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  1. #1
    I would like a sanity check on the way a second thermostat controls the blower on my furnace. The set-up is that a second thermostat was installed with a new HVAC system to turn on the furnace blower in order to circulate warmer air from that part of the house for more even heating. The blower does turn on at the set temperature, but the AC unit also comes on which of course results in cold air blowing through the ducts. Is there a way to control the blower operation from a second thermostat without having the AC or heater function kick on or am I asking for something that cannot be done? The quick answer from the intaller was to throw the breaker on the AC circuit, but there is still a "hum" at the outside unit when the 2nd thermostat calls for the blower. Thank you.

  2. #2
    Join Date
    Nov 2004
    Posts
    2,927
    yes it is possible....wth some modification

  3. #3
    Join Date
    Nov 2004
    Posts
    2,927
    auxillary manual switch W and Y

  4. #4
    Join Date
    Mar 2005
    Location
    burlington county n.j.
    Posts
    9,765
    what happens if you turn fan on at both thermostats?? sounds like a feedback thru other thermostat.

  5. #5
    Join Date
    Dec 2005
    Posts
    1,923
    bpink21901

    please elaborate your application a little more.
    If i understood you correctly: You have one HVAC system. You also have two thermostats. One is to turn on/of the HVAC systems accordingly. The other thermostat's sole function is to turn on the blower ONLY on your HVAC system so it air can circulate throught the area that the second thermostat is located. Am I correct?

    If I am correct then you need to have an isolation relay in conjuction with you second thermostat and G terminal. It will be cheaper if you use mechanical thermostat. If you use electronic thermostat, then you have to add two more relays in addition with the isolation relay mentioned above.


  6. #6
    just_opinion - You are dead on correct in your description of how it is to operate. The second thermostat is to be a 'blower only' switch to circulate the air through the house. I'm sorry if I am not using technically correct terms in this expanded laymans description.

    The second thermostat is a mechanical thermostat (simplistic model with only a temperature slide control) with two wires connected (red / green) to the R & G terminals on the furnace control board. It is wired directly to the terminals, there is presently no isolation relay. The main thermostat is also mechanical but has the heat / cool / off switch along with the fan - auto / on switch. The red / green wires of the main thermostat are also connected to the R & G terminals on the furnace board.

    t527edThere is no fan control switch on the second thermostat. When I put the fan control switch to 'on' at the main thermostat the blower works fine, it doesn't engage the A/C unit and there is no "hum" at the A/C compressor when the A/C breaker is 'off' (unlike when the second thermostat calls for the blower when the A/C breaker is 'off' there is a 'hum' at the A/C compressor).

  7. #7
    Join Date
    Mar 2005
    Location
    burlington county n.j.
    Posts
    9,765
    g circuit is backfeeding thru main thermostat due to power being applied to g while fan switch is in auto position. relay would have to be installed to break g circuit to main stat when 2nd stat is bringing on fan.

  8. #8
    Join Date
    Dec 2005
    Posts
    1,923
    bpink21901

    If you are the home owner and HAD a company out to do this then I am very sorry. You need to can this company and get another company who can spell "CONTROL" to do this correctly and you can have more time to surf the net for something else more useful in you personal life. I am not trying to be rude

    If you are the PRO, then please go to the PRO site and we can help.

    Due to the site rule, we cannot show you step by step.

    But the way it got set up is VERY INCORRECT and it must be correct

    Something for you to think too - WHAT IF THE MAIN THERMOSTAT IS CALLING FOR HEAT AND THE OTHER ZONE NEEDS CIRCULATE AIR FOR COOL

  9. #9
    just_opinion....
    It is sad but true that I am the homeowner, not the hvac "pro" I hired(I would name them, they are in Rising Sun, MD, but thought it might break a site rule) to install it.
    I am not looking for step by step instructions - I am not interested in fixing it myself at this point since it is only a 2nd thermostat feature that we can live without. I imagine it would be hard to get anyone interested in coming out to fix it (I called seven companies for estimates when installing the system, five came out to look and only received two written replies).
    When I discovered the 'hum' at the compressor before Christmas while hanging lights I just set the temp slider to 90' on the second thermostat so it won't ask for the blower to come on. It was recently suggested by another non-pro that I post the question here to see if it was supposed to work that way. I appreciate the time taken to reply to my question.

    I am not understanding the impact of the scenario in your question but would like to in case I ever get brave enough to hire a "pro" again. If it is warm enough at the 2nd thermostat to switch the blower, is it a bad thing to have the first thermostat call for heat?

    [Edited by bpink21901 on 01-25-2006 at 05:08 PM]

  10. #10
    Join Date
    Dec 2005
    Posts
    1,923
    But the RULE did not say any thing about you paying my air fair from TX to MD. I have relatives and friend up there so you don't have to worry about room and board. I garrantee you the hum noise will go away one way or another

  11. #11
    Join Date
    Jul 2005
    Posts
    648
    I'm trying to figure out why you need 2 t-stats to do that. If you have a standard furnace with A/C, why can't you just flip the "Fan" switch to "on"?? Are you trying to say that you have 2 seperate systems, one for A/C and one for cool? Maybe you meant "boiler" instead of "furnace"? The "hum" you hear at the A/C condensor is the compressor contactor energized.

  12. #12
    matt8085....sure, I could turn it on manually from the main thermostat (thats what I did previously and thats what I will do now) but when we had our new system installed, my desire was to have the 2nd thermostat installed to automatically turn the fan on to circulate the warmer air through the house. Because of the layout of the house, one area gets warmer from the sun than where the main thermostat is located and I wanted to move that air around. Call me lazy ( I also use a TV remote rather than walking over to the TV), but I was told by the installer it could be set up that way rather than manually turning it on and off 4-5 times per day.

    There is only one system it is a bryant MAV350 lp fired forced air furnace / A/C unit.

    just_opinion....
    I'm a little leery of your offer....fix the thermostat controls....or....a brick upside my head....both will make the hum go away and I'm not sure which solution you would be inclined to try.....LOL

  13. #13
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,782
    If you call that "pro?" back, tell him he needs to install an isolation relay.

    I use to go to 1 or 2 the bars in rising sun when I was 18, back in the 70's.
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    How many times must one fix something before it is fixed?

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