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    courtne's Avatar
    courtne Posts: 6, Reputation: 1
    New Member

    Jun 29, 2005, 09:50 PM
    24v transformer bad
    My 24v transformer is bad on my gas pack heat and ac unit. I replace the transformer 2 months ago... I replaced it again yesterday. This time it went bad in 4-hours. What is shorted to blow the transformer?

    Also, DC is used to pull the a/c and fan relays, right? Where is the 24v ac converted to DC?
    courtne's Avatar
    courtne Posts: 6, Reputation: 1
    New Member

    Jun 30, 2005, 08:46 AM
    24v transformer bad
    I got a new 24 volt transformer today and a compressor relay... The transformer smoked immediately. The 24v was 1.5-ohms to ground... so all of the relay coils are 1.5 ohms above ground. The coils are about 60-ohms, but the Ignition module is grounded and it has a wire from ground to the 24v transformer! Is this right? The module says GND on the plug... so that is right... but the module attaches the ground to the 24v transformer - directly. I am going to replace the ignition module. Any hints?
    labman's Avatar
    labman Posts: 10,580, Reputation: 551
    Uber Member

    Jun 30, 2005, 08:59 AM
    Add an inline fuse to the transformer circuit, maybe the primary. Check the amp rating for it, and choose a fuse slightly larger. I went through several transformers and fuses before I discovered the chipmunks chewed the insulation off the wires in an inaccessible, for me, area. Fuses are much cheaper than transformers. I found all I needed at Radio Shack.

    The ignitor should show a measurable resistance to ground. You may have found the problem. Some of the solenoids and relay coils use AC. Newer furnaces with control boards, will have a DC power supply on the board. They may use DC to control relays controlling AC. Most devices are marked with the power they use.

    Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.

    In the above paragraph, you can see all the devices controlled by the thermostat need to feed back to the common terminal of the transformer. If it is the A/C season, why is the ingitor powered? You may have a problem with the thermostat or controls. I lost a board to I think lighting. My furnace now has a surge protector.
    courtne's Avatar
    courtne Posts: 6, Reputation: 1
    New Member

    Jun 30, 2005, 09:36 AM
    24v transformer bad
    Thanks! The ignition board puzzles me... But it is not grounded now and the AC is running. I will get the ignition board later today. I will post what I finally find. :)

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