Chiller for Home

[mike8913]

I have lots of questions and I'll try to arrange them some what coherently.

1. Is 20,000 BTU's the correct size for a 2000 sq. foot home?

2. What type of compressor(s) would you need?

3. Which is more concern, HP or displacement?

4. Would a cascade be more efficient than a single compressor?

5. Is a TXV necessary?

6. Does an Autocascade operate more efficiently?

7. Does a larger res equal more efficiency?

8. Should I use the biggest evaporator I can?

9. Does 10,00BTU equal 1 ton (I can't figure that out)?

Premise: I'd like to build a central air system for my girlfriends home. I'm very handy in terms of construction and I have many people to hep me with electrical, so running the vents won't be an issue. They currently run 3 in window A/C's that are extremely inefficient and sized poorly.

Please Answer these questions as though I am a experienced builder. I want to run at about 4 C or less that 40 F so that I am under dew pint fr dehumidification. If running cooler will help efficiency then I am open to that idea as well. I feel as though I could easily build a system withut any faults, but when it comes to refridgerant selection and tuning for loads I am still slightly in the fog. Please explain like I am five years old.

Thank You.

[DetroitAC]

Hi Mike,
1. Load calulations are based on estimating the heat influx by looking at not only the space, but the construction of the house, it's not that hard, but you have a good shortcut available. You already have three window rattlers, that you use to judge how much capacity you need. Add up the capacity of the three, are they undersized for the house? Do they run constantly or cycle on and off (the fan keeps running, so don't be fooled by that).

2. Rotaries

3. You can size them by either measure

4. No, Air conditioning is not an application that would use a cascade

5. No, but they are nice

6. Nope, again not used for an A/C application

7. You don't use a reservoir, and you don't use a chiller to cool a house, you use a direct expansion air conditioner

8. Nope, it should be sized correctly

9. 12,000 Btu/hr = 1 ton refrigeration

[mike8913]

Thanks DetroitAC,

The house is an average of about 2000 sq feet, probably less. Its an older home with weak insulation, but I estimated about 20,000 BTU. Obviously I may be wrong.

I know a chiller is not common for house application, but I have one of those outside chiller units with the wall unit mounted inside and it works really well and energy use is low. And at school they use chillers for every A/C application because, as they explain it, it is much more efficient and cost effective.

Whats your take on that?

Lastly, the three window units are definitely undersized. They run constantly, not just the fan. Not sure on their sizes off the top of my head but they are definitely not suitable for the needs of the home.

I've also been playing with the idea of a passive geothermal A/C system. Where I would bury a heat exchanger for the appropriate size below the frost line and have it fed by a 150 gallon tank of antifreeze/water mix. Pure water should handle 20,000 BTU at 133 gallons, so I figured it couldn't hurt to go a little higher. It also seems that the tanks most widely available are IBC totes that come in 275 gallon size on avg. How does this sound?

With either setup, how would I go about determining evap size?

[DetroitAC]

Mike, you're really out of your depth here.

You don't have a chiller on your house (I'm guessing because I've nver been there :D). What you have is likely a mini-split system, an outdoor condensing unit that is the compressor, condenser and fan, and then an indoor unit that is a fan and an evaporator. A "chiller" circulates a cold fluid. An A/C unit circulates refrigerant only.

There are some very large centrifugal chiller units that are used for large buildings, they are quite efficient and major capital equipment. Nothing like that exists for residential use that I'm aware of.

I'm in the process of building a ground sink/source system for my house. It's a major project, it should take me about 3-5 years to break even but that's only because I'm building it all myself including the refrigeration and controls. My house will require 1600 feet of buried tubing in the backyard, buried 5-1/2 to 6 feet down to meet my heating capacity requirement in heat pump mode. It's a major undertaking, I'm considering hiring the trenching done (can't rent a trencher to go that deep), or maybe buying a trencher or excavator and then reselling it when done. It will be oversized for A/C mode. <--That's the first heat exchanger, ground to propylene glycol/water. Then I'll be using coaxial heat exchangers to exchange between the PGW and the refrigerant. <--Second set of heat exchangers.

The point I'm trying to make...it's a major project, well worth it over a long enough time span...IF I don't screw something up.

There is no need for a tank other than to accomodate expansion/contraction of the fluid when changing over from the seasons.

I'd advise you to stick with normal condensing units and fancoils if you want a central A/C system with ductwork.

[mike8913]

I think maybe i didn't word it correctly. I know what the differences between chillers and normal a/c units are and i do have a chiller at work. It has the whole A/C portion of the unit outside with the reservoir and that chilled fluid is pumped to a unit with a coil and fan. It was like 1200 bucks from the boss and it works great.

Instead of using any refrigerant at all in your ground sink unit, why not just use an appropriate amount of water to handle your heat lad and then bury the appropriate amount of tubing. I keep coming up with the figure of 600 ft per ton of cooling. My logic was this:

If 133 gallons of water can handle approximately 20,000BTU and 600ft of copper tubing can disperse 12,000 BTU then why not simply bury the 600 feet to handle one ton and increase your carrying capacity by say doubling your cooling medium to 266gal ( or 275gal: common size of IBC tote). I can't think of good reasons why this wouldn't work. Come up with anything?