AC control questions

[Snag]

The furnace/AC system at our house in Memphis is going flaky . It has burned out the primary winding on 3 24V transformers ... The control board has been replaced , it doesn't blow the transformer when calling for heat , just AC . Our son that lives there has left the cover over the condenser relay/capacitor off after a start cap replacement , I'm wondering if rust in the relay might be the cause of the problem . What really puzzles me is why is it burning out the primary winding ? The fuse in the board is intact , the last transformer lasted about 20-30 seconds when we turned the AC on . Having already spent over a hundred bucks on parts already , I'm not inclined to spend more until I can figure this out .

[hubops]

Just a quick google search - links below seem to suggest shorted wires somewhere < ? >

John T.

[Snag]

I should have mentioned that we did find some bare/shorted wires and repaired them . The son is/was supposed to do a check to be sure we corrected the short , but I doubt he actually did what I asked . This is the same son who burned up the trans in my truck because it was too much trouble to monitor the oil level - after he was told that the rear seal was leaking a little and to watch it . That cost me a motor too , caused by a failed oil pressure sensor that was damaged during the trans swap .

[Marilyn Manson]

Well, it seems to me that you have found the root cause of the problem.

Maybe you should throw some dollars towards fixing your son. ;-)

[Dean Hoffman]

Maybe you should unhook the wires right at the relay coil. See what happens when you turn the AC on. Can you hook the relay to 24 volts from another source? Fuse it first.

[Snag]

After repairing the bare wires and replacing the 2nd transformer I checked voltage at the AC condenser unit before hooking the wires to the relay . 24V on the nose . I don't really think I have another short in the control wire , I suspect excessive current draw on the relay because there is rust in there preventing it from completing the magnetic circuit . But I've still never seen the primary winding blow like these did . I'm riding down tomorrow (on my Harley) to help install a window unit I bought and had delivered . Gotta make sure it's done right ... he's too likely to take short cuts .

[Dean Hoffman]

There are some specs here of a 24 VAC relay for what good it does . 8.7 ohms coil resistance. it's supposed to draw 375mA. There are some inline automotive type fuses that might help protect things upstream.

[Dean Hoffman]

Amazon has relays for sale. I didn't look at Ebay

[Snag]

I still have a cash account at a Memphis wholesale heating and air supply house from my handyman days .

[Snag]

WE installed a window unit yesterday ... I'm not planning on going back up to that attic until the weather cools . And then probably only to insure the furnace is going to work .

This is exactly what I suspected was happening . When it cools off and I get back to Memphis (I rode the Harley 350 miles round trip down and back yesterday) to look into it more I think the first thing - after testing current draw - after replacing the burned out transformer will be to replace that condenser unit relay . And then feed the old one to my son for not replacing the cover over the 'lectrics on the condenser unit after he replaced the start cap .

[bud--]

If the transformer connects to the thermostat it is "class 2", which is limited power. I never tired shorting one, but I can't imagine only lasting 30 seconds if shorted.

I haven't seen an HVAC transformer larger than 40 VA. At 40VA that is

1/3 A on the primary (plus something for inefficiency). Would be more for a short, but it is limited power. Doesn't seem like much to burn out the primary. 40VA - at 24V that would supply 1.7A to equipment (more if shorted). Measure current from transformer, particularly when AC is started (temporarily)? If you have a clamp-on ammeter, if you run the wire around and through the window 10 times it will read 10 times the current. You could get some similar ideas with low value fuses.

Measure the transformer voltage when you start the AC?

If the compressor relay closes there is a continuous magnetic path and the relay current is limited by the inductance. If the relay can't close there is an air gap in the magnetic path and the inductance will be less

- higher coil current. (AC relays close faster because of high coil current until closed.)

[Clare Snyder]

Sounds like Snag found something he can't fix or even jury-rig - It's called "stupid". If it is fixable at all it needs to be started at about age 4 - - - - -

[Snag]

No Clare , it's called "lazy" . My wife says he's so much like her father it's scary . Thing is he didn't learn that from us , we both worked our asses off - often working 2 jobs to make ends meet - while the kids were growing up .