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.

Thanks a lot. The new transformer blew the lights out in the house 30 sec after I powered it on. The load side was connected at that time. Do you mean that there is a chance that it will not burn if I do not connect the load side, and I will have time to read the voltage?

Terminals look like figure 2 on this page:

http://onlinecatalog.digikey.com/WebProject.asp?BookCode=dus08flx&SectionIndex=0&PageIndex=361#

A transformer of this size is incapable of blowing the lights. But the covers touching the primary side might cause some sparks if there are mechanical problems.

Inspect all wiring. Make sure the transformer will be secure.

You need to tape the ends of the wires that would go on the secondary and put your meter safely where the primary would connect and measure the voltage before permanently connecting the transformer.

Wires should pass through grommets and wires should not be pinched.

Thanks for the important info and the image, H.

Still, cannot get the furnace running. I have measured the input voltage from the line - it is 122V. I installed two inline fuses. For the primary line the fuse rating is 0.5A, 250V. For the secondary: 3A, 32V (it's a car fuse). As soon as I turn the power on, the primary 0.5A fuse burns, the secondary fuse is OK. I am hesitant to install a stronger fuse as the power given to the primary line of transformer is already more than 40W (0.5A*122V). What else may be wrong? How come the power is so high? Appreciate any suggestion. H.

I would start looking at something else than the transformer. The transformer problem is just the end product of the problem.

First make sure the line and the neutral are not reversed both in the furnace junction box and in the house panel. Do not trust color of wires (Black/white) but check with a meter to be positive it is correct.

If that is OK then disconnect all other 120 volt circuits that feed off the incomming line in the furnace except the transformer. Now activate the unit and see if the fuse still blows.

If the transformer does not pop the fuse then the problem lies elseware in the furnace.

Over the years I have seen circuit boards/motors both vent assist and blower/controls and relays cause similar problems and you will have to fish through them all till you find the culpret.

I am sure by now you have disconnected all the thermostat wires at the thermostat itself and also disconnected all low voltage wires at the furnace to eliminate a low voltage wiring/thermostat problem. If not you may try that but when you said the short took out the circuit breaker for the furnace that would somewhat eliminate low voltage wiring since a short of that magnitude would have burnt that low voltage wiring to a crisp by now.

Something is shorting to ground in that unit or circuit. These are the kind of problems I loved when I had the shop especially after 2 or 3 other companys could not figure it out. Over the years I have camped out at customers homes drinking coffee, smoking cigs and testing circuits till I finally figured it out.

You will figure it out it is just a matter of time. Just isloate one component at a time.

I used to carry mini circuit breakers used in the electronics industry so I could reset them and not keep blowing fuses.

You don't need fuses in both the primary and secondary. There is an inrush current when the transformer is connected. The primary fuse MUST be of the slo-blow or time delay fuse if you decide to use one.

Get yourself two 211-2 lamps from an auto store. These draw about 1A at 12V. Put them in series. And put the series connected lamps in SERIES with the secondary.

This means that if connected in this fashion and connected to your furnace. If something was shorted in the furnace, the WORST that can happen is the bulb will light.

BUT, remember that a bulb is worse than a transformer because it's inrush current is 10x times the normal draw.

Trust me and connect the primary directly like it supposed to be connected, no fuse and connect the secondary to the two bulbs in series. Any bulb that draws from 1-1.5 at 12V will be fine. 1*24 = 24 W, 1.5*24=36 W.

Complete the circuit. Show that this works. It will.

Now cut the secondary circuit and connect the two ends where the secondary would go in the furnace. You just proved that the transformer can light the bulb with no problems so nothing bad will happen if something in the furnace is shorted.

If something on the series connected secondary+2 bulbs+furnace shorts, the bulb lights.

Start disconnecting things UNTIL the light goes out or severely dims.

The bulb is a current dependent resistor. Since it's resistance when off is about 10X lower than when on, at low currents it's essentially a short circuit.

IT WILL WORK!

hvac1000 and KeepItSimpleStupid: Really appreciate your input. I am in business again as there are several more things to check and work on. H.

I have done some checks with two 12V bulbs in series (as suggested by an expert in one of the previous answers) and found couple of things I do not know what to do with, but think it might be important. I first connected all the elements in the secondary line in the following order: Transformer - Furnace - Lamps - 3A car fuse - Transformer. The bulbs were brightly lit. When connected in different order, Transformer - Lamps - Furnace - 3A car fuse - Transformer, the bulbs were out... because the 3A fuse burns right away. I do not know if this finding is of any use. When reconnected in the first order with a new fuse the bulbs are on again. I then disconnected the thermostats (two, connected in parallel) and the bulbs went out - the shortage eliminated? I am not sure if replacing thermostats will make it work, would it help? H.

The order thing is a fluke. Bet the wires were touching at the bulb.

Do the same thing, but disconnect the t-stats one at a time at the tstat to eliminate a wiring issue between the furnace and the stat.

Well, I guess the problem has been located. As it was suggested in many responses, it turned out that the two-wire line of one of the thermostats was cut by and in contact with a sharp edge of the furnace's metal cabinet, hence the short circuit. Simply disconnecting that thermostat's wiring got the furnace running again. Thanks for all the help. H.

Glad you got it going

Well, I guess the problem has been located. As it was suggested in many responses, it turned out that the two-wire line of one of the thermostats was cut by and in contact with a sharp edge of the furnace's metal cabinet, hence the short circuit. Simply disconnecting that thermostat's wiring got the furnace running again. Thanks for all the help. H.

I do believe I told you this a week ago lol. Glad you got it up and running.