AC Air Handler Transformer - Smoked?

Gman66

So Monday I had what I suspect was a bad thermostat causing intermittent cooling problems and being fundamentally lazy I tried to swap out the thermostat without throwing the breaker (please note that lecture is no longer needed!).

Long story short is that there is now no voltage present at the thermostat. Unfortunately the air handler is in my disaster of an attic (low pitched roof w/ AC hoses running all over like a plate of spaghetti). The is a really old AC system w/o so much as a control board; really just a transformer and I guess a fan relay (no heat in the air handler, I have BB Hot Water). No separate fuse that I can see anywhere in the control cabinet but the transformer secondary has a button which I suspect is either a breaker or a fuse which I have attached a picture. I tried pushing the button but nothing happened, still no 24v from the secondary. I did verify that I have 240v on the primary and the transformer does the normal barely audible hum when powered on.

I stupidly left the transformer in the cabinet (the heat was getting to me and my judgement was not to be found). I am hoping that someone will recognize this button and explain how to simply reset or replace a fuse. One can hope, right?

Should it matter, the air handler is an old Lennox unit which I guess is 25-30 years old (I have lived here for 21 years).

pecmsg

If you fried the primary side it needs replacement!

JUGHNE

I think that button may be the top of a fuse....push and a 1/4 turn and it may pop out.

How many wires are at your T-stat?

mattmia2

Easy enough to test continuity of the primary and secondary with an ohmmeter. Is that a breaker, it sort of looks like metal version of a glass fueseholder. Also try resetting with the secondary disconnected, might still have something shorted at the t-stat.

Gman66

pecmsg said:

If you fried the primary side it needs replacement!

I was hoping that because it hums under power (nothing abnormal, just that barely audible transformer hum) that the primary was still good, I guess I will find out later tonight when I have another go around with it.

Gman66

JUGHNE said:

I think that button may be the top of a fuse....push and a 1/4 turn and it may pop out.

How many wires are at your T-stat?

Looking at the pictures I am thinking the same; the metal cap is the holder for a glass fuse and that it will pop off with a quarter turn of the octagonal retainer. Had I been thinking more clearly while in the hot attic I might have given this a try.

I have 5 wires at the thermostat:
Rc - Red
C - Blue (common)
G - Green (fan)
Y - Yellow (first stage cooling)
Y2 - White (second stage cooling)

This is AC only and I have a two speed compressor.

JUGHNE

Shorting that red and blue will do it.
On some systems the common side (blue) is grounded to the equipment. So shorting anything to ground may take the fuse also.

Your newish tstat cable was probably run when new outside AC was installed.

This would happen more often to homeowners if the common (your blue) was taken to the Tstat. Most are lucky and don't short the transformer out.

Gman66

JUGHNE said:

Shorting that red and blue will do it.
On some systems the common side (blue) is grounded to the equipment. So shorting anything to ground may take the fuse also.

Your newish tstat cable was probably run when new outside AC was installed.

This would happen more often to homeowners if the common (your blue) was taken to the Tstat. Most are lucky and don't short the transformer out.

Why is it that stupidity so quickly rears its ugly head?? Lets hope I never make the mistake of not powering down again.

One leg of my secondary is grounded to the air handler. Interestingly it is the yellow wire coming from that transformer that is the common and grounded to the frame and connected to the blue wire in the tstat cable. Perhaps even more interesting is that the hot coming from the secondary is blue which then connects to the red wire in the tstat cable. I can't imagine how anyone would screw this up!!

JUGHNE

Tstat cable colors and letters such as R, W etc are not universal, especially with Lennox at that time, IDK about now.

HVACNUT

You're not sure if it was you who blew the fuse. If it is in fact blown.
Is there a common at the thermostat?
Have spare fuses and check the relay and contactor coils.

Big Ed_4

Looks like a circuit breaker , try pressing to regain 24V ..

Gman66

Just to close out the mystery: That silver button is indeed a fuse (not a circuit breaker). As it turns out it is a glass fuse, what you see in the picture is one end of the glass fuse. It is a special, long discontinued, version of a typical glass tube fuse (C Type) where one of the metal end caps has two small ears that slide into the two slots on that octagonal cap and turn 90 degrees to remain captive.

A quick resistance test of both primary and secondary windings show that they are still intact. As I hold no hope in finding a long discontinued glass fuse, I will "defuse" the transformer and reinstall with a simple inline fuse (2A as the transformer is 50VA) after bench testing the transformer when I get a chance later today.

Interestingly, the fuse is on the hot side of the secondary and the neutral side is grounded to the frame of the air handler such that, when powered up, there should be a 24vAC potential between that exposed button and the frame of the air handler.

Thanks to all that took the time to reply!!

mattmia2

I'm surprised that that isn't just a cap that slides over the end of the fuse.

Gman66

mattmia2 said:

I'm surprised that that isn't just a cap that slides over the end of the fuse.

That was my thought as well. Having guessed there was a glass tube fuse in there I had gone into the attic armed with a shiny new 2A fuse. The tube broke off in my hands trying to peel off what I was sure was separate cap. After taking the transformer out and getting out of the attic was able to see writing on the fuse and I saw the "C". Quick google look up found the damn thing; looks like it went out of style in the 70's suggesting my AC is even older than I thought.

ChrisJ

Gman66 said:

mattmia2 said:

I'm surprised that that isn't just a cap that slides over the end of the fuse.

That was my thought as well. Having guessed there was a glass tube fuse in there I had gone into the attic armed with a shiny new 2A fuse. The tube broke off in my hands trying to peel off what I was sure was separate cap. After taking the transformer out and getting out of the attic was able to see writing on the fuse and I saw the "C". Quick google look up found the damn thing; looks like it went out of style in the 70's suggesting my AC is even older than I thought.

Can you provide a link to what you found?
I've never heard of a C fuse and you have me curious.

2AG, 3AG etc sure, but C?

Gman66

ChrisJ said:

Gman66 said:

mattmia2 said:

Can you provide a link to what you found?
I've never heard of a C fuse and you have me curious.

2AG, 3AG etc sure, but C?

https://www.elliottelectronicsupply.com/fuses-holders/other-specialty-types/c-332-series-type-fuses.html

https://www.elliottelectronicsupply.com/fuses-holders/other-specialty-types/c-332-series-type-fuses.html

Gman66

ChrisJ said:

Gman66 said:

mattmia2 said:

I'm surprised that that isn't just a cap that slides over the end of the fuse.

That was my thought as well. Having guessed there was a glass tube fuse in there I had gone into the attic armed with a shiny new 2A fuse. The tube broke off in my hands trying to peel off what I was sure was separate cap. After taking the transformer out and getting out of the attic was able to see writing on the fuse and I saw the "C". Quick google look up found the damn thing; looks like it went out of style in the 70's suggesting my AC is even older than I thought.

Can you provide a link to what you found?
I've never heard of a C fuse and you have me curious.

2AG, 3AG etc sure, but C?

https://www.elliottelectronicsupply.com/fuses-holders/other-specialty-types/c-332-series-type-fuses.html

ratio

That website shows them as in stock…

JUGHNE

ChrisJ, there are more fuse nomenclatures/sizes/voltages then paint colors available in a big box store.

I was very glad when circuit breakers took over.....especially HACR circuit bkrs.
A set of time delay 30 amp fuses approach the price of a double pole CB....and you still need the CB.

I have a couple hundred dollars of fuses in stock.....never the right ones, if needed. Some 600 VAC fuses which are spendy.
Very glad when the local hardware store put in an extensive supply.......the worker there was proud of it......I told him that it would not be good to extend credit to the few people around here who still need fuses in their house. I have dealt with them and learned to stay at arms length.
In the past they have wanted to return fuses that they tried....wrong color of plug type, etc.

All that being said, fuses are supposed to be safer and quicker than circuit breakers. CB's need 2 cycles of the 60 Hertz to respond and apparently fuses do not.

Jamie Hall

I can vouch for the fact that fuses are faster than circuit breakers. The other day I was working on a hot circuit (sometimes you do what you have to do) and made just a brushing arc between one phase (120) and ground (shame, shame). The 60 amp main fuse on that phase on that branch box went boom (gotta love it when they do that) but the 20 amp circuit breaker on the specific line never quivered.

And as you say, there are more sizes and characteristics of fuses than paint colours at the big box -- and you really do want to put in the correct type. And also, as you say, the big ones tend to be a bit spendy...

ChrisJ

@Jamie Hall I think that depends highly on how the fuse is sized and the type of fuse. If fused close to it's limit it'll blow for no reason.

Magnetic type circuit breakers can be incredibly fast. However circuit breakers used in houses are intentionally delay type unless you pass a certain threshold. That's engineered that way.

JUGHNE

I believe the trade off of safety is justified to use CB's.

I have found the "renewal link" fuses with #6 copper wire installed. Copper pipe hammered flat to fit into the 100 amp flat jaws. Of course the standard copper penny behind the 30 amp plug fuse. Then the Fusestat insert chopped out to put the 30 amp plug fuse on a 15 amp severely overloaded circuit....also so the penny would fit.
Then the arsonist favorite was to soak the fuses in water then over load the circuit. Evidence of the water would be gone perhaps in the fire, but the label inside might show having been wet...could have been water from the fire dept???
Have found the Square D CB disconnect only (yellow label) in a panel feeding a 2 pole ckt of 30 amps. Never a trip on that one.
Found a standard 15/20 amp outlet connected to a 100 amp main.

EBEBRATT-Ed

I used to have buckets of fuses tossed them all.

Fuses are safer in my book.

If you get involved with big stuff you can get much better interupting capacity out of fuses for less $$than you can CBs

Jamie Hall

I enjoy (?) @JUGHNE 's comments. And he is so right -- there are many ingenious ways to defeat a fuse installation. From that standpoint, I sort of agree that circuit breakers do make sense -- a lot of sense -- particularly in residential use (the insurance companies agree -- it's almost impossible these days to get home insurance on a house with fuse panels, even with Fusetstat inserts). That said, at the risk of inspiring some idiot, one can go to the big box store and buy a 40 amp CB which fits in the breaker box instead of that pesky 15 amp one which always blows and change it in about 2 minutes.

But fuses make sense in some applications as well -- and I have to think that a properly sized fuse is perhaps a wee bit more reliable. At least I've never heard of one that failed to trip... unlike some breakers...

As @Steamhead says in another context, "you can't fix stupid".

ChrisJ

Jamie Hall said:

I enjoy (?) @JUGHNE 's comments. And he is so right -- there are many ingenious ways to defeat a fuse installation. From that standpoint, I sort of agree that circuit breakers do make sense -- a lot of sense -- particularly in residential use (the insurance companies agree -- it's almost impossible these days to get home insurance on a house with fuse panels, even with Fusetstat inserts). That said, at the risk of inspiring some idiot, one can go to the big box store and buy a 40 amp CB which fits in the breaker box instead of that pesky 15 amp one which always blows and change it in about 2 minutes.

But fuses make sense in some applications as well -- and I have to think that a properly sized fuse is perhaps a wee bit more reliable. At least I've never heard of one that failed to trip... unlike some breakers...

As @Steamhead says in another context, "you can't fix stupid".

I don't know.
Swapping a 15A breaker for a 40A breaker has a tendency to fix stupid.

Years ago I wanted circuit breakers in series with fuses.
For example, a 15A circuit breaker with a 20A fuse on 12 gauge romex.

These days I realize more if things are planned and installed correctly it's unnecessary. I do prefer Square D QO breakers though....

ratio

IDK about li'l breakers, but modern larger breakers (100A & up) are pretty amazing. Some are programmable & communicating, the downstream breakers will talk to the upstream breakers to let them know that they'll be tripping soon, so the upstream breaker will wait to see if the smaller one can clear the fault.

hot_rod

More and more you see transformers fused with these reseting type "links" They "pop" quickly and reset after about 1 minute. This gives you time to find and correct the fault and power back up.
We have tested them over and over, dozens of times in a row and they keep working.

ChrisJ

> @hot_rod said:
> More and more you see transformers fused with these reseting type "links" They "pop" quickly and reset after about 1 minute. This gives you time to find and correct the fault and power back up.
> We have tested them over and over, dozens of times in a row and they keep working.

Resettable fuses have been around a long time.
The issue with them is they just keep going and going and can damage the circuit similar to a compressor overload. I suppose it's like anything else, they have their place.

An example of a resettable fuse is : https://www.digikey.com/product-detail/en/bourns-inc/MF-MSMF250-16X-2/MF-MSMF250-16X-2CT-ND/3437789?utm_adgroup=Circuit Protection&utm_source=google&utm_medium=cpc&utm_campaign=Dynamic Search&utm_term=&utm_content=Circuit Protection&gclid=Cj0KCQjwudb3BRC9ARIsAEa-vUsbpC1iHCmzYN8J7uqpxOvZIpsOXTYQJRqgAZVS5AwuyyqWctecyQAaAgFxEALw_wcB

Gman66

ratio said:

That website shows them as in stock…

Wife and daughter were not so keen on waiting a few more days for the right glass tube fuse to be shipped. Much quicker to just replace with a 2amp inline fuse.

EBEBRATT-Ed

Blew a 400 amp fuse once. I was moving some wires around in the bottom of a switchboard and the original electrician had stuffed the wire in the lug with a strand or two outside the lug which also was not very tight so it shorted, the sparks flew and took out the fuse.

Scared the crap out of me!

Lucky for me they had some spare fuses as I had shut down the whole mechanical room for a hotel.

I can still remember bolting the new fuse in the disconnect while my hands were shaking

hot_rod

ChrisJ said:

> @hot_rod said:

> More and more you see transformers fused with these reseting type "links" They "pop" quickly and reset after about 1 minute. This gives you time to find and correct the fault and power back up.

> We have tested them over and over, dozens of times in a row and they keep working.



Resettable fuses have been around a long time.

The issue with them is they just keep going and going and can damage the circuit similar to a compressor overload. I suppose it's like anything else, they have their place.



An example of a resettable fuse is : https://www.digikey.com/product-detail/en/bourns-inc/MF-MSMF250-16X-2/MF-MSMF250-16X-2CT-ND/3437789?utm_adgroup=Circuit Protection&utm_source=google&utm_medium=cpc&utm_campaign=Dynamic Search&utm_term=&utm_content=Circuit Protection&gclid=Cj0KCQjwudb3BRC9ARIsAEa-vUsbpC1iHCmzYN8J7uqpxOvZIpsOXTYQJRqgAZVS5AwuyyqWctecyQAaAgFxEALw_wcB

I think the key is to find the fault after the first fuse trip, regardless of the type. The fuse "opening" and resetting is treating a symptom, not the problem.

I suppose knowing this is what separates the knowledgable troubleshooter from the trial and error DIYers

Resettable fuses help prevent folks for using pennies, bolts or cigarette box foil to "homemake" a fuse.

JUGHNE

The "Little Popper" after market circuit breaker, 2 or 3 amps, helps a lot on intermittent trouble some faults.
For instance put one on just the Y going to outdoor unit to isolate the portion that is sometimes faulty.

ChrisJ

hot_rod said:

ChrisJ said:

> @hot_rod said:

> More and more you see transformers fused with these reseting type "links" They "pop" quickly and reset after about 1 minute. This gives you time to find and correct the fault and power back up.

> We have tested them over and over, dozens of times in a row and they keep working.



Resettable fuses have been around a long time.

The issue with them is they just keep going and going and can damage the circuit similar to a compressor overload. I suppose it's like anything else, they have their place.



An example of a resettable fuse is : https://www.digikey.com/product-detail/en/bourns-inc/MF-MSMF250-16X-2/MF-MSMF250-16X-2CT-ND/3437789?utm_adgroup=Circuit Protection&utm_source=google&utm_medium=cpc&utm_campaign=Dynamic Search&utm_term=&utm_content=Circuit Protection&gclid=Cj0KCQjwudb3BRC9ARIsAEa-vUsbpC1iHCmzYN8J7uqpxOvZIpsOXTYQJRqgAZVS5AwuyyqWctecyQAaAgFxEALw_wcB

I think the key is to find the fault after the first fuse trip, regardless of the type. The fuse "opening" and resetting is treating a symptom, not the problem.

I suppose knowing this is what separates the knowledgable troubleshooter from the trial and error DIYers

Resettable fuses help prevent folks for using pennies, bolts or cigarette box foil to "homemake" a fuse.

That was my point.
With a resettable fuse, you're not going to find the fault after the first trip. Or the second etc. It'll keep automatically resetting until someone shuts power down to the circuit.

HVACNUT

> @EBEBRATT-Ed said:
> Blew a 400 amp fuse once. I was moving some wires around in the bottom of a switchboard and the original electrician had stuffed the wire in the lug with a strand or two outside the lug which also was not very tight so it shorted, the sparks flew and took out the fuse.
>
> Scared the crap out of me!
>
> Lucky for me they had some spare fuses as I had shut down the whole mechanical room for a hotel.
>
> I can still remember bolting the new fuse in the disconnect while my hands were shaking




I probably wouldn't have made it. 500 MCN cable? Careless of the electrician. I remember pounding the brass bug nuts with a lump hammer, tighten, pound, tighten, pound. Rubber tape. I was a kid and the switch gear looming above scared me. Forget being near it if something shorted.

ChrisJ

I am not saying I am totally innocent in this, but.......

JUGHNE

Auto reset fuses or CB's are used on long remote power lines in the rural area here. 3 strikes and they stay off.
Usually lighting will trip them. You get 2 more chances to keep your lights on.

Or the tree branch or squirrel between phases burns up and falls to the ground, often starting a grass fire.

This may be my caboose entry for this posting, been interesting, can't get this conversation at the local coffee shop!