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Thread: lo vo intermittent open heat pump stat

  1. #1
    Join Date
    Jul 2008

    lo vo intermittent open heat pump stat

    question: why does the 24vac control circuit intermittently blow the 5amp fuse?
    overcurrent to ground [rubbing to the metal], overcurrent to load [stuck open], or over voltage?
    answer: looks like it must be a ground fault in the stat wires since they were the last moving part
    ==>coincidentally a low voltage fault at one of the components? what else to check for<==

    I'm a commercial Electrician I'm at a job and they have trouble with their heat pump.
    Seemed simple enough, they had swapped out stats hot and presumably shorted the LV leads - I found an open 24vac 5a fuse and replaced it at their Tempstar PHF042.

    Job complete so I thought.
    Got a callback a week later, no AC
    Hey, I'm mostly interested in finding voltage there, more than that, call specialist.
    I did work 20 years as district electrician for the state and am qualified to do most common maintenance tasks esp regarding electrical part of it.

    Ok, well, I thought maybe its a different trouble since nobody messed with the stat.
    But I'm nearby, see what I can do.

    5amp fuse is open again. I don't see any other trouble.
    Stat Wires look Ok,
    I didn't megger them but its been a week and they hadn't grounded out yet.
    Secondary on the TX doesn't over voltage.
    28v unloaded and 24v with current flow.
    32v DC fuses rated 250vac
    Primary seems steady 244v on a 230v nameplate.
    Amp draw is 0.9a

    another 5a fuse in and it worked, got to go.

    Wondering if I will hear back don't know how to check if the control board is damaged

    3rd week, 3rd call. On my way over there now

    here's some photos of the unit In the photos you can see there is a 1.5 kohm resistor looks like it gets warm, but it was not warm to the touch when I was there. I'm going to check resistance and voltage drop across that. And IR the whole board

    I need help troubleshooting why else would 24vac LV control fuse open given that line voltage components all seem to be ok.

    Sorry hospodi, you are not allowed to post links in the forums. You must have a minimum of 7 posts in order to post links. You currently have 0 posts.

    searpartsdirect has the manual I'm looking at 16 pages with no troubleshooting info

    1. Visual inspection;
    one hot resistor,
    => recheck wires at stat, megger
    a short to ground

    2. Test using sequence of operation;
    not useful, takes a week to open fuse

    3. Use system voltage and ammeter to locate problems when possible;
    operation normal when I am there
    I could set a data logger

    4. Double check your diagnosis visually and with an ohmmeter when possible.
    megger is better
    Could attach the thermostat wires together R, G and Y. Red is the hot wire from the transformer, G controls the fan relay on the furnace and Y controls the contactor on the outdoor AC
    I didn't see any low voltage coils or relays that could have grounded out
    , but they may be part of the control board
    "A problem clearly stated is a problem half solved."

  2. #2
    Join Date
    Apr 2008
    Ann Arbor, Michigan
    181017-2057 EDT


    Where is the 5 A fuse located? I agree the 1.5 k resistor looks to be overheated. Where is the circuit diagram for the PC board?

    Is the 24 V AC secondary side and its circuitry isolated from earth or EGC?

    What are the two diodes next to the 1.5 k resistor, and how are they connect to whatever or the 1.5 k resistor?

    To do much analysis a schematic is needed and measurements during normal operation.

    If the 1.5 k is 1/2 watt and maximum RMS voltage (AC or DC) across it was 25 V, then power dissipation would be 625/1500 = 0.42 W. I would never design a circuit with a resistor loaded to that high level. I don't think the resistor is anything other than an indicator of possibly where to look in the circuit for the problem.

    Where in the circuit the 5 A fuse is located is important.

    Is the reversing valve at the 24 V level? If so something related to it might be a problem because it is going to be a high current load.

    Quite obviously much more information is needed. Use a scope and appropriate current sensor to monitor normal current variation thru the 5 A fuse as the system cycles.


  3. #3
    Join Date
    Jul 2007
    Clark County, NV
    Well, I fix HVAC systems everyday and haven't seen that, so don't feel bad about not knowing what would do that. I find that if the fuse blows, it will blow right away until you fix the problem. I have an inline 5A circuit breaker that I use for testing so I don't have to keep replacing fuses until I know I fixed the problem.

  4. #4
    Join Date
    Jul 2008
    awesome! I just ordered up a couple poppers... thanks!
    "A problem clearly stated is a problem half solved."

  5. #5
    Join Date
    Jul 2008
    going to fuse the y g & o with 1a fuses first
    "A problem clearly stated is a problem half solved."

  6. #6
    Join Date
    Jul 2008
    according to 1(800)what-the-#?@!

    there are two other lv loads:
    1. reversing valve which comes off the heat [o]
    2. and there is a lv signal to the defrost stat

    so I'll just narrow it down by fusing the tributary loads w little poppers

    and then refuse the troubled channel again till I find it

    but how to bill for that time consuming work
    "A problem clearly stated is a problem half solved."

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