Bench testing a 24v transformer

[Trapper]

Good morning,
I am working on a 60+ year old furnace that has been working great for us and is on the list to be replaced but trying to get one more Michigan winter out of it.
I am trying to test the transformer out of the furnace. It has to leads going in black/hot and white/neutral. I put 120 volts in and check the two screws and I have nothing.
So I went and purchased a new one and of course wanted to bench test it before putting it in and I have the same results.
I have my meter set to 200v DC because it goes from 20v then to 200v, I also did the test with the sound and I get a beep that it's good on both sides.
Also I have returned the first one and got another thinking it was defective.
So am I doing something wrong or do I have another bad one?
I watched a video that said if I had the tone on both side that it was good, so I just don't know what I am doing wrong.
Thanks for any help.

 

[Whatmeworry]

Lots of different transformers out there so getting the correct one is vital but then I mi8ght question what you have??
Are you sure you are wanting DC out? Normally that isnotcalled atransformerbut a powersupply as itchangesfrom ACin to DC out. A transformer doesn't change the output, so perhaps AC out is what you need to look for?

 

[Trapper]

Lots of different transformers out there so getting the correct one is vital but then I mi8ght question what you have??
Are you sure you are wanting DC out? Normally that isnotcalled atransformerbut a powersupply as itchangesfrom ACin to DC out. A transformer doesn't change the output, so perhaps AC out is what you need to look for?

Ok, I am not a furnace guy but it does say on the box 24v secondary on the box so I am guessing it should output 24v. and it is just in AC.
Thank you for helping me figure that out.

 

[Trapper]

Ok looks like I am going to replace the gas valve as it will not hold the pilot even after replacing the thermocoupler.

 

[Whatmeworry]

Little mistakes are totally human and no big deal as it cost you very little!
The last time I had a "transformer" of unknown/unlabeled type, I thought I wanted to test the output.
The last of my meter looked like a glowing volcano with the center adjustment glowing bright red and then a large puff of smoke!
I "think" it was a high voltage transformer from an old TV!
But I threw it and the meter in the trash!

 

[Trapper]

Little mistakes are totally human and no big deal as it cost you very little!
The last time I had a "transformer" of unknown/unlabeled type, I thought I wanted to test the output.
The last of my meter looked like a glowing volcano with the center adjustment glowing bright red and then a large puff of smoke!
I "think" it was a high voltage transformer from an old TV!
But I threw it and the meter in the trash!

I have blown a fuse in my meter before, but never smoked one lol

 

[ekrig]

From what I have seen, most furnace systems work with 24V AC. Actually, in my house there is a 24V AC line in the connections to the thermostat and apparently some thermostats even require this.

To test a transformer first set the multimeter to measure resistance and when connecting on the two primary (i.e., input) ends you should get a low resistance. (The value doesn't matter much because it is an inductance actually.) Check the same on the secondary (i.e., output) ends. This checks that you have connectivity and the wires aren't somehow broken somewhere. Then apply power (110V AC) to the primary and check if you get _around_ 24V AC on the secondary (variations of +/- 3V AC are common, maybe even slightly higher). Make sure the multimeter is set for AC or you'll get measurements that are very much off. I recommend always setting to the highest voltage reading on the multimeter to prevent issues.

Regarding the replacement make sure that you bought just a transformer (the part with windings) and not a power supply, which we sometimes casually also refer to as a "transformer", but in really is a transformer plus a rectifier. That would be a problem because with 24V AC one can higher than 24V DC voltages after rectification. In addition, the furnace may have parts specifically needs the AC signal to work.

 

[BuzzLOL]

Ok looks like I am going to replace the gas valve as it will not hold the pilot even after replacing the thermocoupler.

The thermocouple needs flame heat on it manually for about 30 seconds before it will keep the pilot going

 

[BuzzLOL]

Ok, I am not a furnace guy but it does say on the box 24v secondary on the box so I am guessing it should output 24v. and it is just in AC.
Thank you for helping me figure that out.

Transformers are AC in and AC out... transformers don't work on DC...

 

[BuzzLOL]

I have blown a fuse in my meter before, but never smoked one lol

On older analog meters we usually tried to leave the switch set a 1,000 volts so we don't fry the meter before we realize what we are touching the leads to...

 

[Trapper]

Thanks for all the reply's.
When I saw 24v I just assumed DC, my bad.
I have since retested the original transformer and is in good working order.
From what I have gone through and tested, it looks like it is probably the solenoid in the gas valve.
I found a replacement but have to wait till Monday morning.
In the meantime we have will have to use the electric heater to keep it in the mid 60s.

I have not had the time to check out the whole site but looks like a great site with knowledgeable people.
Thanks again for all the help.

 

[BuzzLOL]

When I saw 24v I just assumed DC, my bad.
.
From what I have gone through and tested, it looks like it is probably the solenoid in the gas valve.
I found a replacement but have to wait till Monday morning.

I think some airplanes work on 24 V DC and have 24 volts in DC batteries... but usually no battery setups in furnaces...
.
I had the main solenoid go bad on my gas valve and they want us to buy a whole new gas valve for $350 but I was able to get a good solenoid assembly off a $38 used gas valve...

 

[Trapper]

.
I had the main solenoid go bad on my gas valve and they want us to buy a whole new gas valve for $350 but I was able to get a good solenoid assembly off a $38 used gas valve...

Yes I did look for a replacement solenoid but no luck.
Fortunately I was able to find a new replacement gas valve for $125 at Granger, and we have one 20 mins away. Online most places wanted $149 or more.

 

[BuzzLOL]

Yes I did look for a replacement solenoid but no luck.
Fortunately I was able to find a new replacement gas valve for $125 at Granger, and we have one 20 mins away. Online most places wanted $149 or more.

Have a buddy who worked for Granger and he got me a discount on things, but I thought they were high priced, otherwise... and preferred to only deal with businesses...

 

[Whatmeworry]

How does it go in your area on Grainger? Are they now more open to walk in or is it hard for the general public to deal?
When in business I had an account and they never questioned any deal but once retired they did not want my business?
Now they do seem to be my last choice for pricing being easy to get things, so I have not been there in several years.

 

[Trapper]

Have a buddy who worked for Granger and he got me a discount on things, but I thought they were high priced, otherwise... and preferred to only deal with businesses...

Times may have changed, they were $25 cheaper than online and $90 cheaper than local HAVC shop.

How does it go in your area on Grainger? Are they now more open to walk in or is it hard for the general public to deal?
When in business I had an account and they never questioned any deal but once retired they did not want my business?
Now they do seem to be my last choice for pricing being easy to get things, so I have not been there in several years.

I called 800 # and told her cash no account and I go in and pickup Monday morning and pay then.

 

[Whatmeworry]

It may be a store issue each place or possibly changing?
What I met was no account, no deal and to set up an account, I had to go through the office and be billed. No walk in with cash! So I really needed what they had as it was somewhat special, air handler filter media, and to get it I had to use my son's account, have the bill go to him several states away, pick it up local and then pay him. Just more trouble than we want to do again!
They do have some things that I would buy just because they DO have it but it is a real drag to have to fight to do business with them.
I'll try it again if it looks right!

 

[BuzzLOL]

I missed a great deal on a furnace at Granger. They were closing out their house brand Dayton Fuel Trimmer 100K BTU 90+% efficiency furnaces for $400... same furnace as $2,000 at the time Sears Best, Heil Quaker Energy Marshal II, TempStar 90+, and Whirlpool 90+... but they were all gone by the time I heard about it...

 

[Trapper]

It may be a store issue each place or possibly changing?
What I met was no account, no deal and to set up an account, I had to go through the office and be billed. No walk in with cash! So I really needed what they had as it was somewhat special, air handler filter media, and to get it I had to use my son's account, have the bill go to him several states away, pick it up local and then pay him. Just more trouble than we want to do again!
They do have some things that I would buy just because they DO have it but it is a real drag to have to fight to do business with them.
I'll try it again if it looks right!

Went in and picked up and paid with CC but he said they would take cash.
I am guessing times have changed and you can purchase as guest or make an account.

 

[maxdad118]

Ok looks like I am going to replace the gas valve as it will not hold the pilot even after replacing the thermocoupler.

What does your pilot flame look like? It should be a strong(not necessarily big) flame directed towards the thermocouple. If it’s a lazy, candeling flame that may be a problem. Shoot a burst of compressed air down the pilot hood where the flame begins. You can dislodge some debris and usually get a much better flame. I often will rap on the pilot assembly with a small wrench and that will sometimes be all it needs.