Gas furnace, digital tester... No voltage on the secondary

[jaylectricity]

Taken from this recent thread. (http://forums.mikeholt.com/showthread.php?t=96768) Click the last few words.

I trouble shoot stuff like that with a wiggy. The digital voltage can be misleading.


I'm having a hard time troubleshooting a gas furnace. I've read the sequence of events and it won't fire. It goes through most of the motions, but the gas doesn't ignite. I called the tech help guy and he said to check for 24V at the gas valve.

So I tested that and the meter says 11.2 volts. So then I pull the two secondary wires from the transformer and test those. The meter bounces between 0.0 and 0.1 volts. The tester is brand new. The furnace does NOTHING with those secondary wires pulled, but even though they test as no voltage, when I plug them back on the blade the furnace comes to life.

What am I doing wrong?

[480sparky]

Meter set to DC instead of AC? Or vice-versa?

[jaylectricity]

Meter set to DC instead of AC? Or vice-versa?

You have no idea how much I would like the answer to be that easy.

[480sparky]

You have no idea how much I would like the answer to be that easy.

You have no idea how simple a mistake like that can be, either. Trust me, I've been there. Spent many an hour scratching my head, cussing myself for being so stupid, and wasting valuable time.

But I'm curious, you state the furnace will not fire up, but you also state the furnace comes 'back to life'. I'm confused.

[jaylectricity]

You have no idea how simple a mistake like that can be, either. Trust me, I've been there. Spent many an hour scratching my head, cussing myself for being so stupid, and wasting valuable time.

But I'm curious, you state the furnace will not fire up, but you also state the furnace comes 'back to life'. I'm confused.

Sorry...I meant the circuit board with the relays et al goes back to life going through the sequence, TRYING to fire the burner up. All the checks through the safety switches that are run in series from the transformer through the circuit board.

[cadpoint]

If your dealing with a I/o circuit board or Master control unit with multi plug ins you probably will never read anything correctly if it has Failed inside.

I'd check and see if blower motor is sound and assure that any contactors are pulling in.

If anything if off, failed to signal or be sensed by the Master Unit it just will not let any or all the circuits have the correct power or work properly.

[jaylectricity]

If your dealing with a I/o circuit board or Master control unit with multi plug ins you probably will never read anything correctly if it has Failed inside.

I'd check and see if blower motor is sound and assure that any contactors are pulling in.

If anything if off, failed to signal or be sensed by the Master Unit it just will not let any or all the circuits have the correct power or work properly.

But why wouldn't the meter be able to read 24 volts off the secondary wires from the transformer?

My BX feed of 120 V attaches to the transformer directly...and when I test the two wires coming out of the transformer I get no DC voltage. Seriously, there is a transformer with 4 wires coming out of it. 2 primary wires from the AC feed and 2 secondary wires that send DC to the circuit board

[infinity]

But why wouldn't the meter be able to read 24 volts off the secondary wires from the transformer?

My BX feed of 120 V attaches to the transformer directly...and when I test the two wires coming out of the transformer I get no DC voltage. Seriously, there is a transformer with 4 wires coming out of it. 2 primary wires from the AC feed and 2 secondary wires that send DC to the circuit board


It likely that a transformer would have an AC output not DC. Most of the boilers/furnaces I've seen operate on AC and the transformers do not have rectifiers built in.

[goldstar]

Just curious, are you using the transformer inside the aquastat control ? If so, that has a limited capability especially if your furnace has multiple zones. From what I remember it won't drive more than two zone valves in addition to whatever it normally controls. If yoy have a separate transformer you should have 24vac on the secondary no matter what type of meter you're using. If you're using a Wiggy obviously you won't read 24 vac.

Anyway, if you can find the 24vac source inside the aquastat, begin tracing the control output while the T-stat is calling for heat. You'll find that it goes through several limit switches or overloads before getting to the gas solenoid (usually last on the run). Check to make sure the 24vac is running through each one of those limit switches. The last time I had a situation like this the plumber had the water temp set too high and the over-heat T-stat inside the exhaust compartment opened the control circuit and wouldn't re-fire the furnace until it cooled down completely. The furnace will also not fire if the flue damper isn't opened completely.

[jaylectricity]

Just curious, are you using the transformer inside the aquastat control ? If so, that has a limited capability especially if your furnace has multiple zones. From what I remember it won't drive more than two zone valves in addition to whatever it normally controls. If yoy have a separate transformer you should have 24vac on the secondary no matter what type of meter you're using. If you're using a Wiggy obviously you won't read 24 vac.

Anyway, if you can find the 24vac source inside the aquastat, begin tracing the control output while the T-stat is calling for heat. You'll find that it goes through several limit switches or overloads before getting to the gas solenoid (usually last on the run). Check to make sure the 24vac is running through each one of those limit switches. The last time I had a situation like this the plumber had the water temp set too high and the over-heat T-stat inside the exhaust compartment opened the control circuit and wouldn't re-fire the furnace until it cooled down completely. The furnace will also not fire if the flue damper isn't opened completely.

It's a central air set-up. Just one zone. Brand new installation it has never fired. And for some reason I don't get the 24 volts directly off the secondary but I was testing for DC, I will try testing it as AC next time I'm over there.

Thanks for the responses.

[Lxnxjxhx]

"So I tested that and the meter says 11.2 volts."

There's a wide tolerance on the voltage from a 24 vac xformer, but I think 11 volts is too low.

Could be. . .
1. Bad gas valve overloading a good xformer.
2. Xformer cannot support its rated load; bad xformer.
3. Something else loading down the xformer in addition to the gas valve.
4. High resistance in series with the xformer primary winding.
5. Some combo of those above.

When things don't make sense according to your mental image of how it should work, it means you are assuming something that's not true. The hard part is to figure out what (unconscious) assumptions you are making here.

[kspifldorf]

Is that with secondary hooked up??

[electricman2]

But why wouldn't the meter be able to read 24 volts off the secondary wires from the transformer?

My BX feed of 120 V attaches to the transformer directly...and when I test the two wires coming out of the transformer I get no DC voltage. Seriously, there is a transformer with 4 wires coming out of it. 2 primary wires from the AC feed and 2 secondary wires that send DC to the circuit board

Unless the transformer has a built in rectifier which is unlikely, it's output will be 24V AC

[jaylectricity]

Is that with secondary hooked up??

Unless the transformer has a built in rectifier which is unlikely, it's output will be 24V AC

I get 0 reading whether or not the secondary is hooked up.

electricman2 I think I'll take a ride up there tomorrow and test for AC, but that begs a new question for me:

If I test DC voltage with my tester on AC will that damage my tester? If the voltage is AC and I've been testing for DC it doesn't seem like it has damaged my tester, but that's only if there was actually AC there.

[electricalperson]

is the gas turned on :grin:

[electricalperson]

try swamping out the transformer with a new one. if your getting 11 volts and the gas valve is 24 volts then that could be the problem

[Pierre C Belarge]

I get 0 reading whether or not the secondary is hooked up.

electricman2 I think I'll take a ride up there tomorrow and test for AC, but that begs a new question for me:

If I test DC voltage with my tester on AC will that damage my tester? If the voltage is AC and I've been testing for DC it doesn't seem like it has damaged my tester, but that's only if there was actually AC there.



It should not have damaged your tester.

I am curious what your reading will be when you use the tester for AC. Let us know.

[goldstar]

It's a central air set-up. Just one zone. Brand new installation it has never fired.Sorry. When you said gas furnace I made the asasumption that it was a gas boiler.

No matter how you look at this you need a 24vac output in the AHU to both energize the gas solenoid in the heating mode or pull in the relay inside the AC condenser in the cooling mode.

Not trying to be a smart a** here but these are some of the things you should check :
Line voltage to unit. Do you have 120vac ?
Operation manual - read thru and see if you can determine whether you have the correct AHU
Operation manual - see if you can determine whether there should be a 24vac output with the unit you have
Check for any blown fuses on the PC board
If the xfmr has a molex connector that connects to the PC board, remove it and see if there is a 24vac output present on the secondary of the xfmr

Chances are that you may have a bad PC board or component on the board right out of the factory. You may have to call the mfr's tech support for add'l help.

[Lxnxjxhx]

Can you e-mail me a schematic? I did the same tracing and had to get a $400 gas valve. Voltage was there but no valve actuation.

[George Stolz]

Jay, I don't understand what the other thread had to do with this?

I would say that the wiggy would be of no use in this case, the voltage trying to be found is too low. Your DMM is the right tool.

I'll betcha 24VAC would look like about 11VDC.

It's not clear from your description what is being tested when you take the conductors off the secondary and find zero volts.

Remember that there are many limit switches on the furnace, giving the processor information about the unit. If one has failed, then at some point in the operation the furnace will shut down. Depending on the unit, there should be an LED blinking a trouble code to indicate at what point in the production the wheels fell off and it aborted.

In my house, I had a weird situation where dimming the kitchen cans caused my furnace to freak out. Moving the furnace to another
phase in the panel fixed it, but I never really figured out what the heck brought it on. :D

[mivey]

...I'll betcha 24VAC would look like about 11VDC...Why would you bet that? I thought the RMS value was the equivalent DC voltage that would produce the same power as the ac voltage. Since the wave is positive and negative instead of some 1/2 wave rectified signal, why would you guess 11 VDC? Why not just zero, except for some minor DC offset? Just asking.

[BryanMD]

It could also be an interlock circuit like the gas pressure switch (and even air in the line being a new install) or the micro on the sheetmetal cover or a poor connection or cross connection in the thermostat too.

LOTS of things can keep a gas burner control circuit from closing.

[jaylectricity]

I haven't gotten a chance to get over there, I'll post the results as soon as I get them. Maybe tomorrow.

[mivey]

Well hurry up man! This is worse than "to be continued next week...":grin:

[steveng]

you should be able to read the secondary voltage of 24 vac at the xfmr. if you are not reading the 24vac at the sec, most likely you have a bad xfmr

if your xfmr tests ok, i would trace the 24vac hot wire thru the wiring and look for continuity thru the switches,



possible the i/o board is bad .

[George Stolz]

Why would you bet that?
No particular reason. Gut instinct.

Sorry it wasn't more profound than that. :D

[electricalperson]

I haven't gotten a chance to get over there, I'll post the results as soon as I get them. Maybe tomorrow.
how did it work out?