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  1. #1
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    Want to use my existing central A/C to chill water tanks

    Hi, I'm working on an HVAC project, and am trying to figure out the best way to chill some barrels of water. My current plan is to buy an air-to-water heat exchanger (like a car radiator), and install it in an air box next to the air handler in my attic. I would connect that box to the air handler with 2 flex ducts (1 to the return side and the other to the supply side). I would also put a motorized damper in one of those ducts to prevent air flow through the air box during normal operation.

    The damper would be closed almost all the time, but whenever the AC is running AND the thermostat is about to turn the system OFF, I would have a "piggy-back" controller take control of the system. If the temperature of my water tanks is higher than I want, then the controller will keep the AC running while the motorized damper is opened up and water starts pumping through the heat exchanger. While running like this, I believe there will be very little air flowing through the rest of the ducting, since most of the air should be "short circuited" through the heat exchanger.

    I haven't done the math about how large the ducts and heat exchanger need to be, but it seems that as long as I can pump enough heat through the heat exchanger to maintain 15-20F across the evaporator coil, then I should be able to chill the water down near freezing. That might take a large sized exhanger and water pump, but it seems possible.

    Do you guys agree? Am I correct that the air looping over and over between the evaporator coil and exhanger can be super-cooled (at or below freezing), as long as a 15-20F temp drop is maintained across the evaporator?

  2. #2
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    Mar 2013
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    the central AC condenser, mainly the compressor, isnt designed for operation that low. it'll trash it.

    you'd be better off with an air to water heat pump and a water to water exchanger.

    or do this:

    http://www.drsfostersmith.com/produc...FYgCaQodLVkPog
    Experience - knowing when to get the hell out of the way and plug your ears. "Don't be a sissy. Turn it on!"

    Glennac - Failed my biology test today: They asked, "What is commonly found in cells?" Apparently "BL*** people" wasn't the correct answer.
    Poodle Head Mikey - "the world is well populated with the unknowing and the uncaring and the stupid."

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  4. #3
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    Thread Starter
    If this does seem possible, then my next question would be just how cold the looping air get? If I put antifreeze in the water flowing through the heat exchanger, would the system be able to chill it even further below freezing?

  5. #4
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    Thread Starter
    Thanks for the reply.

    What would be the lowest temperature that the evaporator coil can be?

    I actually have an aquarium chiller, but I would guess that it would be very inefficient. I should do some testing with it though. Thanks for suggesting it.

    The air to water heat pump is also interesting, but all the systems I've seen look like they are focused on heating. I haven't been able to find much info at all about their ability to chill water.

  6. #5
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    A small air cooled process chiller would make more sense.

  7. #6
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    Quote Originally Posted by Gernby View Post
    Thanks for the reply.

    What would be the lowest temperature that the evaporator coil can be?

    I actually have an aquarium chiller, but I would guess that it would be very inefficient. I should do some testing with it though. Thanks for suggesting it.

    The air to water heat pump is also interesting, but all the systems I've seen look like they are focused on heating. I haven't been able to find much info at all about their ability to chill water.

    The aquarium chiller is far more efficient than converting a central ac unit. plus, it's designed for it. the air-water heat pump works for both heating and cooling. the reversing valve changes whether or not you are heating or cooling. Like this:

    http://www.waterchillers.com/air-cooled-chillers.html

    But, it's basically a large aquarium chiller.
    Experience - knowing when to get the hell out of the way and plug your ears. "Don't be a sissy. Turn it on!"

    Glennac - Failed my biology test today: They asked, "What is commonly found in cells?" Apparently "BL*** people" wasn't the correct answer.
    Poodle Head Mikey - "the world is well populated with the unknowing and the uncaring and the stupid."

  8. #7
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    Your air conditioner isn't designed to cool anything off to near or below freezing conditions.
    The setup you describe would be best suited for a small tonnage process chiller.
    The setup you described will not work.
    I havent failed. Ive just found 10,000 ways that wont work. - Thomas Edison

    Its not whether you get knocked down, its whether you get up. - Vince Lombardi

    "In this house we obey the laws of thermodynamics" - Homer Simpson

  9. #8
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    Thread Starter
    I appreciate your input about this. However, the point of this project is to increase efficiency of my HVAC system using chilled water as "cold batteries". My system currently runs many short cycles throughout the day, but I believe I could extend those cycles during the cooler parts of the day / night while chilling water, then run fewer of the cycles during the hot part of the day (using the chilled water to cool the house).

    I understand that my AC unit may not be able to chill the water to freezing, but it could surely chill it to 50F, since that's the temp of the air coming out of my ducts. So I guess I'm really just wanting to know how much colder it could run than that? I haven't found anything in the documentation about the lowest achievable temperature.

    FWIW, I have a Goodman GSX16-0481F compressor with a CHPF4860D6D coil (with TXV kit), and a GME80805D furnace. The blower is integrated into the furnace.

  10. #9
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    What your describing is referred to as ice banking, and it's been around for many years in the industrial section of the trade.
    In order to prevent damage the incoming air across the evaporator would have to remain above 64-68*. There are modifications that can be done but that's way past diy level.
    This would be many times more efficient using a refrigerant to water heat exchanger
    I havent failed. Ive just found 10,000 ways that wont work. - Thomas Edison

    Its not whether you get knocked down, its whether you get up. - Vince Lombardi

    "In this house we obey the laws of thermodynamics" - Homer Simpson

  11. #10
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    Not gonna work...

    This would be like using a Corvette engine in a dump truck...
    Totally WRONG application of equipment...
    GA-HVAC-Tech

    Your comfort, Your way, Everyday!

  12. #11
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    Quote Originally Posted by ga-hvac-tech View Post
    This would be like using a Corvette engine in a dump truck...
    This would work with proper gearing
    Experience - knowing when to get the hell out of the way and plug your ears. "Don't be a sissy. Turn it on!"

    Glennac - Failed my biology test today: They asked, "What is commonly found in cells?" Apparently "BL*** people" wasn't the correct answer.
    Poodle Head Mikey - "the world is well populated with the unknowing and the uncaring and the stupid."

  13. #12
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    Thread Starter
    That's great to know! Since I've been cooling my house down to 68F every day, and have measured temps as low as 50F coming out of my ducts, I figured that 50F might be the coldest I could chill the water. I was just hoping there might be some product specification about how much colder it could go. I haven't tried setting my thermostat any lower than that, but it seems that I could set it lower. If I set the thermostat to 60F, would the system shut itself down before self-destructing?

  14. #13
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    Quote Originally Posted by ga-hvac-tech View Post
    Not gonna work...

    This would be like using a Corvette engine in a dump truck...
    Totally WRONG application of equipment...
    It's really very much like adding a battery pack, larger alternator, and a pair of electric motors to a corvette, so that it gets better gas mileage, more torque, more power, and all-wheel drive.

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