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    goodvibe's Avatar
    goodvibe Posts: 3, Reputation: 1
    New Member

    Apr 29, 2005, 01:11 PM
    Finding the 24V AC and ground for elec. Filter

    I want to install an electrostatic filter in my furnace. The filter's power source requires 24 V AC.

    The wire is split with black (for the 24 V AC) and white for the ground.

    I'm not experienced at all with wiring, so when I open the panel I have no idea where to connect these two wires.

    Can you help?
    tkrussell's Avatar
    tkrussell Posts: 9,651, Reputation: 724
    Uber Member

    Apr 29, 2005, 02:34 PM
    Any 24 volt power source found in a furnace will be sized to handle the 24 load of the furnace components only. Your filter instructions should tell you what size transformer needed , rated in VA (volt-amperes) to power the filter, and I am fairly sure the transformer in the furnace will not handle the load of the filter.

    You will need to provide a separate transformer as a power source for the filter. This will require you to find a 120 volt source to connect the primary of the transfomer to. If you are not sure how to wire a 24 volt circuit,I doubt you will be able to handle a 120 volt circuit safely.

    If you decide to tackle this project, I need to know the load of the filter, to determine the size of the transfomer. Should you manage to find 24 volts in your furnace, and decide to connect the filter to it because someone helps you without concern to the size of the transformer, the furnace transformer will probably fail, causing the furnace to stop working completely. Get back here with that situation and perhaps we can help you replace the transformer and we can start all over again.
    goodvibe's Avatar
    goodvibe Posts: 3, Reputation: 1
    New Member

    May 2, 2005, 08:18 AM
    Filter load - tkrussell
    Thanks tkrussell,

    The specs for the filter (Cimatec's Aircleen 1500) are:
    Nominal input voltage: 24 volts (floating)
    Input voltage range: 18 v min to 30 v max
    Power consumption: <2 watts
    Apparent power: allow 2.0 VA

    If I understand your comments, I have to get a transformer and attach it to a 120 volt power source so it can convert the power to 24 volt for the furnace. However, I the transformer needs to be the right size - How is that determined?

    Thanks again.
    labman's Avatar
    labman Posts: 10,580, Reputation: 551
    Uber Member

    May 2, 2005, 09:02 AM
    This is no big deal. I am sure the existing transformer in your furnace will handle the additional <2 watts the filter control draws. I am not sure of the size of the one in my furnace, but it is protected with a 3 amp fuse. Normally electronic filters are wired so that they only are on when the furnace or blower are on. If you have A/C, I am sure you will want the filter on when it runs to. My furnace actually has contacts to wire in an electronic filter. If you don't have the luxury of a booklet on your furnace, it should have a schematic on or inside. Many HVAC units have instructions in an envelop inside. You may find it when you open it up.

    As general background, 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 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. 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.

    If your furnace does not have auxiliary contacts for a filter or humidifier, you want to operate the filter from the contacts for the blower which operates the relay controlling the 115V to it. Connect the black wire to the likely green, and the white to the likely blue. Actually it doesn't make much difference. The 24 volts are a floating AC with no real ground. Get back with me if your blower operates at different speeds.

    Does the 24V run the filter, or just power a relay switching 115V AC? If so, you will want to connect the relays 115 V black, white, and ground (green or bare), to the furnace's black, white, and ground. The 12 gage wiring and 15-20 amp breaker for the furnace should be adequate to carry the additional amp or 2 for the filter.
    goodvibe's Avatar
    goodvibe Posts: 3, Reputation: 1
    New Member

    May 2, 2005, 10:59 AM
    Thanks labman
    Thank you labman - I'm going to go home tonight and look it over, to see if I can make sense of it, from your comments.

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