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GSM Control of Furnace

Discussion in 'General Electronics Discussion' started by sparticus, Apr 23, 2013.

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  1. sparticus

    sparticus

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    Apr 23, 2013
    Hello all. I'm a new user and just had a (hopefully) simple question for you. I've got a furnace at a cabin that I'd like to control remotely. I've come across some products that allow you to remotely control a relay via SMS or phone call. I was wondering if anyone had any ideas on how I could hook this up? I assume the furnace would need to continue to run, and I'd wire the relay to the thermostat input or something to trigger it to turn on?

    I read in the FAQ not to post links so I will not link to the product in question, but can anyone give me some pointers to get going in the right direction? Is control via GSM a bit overkill?
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    You could use the relay from the GSM receiver to control the mains supply of the furnace - assuming the relay is rated for the voltage and current required by it.

    But: Do you really want to remotely turn on the furnace while no one is there to keep an eye on it? I assume the furnace is not in your house - then using GSM would be overkill indeed, but out somewhere, e.g. in a cabin. With bad luck, you arrive at the cabin and the cabin is gone, burnt down by some failure of the furnace.

    In case my assumption is wrong and the furnace is in your house or at least rather near, use a short range radio technology (e.g. Wifi, Zigbee or similar) , not GSM.
     
  3. sparticus

    sparticus

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    Apr 23, 2013
    Thanks for your fast response.

    I was thinking of keeping the thermostat in the loop, and interrupting the wire from it to the furnace. So the thermostat keeps saying, "more heat more heat", but the relay interrupts that. Then, if you turn the relay on, it would turn on the furnace.

    I am sometimes away for a few days at at time, so if safety is an issue, then perhaps it's not the best. How do I know how many amps it is rated for, and how many amps do I need?
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    11,513
    2,651
    Nov 17, 2011
    The rating of the relay should be stated in the GSM receiver/relay unit's manual. Or look up the type of relay used and find the datasheet.
    Similarly, the manual of the firnace (or at the very least the type label of the furnace should state the amperage needed for switching the mains (note that in that case the inrush current of a cold firnace is many times the operating current of the warm furnace). This is irrelevant, if you switch only the thermostat wire. The current there won't be more than a few milliampere.
     
  5. sparticus

    sparticus

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    Apr 23, 2013
    You're making this sound so much easier than I first expected...so I just splice the wire from the thermostat to the furnace? Or do you think it would be better to simply "switch the mains" as you say.

    If I wanted to switch the mains (so I can ensure a full shut off), you said that the inrush could be very high. I have found a device that claims it is "high amperage", but I don't know what that is in regards to. High amperage for a blender? High amperage for a furnace? I will have to dig out the manual for the furnace, but is there a safety margin I should watch for? You're a mod, so feel free to remove this link if it violates any rules; I just want your feedback on what "high amperage" is.

    High Amperage?
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    2,651
    Nov 17, 2011
    High amperage is relative. 30A is nothing if you want to operate e.g. an aluminum smelter. 30A makes 7.2kW which would be quite a bit for a household appliance.
    You'll have to check the ratings of the furnace.

    As to whether it is better to cut the mains or the sense wire: I frankly don't know. Cutting the mains will safely disconnect the furnace and minimize the risk, but it is also dangerous to fiddle with mains if you lack experience! You should get the help of someone who knows how to handle this.
    Cutting the sense wire to the thermostat is probably less dangerous, but you'll have to make sure that this works as expected. It would be a bad thing to do if for example the furnace would turn ON without a signal from the sensor (unlikely, but who knows?).
     
  7. sirch

    sirch

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    1
    Dec 6, 2012
    I'm guessing that you are in the USA? It may be helpful to know if the furnace uses electricity for heating or just for control and some other fuel (gas, oil) actually produces the heat?

    Here in the UK most people use gas "boilers" for heating. The thermosat is normally run at mains voltage but only has to handle the current for the diverter valve, the diverter valve switches on the circulating pump and ignites the boiler when it is open.
     
  8. sparticus

    sparticus

    4
    0
    Apr 23, 2013
    I'm in the USA, and it's an old electric furnace at a cabin. No natural gas lines out here unfortunately.

    I think I'm going to go the route of the thermostat first and test it (try to learn how to use a multimeter and do some testing to ensure that when I turn the relay off that the furnace stays off, etc.) during the warmer months.

    Thanks Harald and Sirch for your help :) I'll post back here with progress some time if anyone is interested.
     
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