Question about Ruud UBHC Air Conditioner

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Thermostats being replaced often

One customer has had 5 new stats on 2 different systems. What should I look for? Transformer or house voltage issues or grounding issues?

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I have repaired A/C commercial and in dustrial for 40+ years. ( ot of ten times replacing one item as a t-stat 5 times pretty much proves it is not the thermostat. The VA rating on your transformer has to ne able to carry the total load of your circritry.

Yes, I have als seen cases wher a intermitent short occurs in the thermostat cable that has up to 6 different wires in it.

Also "Heat anticipators" in all t-stats (Unless they are automatically controlled) need to be set properly.

You need a AMP reading meter with a Volatage and Resdistence Meter.


I had one job wher a painter had nailed through the t-stat wire inside the wall and caused a short between the Red (Hot wire) and the ground wire ( in this case it was the blue wire) Depending on the t-stat ... There ar up to eight terminals that need to be i9dentified and then properly wired.

Heat pumps use at least 5-6.

5 t-stats failing is gettin close to $500 of repairs if not more.

Posted on Feb 09, 2008


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I have no voltage to my thermostat

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Here's a picture of a typical low voltage transformer found in an A/C system:

If you replace the transformer, add a 5 amp fuse on one of the 24 volt leads......this will save you from having to replace another transformer in the event of future "short" in the low voltage circuit.
Contact me with any questions or concerns.

I hope you find this to be very helpful to you moving forward,

Please leave a rating. :-)f0b3aee.jpg

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2 Answers

The heat was working and then i turned it to cold and now the thermostat wont work and it is new cant get the heat or air to come on

Sounds like you had a direct short on the 24v side
of the transformer. Usually there is a inline fuse that
will blow so it protects the transformer..

Turn power off to unit
-Take doors off unit
-Look for a little fuse that may be located on the
control board where the thermostat wires go.

-Check the fuse, if it is blown then replace. However
you need to check the thermostat and the wires
that go to it to make sure they are not touching.

-If the fuse is not blown, then at the thermostat put
the fan to the on position. Go to the unit and turn the
power on, if the fans starts then things are at least
looking better. If the fan doesnt start, check to see
if the 110/220 fuse or breaker that you have isnt tripped

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