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

  1. #41
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    Quote Originally Posted by Juan Madera View Post
    For the money, time and energy involved in what you want to do, you could have a good tech install the right sized A/C system with either a 2 speed or variable speed condensing unit and save a whole lot more on energy than going down this path. That said, the next best bet is to add thermal mass to the envelope. Google water wall, trombe wall etc.
    I'm not trying to reduce my own electric bills much further than I already have. I'm just trying to perfect a process (I'm an patent-holding engineer). My electric cost over the last 12 months was 45% lower than the prior 12 months ($2971 down to $1608). I don't really think I could lower my cooling cost all that much more, but I think I could make my house more comfortable while doing it. I prefer being cold than hot, but I'm not always comfortable watching TV in a 68F room. I think it would be much better to cool the interior air down to 73F (instead of 68F), but cool some water down to 50F to achieve the same cost savings.

  2. #42
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    Quote Originally Posted by Gernby View Post
    Okay. I definitely agree that it's best to use the "right equipment for the job", but even then, where is the cross-over temperature that makes a refrigeration unit better than a residential central A/C unit (in terms of target temperature)? Also, if a residential central A/C unit is already in place, what would be the absolute coldest it could go, even if it isn't "optimal"?
    That will vary by the amount of air the indoor unit is actually moving, per ton of cooling. At 350 CFM per ton, the coil may start to freeze when the indoor temp is between 68 and 70. At 450 CFM per ton, might be able to get the temp down to 65.

    A/Cs with a hydro coil added to them usually have low air flow rates after the hydro coil is added.
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  3. #43
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    Quote Originally Posted by Gernby View Post
    That's a good point, but I am not sure I would need to worry about humidity after cooling all the water down. It seems that the air inside my home would be ULTRA dry after going through the water cooling process. My home is very air-tight, so it seems that thoroughly dehumidifying the house in the morning should help the humidity down all day, right?
    So what RH is your house at when you cool it down to 68? A tight house can actually have a very high humidity.
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  4. #44
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    Quote Originally Posted by Gernby View Post
    Please don't take this as argumentative, but I don't think you are looking at this from my perspective. I'm actually not trying to convince anyone here that my project will be successful. I'm just trying to get some answers about the capabilities of the equipment I have.

    It does seem that my units might be grossly over-sized, but we do have a very large temperature swing throughout the year. It gets over 100F, and humid here in the Dallas area. There's only about 2 months when I don't run my A/C, so it winds up running very inefficiently about 6 months of the year.
    So how long does your A/C run when its 100F or hotter outside.

    Even figuring the water being chilled from 70 to 50. And then using the water until it reaches 70F again. You are only going to get 33,320 BTUs of cooling stored, only having a 200 gallon storage system.

    Due to heat transfer inefficiencies, and circulator heat being added to the water. You will use 3 to 3.1 tons of capacity to store a 2.77 ton capacity, using the water until it reaches 70.
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  5. #45
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    Quote Originally Posted by beenthere View Post
    That will vary by the amount of air the indoor unit is actually moving, per ton of cooling. At 350 CFM per ton, the coil may start to freeze when the indoor temp is between 68 and 70. At 450 CFM per ton, might be able to get the temp down to 65.

    A/Cs with a hydro coil added to them usually have low air flow rates after the hydro coil is added.
    Funny you should mention that. I just discovered a few days ago that BOTH of my furnaces are running at the max blower speed, which the documentation shows is way too high. According to the documentation, it looks like the downstairs unit needed to be between speed 1 and 2 (out of 5), and the upstairs unit needed to be on speed 1 (out of 5). I decided to set both of them to speed 1, and it seems to be running much better. The run times are longer, but power usage is lower. The temperature drop across the evap and blower is about 20F (68F return / 48F supply). Hopefully that isn't too cold...

    I don't know how accurate my RH reading is, but my Ecobee3 stats both show RH dropping down to about 50%. I don't really know what ideal would be, but my aquarium puts quite a bit of water into the air.

  6. #46
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    Quote Originally Posted by Gernby View Post
    Funny you should mention that. I just discovered a few days ago that BOTH of my furnaces are running at the max blower speed, which the documentation shows is way too high. According to the documentation, it looks like the downstairs unit needed to be between speed 1 and 2 (out of 5), and the upstairs unit needed to be on speed 1 (out of 5). I decided to set both of them to speed 1, and it seems to be running much better. The run times are longer, but power usage is lower. The temperature drop across the evap and blower is about 20F (68F return / 48F supply). Hopefully that isn't too cold...

    I don't know how accurate my RH reading is, but my Ecobee3 stats both show RH dropping down to about 50%. I don't really know what ideal would be, but my aquarium puts quite a bit of water into the air.
    While 50%RH is okay. Its not considered a low RH.

    See how the house feels after a day with the blowers on a lower speed.
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  7. #47
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    Quote Originally Posted by beenthere View Post
    So how long does your A/C run when its 100F or hotter outside.

    Even figuring the water being chilled from 70 to 50. And then using the water until it reaches 70F again. You are only going to get 33,320 BTUs of cooling stored, only having a 200 gallon storage system.

    Due to heat transfer inefficiencies, and circulator heat being added to the water. You will use 3 to 3.1 tons of capacity to store a 2.77 ton capacity, using the water until it reaches 70.

    It's a bit hard to say what my run times are when it's 100F outside, since I didn't start capturing run times and temperatures until September of last year. However, when the temperature was in the 90's back then, my run times were longer than 20 minutes while maintaining indoor temp in the low 70's.

    A couple things that I'm hoping will improve efficiency are that I plan to use a variable speed DC water pump, which only uses 20 Watts while flowing more GPM than I need. I also DO NOT plan to use the central AC blower to circulate air while in "water mode". I am going to mount an MBVC1200 modular blower in my garage, with a single return duct going to 2 upstairs rooms (with a Y), and a single supply duct going into the garage hallway. There will be another air-to-water heat exchanger in the supply duct to cool the air using the chilled water.

    I'm not sure how much efficiency I'll gain from it, but it seems likely that I currently lose a ton of cooling efficiency through the ducts in my attic. At the hottest time of day, my attic is VERY hot, and I have hundreds of feet worth of flex duct running up there. However, my garage stays just a few degrees warmer than my house, since it only has 2 exterior walls, which are insulated.

    BTW, I used 200 gallons as a talking point, but I could easily increase my water capacity well beyond that. My plan is to setup a bare-bones POC, just to determine its effectiveness. If the effect is measurable, I'll be able to determine exactly how much water would be required. I think it's likely that water enhancement might just make it so that I only have to over-cool my house 2 degrees, instead of 5 degrees.

  8. #48
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    You would need 1,000 gallons of water. To get you 4 tons of cooling for 4 hours.
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  9. #49
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    Quote Originally Posted by Gernby View Post
    However, the point of this project is to increase efficiency of my HVAC system
    The one thing I can assure you won't happen with a set up like you envision is increased system efficiency!

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    I think the ideal amount of chilled water would be whatever it takes to keep the house comfortable from 3:00 PM to 8:00 PM without supplemental AC usage on all but the ~10 hottest days of the year. If one of my AC units needs to kick on for some amount of time on those hottest days, I would be fine with that.

    On a side note, I'm doing a test with a ~10 year old aquarium chiller that I had out in the garage, and am pretty impressed by the results so far. I am using it to chill 25 gallons of water in my kitchen, and it's making quick work of it. It chilled the temp from 65F to 56F in 60 minutes while consuming 400 watts.

  11. #51
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    Exactly. A dedicated chilling device is far better suited for the task.

    Do the math though...

    1000 gallons of water is 8000 pounds of water, and in a perfectly insulated container, 8000 pounds is going to need 8000 btuh per degree of temp drop... per hour.
    So... 80F water is going to need 400,000btuh or 33 TONS to cool to 30F... in an hour... 15 in 2 hour 7.5 in 4 hours... assuming zero heat gain... 3.25 tons of cooling to drop it in 8 hours... constant 3.5 tons of chill power.

    It is NOT cheap to chill water. However, chilled water is VERY efficient ad absorbing and transferring heat...

    So, a dedicated chilling device is more efficient at getting the heat out of the water as it is absorbed in from water coils.

    Which is why large buildings often use chillers...

    But a small building (under 20,000 sqft) it really isn't practical.
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  12. #52
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    Quote Originally Posted by Gernby View Post
    On a side note, I'm doing a test with a ~10 year old aquarium chiller that I had out in the garage, and am pretty impressed by the results so far. I am using it to chill 25 gallons of water in my kitchen, and it's making quick work of it. It chilled the temp from 65F to 56F in 60 minutes while consuming 400 watts.
    Told you so...

  13. #53
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    Quote Originally Posted by Gernby View Post
    I think the ideal amount of chilled water would be whatever it takes to keep the house comfortable from 3:00 PM to 8:00 PM without supplemental AC usage on all but the ~10 hottest days of the year.

    Then you should time how much run time your A/C has during one of those hot times you want the chilled water to do all the cooling. And then compare that run time to the size and EER of your A/C to determine the amount of cooling you need from the chilled water. It will probably come out to 1000 gallons give or take 50.

    If one of my AC units needs to kick on for some amount of time on those hottest days, I would be fine with that.

    On a side note, I'm doing a test with a ~10 year old aquarium chiller that I had out in the garage, and am pretty impressed by the results so far. I am using it to chill 25 gallons of water in my kitchen, and it's making quick work of it. It chilled the temp from 65F to 56F in 60 minutes while consuming 400 watts.
    You removed 1,874 BTUs of heat from 208 pounds of water in an hour using 400 watts. 1874/400=4.685. So a COP of 4.685

    So 8,330 pounds of water(1,000 gallons) cooled 9F by that same chiller would use 16.019 KWHs of electric.

    The 16KWs to cool the water 9 degrees is what you would be adding to the electric usage of your A/C to cool the water while also cooling the house. And thats just cooling it 9 degrees. To cool it 18 degrees will use more then double that. Since the colder the water is, the less heat the A/C will remove from it.
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  14. #54
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    Quote Originally Posted by vstech View Post
    Exactly. A dedicated chilling device is far better suited for the task.

    Do the math though...

    1000 gallons of water is 8000 pounds of water, and in a perfectly insulated container, 8000 pounds is going to need 8000 btuh per degree of temp drop... per hour.
    So... 80F water is going to need 400,000btuh or 33 TONS to cool to 30F... in an hour... 15 in 2 hour 7.5 in 4 hours... assuming zero heat gain... 3.25 tons of cooling to drop it in 8 hours... constant 3.5 tons of chill power.

    It is NOT cheap to chill water. However, chilled water is VERY efficient ad absorbing and transferring heat...

    So, a dedicated chilling device is more efficient at getting the heat out of the water as it is absorbed in from water coils.

    Which is why large buildings often use chillers...

    But a small building (under 20,000 sqft) it really isn't practical.
    While I was initially impressed by the effectiveness of the aquarium chiller, that test proved how ineffective that approach would be. As expected, the chiller could not chill the water nearly as fast the second hour as it did the first hour. Each hour that passed brought the temperature of the water somewhat closer to the minimum achievable temperature, which I think would eventually be about 30 below the ambient temperature of the room. I let it run from 8AM to 3PM in a room that was cooled to 68F. It only dropped 1F in the last 90 minutes, and ended at 38F. I recorded the temperatures, power draw, and power factor about twice each hour. After shutting off the chiller, I continued recording the temperature until 8PM. As expected, the rise in temperature was similar to the drop in temperature. It gained temperature very fast at first, then slowed over time. Basically, it acted just like a capacitor in an electric circuit, which is what I want!

    There are several reasons why the dedicated chilling unit would not help me achieve my goals, but I think it would be best to explain them with an automotive analogy. The analogy I would use is that of a hybrid car. If hybrid cars didn't exist yet, and an engineer proposed the idea (on a car forum) of adding hundreds of pounds of lead-acid batteries into a small car, then use the car's gasoline motor to charge those batteries using a large alternator, so that the batteries could be used to power some electric motors on the wheels, I feel certain that very few (if any) people on the forum would consider the idea "good" (myself included). I think people would say that there way more efficient ways to charge a battery than an alternator and a gasoline engine, and they would probably also say that the extreme weight of the batteries would eliminate any possible benefits of the system.

    However, we know now that hybrid cars are more efficient than conventional gasoline cars. The reason is that gasoline engines have to be able to deliver MUCH more power in short bursts to accellerate than it needs to cruise at a constant speed, so the gasoline engines wind up running at very low load most of the time. Since an engine at 20% load is WAY less efficient than the same engine at 60% load, some genius came up with the idea to artificially increase engine load to charge batteries, then to use electric motors INSTEAD of gasoline to move the car at times that it makes more sense to do so (stop and go traffic, etc.)!

    The AC strategy I'm going for is JUST LIKE the hybrid car strategy. Even though there are more efficient ways to cool water, I believe that using my AC system to chill a "cold battery" during the coolest part of the day (a time when the AC system is very UN-loaded and inefficient), then using that battery to cool the house during the hottest part of the day (when electricity is 3X the price) would provide at least as much benefit as the hybrid car strategy.

    I don't think it would matter if there is a 10-20% increase in total BTU's due to inefficiencies, since I believe the refrigeration equipment will be much more efficient running a couple long cycles during the cool part of the day, instead of a mix of short cycles while cool, and longer cycles while hot.

    On a side note, after spending so many years participating heavily on automotive forums, where the contributors are often teenagers, I really appreciate how much more courteous this forum is.

  15. #55
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    Quote Originally Posted by beenthere View Post
    You removed 1,874 BTUs of heat from 208 pounds of water in an hour using 400 watts. 1874/400=4.685. So a COP of 4.685

    So 8,330 pounds of water(1,000 gallons) cooled 9F by that same chiller would use 16.019 KWHs of electric.

    The 16KWs to cool the water 9 degrees is what you would be adding to the electric usage of your A/C to cool the water while also cooling the house. And thats just cooling it 9 degrees. To cool it 18 degrees will use more then double that. Since the colder the water is, the less heat the A/C will remove from it.
    Since the chiller was located inside the house, all of the heat that was being extracted from the water was being transferred into the air, which then needed to be removed by the A/C, so unless I moved the chiller unit outside the house, the A/C will have to move those BTU's anyway.

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    BTW, I did change my overall strategy a bit, since I discovered that the LG mini-split system I have in my garage seems to be a MUCH better option for cooling water than my central A/C units. The LG unit achieves an amazing 30F degree delta, with a supply temperature near 40F! The DC inverter system with variable speed fan also seems like a much more "plug and play" option, with far less risk to the equipment.

    I was already planning to install the new variable speed air handler and ducting in my garage, so it won't require much extra ducting to include the LG unit in the circuit. By doing so, I'll be able to use the LG unit to directly cool the inside of the house too, whenever it's not cooling water.

    It is only 23,000 BTU/hr (about 1/4 of the total indoor capacity), but it would have 19 hours each day to do its job.

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    Its a code violation for the garage air to be able to communicate with the homes air. So by code you won't be able to use the garage mini split to cool the house directly. Plus any and all duct work in the garage that moves air through the house, Must be sealed so that no garage air can leak into it.

    So where is the practicality of doing what you want to do. If your keeping the/A chiller in the house, and adding a large load to the A/C. Your going to use more then twice as much electric, since you also have to remove the heat from the compressor, not just the water.

    Remember, a 4 ton condenser rejects not only the heat that is removed from the house, but also the heat of the compressor moving the refrigerant. So if you have a 12,000 BTU chiller, the heat its condenser rejects is more then 12,000 BTUs because it also has to reject 2,550 BTUs of compressor heat.
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  18. #58
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    Quote Originally Posted by beenthere View Post
    Its a code violation for the garage air to be able to communicate with the homes air. So by code you won't be able to use the garage mini split to cool the house directly. Plus any and all duct work in the garage that moves air through the house, Must be sealed so that no garage air can leak into it.

    So where is the practicality of doing what you want to do. If your keeping the/A chiller in the house, and adding a large load to the A/C. Your going to use more then twice as much electric, since you also have to remove the heat from the compressor, not just the water.

    Remember, a 4 ton condenser rejects not only the heat that is removed from the house, but also the heat of the compressor moving the refrigerant. So if you have a 12,000 BTU chiller, the heat its condenser rejects is more then 12,000 BTUs because it also has to reject 2,550 BTUs of compressor heat.
    Good to know about the code violation. I'll have to consider what real impact that would have regarding home owner's insurance coverage, or something, but I'm sure I probably have quite a few other violations now. I do plan to seal the system from the garage, but would eventually do something to allow it to cool the garage occasionally.

    Regarding the heat load, it seems that you missed my post about the aquarium chiller NOT being part of my plan. Since the air coming out of my mini-split system is 40F, I'm just going to put an air-to-water heat exchanger somewhere in the supply duct. Doing this will make it so that all of the air flowing through the system will be cooled by the mini-split during off-peak hours, but all of the air will also flow through the water coil. I will be able to control the flow of water through the coil, so that I can vary the rate of heat exchange between the cold air and warm water. A high water flow rate would result in the air being warmed up greatly before it returns to the house, but a slow water flow rate would jut allow the cold air to stay cold. Obviously, I can't rely on the mini-split to move all the air through the system, so I will use the Goodman MBVC1200 to push or pull the air through the system.

    An advantages to this approach is that it should result in a much higher level of dehumidification, since the air flowing through the system will be cooled down to 40F before being warmed up again by the water coil.

  19. #59
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    Quote Originally Posted by Gernby View Post
    However, we know now that hybrid cars are more efficient than conventional gasoline cars. The reason is that gasoline engines have to be able to deliver MUCH more power in short bursts to accellerate than it needs to cruise at a constant speed, so the gasoline engines wind up running at very low load most of the time. Since an engine at 20% load is WAY less efficient than the same engine at 60% load, some genius came up with the idea to artificially increase engine load to charge batteries, then to use electric motors INSTEAD of gasoline to move the car at times that it makes more sense to do so (stop and go traffic, etc.)!

    The AC strategy I'm going for is JUST LIKE the hybrid car strategy. Even though there are more efficient ways to cool water, I believe that using my AC system to chill a "cold battery" during the coolest part of the day (a time when the AC system is very UN-loaded and inefficient), then using that battery to cool the house during the hottest part of the day (when electricity is 3X the price) would provide at least as much benefit as the hybrid car strategy.
    The gain in fuel efficiency with a hybrid car comes mostly from regenerative braking. The system recovers energy during deceleration, instead of wasting it heating up the brake pads.
    That is why their city mileage rating is higher than their highway mileage, and there are any number of efficient gasoline and turbo diesel engine cars that get better highway mileage than any hybrid can achieve.

    AC systems are actually more efficient under partial load than under full load, but their cyclical efficiency isn't very good because it takes time for the system to reach steady state when it turns on.
    An AC system that is cycling off and on a lot is very inefficient, one that is able to run long cycles at part load is very efficient.

    I have not read every post since my last reply, but what you want to do transferring heat back and forth with an air to water exchange will introduce multiple inefficiencies to your system.
    You won't get the water down to the temperature you need without running the evaporator coil significantly below freezing, which will itself make the system less efficient, and require additional system controls to protect your equipment from damage.

    What you are talking about doing is going to cost you quite a bit of money, won't work the way you think it will, and won't save you energy.

    Take the money you are about to throw in the trash on this project, get you a nice variable capacity HVAC system, and find something else to tinker with.

  20. #60
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    Quote Originally Posted by mark beiser View Post
    The gain in fuel efficiency with a hybrid car comes mostly from regenerative braking. The system recovers energy during deceleration, instead of wasting it heating up the brake pads.
    That is why their city mileage rating is higher than their highway mileage, and there are any number of efficient gasoline and turbo diesel engine cars that get better highway mileage than any hybrid can achieve.

    AC systems are actually more efficient under partial load than under full load, but their cyclical efficiency isn't very good because it takes time for the system to reach steady state when it turns on.
    An AC system that is cycling off and on a lot is very inefficient, one that is able to run long cycles at part load is very efficient.

    I have not read every post since my last reply, but what you want to do transferring heat back and forth with an air to water exchange will introduce multiple inefficiencies to your system.
    You won't get the water down to the temperature you need without running the evaporator coil significantly below freezing, which will itself make the system less efficient, and require additional system controls to protect your equipment from damage.

    What you are talking about doing is going to cost you quite a bit of money, won't work the way you think it will, and won't save you energy.

    Take the money you are about to throw in the trash on this project, get you a nice variable capacity HVAC system, and find something else to tinker with.
    Good point about regenerative braking, but I disagree that a hybrid's efficiency is mostly due to recuperation while braking. Here is an informative diagram from Wikipedia about energy consumption by a hybrid during urban driving and highway driving. There is definitely a difference due to braking, but there are also significant differences in engine losses, standby (battery charging), etc.

    Even if regenerative braking could claim all of the credit for hybrid efficiency, that analogy would support my hybrid cooling strategy too! Cars and AC's both lose efficiency when they stop and start. Hybrid cars reduce those losses by charging batteries with regenerative brakes, but my goal is to reduce those losses by reducing the number of stops and starts. You said it very well, "An AC system that is cycling off and on a lot is very inefficient, one that is able to run long cycles at part load is very efficient." I plan to achieve long cycles at part load by using a thermal battery.

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