Need help sizing AC for shop

[Joe Jensen]

I have a typical 3 car garage. 29 wide by 20 deep with 9 foot ceilings, or about 5200 cu feet. I live in the Phoenix area where it's super hot, regularly over 110 F in the summer. The ceiling is well insulated, but the sidewalls only have drywall, 2 by 4 studs, and then 1" of cheap foam and then 2 coats of stucco. I will insulate the garage doors, but the top panel is mostly glass and I don't intend to insulate that.

I plan to use the AC or heat only when working there. I will set it up to run before I plan to start working, ideally I'd like it to do a reasonable job cooling 110F down to say 90F or less in an hour.

Anyone know how many BTUs I need? I'll be doing a mini-split Heat Pump, and I'd like to do 18,000 BTU..joe

[Thomas Pender]

Recommend you get some HVAC contractors out there and give you an estimate. Get a Carrier, an American Standard/Trane, and some lower grade guy out of the book and just ask them to estimate it (estimates are free - usually). Some will put an infrared camera on the walls and ceiling so you can see where the enrgy loss is - (you can also pay for this kind of service and perhaps you should). HVAC is not usually do it your self and there will be codes for new work to comply with, etc., especially if you ever want to sell the house. There are various helper units out there (they put the coils into a console on the wall and blow air through it with the compressor outside) and SEER has really increased, which is very important given electric rates.

Alternatively, is the humidity low enough where you live to use a swamp cooler?

The lack of insuallation in your walls is problematic as is the garage door(s), so while normally you can count on one ton per 500 sq feet in a modern house, that may not do it for you. It may pay your to re-insullate. Also - do not forget if you open the door to the garage, you have issues.

Best of luck.

[Ted Jay]

I have a typical 3 car garage. 29 wide by 20 deep with 9 foot ceilings, or about 5200 cu feet. I live in the Phoenix area where it's super hot, regularly over 110 F in the summer. The ceiling is well insulated, but the sidewalls only have drywall, 2 by 4 studs, and then 1" of cheap foam and then 2 coats of stucco. I will insulate the garage doors, but the top panel is mostly glass and I don't intend to insulate that.

I plan to use the AC or heat only when working there. I will set it up to run before I plan to start working, ideally I'd like it to do a reasonable job cooling 110F down to say 90F or less in an hour.

Anyone know how many BTUs I need? I'll be doing a mini-split Heat Pump, and I'd like to do 18,000 BTU..joe

580 square feet comes out to a 1.5 ton unit..... according to the HVAC sizing estimator for your area.
http://www.hvacopcost.com/equipsize.html

[Joe Jensen]

I did some more research and found that the Sanyo 17 SEER mini split 2 ton draws the same amps as most 1.5 ton units. Even though it's much more expensive I'll feel less guilty running it :rolleyes:

I will hire a licensed installed to pump out the lines, and the unit comes pre charged. I had an electrician out today to put in a larger panel in the shop so I now have enough additional power to the shop.

[James Hart]

The guy I'm working with cut a 3" channel in the drywall across the top of each of my garage walls that weren't insulated. Only the walls that have living space on the other side had additional insulation.

These channels also were used by the electricians to add outlets. I've gone from 2 outlets in the garage to 24.

they then pumped insulation into the walls, taking them from about R4 to somewhere around R19.

Even with the AC not yet installed the difference is amazing.

Jim

[Don Bullock]

580 square feet comes out to a 1.5 ton unit..... according to the HVAC sizing estimator for your area.
http://www.hvacopcost.com/equipsize.html

Ted, thanks for the link.:D

[David Giles]

Joe, I just installed an 18000 Btu minisplit in an oversized 2 car garage in Houston. It blows directly on the bench / saw work area, so I can't say how well it cools the entire garage, but others have commented on the improved comfort level. It's a joy not to drip sweat over the work.

If you haven't already purchased a unit, may I recommend www.ductlesshvacsupply.com They buy container loads of minisplit systems and resell them at a reasonable markup. No sales tax and free shipping. The Ambiance unit came promptly and undamaged. Customer service has been superb. It took an afternoon to install the equipment and run the lines. My normal AC guy charged me $150 to complete the tubing, wiring and charge the unit.

[Ted Jay]

Ted, thanks for the link.:D

:D:D:D:D:D
Your Welcome

[Joe Jensen]

Joe, I just installed an 18000 Btu minisplit in an oversized 2 car garage in Houston. It blows directly on the bench / saw work area, so I can't say how well it cools the entire garage, but others have commented on the improved comfort level. It's a joy not to drip sweat over the work.

If you haven't already purchased a unit, may I recommend www.ductlesshvacsupply.com (http://www.ductlesshvacsupply.com) They buy container loads of minisplit systems and resell them at a reasonable markup. No sales tax and free shipping. The Ambiance unit came promptly and undamaged. Customer service has been superb. It took an afternoon to install the equipment and run the lines. My normal AC guy charged me $150 to complete the tubing, wiring and charge the unit.

The Ambience looks really good for the price, but the largest is 18000 BTU, and I think with my situation I really need more. Hmm, should I spend $1000 more to get 24000 BTU instead of 18000. Tough one...joe

[David Giles]

Well, I don't know of any disadvantage to an oversized AC, except that you might have to wear flannel shirts if it gets too cold. Will you only use it 2-3 hours per day? That's my situation and it seemed to call for a less efficient, but less expensive system. If you run it around the clock, I definitely recommend a high efficiency unit. When the AC is initially started, much of the load goes to cooling all the mass in the shop. Once it is cool, it shouldn't take a much AC tonnage to keep it cool. Maybe what you want is a timer for the AC instead of a larger AC unit.

[Jim Andrew]

I'm looking at AC too. A friend had two units that he got stuck with from his rental building. One is a 2 ton heat pump, and the other appears to be a 5 ton unit. Both have been used just one season. And the 5 ton is a high efficiency unit. My shop is 26 x 50 with 9' ceiling. Walls and ceilings are insulated, as well as garage doors. I want to just turn the AC on when I have time to use the shop, do you think I can get by with using the 5 ton? My local hvac guy says it is too big, but I think the 2 ton would have to run all afternoon to cool the shop off when it's hot out. Jim

[Joe Jensen]

I'm looking at AC too. A friend had two units that he got stuck with from his rental building. One is a 2 ton heat pump, and the other appears to be a 5 ton unit. Both have been used just one season. And the 5 ton is a high efficiency unit. My shop is 26 x 50 with 9' ceiling. Walls and ceilings are insulated, as well as garage doors. I want to just turn the AC on when I have time to use the shop, do you think I can get by with using the 5 ton? My local hvac guy says it is too big, but I think the 2 ton would have to run all afternoon to cool the shop off when it's hot out. Jim

Jim, I talked today with the mini split dealer. He said that for AZ, a 1.5 ton is good for up to 700sq ft of normal house. He thought it would take at least 2 hours to drop my garage from 100 to 80.

Does anyone know how to calculate BTUs required to lower a given volume of air a degree?

[Ted Jay]

Jim, I talked today with the mini split dealer. He said that for AZ, a 1.5 ton is good for up to 700sq ft of normal house. He thought it would take at least 2 hours to drop my garage from 100 to 80.

Does anyone know how to calculate BTUs required to lower a given volume of air a degree?

Here is another good site that explains alot. Lots of info.:eek:

http://www.inspect-ny.com/aircond/AirConditionerChart.htm

ALso:
http://www.inspect-ny.com/aircond/aircond03.htm
Scroll down to "COOLING RULES OF THUMB" some of the info overlaps.

Also check out all the info links on the left, bring drinks and popcorn:)

Have fun, enjoy
Ted

[Ted Jay]

Jim, I talked today with the mini split dealer. He said that for AZ, a 1.5 ton is good for up to 700sq ft of normal house. He thought it would take at least 2 hours to drop my garage from 100 to 80.

Does anyone know how to calculate BTUs required to lower a given volume of air a degree?

I also wanted to add.
Being in Phoenix, which has an average humidity of around 20%, even with a 10 degree drop is gonna make a huge difference.

Some places that have high humidity of around 70% and the temp is around 85* it's killer. That old saying "it's not the heat, it's the humidity", is true.

It took about 3 months to get used to the heat when I lived in Phoenix in '75, but then, we just came in from Germany with frost on the ground.

Later....

[Samuel Brooks]

Please remember to keep the humidity change in effect. With AZ being low in humidity to start with if you have to run an AC unit for extended periods of time to lower the temp it will also remove humidity from the air. In my server room at work, I have a 10x10 room with a 24000 btu cooler. It runs continuously in the winter because of the qty of servers in the room (18) and it fights with the other office rooms that have heat pumped into them, no insulation in the walls. Because it runs so much the humidity runs in the 8-10% range which is not good but cannot be helped because a side effect of AC is that it removes humidity and when it runs allot you end up with low humidity.

In a home with high humidity they say to put in the right size or a little smaller which means the unit will run longer which pulls more moisture from the air. If you oversize the unit then it will cool the area quickly, but since it does not have time to remove the moisture it does not feel cool but in the case of AZ you would not want your humidity to drop much.

I would think that in AZ you might want to consider up sizing your unit larger then what a home would calc out to be, because you would not want to remove much moisture and you are not wanting to run it very often. But you will be wanting a fast drop in temperature. You will most likely find that you end up leaving it running because it takes to long to cool off otherwise. So a 1/2 hr session in the shop is to feasible because you need to plan 2 hours in advance.

Also put in a ceiling fan or two to help circulate the air.

Hope this made sense.

Sam Brooks

[Leo Zick]

Jim, I talked today with the mini split dealer. He said that for AZ, a 1.5 ton is good for up to 700sq ft of normal house. He thought it would take at least 2 hours to drop my garage from 100 to 80.

Does anyone know how to calculate BTUs required to lower a given volume of air a degree?


1.45 tons is the back of the envelope calc for your size. id guess 1.5tons is more than enough. 2 tons could wind up short cycling. that actually costs more in the long run, and (though this may not be an issue in AZ) oversizing wont ring out the humidity. its more efficient to have a unit run somewhat continously rather than off and on all the time.

1 ton = 12000 BTU/hr. going back to the days of horses and ice boxes, one ton of ice freezes one ton of water in 24 hours.

so, assuming the unit is correctly sized, you would see an approximate drop of 1 degree in air temp per cubit foot, per hour. give or take, a BROAD estimation. your specific house values are needed to calc this out.

since you mentioned split AC units - if you have the space for a condenser outside, how about 2? one option to consider is putting in two 1 ton units. with more directional airflow, youd feel the effect more where you would need to.

[Joe Jensen]

I've decided to get the 2 ton Sanyo. It uses "inverter" technology to give it highly efficient variable speed. The inverter technology uses electronics to vary the frequency of the AC supply. The unit I'm getting varies between 4,000 BTU and 24,000 BTU. It's more expensive, but it should give me the best of both worlds, 2ton when I need it, and as low as 1/2 ton to hold the temp where I want. Also, depending on speed, it's between 16 and 20 SEER.

Leo, 2 one ton units would be $1499 each, or almost 50% more than on 2 ton. Plus, to really get dispersion benefit, I'd need to run one on the opposite end of the shop.