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  1. #1


    i'm a student currently studying HVAC and have a few questions regarding pneumatic systems. I'm hoping to get some help understanding the subject better from experienced control technicians.

    my questions are:

    what is the output response for direct acting and reverse acting thermostats?

    what are their applications?

    what are some ways of determining if the thermostat or the box it's controling are defective when there is a problem?

    is there a good internet resource i can use to understand how pneumatic control systems operate and how to troubleshoot them?

    thanks in advance... i really appreciate any time you spend anwering my questions...


  2. #2
    I'll take the 1st question,,,,,

    Direct Action is when the temperature goes up, the output goes up. When the temperature goes down the output goes down.

    Reverse Action is when the temperature goes up the output goes down. When the temperature goes down, the temperature goes up. This is the case in pneumatic as well as electronic. Substitute pressure or whatever you are controlling for temperature and the rule still applies. RR

  3. #3
    correction in the direct action,, as the temp goes up the output goes up,, temp goes down the output goes down,, that is direct action,, sorry

  4. #4
    Join Date
    Feb 2005
    our good friend Osiyo was always quite helpful in these matters. Please follow this thread, go down to his post and I suggest you study the honeywell manual that he provided a link to.

  5. #5



  6. #6
    Join Date
    Sep 2006
    I'll take the mixing box question. Mixing boxes essentially have a heating and cooling supply duct ran to a local mixing box. A thermostat proportionally modulates the damper inside the box between the hot and cold ducts. This control strategy enabled precise temperature control. Draw backs include increased installation costs and excessive energy consumption.
    Law Of The Thermostat: He who has the thermostat wins!!!!!

  7. #7
    Join Date
    Oct 2005
    Troubleshooting is relatively easy compared to software based systems because you can plug in air gages and view the pressures. It's hard to watch those bits and bytes.

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