how we measure it, the one and only measurement that counts is **between** the two poles on the secondary side. no reading from either pole (in and out to use your terms) are valid to prove transformer output...

a reading from each pole to ground is used only to prove if a transformer reads below 10vac or so, that is quite rare. we replace in that case.. there is no IN and Out on the secondary side of the transformer, that is AC current it alternates 60 times a second.

The only IN reading that counts is between Line1 and Neutral on the PRIMARY side of the transformer, the only OUT reading that counts is between the two poles on secondary side of the transformer.

Regarding power to the outside unit, it is *not live unless you get 230v **between** line1 and Line2... readings from either line to ground showing 120v do not count.

Its time to locate the "finish and rate" link now, I will appreciate that, and in exchange will hold the question open for any follow up you need in the event the tech you called is not one the more fortunate ones in the top 20% of his field.

Customer: replied 11 months ago.
OK, I'll wait for the HVAC tech seeing that I was informed here that when I replaced my faulty transformer, I should be "Good to Go". Obviously I have a bigger problem than a faulty transformer that is causing me not to have power to my thermostat. Maybe it is a bad wire somewhere, but the thermostat is fine in itself. But thanks anyway.
Expert:  Phil replied 11 months ago.

Hello again, yes indeed it is a more complex issue, it will be interesting what the tech has to say, if you can get that to me verbatim I can assess the veracity for you.

Its time to locate the "finish and rate" link now, I will appreciate that since i work at my own expense here and loose money if you do not rate, in exchange will hold the question open for any follow up you need

Customer: replied 10 months ago.
Tech replaced a 50 amp breaker in the box and kept the transformer I installed. First thing he did was check the breaker in the box in the garage. Thank you anyway