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thermostat circuit counter

Discussion in 'General Electronics Discussion' started by bradslees, Nov 8, 2014.

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  1. bradslees

    bradslees

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    Nov 8, 2014
    Hi All-

    I am troubleshooting my heating system and want to hack something together that will either log the total hours the thermostat circuit is closed (calling for heat) or count the number of times the circuit opens and closes -- or ideally does both.

    I know almost nothing about this sort of thing, so it's all theoretical at this point and hoping you can help me find out what's possible. Maybe this would involve an electromechanical counter? Maybe something off the shelf already exists that just counts up when it sees current? I don't know!

    (oh, and the cheaper, the better -- of course!)

    Thanks.
     
  2. BobK

    BobK

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  3. bradslees

    bradslees

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    Nov 8, 2014
    yes! that is exactly what I want. the wire that runs from my thermostat to the boiler only has 2 connections (red and white). how would I connect this so it would count when the circuit was closed?
    also, do you know of a similar meter that just counts instances of the circuit being made?

    THANKS!
     
  4. BobK

    BobK

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    The counter that I pointed to, one of many available, has a 10V to 80V DC input. Do you know what voltage is on the two wires from the thermostat when it is on and when it is off? Also AC or DC. If have a funny feeling that they are 24V AC. If so a simple circuit would deal with it or you can probably find another hour meter that takes that input.

    Bob
     
  5. BobK

    BobK

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    I did some more research and I find that thermostat circuits, at least in the US, work on 24VAC as I guessed.

    If this is what you have, search for "24VAC hour meter" and "24VAC counter" to find appropriate ones.

    With these you would just wire the two terminals to the R and W wires of your thermostat, in parallel with your heating system.

    Bob
     
  6. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Most thermostats on commercial heating systems operate from low-voltage AC, typically using a low-voltage door-bell type transformer. On an integrated HVAC (Heating, Ventilation, and Air Conditioning) system there will be separate circuits for heating, cooling, and the air circulation fan. It sounds like you only need to intercept the heating contact closure from the thermostat.

    The elapsed-time meter that @BobK suggested requires 10 to 80 V DC to operate. That means you need a small 12 V DC power supply, a small reed relay with a 12 VDC coil and a normally-open (SPST) contact, and the elapsed-time meter, all wired in series with your thermostat "heat" contact. Use the SPST NO contact on the reed relay to replace the thermostat "heat" contact to your furnace. Reed relays are also available with SPDT contacts, which is okay, but make sure you use the normally open pair of contact terminals.

    Counting the number of times the thermostat calls for heat can be performed by wiring an electro-mechanical totalizer in parallel with the reed relay coil. The web site link requires that you submit your name, address, telephone number, and e-mail address before it will allow you to download the datasheet, so I have attached the datasheet to this post.
     

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  7. BobK

    BobK

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    hevans,

    I wasn't recommending that specific device, it was just to show an example.

    It turns out there are such meters that operate off 24VAC, which is what he really needs.

    Bob
     
  8. hevans1944

    hevans1944 Hop - AC8NS

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    See my previous post #6. You need to separate the thermostat contacts from the two wires going to the furnace and replace them with a relay contact. The thermostat will then operate the relay and the elapsed-time meter as well as the totalizer without these devices interfering with normal furnace operation.
     
  9. bradslees

    bradslees

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    Nov 8, 2014
    thanks guys, this is really helpful.
    these are the cheapest I could find:
    http://www.zoro.com/i/G5151151/
    http://www.zoro.com/i/G1908365/

    would they work?

    as for the wiring, the current setup is a single red and white wire that runs from the t-stat directly to the boiler controller. I would splice this wire into 3 and wire in these devices on 2 distinct wires? do they have an R terminal and a W terminal? I have zero experience with this, but I realize this is a newbie question...
     
  10. hevans1944

    hevans1944 Hop - AC8NS

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    You are free to wire it up any way you like, but if you want it to actually work, you have to use the thermostat contacts to operate a relay... which is probably what they already do in the furnace. The two wires going to the boiler controller are NOT a source of power for your elapsed time meter and counter.
     
  11. BobK

    BobK

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    Well, the counter uses 90mA and the hour meter has not indication of the current used. It might work, it might not. For that matter, there might not be enough power for another relay.

    To do it right, you would need to see how much current the transformer can supply and how much is used by everything powered by it. Or you could do it by trial and error.

    Bob
     
  12. hevans1944

    hevans1944 Hop - AC8NS

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    The point is this: the transformer supplies power to whatever load the internal control circuitry of the furnace presents to the transformer through the thermostat switch. This is a series circuit consisting of the transformer providing power to the internal control circuitry of the furnace in series with the thermostat contacts.

    In the discussion below I use the terms "common" and "hot" to distinguish the two transformer leads. This is arbitrary, so feel free to replace "hot" with "common" and "common" with "hot" without any other change.

    One wire from the transformer 24 V AC secondary winding, call it the "common" wire, is one of the wires (probably white) going to the thermostat. This "common" wire also connects to the internal control circuitry of the furnace to provide a return for other components in the internal control circuitry.

    The other wire from the transformer 24 V AC secondary winding, call it the "hot" wire, also goes to the internal control circuitry of the furnace. These two wires from the transformer, "hot" and "common," together power the internal control circuitry of the furnace, but only one of them is available at the thermostat.

    The transformer "hot" wire does not connect to the thermostat, and therefore no power is available on the other (probably red) thermostat wire. So where does the, probably red, thermostat wire connect? It connects to the internal control circuitry of the furnace, probably to one terminal on the coil of a relay. The other coil terminal of that relay connects to the "hot" transformer wire. When the thermostat contacts close, they complete a circuit in the internal control circuitry of the furnace, said circuit being powered by the transformer.

    When the red wire is connected to the white wire by the closing of the thermostat switch (calling for heat), it completes a circuit in the internal control circuitry of the furnace. This action will eventually actuate the gas valve, or electrical heater contactor, or heat-pump, according to the "logic" inside the internal control circuitry of the furnace.

    The thermostat contacts don't have to energize a relay coil in the internal control circuitry of the furnace. The thermostat contacts could be connected to anything that provides a command-for-heat signal to the internal control circuitry. Whatever it is, it is just a load on the transformer secondary that is switched by the thermostat contacts. It could very well be a high-impedance load, such as a resistor in series with a pair of diodes on an optical isolator, meaning very little power would be available from the "red and white" wires connecting to the thermostat. In any event, placing a load across the red and white wires will divide the transformer voltage between this additional load and the intended load in the internal control circuitry of the furnace.

    The additional load of the elapsed time meter and event counter across the red and white wires could even be mistaken by the internal control circuitry of the furnace to be a thermostat contact closure, calling for heat when the thermostat contacts are actually open. Even if this does not occur, and the load presented by the internal control circuitry of the furnace is a low impedance (such as a relay coil), there may not be enough voltage dropped across the elapsed time meter and event counter to reliably operate them.

    In any event, this is not what the OP wanted: the elapsed time meter and event counter are supposed to operate when the thermostat contacts close, not when they are open.

    I believe the easiest way to do this is to use a small transformer with a low-current 24 V AC secondary connected in series with the thermostat contacts and the coil of a relay. The the elapsed time meter and event counter can be powered from this transformer through normally-open contacts on the relay. A second set of normally-open contacts on the relay are connected to the red and white wires formerly connected to the thermostat. Clearly this requires a relay with a pair of DPST normally-open contacts, or a pair of DPDT contacts. I would not use the thermostat contacts to operate the elapsed time meter and event counter directly, because of the limited current capacity of the thermostat contact, but that is just my personal preference.
     
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  13. BobK

    BobK

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    Okay, I see your point now. They cannot connect to the R and W wires from the thermostat directly.

    You need access to the other side of the transformer as well.

    Bob
     
  14. hevans1944

    hevans1944 Hop - AC8NS

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    I was going to suggest finding the furnace transformer and running a twisted-pair from its secondary along side the thermostat pair, just to avoid having to install a transformer to operate the relay. Technically, since one of the thermostat wires is already connected to one terminal on the transformer, only one additional wire is necessary, but on reflection I decided it would be a real PITA to pull that wire, whether just one or a twisted-pair, through the wall(s). The new transformer used by the thermostat to operate the relay could be mounted in the same enclosure as the elapsed-time meter and the event counter, with a zip cord to plug it into a convenience outlet near the thermostat, and a 4-pin jack and plug to interface to the thermostat contacts and the existing red/white pair of wires. The OP might even figure out a way to mount the enclosure in the wall, but I wouldn't take that skill for granted.
     
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