No power at thermostat

We just moved and I'm dealing with a 20 year old American Standard heat pump. Everything worked peachy for about 2 weeks and then then my Nest thermostat said it wasn't getting power. So I checked the voltage on Rh and sure enough no voltage. I went down to the air handler and pulled the fuse on the 24 volt transformer and it was blown, so I bought more odd ball fuses online and plug one in and still nothing. I opened the box only to find they have taken the factory fuse out of line and installed a 5A automotive fuse, and that fuse is not blown. So I get out my multimeter and check the voltage running to the transformer. It tested around 120V, but on the output side before the fuse no voltage. So I assume I have a bad transformer. The thing that gives me pause is that the sticker on the transformer says "Pri: 60Hz WHT-COM, DRM-200V, RED - 230V, SEC 24V, 75VA". This makes it sound to me like I should be getting 220V at the transformer primary? I have no idea what kind of failure could occur to have me 110V instead of 220V? Or it is the wrong transformer? Am I missing something here. The air handler is an American Standard TWE065E130A0. Is there anyone that can help me trouble shoot this?

 

Welcome to the forums.

Yes... that is a high voltage in transformer. You should measure 240vac on the input side to it which is probably red and white. If you don't have the high voltage there you'll need to go back to the service disconnect and test for it there.

 

No Pro, but if you were really getting 120VAC then I would think your output would be half of spec, not 0V. How did you test? Across the input connections? Or each side to ground?

I agree based on the specs it's HV, but before you leave the xformer, make sure you are checking correctly.

Btw...before someone else says it...common US voltages are 240 and 120...not 220 and 110. I can't remember the tolerances, but I think it's 5% +/-? Personally I've never seen that from a good utility, too many built in safeguards. More like 2V is common in my experience.

 

I tested relative to ground. When I tested across the top and bottom breakers it was 240V. And 240V going into the transformer. And transformer output relative the bottom breaker floated, but settled out at less than a volt. So it appears to the be transformer, unless there is something I'm missing. Is there anything I need to be careful of when buying a replacement? Other than 240V primary and 24V secondary. How important is the VA rating? I've seen anywhere from 40 to 75VA. The current unit is 75VA. Thanks.

 

The va rating is the physical size and needs to be matched.

When you test the output of a transformer...... it's across the 24vac output wires or terminals.... not referenced to anything on the input side.

Are you saying you had a fuse plugged into the transformer ?




Midwest HVAC Parts. Transformer - American Standard, Trane TRR01581

 

The Secondary wires are the Blue/Black and Red/Black wires coming out of the transformer that run to the control board. There is no measurable voltage between the two secondary wires at the control board terminals or that I can measure at the wire nuts immediately after the transformer. But there is 240V at the wire nut going into the transformer. Man that is an expensive transformer. I was hoping for like $30.

 

I'm saying there is a 5V fuse between the the red secondary wire and the control board.

 

Can you see why I'm confused.

 

Yeah, to clarify the fuse installed on the transformer kit was an odd twist and lock style cartridge fuse. I assume at some point the HVAC guy just didn't have the right fuse on him when it blew, so he cut out the original fuse, and installed a 5A yellow automotive fuse in it's place.

 

So the fuse appeared to be blown because it was never replaced and a new one was installed inside the box where I couldn't see it. The new fuse was not blown. It was a bait and switch.

 

So when you went to check the transformer it was just dead. That would be a power line related spike and just a normal failure and replacement.

If you have problems with the 5A control fuses blowing out.... that needs to be investigated.

 

Is there any downside to purchasing a transformed that is equivalent but not train branded? This seems like a much cheaper option.


http://www.supplyhouse.com/Honeywell...5VA-11016000-p

 

No problem.... that transformer will work just fine for your unit.

 

Thanks so much for the help. Ordered a new transformer, and hopefully we have heat again by tomorrow.

 

Back to the drawing board. I got the new transformer. Swapped it out, and still the same problem. So I guess it is further up steam and I missed something. I have three pairs of breakers where the main power comes into the air handler. There is 240V over the bottom pair, 240V over the middle pair, and no voltage between the top pair. But there is 240V if I measure the red wire on the top breaker relative to the black wire on the bottom breaker. Following the wires back it appear that this is where my power and common for the transformer are coming from (the top pair breaker). Should be be getting 240V measured top and bottom at this breaker? Does this imply my issue is back at the fuse box? Any help would be appreciated.

 

It would appear the problem is before it gets to your air handler. You'll need to go back to the main panel and check there.

 

Yup, figured that must be. Looks like a bad connection on the neutral wire at the breaker. Slammed it on and off a few times and it started working again. That was a lot of extra work for nothing. Guess, I'll have to check out the breaker when I have some daylight to flip off the main power. Thanks again.

 

The wire on the breaker may be white but it is not a neutral wire. Both wires are combined for 240v power. You may have to cut and retrim the wire if it's frayed.

Good move on turning the main off when working on those big breakers.