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  1. #1
    Join Date
    Nov 2010

    Default Programmable Timer switch

    I travel a lot and when I get back home it smells musty because it is shut up. I have what is called a 'summer switch' on the heating system that lets the fan run without the heater going on. It is a simple on-off toggle switch that turns the fan on and off, bypassing the heater.

    So I came up with an idea to fix a timer switch instead of the toggle switch, and let it turn on and off automatically a few times a day.

    I expected 110V but found that the ‘summer switch’ wires were only carrying AC 25V. But I fixed the programmable timer anyway and it runs fine. The house smells clean all the time now and the switch is not malfunctioning or over heating or any thing.

    Here is question:
    The switch I put in is designed to operate in a 110V circuit. Here, I have put it in a 25V circuit.

    Can you tell me how these things work? And if I am doing something dangerous?

  2. #2
    Join Date
    May 2008
    Pacific Northwet

    Default Re: Programmable Timer switch

    Is it an electronic (digital) or mechanical (dial) switch?

    If it's electronic, it's not entirely surprising that it works. Some electronic devices can operate on a wide range of voltages. If the timer has an internal battery, that battery is what operates the timer and the "120V" refers to the maximum voltage the switch can handle.
    The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.

  3. #3
    Join Date
    Nov 2010

    Smile Re: Programmable Timer switch

    Thank you so much for replying. I hope you don't mind if I bounce a couple of thoughts I have had off you.

    Yes, it is electronic.

    I too realized it has some kind of a capacitor or battery inside to let it work for a few hours in case of failure. I suspected initially that it was working on its battery and would stop working after a few hours/days when it would be fully drained. But it is working for the last week and looks fine - so it must be re-charging also.

    I think your surmise that this can work over a wide range of Voltage is probably correct. I was worried that, because I was using it on such low voltage it might draw a huge current to compensate and over heat.

    But I think this part of the circuit has Zero Watts in load because it does not directly connect to the Fan. I think it connects to a logic circuit that actually turns the fan on and off.

    In other words the logic may be something like this:
    If the Summer switch 'on'; Turn on the fan and air filter.
    If the Summer switch is off, but heater is on, turn on the fan and air filter.
    If both Summer Switch and Heater are off, turn off the fan.

    Does this sound like a crazy theory to you?


  4. #4
    Join Date
    Aug 2008

    Default Re: Programmable Timer switch

    The 24 volt control wires on a furnace operate a relay to start the fan in the cooling mode and it does use a small amount of watts. There are hundreds of different systems for new systems, but the old standard is this:
    1. With a call for cooling, the compressor contactor and the indoor fan relay are energized.
    2. With a call for heat, the gas valve is energized and the fan thermostat (not the fan relay) will energize the indoor fan motor when the correct temperature in the heat exchanger is reached (aroung 120 degrees).
    3. With a call for fan the indoor fan relay is energized. The "summer switch" and the "fan" switch on the 'stat are using this method. It is one and the same.

    A timer has two circuits. One to operate the timing control, and a seperate set of contacts to control a load. They don't have to be the same voltage.
    Last edited by ZZZ; 11-10-2010 at 11:54 AM.
    "Lead by Example"

  5. #5
    Join Date
    Nov 2010

    Default Re: Programmable Timer switch


    So it means the switch can use 24V and function fully as far as the clock is concerned? I was under the impression that since it would normally get 110V, there would be some kind of step-down mechanism built-in to feed the clock and other circuitry inside. And because of that I wondered how the step-down would fare with a 24V input.

    Thank you for replying. Much appreciated!

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