Stray Voltage
- Thread starter edw
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Ed MacLaren
Senior Member
Re: Stray Voltage
Edw, induced voltage wouldn't produce enough current to run the valve motor.
Ed
Edw, induced voltage wouldn't produce enough current to run the valve motor.
What I meant by that - Are you sure the cable arriving at the valve in question, the one with the "mystery" voltage is not connected to someone else's thermostat?
Ed
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- Massachusetts
Ed MacLaren
Senior Member
Re: Stray Voltage
Edw, if you prefer, you can e-mail the diagram to me, and I will post it for you.
I sent you the e-mail address by Private Message.
Ed
Edw, if you prefer, you can e-mail the diagram to me, and I will post it for you.
I sent you the e-mail address by Private Message.
Ed
hurk27
Senior Member
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- Portage, Indiana NEC: 2008
Re: Stray Voltage
I would check your "W" connection at the furnace and see if this phantom voltage is being back fed through the main gas control board. You might have a bad control board or mis-wired furnace. Or if there is a heat pump then a "R" (hot) landed on the common at the terminal strip in the control modual will have the same effect. It will back feed the "W" wire with the voltage drop across the valve from the common side of the valve. Double check that no one has mis-connected the low voltage in the unit. This sounds like this is what has happened. Many of the transformers just have stab on connections and will fit on any of the terminals on the control board, and it's hard somtimes to see which terminal it go's on, so look realy close to where these connections are made.
I would check your "W" connection at the furnace and see if this phantom voltage is being back fed through the main gas control board. You might have a bad control board or mis-wired furnace. Or if there is a heat pump then a "R" (hot) landed on the common at the terminal strip in the control modual will have the same effect. It will back feed the "W" wire with the voltage drop across the valve from the common side of the valve. Double check that no one has mis-connected the low voltage in the unit. This sounds like this is what has happened. Many of the transformers just have stab on connections and will fit on any of the terminals on the control board, and it's hard somtimes to see which terminal it go's on, so look realy close to where these connections are made.
Ed MacLaren
Senior Member
Re: Stray Voltage
Edw's sketch of the circuit in question.
Edw's sketch of the circuit in question.
Re: Stray Voltage
Have you actually measured 24V at the xfrmr?
The possbility exists that the xfrmr is actually a 240/24 ratio. If we connect to 208V, the secondary voltage would be about 21V. If that is the case then you have a simple short.
Capacitive coupling in the shielded cable could easily account for the 21V reading on a hi-Z meter, but would not provide enough current to operate a gas valve.
Have you actually measured 24V at the xfrmr?
The possbility exists that the xfrmr is actually a 240/24 ratio. If we connect to 208V, the secondary voltage would be about 21V. If that is the case then you have a simple short.
Capacitive coupling in the shielded cable could easily account for the 21V reading on a hi-Z meter, but would not provide enough current to operate a gas valve.
physis
Senior Member
Re: Stray Voltage
Second, The best I can come up with given the available information is:
21 volts is 7/8 of 24 volts.
With no current measurements, there will be no mathmatically based solution.
I can presume a 1/8 ratio voltage divider.
But I don't know the available nor the drawn current.
edw, you've got some signifigant time invested already, (unless I've missed where it's posted) Can't you measure the current that's not supposed to be there?
That should provide the missing information needed to complete the circuit mathmatically. Or determin that there is another sourse involved.
No?
Rattus, first, you can call me Sam if you'd prefer.
Second, The best I can come up with given the available information is:
21 volts is 7/8 of 24 volts.
With no current measurements, there will be no mathmatically based solution.
I can presume a 1/8 ratio voltage divider.
But I don't know the available nor the drawn current.
edw, you've got some signifigant time invested already, (unless I've missed where it's posted) Can't you measure the current that's not supposed to be there?
That should provide the missing information needed to complete the circuit mathmatically. Or determin that there is another sourse involved.
No?
Re: Stray Voltage
First off my Thanks to Ed MacLaren and Bob Badger for their kind offers to post the diagram.
It was a multitap transformer which did have a 208 to 24 V tap. We sort of eliminated the transformer by changing it out. We used a different one with specific taps 208 to 24 V and guess what...same 21 V just like in the diagram above. The only thing we didn't try was to ground the shield of the control wire which I guess could have been transfering the voltage and approx. 30 ma of current to the W wire. I really don't know at this point. This makes the most sense to me. Thanks to all who participated
First off my Thanks to Ed MacLaren and Bob Badger for their kind offers to post the diagram.
It was a multitap transformer which did have a 208 to 24 V tap. We sort of eliminated the transformer by changing it out. We used a different one with specific taps 208 to 24 V and guess what...same 21 V just like in the diagram above. The only thing we didn't try was to ground the shield of the control wire which I guess could have been transfering the voltage and approx. 30 ma of current to the W wire. I really don't know at this point. This makes the most sense to me. Thanks to all who participated
Ed MacLaren
Senior Member
Re: Stray Voltage
Edw, you have likely already tried this, but this is what I would do first.
Remove the wires from the transformer and connect them together. Check with ohmmeter where indicated on sketch below. It should be open. (with the thermostat not connected)
If not, the cable is shorted.
You've already checked for staples driven too tight, etc.?
Once, I have found a new cable shorted due to a manufacturing defect.
Ed
[ January 14, 2005, 08:45 AM: Message edited by: Ed MacLaren ]
Edw, you have likely already tried this, but this is what I would do first.
Remove the wires from the transformer and connect them together. Check with ohmmeter where indicated on sketch below. It should be open. (with the thermostat not connected)
If not, the cable is shorted.
You've already checked for staples driven too tight, etc.?
Once, I have found a new cable shorted due to a manufacturing defect.
Ed
[ January 14, 2005, 08:45 AM: Message edited by: Ed MacLaren ]
Re: Stray Voltage
You are correct we did Ohm the shielded cable for shorts between each lead and to ground as well.This was done with the wires open on both ends. On top of that the stat wire was replaced once already before I got there by the building tech. The building guys worked on this for 3 days before I even got there. They tell me they ran a new cable I've got to believe they did.
Question: If there is a stray voltage in a control circuit does grounding the secondary of the transformer help??
You are correct we did Ohm the shielded cable for shorts between each lead and to ground as well.This was done with the wires open on both ends. On top of that the stat wire was replaced once already before I got there by the building tech. The building guys worked on this for 3 days before I even got there. They tell me they ran a new cable I've got to believe they did.
Question: If there is a stray voltage in a control circuit does grounding the secondary of the transformer help??
Re: Stray Voltage
I see no reason that grounding the secondary would help, and I see no way that you would get enough capacitive or inductive coupling to operate a gas valve. Your Volt-Ohmmeter should tell the whole story. You should also measure the voltage at the gas valve when it is on to see if the voltage drops from 21V.
You didn't say that the voltage at the xfrmer is 24V like it should be.
Sam, small gas valves require current in the order of 1A at 24V to operate. A drop of 3V at 1A would be 3 Ohms.
[ January 14, 2005, 09:30 AM: Message edited by: rattus ]
I see no reason that grounding the secondary would help, and I see no way that you would get enough capacitive or inductive coupling to operate a gas valve. Your Volt-Ohmmeter should tell the whole story. You should also measure the voltage at the gas valve when it is on to see if the voltage drops from 21V.
You didn't say that the voltage at the xfrmer is 24V like it should be.
Sam, small gas valves require current in the order of 1A at 24V to operate. A drop of 3V at 1A would be 3 Ohms.
[ January 14, 2005, 09:30 AM: Message edited by: rattus ]
Ed MacLaren
Senior Member
Re: Stray Voltage
The thing is, that "stray voltage" shouldn't be there. And, you said it actually operated the actuator motor. Did you see that, or did the "building guys" say that? Stray or induced voltage wouldn't do that.
There is more to this than meets the eye.
I would have a real close look at the work these "building guys" did. There are mixed up connections somewhere.
Ed
[ January 14, 2005, 09:26 AM: Message edited by: Ed MacLaren ]
Without seeing the control setup, it's hard to say for certain. You should be able to ground one terminal of the transformer secondary without causing problems.
The thing is, that "stray voltage" shouldn't be there. And, you said it actually operated the actuator motor. Did you see that, or did the "building guys" say that? Stray or induced voltage wouldn't do that.
As Rattus said, this should have been a routine trouble-shooting job.
There is more to this than meets the eye.
I would have a real close look at the work these "building guys" did. There are mixed up connections somewhere.
Ed
[ January 14, 2005, 09:26 AM: Message edited by: Ed MacLaren ]
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