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.

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

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.

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.

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.

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

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?

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

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

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.

Appreciate your suggestions! Well, the new transformer (which is a copy of the old one) has four wires total: two for line, and two for load. Please see the image - this is exactly my replacement transformer. Not much space for experiments! Yes, this is one of my worries that one of the line wires arches against the metal wall. Thanks again.

How do I explain this. Power is conserved in a power transformer: Pp=Ps or VpIp=VsIs, so in reality it doesn't make any difference which one is fused. Generally a fuse size of about 1.5 * the expected current is good enough.
So 40 Watts / 24 is about 2 and the closest fuse is 3 Amp, so that's how we come up with a value of 3. Not a critical calculation.

The primary would be 40 W / 120 V * 1.5 or about 0.5 A. But now things start getting messy with the wiring. It's not low voltage anymore and automotive fuses don't work anymore because they are rated for 32 V.

If the secondary is fused it will protect against an overload on the secondary, but not things like 240 V applied to a 12 V xformer, nor shorts of the primary to ground.

The secondary fuse is a good compromise. It's easy, simple and standard practice. The 3A blade fuse is also less likely to nuscence trip.

I would focus your search on the secondary side first. A short on the secondary side could be causing the primary to draw more amps then it is rated to withstand.

Use the fully insulated female terminals and you will be fine.

The primary going against the side isn't a concern, the main fuse/circuit breaker will blow.

Thanks for all the info! Is this true that the current is higher in the secondary coil? (Ipr*Vpr=Iscnd*Vscnd=40va; Vpr>Vscnd -> Iscnd>Ipr ) If this is, then the secondary would blow first, if both primary and secondary can tolerate comparable currents. I have the primary (line) side burned.

Could you direct to a picture of completely isolated female terminals? I do not know what this is. Thanks.

What's the voltage reading going to the transformer(load side).

I understand what you are saying and when my old electrical instructor talked about it, it didn't exactly make sense until I actually thought about it. I have had a few systems in a new building that blew the primary sides and when I ohmed out the control lines (secondary side) I found a direct short across the 18-6. Replaced the wire and no further issues.

I have not measured it yet as I have to know first what I am doing and for what. I assume that it is 110V as it should be (has been before). Also, a general question: is there a problem in connecting two thermostats to one gas furnace? Appreciate your help!

You need to check the voltage line side. You can do this with out the load side wires instaled(low voltage). If you have your 115 or 123 volts on the line side and 24 volts on the load side with out the low voltage wires on the transformer the shorts on the load side. Make sure your furnace is polarity corect for line voltage.