NO low voltage?


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Old 07-28-11, 08:05 AM
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NO low voltage?

The thermostat isn't getting 24 volts. I know the fan and compressor on the unit is good because I pressed the contactor in and the outside unit started up and my Amp meter jumped. The air handler under the house isn't getting any voltage at the thermostat wires. I only have 13.5 volts at the 5 Amp Fuse. The fuse looked good but in my infinite wisdom I didn't Ohm it out ! There is no voltage at the 4 thermostat wires. There are 4 wires coming off the transformer. Purple labeld Primary is getting 60 volts, Orange labeled COM or common is getting 0 Volts, Blue is spliced in with a green ground wire, and Red labeled secondary is getting 13.5 volts. I'm thinking transformer, but I'd like to be sure before I buy one. Can anyone help me out here?

Outside unit is a 2 year old Payne Heat pump. Inside unit/air handler is a Tempstar I believe. Model number on the air handler is NFCP2400C2.
 

Last edited by hvactechfw; 07-28-11 at 03:21 PM. Reason: moved posts to a thread of its own.
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Old 07-28-11, 09:41 AM
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What kind of meter are you using? If digital, the 60V is most likely phantom voltage. Also...are you reading between Purple and Org or Purple to some other point?

Unlikely you would only get 60V on a 120V circuit....in fact I'm not sure how you could without adding components.

Disconnect the 120V supply lines and read across the black and white......should be 120V +/- 5%. If not...you need to trace that line back and make sure all breakers and fuses are ok. If so, its prob the transformer, assuming connections are good.
 
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Old 07-28-11, 09:47 AM
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Thanks for the response. I'm definitely using a digital meter. I read Purple to ground and came up with 60 volts. So you say if I read Purple to orange, which is primary to common, I should get 120V? Correct?
 
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Old 07-28-11, 09:54 AM
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If it's on the Primary side of the xfmr yes. That's why I suggested disconnecting the supply and measuring directly (normally white and black if its connected directly to house wiring).

Not an HVAC Pro...I can do things when they are right in front of me...but don't have the HVAC stuff stored in my brain.

I learned a long time ago in the Navy...the more you can disconnect, the easier it often is to find the actual problem.
 
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Old 07-28-11, 10:51 AM
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I hear you on that. This unit is 220 and I read both legs coming in and got 240, so I think i'm good there. I'm willing to bet the transformer has taken a crap on me. Thanks for clarifying for me!
 
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Old 07-28-11, 03:24 PM
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When replacing the transformer add an inline fuse (no more than 5 amp) to the secondary side of the circuit. This will keep the transformer from blowing again. Fuses are much cheaper than transformers.
 
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Old 07-28-11, 04:24 PM
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shut the furnace off....remove the wires on the primary and secondary of the TR,and ring each pair out not looking for a certain OHM reading... just which shows open or continuity
 
 

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