The HVAC transformer

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I discuss the low voltage transformer design for heating and air conditioning systems. The 24v transformer is used on most residential and commercial systems for thermostat power.
This video is part of the heating and cooling series of training videos made to accompany my websites: www.graycoolingman.com and www.grayfurnaceman.com to pass on what I have learned in many years of service and repair. If you have suggestions or comments they are welcome.
If you are a homeowner looking to repair your own appliance, understand that the voltages can be lethal, the fuels are highly flammable and high pressures are used. Know your limits.

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Опубликовано:

22 июл 2012

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Комментарии : 46

@PBS-nm1uu 6 лет назад

great demo please keep them coming thanks !!!!

@jb-ik8sj 7 лет назад

ok. that clears things up. thank you so much

@madcyborg 8 лет назад

Thank You for clarifying VA.. good video.

@grayfurnaceman 8 лет назад

Welcome GFM

@randymarsh6564 Год назад

Ok thanks. watched a bunch of videos but this helped me a lot. just little things like short to ground is something laymen like me can be ignorant to. Now i know. Thanks again.

@zakiization 10 лет назад

my air conditioner is under 220v and i want to use a thermostat Nest that work with 24v, so what should i do to get both wired without damage, thank you.

@geojor 9 лет назад

great, thanks...

@bencmoreno 11 лет назад

thank you...............

@brianellsworth4767 7 лет назад

I lost neutral in my house. Some outlets 175 volts and some had 50 volts ..All my 120 volt primary transformers fried .. Good tip on the inline fuse....They can save from replacing transformer twice ..or until you find the cause...although it would not have saved me from a power company fault.

@SombraLocs 9 лет назад

Where can you normally hook up a resettable fuse to troubleshoot a short? My Heat Pump doesn't have a fuse, but I want to install an inline fuse.

@grayfurnaceman 9 лет назад

SombraLocs On the R wire. GFM

@jkbrown5496 5 лет назад

So that's how they make a class 2 transformer current limited. They incorporate the fuse on the secondary side. Recently I dug into code for low voltage circuits, discovering the class 2 is considered inherently safe because of voltage and power, i.e., VA/wattage, design limitations and self limiting. (60VA for 12v, 96VA for 24v) A DC power supply can be designed that self limits the current, but I wondered how they did it with the transformers. Class 1 or 3 power supplies/circuits require inline fusing to limit overcurrent.

@SombraLocs 9 лет назад

Should I be reading 24 volts on both leads of the transformer coming out the 24 side? I am checking one leg to ground on the low voltage and reading 28 volts and I checked the other leg to ground and getting 0, but if i check both together i read 28 volts. will the common side of the transformer read 0 if i check it to ground?

@grayfurnaceman 9 лет назад

SombraLocs The transformer should read voltage across the 2 terminals. The transformer usually has one side connected to the chassis of the unit. Hope this helps. GFM

@arivaldiv4078 7 лет назад

Furnace man how many thermostats can one transformer power?????

@jb-ik8sj 7 лет назад

my unit is a 240v system. how do I wire the 240 tap with one wire? this system has no return and only a ground. I have one leg from L1 going to the 240v tap so my com is ground. and my 24v side only puts out 11 volts. so do I hook up the L1 leg and a leg from say T1 or T2 to the 240v tap? thanks. I'm stumped on this one

@grayfurnaceman 7 лет назад

The primary of the transformer is just wired to each side of the 240 volt power source. The use of a common does not mean it is ground. GFM

@EurekaRecycler 8 лет назад

I have a forced air gas furnace, the 4 wire transformer is dead. The output wire are colored. I went to the hardware store and they said a "24VAC door bell transformer would work. They told me the output is non polarized so the 2 wires that go to the control board are not polarized and it did not matter which wire goes where. Is this true? Thank you

@grayfurnaceman 8 лет назад

+EurekaRecycler Unless the bell transformer is 40 VA capacity, it will not work. Polarity is not a problem. GFM

@greatestcantor 9 лет назад

Thanks for the posting Did you ever hear of grounding the neutral on a on the output side for boilers?

@grayfurnaceman 9 лет назад

eel1452 If there are any unions between the boiler and the piping, it should be grounded. GFM

@greatestcantor 9 лет назад

I'm talking about taking one of the wires from the 24 volts and bonding it to the box thereby creating forced polarity?

@RalphsHeating 7 лет назад

Are the white and black on the HV side actually interchangeable? Or it the white some how tied to the body or if reversed will it not operate properly or safely? Thanks

@grayfurnaceman 7 лет назад

They are interchangeable. White is not connected to the chassis of the transformer. The only reason for the different colors is if there is a fuse integral to the transformer, it will be on the black side. GFM

@RalphsHeating 7 лет назад

Thanks Good to know.

@macster1457 7 лет назад

I have a transformer that I can wire either as 16v or 24v. how do i wire it for 24v?

@grayfurnaceman 7 лет назад

There should be a listing on the side of the transformer with color codes. One line will be common and the color code should tell which to use. GFM

@jeremymass2770 5 лет назад

I was swapping thermostats and I think I hit the blue to red wire. Would that smoke the transformer?

@grayfurnaceman 5 лет назад

I can't be sure as the color code is not always followed. Generally, if the blue is used as common it will blow the fuse in the transformer. GFM

@aol11 Год назад

I noticed a humming coming from my transformer when the furnace is off. It is warm to the touch even when the furnace has been off for a long time. Is this normal? This is happening just after 3 doors down our neighbor installed a Tesla charger. My duplex and the one next door keeps burning circuit boards in the water heater also would these be connected?

@grayfurnaceman Год назад

The transformer hum is common. I won't say it's normal, but unless it is is loud enough to be a problem, it's ok. I can't see where a vehicle charger could affect other locations. I have one in my garage and it has no effect on anything. GFM

@Pretzel114 10 лет назад

Hey GFM, Thanks for the video and explaining the Volt Amps that helped out a lot. I wanted to ask, if you could replace a 40VA with a 60VA if that is all you had in your truck at the time or if it needs to be the exact match? Thanks for your help.

@grayfurnaceman 10 лет назад

Bigger will work, smaller may not. GFM

@Pretzel114 10 лет назад

grayfurnaceman Thanks for the response I appreciate it. Take care.

@bardinkurtis 8 лет назад

on heat pumps what are the 2 24 volt wires hooked up to usually?

@grayfurnaceman 8 лет назад

+kurtis bardic Lots of stuff. Contactor, reversing valve, defrost board are some places. This probably does not answer your question, but there is no "usually". I have been meaning to make up some wiring diagrams that are commonly used in heat pumps, but have not completed it yet. GFM

@bardinkurtis 8 лет назад

+grayfurnaceman that actually helps a lot your the man! im a instant subscriber😆 hopefully you can get to that soon

@jsp146 9 лет назад

I really enjoy your videos, they are on point and very informative. I teach an HVAC Class at a state prison in Texas. Under the circumstances I cannot access the web in my class, but I would really like to show your videos to my classes. However, I am able to take a flash drive in with me. Would you be willing to authorize the downloading and use of your videos as training aids in my class? Thank you very much for your consideration!

@grayfurnaceman 9 лет назад

Shane Pritchard No problem. Go for it. Tell them its a great profession that needs good people. GFM

@achilles1373 3 месяца назад

I dont have an inline fuse on my system and we had the powerline running the the house from the pole catch on fire. Also something chewed the 24v wire going to the ac unit. I replaced the wire, checked all fuses, checked the contactor and capacitor and all is well. I am not getting 24v at the contactor. I believe I have a bad transformer

@grayfurnaceman 3 месяца назад

Check voltage coming into the transformer. Then check voltage coming out. Voltage in, no voltage out, transformer failed. GFM

@mts7274 Год назад

7:25 ...actually, transformers WILL cause failure on the furnace side in a split system when they burn out. A sticking contactor on the outside AC unit can contribute to this, all while the outside unit will still operate.

@grayfurnaceman Год назад

A sticking contactor is what is causing the failure because the contactor is overdrawing from a failing armature. Overdraw of the circuit is not the fault of the transformer. The point is the transformer seldom fails. It is damaged by an overdraw of the circuit. When you find a failed transformer, you need to look farther to determine if there is something that caused it to fail. GFM