24 volt false?

Paul_69

I had a goodman gmpn gas furnace that was 1 blink on code. Tested thru circuit and found 24 volts @ gas valve but wouldnt open. I could see it was struggling to open and was certain it was a bad gas valve. I called goodman to verify a few things and told them what i found. He said to try 24 volts strait from transformer as he said sometimes on old units a switch or the board could be sending proper 24 vlt but not proper amperage. I put it direct and the gas valve worked fine. Bypass all swiches and looks like it might be a faulty board. I have never seen this b4. Anytime i had 24 vlt to a valve , motor etc. Yiu would replace itbecause you had voltage proper. Any commentd on thid new find would be good. I am a tech.

Zman

What you are seeing is a bit unusual but makes perfect sense.
My favorite auto mechanic showed me a trick that helps pinpoint problems like this.
You put a load on the circuit in question then put one lead of your voltage meter on the battery positive (Transformer in your case) and the other lead on the suspect positive at the device (gas valve). If the voltage meter shows voltage, from positive to positive, it means there is a restriction,the power will use your meter as a path if it does not have an adequate path through the circuit.
You should be able to use this technique to find the weak spot. It could just be a bad wire or connector.

Carl

icesailor

Mitco makes a thing called a "Amp-Mate" which is just a low amp meter. For testing current draw through controls to set heat anticipators on old thermostats. If you wire the valve direct through a 24 volt transformer and check the amperage and voltage both. Then, put the Amp-Mate between the control and the valve. The amps and Voltages should read close to the same.

Handy little tool/device.

Tim McElwain

What is the Make and number of the board. Most EFT boards power the transformer after the board is powered and then send 24 back to the board on say and "X' and "C" terminal which then is sent out through pressure switch, blocked vent switch and roll out switch and maybe even an aux switch located in the blower compartment. Look at the gas valve it should have the required amperage draw on the valve. Hook your meter up in series with the control circuit to the valve and read the amperage. Then run the valve directly from the transformer and see what the amperage is, it should be the same if not then something is wrong with the board.

Paul_69

Thanks for the comments.tim i dont have the board number.very strange situation. How can u have 24 volts to gas valve thru control circuit but valve wont open and 24 volts measured strait from a 24 volt transformer direct and valve opens. Both registered 24 volts and i would have changed gas valve and new one would do same thing. I didnt change it. I checked with tech support but i still dont understand. I have never seen anything like it. The customer is going to replace furnace as ig is 18 yrs old and has other things wrong. But i just want to learn more. Thx

icesailor

As I understand it and have experienced it, you can have 24 volts and no amps.

Tim lays it out.

Tim McElwain

What happens is the back feed through the board is not complete. If you ground one leg of your multimeter to "C" on the board and use the other lead for troubleshooting you will not show 24 volts.

BobC

Don't assume just because you have 24v on one terminal of something that it should work, the other side might not be going where you think it is. Check across bother terminals to see if there is 24v across whatever you are trying to check.

Bob

Paul_69

Thanks.i did have both probes on both terminals to gas valve.what tim said about grounding your meter to c and making your tests thru circuitry is key. Or else you could have false volt readings it sounds like.but to find out if it has amps or not i am not getting like ice said. Nice discussion..thx guys

BobC

Your right about having voltage but no current, that usually means something is open. The coil on a gas valve is very small wire, not inconceivable there is a break in the wire.

Bob

Don_197

Yes....when measuring voltage, you "back probe" or put your meter in PARALLEL with the circuit you are testing......but to measure current (amperage) you have to put your meter in SERIES with the circuit. Don't feel bad.........I have had techs with 20 years experience not know what the hell I'm talking about.......because they always measured amperage with a clamp meter........which works fine on currents of say 1.5 amps or more........but not on anything less than that really.

Jamie Hall

Keep in mind that when you are measuring voltage, the meter has a very very high resistance -- no load on the circuit. If there is a bad connection or bad relay contact in there, that will have considerable resistance, but nowhere near the meter -- and you will read 24 volts. Put any sort of load on the circuit, though, and that bad connection will have a large voltage drop and whatever you are powering will have nowhere near 24 volts. You will see this if you can measure the voltage across the item you are trying to power up -- say a gas valve or whatever.

Of course, then the trick is finding the bad connection or contact...

Paul_69

Thx for all the advice. Bob c,I put 24 volts directly to gas valve from trans. And gas valve opened fine. Is there any resources for better testing low voltage on hvac equipment as it seema to be diffrent then line volt. Thx

BobC

The problem is that the existence of voltage at a point does not mean there will be current flow, that is why it's important to measure the voltage across the device your trying to test.

What you have to do now is figure out what is between each leg of the 24v transformer and your gas valve and what might be open. It could be a open safety switch, a bad wire, or a bad solder joint. I would start with a very careful visual inspection to see if you notice something.

Bob

unclejohn

What you have is most likely a bad gas valve relay on the board. The contacts are dirty or something and they will pass voltage but when you put a load on it the resistance at the contacts goes up and the contact then acts as a load. I think it's called voltage drop or droop.

Tim McElwain

You have determined that with a "load" "no load" test that the board can't handle the gas valve, replace the board.

SherlockOhms

Paul said:

I had a goodman gmpn gas furnace that was 1 blink on code. Tested thru circuit and found 24 volts @ gas valve but wouldnt open. I could see it was struggling to open and was certain it was a bad gas valve. I called goodman to verify a few things and told them what i found. He said to try 24 volts strait from transformer as he said sometimes on old units a switch or the board could be sending proper 24 vlt but not proper amperage. I put it direct and the gas valve worked fine. Bypass all swiches and looks like it might be a faulty board. I have never seen this b4. Anytime i had 24 vlt to a valve , motor etc. Yiu would replace itbecause you had voltage proper. Any commentd on thid new find would be good. I am a tech.

The presence of voltage at times can fool you as you have discovered.it can be ghost voltage or induced voltage,if you apply a load across the legs of power it will vanish or remain existing.
If you have proper voltage but no amp draw, then the load, motors, heat strips, water heater elements is bad.
Newer gas valves wil read open if you ohm them this will also can throw you off at times.

SherlockOhms

The 2 probes of your meter are placed "across" both "legs" or sides of power to read voltage, you can read a voltage drop by placing the probes in series , on the same leg or side of power.
You want to reference from the opposite side of power from which you began, example,

L-1----------------------------------------------- L1 = 115V
ACROSS BOTH HOT LEGS L1 & L2 = 230 volts
L-2------------------------------------------------ L2 = 115V

If you were to put both probes on only 1 leg it will read nothing usualy and IF it does get a reading the voltage read wil be the difference between the 2 probes as in a voltage drop, used in automotive troubleshooting.

I had a strange condition called a RPBG or also known as
Reversed Polarity Bootleg Ground.
Older 2 wire circuits in residential used no ground wire, thus had no ground hole in the receptacle, after they quit making 2 hole outlets, they would often jumper neutral to the ground screw at each outlet to provide a "ground"
at least to fool a 3 prong outlet tester, as they will not sense this deadly condition. With polarity reversed the hot leg is neutral and if a bootleg ground is used then both neutral and ground are hot! So when you go put 1 probe on ground to reference from you will be putting both probes on the same hot leg, the meter reads nothing because both ground and neutral are hot!
And if you reference to the other leg it will read power so you think all is well, I bought a old home with knob and tube wiring that the inspector used the outlet tester aone to test for proper grounding before I bought it, and my being an HVAC man felt I was too good to be tricked so easily but the inspector and his tester are fooled too!

Everything I work on pulls more than 1 amp,the transformer 's low voltage control power is a 3 amp fuse usually, I cannot think of the last time I ever used an amp meter in series to read 1 amp, I never have had to use a meter in series I guess.

Jamie Hall

Those bootleg grounds on older wiring can be flat out lethal. If you are working on, or contemplating working on, wiring in an older building (even something as simple (?) as a light socket), do your family a favour and never, ever touch a wire, plug, switch connections, socket, or what have you until you have verified with a neon test screwdriver or similar doo-dad that the wire or fixture you are about to play with really is dead. ALL of the wires, contacts, and what have yous are dead.

I can't begin to count the number of mis-wired plugs I've seen, or the number of light fixtures with the switch in the neutral and the socket hot (that can even make changing a bulb exciting).

icesailor

Then, there's the old trick with an old appliance that doesn't work correctly. You flip over the plug and it works fine.

I've been "Lifted" from very old refrigerators and flipped the plug. End of the "Lift".

SWEI

Installing GFCI's on 2-wire branch circuits makes them a lot safer.
http://ecmweb.com/content/replacing-2-wire-ungrounded-receptacles

icesailor

As long as Sparky knows how to do it properly.

Paul_69

Thanks for all replys.i would think this would be more common on low volt than line volt. I have been a tech for a long time and never ran into this before. So how do you know when its ghost volts or improper? No amps. And volts? Volt and no amps. Again i was ready condemn gas valve because i measured 24volts at gas valve.it wouldnt open. I put 24 volts from transfmr direct and it opened. How is one not opening valve and one is. Both are 24 volts. This is what i am trying to learn. I agree with tim i proved it to be a weak board. Just want to learn what i have never seen. Must be just luck as i have done alot of service without this senario.maybe i just dont remember !!!! Thx

Jamie Hall

The gas valve needs a certain number of amps to open -- it's a magnetic coil, and the strength of the magnet is determined by the current (amps) and the number of turns. Yours is designed to take a certain number of amps.

It also has a certain resistance, so there is a specified voltage (24 volts in this case) which will cause that current to flow in the coil.

What you are seeing is that somewhere else in the circuit -- on that board? -- there is additional resistance and it isn't allowing enough current to flow to pull in the valve. The difference can be surprisingly small.

KC_Jones

The volts/amps comparison would be just like a car with an almost dead battery that does the "death rattle". There is just enough amps to close the solenoid (magnetic coil like that gas valve), but once the starer tries to run it draws the amps down too low and the solenoid releases. Then the amps jump back up and the solenoid closes again. In your case the amps must have been so low nothing could happen. Doesn't point to your cause, but sometimes analogies like that help people understand a little better.

1234567

I had the same problem,talk to 3 techisions nobodi mentioned the heat sensor on top of the vent i changed it and the gas valve opened.Before that i bought gas valve becouse i mesured 27 volts coming to the velve wires but then i found out that the 27 volts was not crosig over to the valve wich ment that i had voltige to the wires but no amps -no curent.i also didn't have a message fuselink field foult wich would point to the 2 heat sensors.