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Old 10-19-2008, 06:51 PM   #1
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Central Ac transformer question

I have a rheem ac system from 1989 no problems with it only issue was with thermostat i have a ct8775 digital round thermostat thermostat works fine only problem is it was reading wrong inside temperature it was saying that it was 5 degrees warmer in house then it really was but then at times it reads fine thermostat is on inside wall and not near windows. for curiosity called honeywell and they say that this can happen occasionally but should correct itself but then the guy told me to check the voltage going to thermostat because he said i should have no more then 28 volts so i checked the voltage between r and g and got 38 volts! he said this is not normal he said i have problem with transformer. Today i went up to the attic to look at transformer checked voltage from 24 volt terminals in attic and got 38 volts. It also was making a loud humming sound louder then i remember hearing last time i was up there.

i got the numbers off the transformer

Essex 291-421106-10701
Pri 208V 50/60HZ Red-Blu
240V 50/60 HZ Red-Blk
Sec 24V 40 VA Yel- Yel

it is using the red and black wiring so it is wired for 240 i already cut out the old transformer because i am assuming it is bad and also i notice that it is Original from 1989.

I already have the 240 volt wiring figured out but i am alittle confused about the 24 volt wiring because in the process of removing it all the 24 volt wiring pulled out from wire nuts but here is what i do know

Green wire from thermostat going to blue wire from white rodgers blower relay

White wire from thermostat going to other blue wire from blower relay i belive that one of the 24 volt wires from transformer also hooked up to this.

I have a Red and White from Compressor remaining and a Red wire from thermostat remaining where do these wires go do i pigtail the two reds togather and does the other 24 volt wire go there? WHat do i do with white from condensor also. Any help is greatly appreciated.

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Old 10-19-2008, 09:02 PM   #2
Join Date: Oct 2008
Posts: 3

Central Ac transformer question

How many wires are coming from the stat? Should be 4, maybe 5,or 6.Does it have batteries? Is it 1 or 2 stage heat? The yellow wires from the transformer should go 1) to R terminal on the stat, 2) TO COMMON ON THE FURNACE with one wire from the condenser going to common also.WE NEED TO KNOW WHAT TERMINALS THESE COLORED WIRES ARE COMING FROM ON THE STAT. this always varies according to the type of wire.

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Old 10-19-2008, 09:11 PM   #3
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Posts: 17

Central Ac transformer question

This is only Central AC No Heat i have baseboard heating on seperate thermostat

I have a RED White and Green from thermostat. I assume i am suppose to pretend the white is Yellow?

Is it possible for a transformer to fail in the open position and let more voltage through?

So one more time can you tell me where the 3 wires from the thermostat go and the two from the condensor go?
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Old 10-20-2008, 11:30 AM   #4
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Posts: 17

Central Ac transformer question

Went to supply house today and explained my problem he said that it is not normal for a transformer to put out more voltage he said that i may have too much voltage coming into house. I checked and it was about 255 to 258 so I called power company they are going to come check out why i have high voltage. I already bought new transformer so i am going to install it the one that was in there i cut all the wires on it when i took it out. I already hooked up the 240 power to the transformer i used the orange and white. On the 24 volt wires i checked and am getting 32 volts. I just need a hand with the 24 volt wiring there is a blue and a yellow coming out from the transformer. I read somewhere that it is a good idea to put in an in line fuse 5 amp so i don't burn up transformer. I put it on the blue coming out from transformer and i hooked the in line fuse to the blue wire from the white rodgers blower relay and also pigtailed with that is the yellow from the thermostat that is how i believe it was before Next the yellow coming out from the transformer i pigtailed with the red from condensor and red from thermostat. I don't know what to do with the white from condensor so i didn't hook it up yet. Just for curiosity i wanted to see if the air handler would come on so i put the thermostat on and once i did that the inline fuse i installed blew right away what did i do wrong? ANy help is greatly appreciated
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Old 10-20-2008, 08:21 PM   #5
Join Date: Mar 2008
Posts: 16

Central Ac transformer question

"Make Sure power is off first" If it is a cooling, only unit you should have a red and white wire coming from the outside unit, the indoor unit will have a fan relay that has a green wire coming from the t-stats g-terminal, on the other side of the relay it will be the common side and should tie to the white going outside, then tie it to the yellow wire from the transformer. The red going outside should tie to the wire coming from the t-stat that is on the y-terminal. Then the blue wire that you have the fuse on should tie to the wire that goes to the red terminal on the t-stat. Well as confusing as this may sound I hope it helps, the voltage is too high and needs to be lowered, I am suprised you are not blowing a lot of light bulbs. good luck!
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