Hello, I am experiencing problems with a Coleman D.E.S. 80 furnace. Was told by a repair person that the transformer was burned and that it had to be changed. I did just that and the new one burned within 30 sec with a spark and power outage in the house. The old transformer went bad quietly as far as I could tell. Both old and new transformers have broken primary (line) coils, i.e. infinite resistance. Load coils are OK. All the specs match: PRI 120VAC, SEC 24 VAC, 40VA. What could be the problem? Bad line contacts? Would greatly appreciate any suggestion. H.

dac122

Yes, I would look at bad contacts somewhere, and check your line side voltage?

harum

In case this matters, the transformer burned about 30 sec after the power was turned on, sooner than I could turn the thermostat on. When powered on, the transformer gave a humming sound and became hot after it burned. No visible signs of burning though on either wires or the transformer.

harum

Thanks! Appreciate your answer. Could you hint on why this is important or why would voltage suddenly change from a week ago? I would guess that the line side voltage is around 110V as it should be (or it has been for a long time when everything was working). What I suspect has happened is that the old transformer (15-20 years old) burned because of its age, and the new one burned because I made some contacts loose during replacement.

dac122

If your voltage is too high that might burn out the transformer. I've seen cases where that happened and then people discovered problems with their service. Its probably not the case but worth checking. A bad contact could do it. Some humming and heat is normal. It is possible you got a bad transformer.

wmproop

are you operating just a gas furnace only? or do you also have a heatpump?

on next try install a inline fuse,this could save another transformer,,,for about a buck or so extra

harum

yes, this is just a gas furnace. What is the appropriate spec for this kind of fuse for a pri110v/sec24v/40va transformer for the line circuit and for the load circuit? Is 40va maximum power for the primary coil or for the load?

KeepItSimpleStupid

A 3A blade automotive fuse on the secondary is fine.

harum

Thanks! What about the primary? I worry more about the primary coil as both transformers have them burned, not the secondary.

letmetellu

I would like to know about the new trans. How many wires did it have coming out of it. If it only had two, that is two primary and two secondary, then I think you have a problem with your line wires other than in the transformer, like you may have shorted out a wire against the cabinet some where.
If it had Four wires or more for the primary side it is possible that you used the wrong wire somewhere, Some of these transformers are a little confusing.

I think you need to do some checking with a meter to see what you voltage is going into the trans. and also do some ohm testing on all of the wires, testing from a wire to the cabinet.