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# AC BTU size for ice bath.

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Member Posts: 3
Looking to convert a window AC unit into a chiller for a 150 gallon ice bath. Trying to figure out how many BTUs the unit needs to be to cool the 120-130 gallons of water to 38 degrees. The unit will be outside in South FL with insulation so external heat will definitely be a factor.

• Member Posts: 4,955
edited January 2023
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How do you plan on cooling the Water with a window shaker?

Why 38°F water, what's it for?
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BTUs = pounds water x degrees f raised or lowered. Water is heavy, 8.34 pounds per gallon. 150 gallons is 1251 pounds. If you are lowering the temp 30 degrees you would need more than 3 tons of air conditioning capacity. So a 1 ton unit would have to run 3 hours, discounting outside heat gains.
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vhauk said:

BTUs = pounds water x degrees f raised or lowered. Water is heavy, 8.34 pounds per gallon. 150 gallons is 1251 pounds. If you are lowering the temp 30 degrees you would need more than 3 tons of air conditioning capacity. So a 1 ton unit would have to run 3 hours, discounting outside heat gains.

But that's based on a 40°F evaporator. To get 38°F water you're going to need a 18° - 23°F evaporator. That #-Ton a/c won't do it!
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pecmsg said:

But that's based on a 40°F evaporator. To get 38°F water you're going to need a 18° - 23°F evaporator. That #-Ton a/c won't do it!

But I could, Dream Can I

Edward Young Retired

After you make that expensive repair and you still have the same problem, What will you check next?

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pecmsg said:
How do you plan on cooling the Water with a window shaker? Why 38°F water, what's it for?
I’m pulling the fan components out and dropping the coil into a sump to act like a chiller. I’m guessing there may not be perfect math for that but I’m looking at an 18k btu unit.
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Why 38°F Whats going to be in the water?
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pecmsg said:
Why 38°F Whats going to be in the water?
People. There are a ton of mental and physical health benefits to doing 3-7 minute cold plunges a day. I’ve been doing it with 16lbs of ice which gets it to 48 but it’s not something you can logistically do everyday with just ice bags.
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Either get a small ice maker or a Chiller.
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A dairy barn milk tank would probably do it.

But usually only one man hole on the top.

I think some older ones had full size lids.

Plenty of used smaller ones available, at least 2 HP compressor unit IIRC.
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Forget the window ac. You need a medium temperature chiller. Everything for AC is a high temp.

The chiller will need glycol and your chiller evap temp would need to be 28 degrees minimum. Lower if you want a faster pull down. The window ac will never cut it. You need 3 tons at medium temp just for the water. If you tank is insulated the you will have to allow for the surrounding environment
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What about a 3 ton air to water heat pump? How low can those chill the water, I think 32F?

Then you could reverse it for a hot tub
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
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Re-engineering a window unit is going to be problematic at best. I would suggest looking at products that have been designed for the application. The dairy chiller mentioned above, or an air to water chiller like the ones used for beer tap lines would be a good place to start.
As for the sizing, the heat gain is the biggest factor. Are you planning on leaving it cold all the time? How is the tub insulated? Will it be covered? The dew point condensation on the outside may be a problem as well.
"If you can't explain it simply, you don't understand it well enough"
Albert Einstein
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Zman said:
Re-engineering a window unit is going to be problematic at best. I would suggest looking at products that have been designed for the application. The dairy chiller mentioned above, or an air to water chiller like the ones used for beer tap lines would be a good place to start. As for the sizing, the heat gain is the biggest factor.
Also the safety issues. That window shaker was never designed for that application.

Salt water fish tank chiller is one consideration.
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I'll ad to this If an ice-cold drink sweats in that bathroom so will the entire tub!
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Even my trash-picked dehumidifier has no problem frosting-up its coils. So 32 easy. While 38 may not be an optimum efficiency for A/C rigs, Cauley has not complained about the electric bill (yet). If the health benefits are there, running-cost may be minor concern.

As shown, several tons will cool the water in an hour, so a half-ton (yes, a window-rattle) may be ample on the first day. Then it sits 23.8 hours absorbing room warmth. Manual Sch J is appropriate but may be overkill. Then you throw a warm body in for 7 minutes... I'm sure this data exists from cold-water survival studies (or bird slaughtering?) but you may be translating furlongs to hectares or joules. Ah: I doubt 98.6 should drop below 97.6. Body is water. 100 pounds 1 degree is 100BTU, or two people per day @ 300lb total is 300BTU, essentially "nothing". Even a deep-chill 10deg body temp dip is 3,000BTU so even the \$99 A/C can recover in an hour.

23.8 hour ambient temp may be the main accountable loss. (There's always a lot of "minor" losses that add-up to a big suckage, but you may have to build to get a sense.) Say a 4'x4'x2' "hot-tub". Insulated lid. 64 square feet total. Assume R1, need 64BTUh per degree drop(rise). 68 room to 38 water is 30deg so 2,000BTU/h all the time. At this point, insulation is quickly cheaper than fire(ice). Say 2" foam, hope for R10, now 200BTUh, not a heavy loss. The window AC would come on an hour+ a day.

The huge unknowns are the loss in Freon plumbing. I'm sure you can't drop a de-fanned window A/C in the water. Also the hot air on the back side really wants to short-cut into cold water. Given an abundance of 2000-9000BTUh window A/Cs, even as \$10 yard-sale take-mes, the real question is not how much BTU but how to harness them.

I have a chest-freezer. The inside walls get cold. To me, water-plumbing is easier and safer than messing with Freon. Run PEX into the chest, zig-zag against the walls, prop with milkcrates or turkeys for good contact? I guess you can't let the PEX freeze so set the chest for 33 degrees and only keep unfroze food?
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most evaporators in a/c are designed to operate around 40 f. Because you really don’t want to freeze the condensation on the evaporator. So with a normal a/c system 38 f ain’t happening
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PRR said:

To me, water-plumbing is easier and safer than messing with Freon. Run PEX into the chest, zig-zag against the walls, prop with milkcrates or turkeys for good contact?

How about brine instead of water? When my Dad would design cold chambers he'd frequently use brine. I don't know if PEX is compatible with brine though.

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ron said:

a small mini freezer, amazon, \$300 range, 1 to 3 cubic foot, what the BTU rating is I don't know.
But drill a hole through it and use some 3/4" copper pipe and make your own coil, have one coil within the freezer and another within your ice bath tub. Locate the mini freezer right next to the tub if possible, could use vinyl tube between the freezer and tub; minimize that line length and insulate if possible. Use water with a minimal mix of antifreeze; more water is better for heat transfer vs 50/50 mix like what's in your car radiator. And there's RV non toxic propylene glycol too.

Finding a mini freezer to give an acceptable amount of cooling capacity would be the question. And also what to use as a pump... preferably a delta-T circulator

actually 1/2" copper and make as much coil would be better i think vs using 3/4" or larger diam tube; use the smallest diam tube as possible to make the coil, you want as much surface area as possible per square area slice of fluid within.

With the evaporator lining the interior walls and the condenser lining the exterior walls I wouldn't be drilling any holes in that cabinet!

The small fractional compressor in those is not even close to whats needed.
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I used to run a 900 ton chilled water plant. We ran at 40 f supply temp with an evaporator temp of about 34.
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Polar cool for tubs
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream