2 Thermostats in 1 Zone?
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  1. #1

    2 Thermostats in 1 Zone?

    Is it possible to put more than one thermostat in a zone? Id like to keep providing heat until all thermostats are above their set point and keep providing cooling until all thermostats are below their set point. A really fancy might know to up the fan speed if the difference between the thermostats became large. I know commercial systems work kind of this way. Is it ever done for residential?

  2. #2
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    Not really. You can get a thermostat that uses remote sensors. And have them installed in different areas of that zone. And then average the temps of all of those sensors.
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  3. #3
    It seems like this would be easy to do with an old mercury thermostat. If it is hot enough the circuit is open and if it gets cool the thermostat closes the switch to call for heat. If you hooked two of these together in parallel, when either switch closed, the circuit would close and the heat would come on.

    I know the thermostats are a lot fancier now, but is the communication with the heater really more sophisticated?

  4. #4
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    it can be done. in some ways it is easier with the new stats as the anticipators don't factor. but it can be easy to get both the heat and cooling on at the same time. that is not very efficient.

  5. #5
    Yes, but the humidity would go down

    How is this done. Do you need special thermostats or can you hook up regular ones this way?

  6. #6
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    Sep 2005
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    Stop and look

    Step back and look at what has been done!! Tel the coustomer NO unless after looking at whats been done the second stat has a remote sensor and is only mounted in the other zone because of looks. That could be whats up or you could be under attack from THE HANDYMAN !! LOL just step back and think it through, remember to fix something you gotta know how it is supposed to work.
    Goodluck - Warmaire

  7. #7
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    Quote Originally Posted by David Lang View Post
    It seems like this would be easy to do with an old mercury thermostat. If it is hot enough the circuit is open and if it gets cool the thermostat closes the switch to call for heat. If you hooked two of these together in parallel, when either switch closed, the circuit would close and the heat would come on.

    I know the thermostats are a lot fancier now, but is the communication with the heater really more sophisticated?
    Mercury thermostats use a heat anticipator. Connecting the stats in parallel. Will cause short cycling when both call at the same time.

    A digital using remote sensors won't have that problem. Nor will there be a problem that someone sets one stat too high or too low. Since all the temp settings are done from one unit.
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  8. #8
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    Quote Originally Posted by beenthere View Post
    Mercury thermostats use a heat anticipator. Connecting the stats in parallel. Will cause short cycling when both call at the same time.

    A digital using remote sensors won't have that problem. Nor will there be a problem that someone sets one stat too high or too low. Since all the temp settings are done from one unit.
    other way isnt it? 1/2 the current through each- long cycle? if only one calls then you get a normal cycle? unless youset it to short cycle on one any way does not matter with digital stats. but there are lots of ways to screw it up.
    dont do it unless you know how. and if you know how you know not to do it.

    the remote aveaging stat is good. not real simple or cheap

  9. #9
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    Quote Originally Posted by fearlessfurnace View Post
    other way isnt it? 1/2 the current through each- long cycle? if only one calls then you get a normal cycle? unless youset it to short cycle on one any way does not matter with digital stats. but there are lots of ways to screw it up.
    dont do it unless you know how. and if you know how you know not to do it.

    the remote aveaging stat is good. not real simple or cheap
    No. 2-1500 ohm resistors wired in parallel equals 750 ohms.
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  10. #10
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    Not sure your reason for doing this and I could be wrong, but if you did hook up the two tstats wouldnt you have considerable overheating/overcooling of the stat that satifies first? The temperature between the two stats will never be right unless you split the ductwork up and zone it properley.

  11. #11
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    Quote Originally Posted by beenthere View Post
    Mercury thermostats use a heat anticipator. Connecting the stats in parallel. Will cause short cycling when both call at the same time.
    If each stat has the anticipator set as if there were only one stat, when both call there would be less anticipator effect (half the amperage through each anticipator assuming equal resistance). When one thermostat stops calling the anticipator of the one still calling would become normal. But, the one that satisfied first would cool down and start calling sooner than it would have if it's anticipator had been working normally. But, that might be offset by the overheating of the space by the other thermostat.

    And what about the cooling anticipator on mercury stats? Since they function during the off cycle when the mode switch is set to cool, having two in parallel will lengthen the cycle (half the amperage, less heating, longer time to initiate a call).

    If the boss said I had to wire two thermostats in parallel, first I'd say "really?" then...

    Relay logic to prevent either stat from operating in an opposing mode. So, if a stat is in cooling mode (may or may not be calling for actual operation) the other stat heating mode is locked out, and so on. Would take two relays for each stat and one extra wire at the stat. Some stats may not have the appropriate terminal for this function.

  12. #12
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    Quote Originally Posted by beenthere View Post
    No. 2-1500 ohm resistors wired in parallel equals 750 ohms.
    Heat circuit amperage is a fixed value, not dependant on the anticipator value. With two equal paths, amperage in each path will be half.

  13. #13
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    you see it done around here in newly finished basement situations (forced air)works ok even with T87Fs. were things get screwy is if you try to do the heat and AC through both stats. real easy to get the heat and cool both running.

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