DIY Home Improvement, Remodeling & Repair Forum > Appliances > HVAC > Cant figure this out! Not getting 24v at the board or thermostat




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Old 03-05-2013, 09:10 PM  
jayrod
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Default Cant figure this out! Not getting 24v at the board or thermostat

Its a Carrier split unit. Had the heater on yesterday and at some point when I wasnt around it turned off on its on. Its completely dead at the thermostat, and on the circuit board it doesnt get 24 volts either. So I tested voltage going into the transformer and it has 120 volts on all 3 wires going in but it only has .1 volts on the 24v side. Well thats not right. We have a 2 story house and both units have the exact same transformer so I measured the transformer on the working unit to be sure Im dong it right and it measures 120v on all 3 coming in, and about 27 volts going out. So I go and buy a new transformer, wire it up, and still no 24 volts! Did a resistance check, all wires have some resistance. So now Im stumped. If it has voltage coming in, how is it not coming out!? What should I check next?



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Old 03-05-2013, 09:44 PM  
nealtw
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Switch transformers from the other unit, maybe you got a bad one.



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Old 03-06-2013, 09:58 AM  
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Your on the right track but, nealtw has the most probable answer. What are the odds?
Unfortunately, most of the time, electronic parts are not refundable as the seller suspects that you did something wrong to burn up the component.

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Old 03-06-2013, 10:43 AM  
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Thanks guys. I have switched transformers this morning. I took the new one I just bought that didnt fix my downstairs unit, and installed it on my working upstairs unit. It didnt work! Same thing, power coming in, but no 24v coming out. So I took it out and put it back like it was and it works still so all I know is I did get a bad transformer or either something shorted it out right when I hook it up. but it doesnt smell or hum or anything. Oe thing I did notice is the working uptairs unit only requires the common and the 240v wires of the transformer to be hooked up while the downstairs unit requires the common, the 208v and the 240v wires to be hooked up. (Common goes to XFORMT1, 208v goes to T2, and 240v goes to T3) But thats how the old transformer was wired so I wired the new one the same. I just didnt think it was very likely to get a bad Transformer.

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Old 03-06-2013, 12:21 PM  
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Wait, I believe I should have been measuring across the terminals to get 240v or 208v. I aint geting either when I measure across. I think there must be a short or a safety switch keeping the 240v from coming in. The contactor outside has 240 going in and 240 going out when I manually press it in. T2 and L2 have no voltage on the contactor.

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Old 03-06-2013, 12:36 PM  
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Standing by, with rapt attention, to see how this resolves ....

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Old 03-06-2013, 12:40 PM  
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Ok, now you've got me confused. You originally said that you have a 120V primary and 24V secondary transformer. Now your saying it's 240V or 208V. Did you buy the wrong transformer and smoke it when you hooked it up to a higher voltage?
T2 & L2 are opposite sides of the contactor but, are the same leg.
Sounds like you blew a fuse on the L2 phase.

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Old 03-06-2013, 12:45 PM  
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I dont see any fuses on the outside part of the unit. Its the right transfomer, I just didnt undersand it. Its a 240v setup. It says 240vac on the little circuit board inside the handler.

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Old 03-06-2013, 12:46 PM  
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Should T2 and/or L2 have any voltage when its not turned on?

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Old 03-06-2013, 05:00 PM  
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is this a heat pump unit?
L1 & L2 is LINE power, T1 & T2 is the LOAD.
On a split system, the air handler/furnace components would be 120V and the compressor unit outside would be 240V.
Where exactly are you working (on the 120V or 240V side of the split system)?
If your working outside on the compressor unit where I would expect to find a contactor, then the fuses would be in a service disconnect box. Follow the electrical lines from the compressor unit to the side of the house where you will find a metal box that houses the fuses.
You may have to remove a piece of cardboard to see the fuses (make sure you turn the breaker off to the unit before digging into the box).



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