Ask Me Help Desk - Repeated 24V furnace transformer failures?

Repeated 24V furnace transformer failures?

Twice in two years, the 24V transformer in my furnace has failed. Both times it happened was after we had been away a couple of days and went to switch the heat back on at the thermostat switch. Our house is about 20 years old but this has just started happening the last two years.

I have a heatlhy 121V present at the input side of the transformer but nothing at the output that is supposed to be 24V. These are the same symptoms as 2 years ago.

What could be making my transformers blow like that. :confused: Is there a heavier duty one available? How many amps does this transformer have to handle?

Thanks,

-Jeff

How many amps does this transformer have to handle?
20 VA for a heat only system (simple)
40 VA standard today for a modern system

Next time you replace the transformer get a small fuse holder and put it inline then the fuse will blow instead of a expensive transformer.

Quote:

Originally Posted by hvac1000

How many amps does this transformer have to handle?
20 VA for a heat only system (simple)
40 VA standard today for a modern system

Next time you replace the transformer get a small fuse holder and put it inline then the fuse will blow instead of a expensive transformer.

Thanks, it's a heat only system so you are saying 20 amps? Wow. I'm not sure what "VA" means. Should the in-line fuse go on the 24V side? Thanks again.

That is Volt Amps. If you want the heavy duty one get the 40VA unit. When you ask at the supply house they will know.
The 24 volt side will work for the fuse. You will need to do a amp draw check to be accurate on the fuse size but a 2 amp one should do.

The fuse is only there to protect so the transformer does not burn up again.

Quote:

Originally Posted by hvac1000

That is Volt Amps. If you want the heavy duty one get the 40VA unit. When you ask at the supply house they will know.
The 24 volt side will work for the fuse. You will need to do a amp draw check to be accurate on the fuse size but a 2 amp one should do.

The fuse is only there to protect so the transformer does not burn up again.

Thanks, I appreciate the advice. Only 2 amps? I've got some automotive in-line fuse holders and fuses in my shop, I suppose that would work? Do you have a suggested source for the transformer? I'd prefer to buy locally to avoid waiting for one ordered on-line. The local Lowe's has a 24V one intended for door bells, would that work? I don't know the VA rating though.

No door bell transformer. Auto fuse holder will work.

Quote:

Originally Posted by hvac1000

No door bell transformer. Auto fuse holder will work.

Would this transformer work? It's rated at 2A but I can't find a VA rating for it:

RadioShack.com - Cables, Parts & Connectors: Component parts: Relays & transformers: 25.2V CT 2.0A Heavy-Duty Chassis-Mount Transformer with Lead

Thanks again!

Give it a whirl. It does have a 90 day warranty just in case you smoke it again.

OK, one last question and then I will go quietly away!

I picked up the Radio Shack transformer referenced above at lunch just now. It has two primary wires, both black, but I don't think it matters which way it's wired. The secondary side has three wires however. :confused: Two yellows and a black. The secondary side is embossed 12V - 0 - 12V to match the yellow-black-yellow wire colors. I assume that I need to wire both yellow wires together to get 24V and the black is common?

Is that correct? Thanks again!

I know nothing about that transformer. Call Radio Shack or visit there web site to see if they have a drawing of what to do with it. Sorry

OK thanks for the quick reply. The dweeb in the store had no idea.

The BEST choice would be a 40 VA "Energy Limiting Transformer" with a 120 V primary.
The BE2G on this page. Use without a fuse. A short circuit on the secondary (24 VAC) will not blow these transformers.

The second choice is the fuse. Probably a 2.5 to 3A fuse on the secondary.

In all likeliness you have some worn insulation somewhere in the thermostat wiring. Vibration, temperature will cause it to fail. The wire may have been pulled too hard during installation. A really tough problem to solve unless it stops being intermittant.

There could be an insulation failure in the gas valve too. It's really hard to guess and really hard to find.

If the tstat wire can easily be replaced, I'd consider doing that. Inside and out.
First do an inspection of what you can see.

Do you have the old blown transformer?? By a simple ohms test you can determine which voltage side caused it to burn out... which will guide you to really what the problem may be..?

Quote:

Originally Posted by tsa7man

Do you have the old blown transformer ??? By a simple ohms test you can determine which voltage side caused it to burn out...which will guide you to really what the problem may be..???

The old transformer is out and the new one from Radio Shack is in - let there be heat! I don't know how much resistance there should be across the secondary side but I measured only one or two ohms on the old one. There was no output from the secondary side of the old transformer with the primary wired to a temporary 120V pig tail on my bench.

Turns out the Radio Shack transformer is center tapped on the secondary side so in order to get 24V out of it one has to use the outer two yellow wires leaving the center black one unused. Works like a charm so far. I added a 3A fuse on the secondary side and plugged the furnace into a single outlet heavy duty appliance surge suppressor. Hopefully this is the last time I have to mess with it.

Surges generally damage the primary. The resistances of the windings are low, Definitely less than 200 ohms. I should have been paying attenion. I could have told you about the center-tapped thing. There are some surges associated with the comtrol transformers due to the inductive nature of the relays.

It never hurts to put a wired surge suppressor on the heating system because of the expensive control boards that are sometimes involved.

Quote:

Originally Posted by hvac1000

The 24 volt side will work for the fuse. You will need to do a amp draw check to be accurate on the fuse size but a 2 amp one should do.

The fuse is only there to protect so the transformer does not burn up again.

I'm a little late on this thread, sorry.

Let me ask you this. I also suffered the Revenge of the Chipmunks on my compressor wiring and had to replace the wire and the transformer. I am, of course, assuming that it was the bare wires that caused the transformer to blow, but when I put the new transformer in I put an inline fuse in the primary side since it was the primary side that was reading infinite resistance after the failure, there was still continuity on the 24volt side of the transformer. I used a 1.5 amp MDL type (slow blow) fuse on the primary side, although it should only draw about 300 milliamps once the coil is saturated. The reason I use a larger fuse is because the resistance on the primary side of the transformer is 20 ohms and at 120 volts this gives a potential current inrush of 6 amps. I just used "Kentucky windage" to come up with 1.5 amps.

My questions are:

(1) Should I use a smaller fuse on the primary?

(2) Should I also fuse the 24 volt side? With .75 ohms on the secondary at 24 volts the potential is there for an inrush of 32 amps. Would that blow a 2 amp MDL type fuse?

(3) Who has a good AC ammeter for testing these low current draws. My multimeter only does 300 mA DC and my AC ammeter is 600 amps +/- 2%, which is +/- 12 amps. I would buy one if it is not too awfully expensive.

(3) How warm should the tranformer be when is is just sitting there not juicing the contactors? It is warm just sitting there with power to the primary and warms up a bit more when actually working. It never gets really hot, but if after it supplying the contactors for a while, you try to hold on to it for more that just 10-15 seconds or so it becomes uncomfortable to hold on to it for much longer.

Thanks in advance for your reply.

Sounds like you have a 12 volt stepdown transformer,, that's prob. As high as it will go,, if so it won`t work

You said you put a fuse on the primary yet you want to put a fuse on the primary? I'm confused. The secondary fused at about 1.5x the currentin the secondary is all you need. 40 VA/24 * 1.5 Amps which is about 3. So, go for 3A.

No need to fuse both. Primary fuses may blow more easily and this would be more sensitive to surges. When playing and troubleshooting 24 VAC is safer.

(3). Your reading the specs wrong. It's 2% of the reading. On hall effect probes there is a residual that needs zeroing, but on coil probes no.

(4) Usually they are not too warm, but if they are energy limiting they would be. That's what should be on a HVAC system. You can short them to death and they get hot, but don't die. It is important to heatsink the transformer for reliability reasons.

It also makes a lot of sense to install a wired in surge supressor for the furnace. That control board is expensive.

Quote:

Originally Posted by KeepItSimpleStupid

You said you put a fuse on the primary yet you want to put a fuse on the primary? I'm confused.

Yes, I fused the primary since it was the primary that blew. But I put a 1.5 amp fuse in the primary and was concerned that 1.5 amps was too big and would I be better off with a smaller fuse, say a 1 amp or a 0.5 amp fuse.

Quote:

The secondary fused at about 1.5x the currentin the secondary is all you need. 40 VA/24 * 1.5 Amps which is about 3. So, go for 3A.

No need to fuse both. Primary fuses may blow more easily and this would be more sensitive to surges. When playing and troubleshooting 24 VAC is safer.

Simple matter to just switch the wires around so the fuse is on the 24 volt side and put in a bigger fuse.

Quote:

(3). Your reading the specs wrong. It's 2% of the reading. On hall effect probes there is a residual that needs zeroing, but on coil probes no.

Ahhhh, that does make sense. Actually, it's not MY ammeter but belongs to the company and I only borrow it occasionally to check the odd thing now and then. I was a bit confused by the +/- 2% of 600 amps since I do use it to check 240 volt 3-phase draws and it is usually spot on to the 0.1 amp on those circuits.

Quote:

(4) Usually they are not too warm, but if they are energy limiting they would be. That's what should be on a HVAC system. You can short them to death and they get hot, but don't die. It is important to heatsink the transformer for reliability reasons.

It also makes a lot of sense to install a wired in surge supressor for the furnace. That control board is expensive.

It came from the HVAC supplier so it SHOULD be whatever is correct for an HVAC system. But it did not say energy limiting or anything like that on the box as I recall, and I think I threw the box away.

The original location of the transformer was inside of the electrical box in the furnace where it got no air at all. The new transformer would not fit back into that location and have the cover go back on the box because it has spade connectors on it and the original just had wires hanging out of it. So I mounted it on the outside of the electrical box, using insulated connectors of course. Now it is sticking out in the air and is bolted to the metal electrical box, so it simply must be getting better cooling than the original transformer in it's original location.

I like the surge suppressor idea too. Any suggestions as to which one specifically? Is this something that the friendly local HVAC supply house would sell?

Thanks for the reply. I'm probably doing the overkill thing at this point since I did rewire the compressor and am going to trap the chipmunks and dispatch them to the hereafter. I just want to be covered in case Alvin and his homies come back for another trip to the buffet for an extra helping of insulation. I am seriously thinking conduit for the wire, we are really up to our a$$es in chipmunks here, it might not be possible to keep them away. They get in the house and stuff in spite of putting hardware cloth everywhere you would think they could squeeze through. They are cute and all, but enough is enough.

Quote:

Originally Posted by KeepItSimpleStupid

The secondary fused at about 1.5x the currentin the secondary is all you need. 40 VA/24 * 1.5 Amps which is about 3. So, go for 3A.

Oh, I meant to ask, should I use a slow-blow or a fast blow on the 24V side?