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Thread: Want to use my existing central A/C to chill water tanks

  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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    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

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    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?

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    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.

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    A small air cooled process chiller would make more sense.

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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.

  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

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

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    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

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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!

    GA's basic rules of home heating and AC upgrades:
    *Installation is more important than the brand of equipment
    *The duct system keeps the house comfortable; the equipment only heats and cools (and dehumidifies)
    *The value of comfort, over the long term; leave economic choices behind!
    Choose your contractor wisely!

  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

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    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?

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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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    No, you'd know it was too much after it slugged enough liquid to break something and then needs the compressor to be replaced.
    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

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    Quote Originally Posted by rider77 View Post
    No, you'd know it was too much after it slugged enough liquid to break something and then needs the compressor to be replaced.
    Damn... why wouldn't the documentation specify some numbers regarding the safe operating temp ranges. If a person was designing a very well insulated building that needed to be 50 degrees inside, how would they determine that a particular unit couldn't safely achieve that temperature?

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    Quote Originally Posted by Gernby View Post
    Damn... why wouldn't the documentation specify some numbers regarding the safe operating temp ranges. If a person was designing a very well insulated building that needed to be 50 degrees inside, how would they determine that a particular unit couldn't safely achieve that temperature?
    the documentation DOES. plus, the equipment is selected by a qualified person. there isnt just one kind of equipment

  18. #17
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    You do realize that in order to do this, your going to need a LOT of water. And cooling water is going to result in the house heating up. And that will result in the unit using more energy as it's not only conditioning the house, and not only conditioning the water, but also will cause the unit to run for longer periods trying to cool the house.
    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

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  19. #18
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    Quote Originally Posted by HVAC_Marc View Post
    the documentation DOES. plus, the equipment is selected by a qualified person. there isnt just one kind of equipment
    Glad to hear that. Would you mind telling me what that specification would be called in the documentation? I've been studying the specification sheets from Goodman for days, and haven't found anything that would indicate a minimum safe thermostat setting. My Ecobee will allow me to set the cooling set point down as low as -10F.

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    Quote Originally Posted by Gernby View Post
    Glad to hear that. Would you mind telling me what that specification would be called in the documentation? I've been studying the specification sheets from Goodman for days, and haven't found anything that would indicate a minimum safe thermostat setting. My Ecobee will allow me to set the cooling set point down as low as -10F.

    It would be found in the condenser/compressor specifications. And believe me, a Goodman condenser wont take this kind of abuse.

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    Quote Originally Posted by rider77 View Post
    You do realize that in order to do this, your going to need a LOT of water. And cooling water is going to result in the house heating up. And that will result in the unit using more energy as it's not only conditioning the house, and not only conditioning the water, but also will cause the unit to run for longer periods trying to cool the house.
    I realize that the amount of water necessary to achieve benefit might make it impractical, but that will depend on the achievable temperature of the water. I'm going to start with ~200 gallons of water stored in containers in the middle of my house. Unless I do something very wrong, there shouldn't be a significant increase in total load on the air conditioning system, since the total BTU's per day won't change. Assuming the system stays within it's safe operating parameters, the effect on the system should be fewer run cycles per day, but each run cycle being longer. If successful, the total run time per day will be lower.

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