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  1. #1
    Join Date
    Feb 2005


    I have a hydroair system with an air handler for the main floor in the basement next to the oil burner and an air handler for the second floor in the attic. Since it's regularly below zero here and since we have occasional power outages, I got worried about freezing in the attic loop and had glycol put in the system.

    After that I noticed that the fan in the air handler in the attic was running essentially all the time, even when the thermostat upstairs was not calling for heat and the circulator had been off for a long time. I checked and found that the pipe is hot on the supply side of the attic air handler but cold on the return side. It's slow circulation due to convection, I thought, just enough to make the hydrostat turn the fan on. Sure enough, if I switch off the power to the air handler, in a few minutes, the return side starts to warm up.

    Then I got to thinking -- I wonder why they put the hydrostat on the supply side of the air handler instead of the return side? When the circulator turns off, the temperature drop on the return side is much bigger than it is on the supply side. So, I had a look at the air handler in the basement and discovered that its hydrostat *is* on the return side.

    Now I'm confused. Which side of air handler should the hydrostat be on? Should I just turn up the set temp for the one in the attic until the fan shuts off? Why did this start happening after putting glycol in the system?

    Any help or enlightenment would be most welcome.

  2. #2
    Join Date
    Mar 2001


    The fan control or "B" aquastat belongs on the return side of the coil ,you are correct.

    The over heating problem..... Depends if your system uses flow or zone valves . They may have been left open , need to be cleaned or you need a new one......

  3. #3
    Join Date
    Feb 2005

    Thumbs up Thanks for the information

    Thanks for the information about the placement of the aquastat. It's good to know that I'm not as confused as I thought.

    You were also exactly right about the cause of the air handler fan running continuously. The system has a flow control valve in the supply side of each zone near the boiler, and the tech who put the glycol in left all of them open. The result is convective flow (aka gravity flow). A quick twist of the control, and no more problem.

    As a side benefit, I now know how to get a little heat if a circulator dies.

    Thanks again for the information, it was very helpful.

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