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Thread: Relay controlled fans from furnace

  1. #31
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    Quote Originally Posted by Besoeker View Post
    Just out of interest, what decides when to turn the fan on or off? Is it a manual operation or is there some sensor?
    For my money I'd put in room thermostats that call for the fans to come on if the temp falls outside a certain range.

    Lets say you want the house to always be at between 68 and 74. As long as it is, the thermostat doesn't call for fan on. Otherwise, whether it's too warm or too cold, the thermostat says "Fan on!" The neat thing about that is it doesn't matter if the furnace is set to Heat, Cool, or Auto. If the room is too cold, presumably the house generally is trying to warm, so you won't be demanding cold air when it's cold. Likewise if the room is too warm. The only thing is that the thermostats will need an independent power supply. Probably a 24V transformer off the same circuit powering the fan it controls.

  2. #32
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    Quote Originally Posted by Besoeker View Post
    Just out of interest, what decides when to turn the fan on or off? Is it a manual operation or is there some sensor?
    sounded to me like he would use the 24v from furnace and extend that out to low-v switches in each location. that assumption based on his comment that 1rib per fan was he choice for independent control of each fan. but to do that some other switch/device would need to be installed to switch the rib but only works when furnace is running.

    if that is the case, i would simply rib the 120v right there at the furnace and then run 14/2 like any other ckt, out to switch box(es) then up to fan(s). 1rib & indpendent control.

  3. #33
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    Quote Originally Posted by gadfly56 View Post
    For my money I'd put in room thermostats that call for the fans to come on if the temp falls outside a certain range.

    Lets say you want the house to always be at between 68 and 74. As long as it is, the thermostat doesn't call for fan on. Otherwise, whether it's too warm or too cold, the thermostat says "Fan on!" The neat thing about that is it doesn't matter if the furnace is set to Heat, Cool, or Auto. If the room is too cold, presumably the house generally is trying to warm, so you won't be demanding cold air when it's cold. Likewise if the room is too warm. The only thing is that the thermostats will need an independent power supply. Probably a 24V transformer off the same circuit powering the fan it controls.
    Admittedly I haven't work with HVAC for years, but that doesn't sound like a standard TSTAT from the local Big Box. Maybe with a bit of tweaking.
    Tom
    TBLO

  4. #34
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    Quote Originally Posted by Besoeker View Post
    Just out of interest, what decides when to turn the fan on or off? Is it a manual operation or is there some sensor?
    What he calls the "aux 24 volt relay" is only energized (contact closed) when the furnace is running. These are commonly called the 'X / Y' terminals on the furnace control.
    It is common to add a local thermostat in series so that the booster fan only runs when the furnace is running AND the thermostat is calling for heat.

  5. #35
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    Quote Originally Posted by ptonsparky View Post
    IMO, Boosters are the result of poor design to begin with.
    They're often needed when air conditioning is added to an old house. The duct design that was perfectly adequate for distributing warm air often needs a little boost to get cool air to the second floor, now that gravity & buoyancy are opposing airflow instead of aiding it.

  6. #36
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    Quote Originally Posted by gadfly56 View Post
    For my money I'd put in room thermostats that call for the fans to come on if the temp falls outside a certain range.

    Lets say you want the house to always be at between 68 and 74. As long as it is, the thermostat doesn't call for fan on. Otherwise, whether it's too warm or too cold, the thermostat says "Fan on!" The neat thing about that is it doesn't matter if the furnace is set to Heat, Cool, or Auto. If the room is too cold, presumably the house generally is trying to warm, so you won't be demanding cold air when it's cold. Likewise if the room is too warm. The only thing is that the thermostats will need an independent power supply. Probably a 24V transformer off the same circuit powering the fan it controls.
    OK - furnace as in domestic use...........I should have had my American hat on...............
    Si hoc legere scis nimium eruditionis habes.

  7. #37
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    Quote Originally Posted by Mustwin351 View Post
    So is it allowable by code to mix a class 1 and class 2 circuit in the same box?
    Yes, but generally still need to provide some separation between them. If not allowed in same box we could never place a relay/contactor with power/lighting and class 2 control coil within the box.

    Quote Originally Posted by Mustwin351 View Post
    I was thinking the same. So with that particular RIB where there is no separation between high and low voltages.....what situation or place could you actually use it legally by code?
    24 volts can still be class 1 control circuit in some cases, but in such cases the entire control circuit is class 1. You can not have class 1 and class 2 or 3 "sections" on the same control system - the entire system is the same class.

    Quote Originally Posted by gadfly56 View Post
    For my money I'd put in room thermostats that call for the fans to come on if the temp falls outside a certain range.

    Lets say you want the house to always be at between 68 and 74. As long as it is, the thermostat doesn't call for fan on. Otherwise, whether it's too warm or too cold, the thermostat says "Fan on!" The neat thing about that is it doesn't matter if the furnace is set to Heat, Cool, or Auto. If the room is too cold, presumably the house generally is trying to warm, so you won't be demanding cold air when it's cold. Likewise if the room is too warm. The only thing is that the thermostats will need an independent power supply. Probably a 24V transformer off the same circuit powering the fan it controls.
    If I were to do it that way I would probably look into line voltage thermostat and directly control the fan with it.

  8. #38
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    Quote Originally Posted by kwired View Post
    Yes, but generally still need to provide some separation between them. If not allowed in same box we could never place a relay/contactor with power/lighting and class 2 control coil within the box.

    24 volts can still be class 1 control circuit in some cases, but in such cases the entire control circuit is class 1. You can not have class 1 and class 2 or 3 "sections" on the same control system - the entire system is the same class.

    If I were to do it that way I would probably look into line voltage thermostat and directly control the fan with it.
    Yep, that's a better idea.

  9. #39
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    Quote Originally Posted by Besoeker View Post
    Just out of interest, what decides when to turn the fan on or off? Is it a manual operation or is there some sensor?
    For now manual.

    For tomorrow... Ain't that the beauty? Could become smart home. Could be the kids tablets or occupancy sensor.

    I have dreams but no time. Hence the constant overcomplicating desires.

  10. #40
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    http://forums.mikeholt.com/showthrea...816&highlight=

    I previously had asked about isolating classes. Similar problems. Only 1 box save a big project box did I find

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