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  1. #1
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    Accurate amp clamp for flame sensor or 24v wiring?

    I've been running into alot of thermomstats that are bad. I want to start testing the R wire in the furnsce to see what kinda current draw exists. Does anyone amp clamp 24v wires to see the amp draw. What meter?

    Is it possible to measure flame sensor without disconnecting and testing in series?


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  2. #2
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    Wrap the wire around your clamp 10x then 10

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  3. #3
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    Quote Originally Posted by 503 Hvac View Post
    I've been running into alot of thermomstats that are bad. I want to start testing the R wire in the furnsce to see what kinda current draw exists. Does anyone amp clamp 24v wires to see the amp draw. What meter?
    Current draw to a board is meaningless as a diagnostic tool. Most relay and output loads are mere fractions of an amp. It is the presence of voltage, and the expected response from the device, that is more pertinent.

    You won't make any determinations about thermostats using an amp clamp.

    Quote Originally Posted by 503 Hvac View Post
    Is it possible to measure flame sensor without disconnecting and testing in series?
    Typically, no, because of the low DC value that cannot be coupled using a clamp meter.
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  4. #4
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    Maybe you can help me understand.

    I had a old power passing thermostat that was shorted G to Y and was causing the board to throw pressure switch codes and run intermittently. No batteries. Disconnected all thermostat wires on board and can run the furnace no issues.

    Had nest thermostat with no common wire hooked up on a trane xv95. Gas valve wouldn't open.

    When I take them out of loop furnaces run. Current draw in these cases can't be measured and isnt important?

    On another side note:
    Alot of times when I diagnosis a bad board for a furnace it goes through the whole heating sequence through the igntior but won't open the gas valve, almost like there isn't enough 24v. To confirm bad board I jump 24v to gas valve directly to hear it click and open. Is measuring total 24v load off the transformer anything anyone does? Does anyone experience anything similar?


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  6. #5
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    Quote Originally Posted by Spitz View Post
    Wrap the wire around your clamp 10x then 10

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    Forgot about that I'll start trying it. Thank you


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  7. #6
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    Quote Originally Posted by 503 Hvac View Post
    Maybe you can help me understand.

    I had a old power passing thermostat that was shorted G to Y and was causing the board to throw pressure switch codes and run intermittently. No batteries. Disconnected all thermostat wires on board and can run the furnace no issues.

    Had nest thermostat with no common wire hooked up on a trane xv95. Gas valve wouldn't open.

    When I take them out of loop furnaces run. Current draw in these cases can't be measured and isnt important?

    On another side note:
    Alot of times when I diagnosis a bad board for a furnace it goes through the whole heating sequence through the igntior but won't open the gas valve, almost like there isn't enough 24v. To confirm bad board I jump 24v to gas valve directly to hear it click and open. Is measuring total 24v load off the transformer anything anyone does? Does anyone experience anything similar?


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    Y &G may not have been shorted. Many times, depending on the wiring schematic, Y is a dummy terminal.
    They are "connected" in the stat.

    NEST is junk to start with. If it's doing funky things, remove the NEST.
    If I come across a NEST, I disconnect it when I service the furnace. They can easily be fried with normal stuff others have non-issue with!!!
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  9. #7
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    Y + G not shorted makes sense. Im talking about a old honeywell thermostat with no subase, no common, wired directly into the back. Kinda looked like a chronotherm IV. batteries were corroded and green all over the terminals. Will that cause 24v issues like a furnace board freaking out?

    Had a customer with an old trane duel fuel kit heatpump + gas furnace. Owner wired in his own nest. Tried to kick on furnsce with emergency heat, no such option on thermostat. Auxiliary heat showed up on wiring auto detect. Is this one of the major hickups with nest, can't select auxiliary heat?



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  10. #8
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    Quote Originally Posted by 503 Hvac View Post
    Y + G not shorted makes sense. Im talking about a old honeywell thermostat with no subase, no common, wired directly into the back. Kinda looked like a chronotherm IV. batteries were corroded and green all over the terminals. Will that cause 24v issues like a furnace board freaking out?
    Yes.

    Quote Originally Posted by 503 Hvac View Post
    Had a customer with an old trane duel fuel kit heatpump + gas furnace. Owner wired in his own nest. Tried to kick on furnsce with emergency heat, no such option on thermostat. Auxiliary heat showed up on wiring auto detect. Is this one of the major hickups with nest, can't select auxiliary heat?
    Can't really say. Nest is a solution without a reason.

    When you have green crusty deposits, you start with a new stat right away.

    As I mentioned, trying to diagnose a stat by measuring current is not an accepted diagnostic approach. If you think you have voltage crossover, remove the stat and use some jumpers. If the problem goes away, it is in that pile of junk at your feet.
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  11. #9
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    Quote Originally Posted by 503 Hvac View Post
    Maybe you can help me understand.

    When I take them out of loop furnaces run. Current draw in these cases can't be measured and isnt important?
    Correct. If you think current is compromised, put a load on the wire, such as a type 91 relay....the typical brown DPDT. If it can't pick up, you have a high resistance somewhere acting as a choke (limiting) resistor. Remember, a digital meter has a VERY high input impedance, and places nearly zero load on the circuits it is used to measure.

    Now....if you took a lot of current readings using the 10-turn technique all of the time, could you theoretically develop your own protocol as a tool? Perhaps. But other methods are far faster and easier than trying to re-invent the wheel.

    Quote Originally Posted by 503 Hvac View Post
    On another side note:
    Alot of times when I diagnosis a bad board for a furnace it goes through the whole heating sequence through the igntior but won't open the gas valve, almost like there isn't enough 24v. To confirm bad board I jump 24v to gas valve directly to hear it click and open. Is measuring total 24v load off the transformer anything anyone does? Does anyone experience anything similar?
    You can confirm correct transformer voltage, but looking for the load of a gas valve trying to open by current is another needle in a haystack approach. Instead, put a different load on the wire, like a relay, to confirm that enough power is available.
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist
    Member, IAEI

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