View Full Version : Sizing AC for shop (slighty OT)

05-18-2012, 11:34 AM
Is there any advantage or disadvantage to over-sizing an air conditioner (window unit) for a work shop. The square footage is 1000 with 8' ceiling. The local box has AC units recommended for 1000 sq ft and the next is 1500 sq ft.
I understand for normal use (residential AC) there is little advantage to oversizing the unit, with the disadvantage of cycling off and on too often.
But, with a shop, it's not occupied 24/7, rather the occasion I want to go out to the shop and do some work I'd like the shop to cool off as fast as practical.

Thank you for any opinions on this subject.

Weston Bye
05-18-2012, 11:58 AM
Anecdotal, but I was surprised to see that running my Bridgeport in my 250sq.ft. shop made enough waste heat to keep the furnace from running as often. Haven't had any long runs in summer to see if the AC can keep up.

Another consideration, maybe, with enough machinery iron mass, the AC may cool the air initially, but will have to keep working to remove the heat that the machinery gives up.

05-18-2012, 12:00 PM
Not exactly an answer but two things to consider before you proceed.

First, an A/C unit that is too large has the possibility of not cycling long enough to remove humidity from the air. Longer cycle times (A/C running) allow the unit more time to remove the humidity.

Second, your thoughts on using the A/C only when you are going to use the shop may cause another issue, condensation and ultimately oxidation. If you live in a high humidity area, as I do in South Florida, cooling off all that cast iron and then turning off the A/C may allow humidity to collect on your machines much like it does on the outside of a cold glass of beverage. I've had my glasses fog up when exiting my house to the outdoors on humid days so I believe a similar condition could occur on machinery. If you live in Arizona I doubt this would be an issue because relative humidity is so low.

A partial answer may be to keep fans blowing and have good ventilation in the shop after shutting the A/C down but that may not work either and may, in fact, exacerbate the problem. Another, more expensive possibility, would be to keep the A/C unit running but with higher temperature setting so it would continue to dehumidify the air but not run as much as it would to keep the shop at a comfortable temperature. One last option might be to install a dehumidifier as well as an A/C and run the dehumidifier after the A/C is turned off.

These are issues I have tossed about while considering A/C for my shop. Maybe I worry too much and a good coat of oil or LPS 2 would prevent the condensation I am concerned about. I still haven't decided and probably never will as I plan to head to the mountains of NC where it's cooler in summer within a couple of years.

Bill Pace
05-18-2012, 12:26 PM
This is my experience - no doubt there are many variables involved, area of country, shop construction, etc...

About 5 years ago I decided to AC my shop. Using the common tables for size, I chose a 12,000 btu window unit - shop being some 800 sq ft -- it didnt even come close to handling it. So, 2 yrs ago I bought a 24,000 btu unit and, while much better, when it gets up around the 90-100 degree mark (which last summer was 64 days over 100) it doesnt cope well. My shop is sheet rocked and insulated and has a large oak tree and a hickory giving partial shade. Its mostly just a puzzle now as to why the shop shouldnt have icicles hanging - I am satisfied enough with it to not pursue any further. I use 3 smallish buzz fans to move the air around and that helps a good bit.

Paul Alciatore
05-18-2012, 01:52 PM
My understanding about the size of an AC unit is that it is based on BOTH temperature and humidity control. First, you need a unit that can keep up with the heat load in the hottest weather without too high a cycle time. But, if you go too large, then the cycle time is too short and the humidity can become excessive.

Now, most tables are for residental/office environments. They assume that most of the heat load is from external sources like air leakage at windows and doors and through walls and ceilings/roofs. I have had many facilities where those factors were only the tip of the iceberg (or should I say the heat-berg) and I had to argue with less than qualified AC technicians about the need to include the power consumed in the area. If dozens of Amps are consumed in the room then their charts are completely useless and you need to measure the current consumed in the area and calculate the Wattage and translate to BTUs per hour and ADD that to their charts. A home shop is probably somewhere in the gray area between these two extremes, but power consumed in the shop IS a factor and it should NOT be surprising.

For the situation described and without doing a lot of math, I would probably look for a unit that is roughly twice the size specified by the common tables. I would also look for a bit of heating ability for winter use and I am in south Texas, not NC. I like my comfort.

05-18-2012, 01:53 PM
Don't forget to add the lighting thermal load to the conventional wall and ceiling calculations. In central Texas, I have a 625 sq. ft. shop, well insulated both walls and 8' ceiling with no windows. The lighting load is about 1800W. A 24,000 BTU/hr. thru the wall unit is perfect.


05-18-2012, 02:39 PM
Hi. I don't know what those window units cost, but about three years ago I bought a 24,000btu mini-split heat pump from this outfit: http://www.heatandcool.com/INVERTER-DUCTLESS-SPLIT-A-C-p/ksil024-h216.htm.

It is made in China, but mine used a top-quality Japanese compressor and has functioned perfectly since installation. My shop is 824 sq. ft, 10 foot ceiling, cinder-block and insulated. We routinely see 105-110 deg F. in the summer. It is also very, very quiet and provides all the heat I need in the winter. Installation was dead simple.


05-18-2012, 02:46 PM
The heat load in a shop CAN be staggering. I have two refrigerators and a mondo stereo system in my room. Also the heat created by running lights should be figured. It isn't normally like a lamp or two in the house, it's rows of lights. Machine motors and friction of tools (remember the smoke that comes off your lathe work :) ) all add BTU's.

Shoot, I ain't no help. I just run a big azzed fan and live with it. Sorry.

Ohio Mike
05-18-2012, 02:48 PM
All the comments here are good. Sizing is very important and considers not only sq footage but ceiling height, overall construction, windows, doors and energy loads with in the space (like that Bridgeport). All those impact what is appropriate. Personally I really like the split units, and have long wished to install one in my garage. They also make multi-zone split units too. My brother has one for his garage which has a bonus room above it that is living space.

As noted humidity is another battle and you may be able to setup a humidistat to run the unit when it normally wouldn't based on room temperature. That configuration is often used in vacation homes where you don't need 70F everyday but you won't want mold and mildew to take over.

05-18-2012, 09:09 PM
As noted humidity is another battle and you may be able to setup a humidistat to run the unit when it normally wouldn't based on room temperature. That configuration is often used in vacation homes where you don't need 70F everyday but you won't want mold and mildew to take over.

Forgot to mention that the unit linked in my post above has a 'dehumidify' mode built in.

05-18-2012, 09:30 PM
I do not know the square footage of my shop . But it is 16x20 out side and well insulated. I have a 14000 btu and it is not near enough in hot weather with 75% humidity we have here. the machines put out more heat than you realize and so do the lights. So I would go bigger than the charts say .

05-18-2012, 09:32 PM
Anecdotal, but I was surprised to see that running my Bridgeport in my 250sq.ft. shop made enough waste heat to keep the furnace from running as often. Haven't had any long runs in summer to see if the AC can keep up.

Another consideration, maybe, with enough machinery iron mass, the AC may cool the air initially, but will have to keep working to remove the heat that the machinery gives up.

And that compressor thumping away in the corner won't help much with the heat it gives out.

Cold machines are "heat soaks" ("cold banks"?) as their temperature rises to shop ambient temperature and when those machines are used the heat given out has to be extracted - as you say.

05-18-2012, 09:37 PM
I was able to cool my old 20x24 shop with a 12K BTU window unit mounted high on the wall. It took it a bit, but it could hold things down around 75F when it was over 100 outside. There were only two small double insulated windows and R13 insulation in the walls and ceiling.

The new shop is 960 sq/ft (24 x 40) with part of that sectioned off for tool room/work area - 240 sq/ft. We saved a 10K window unit from the move that I plan to mount in the wall of the work room. The other 672 sq/ft would need at least a 15K or more like an 18K unit. I will be passing on that for a while due to cost, but I ran a wire for it anyway. Insulation in the new building is R13 in the walls and some R13 in the ceiling, to be final at around R19. Still working on the insulation and OSB wall covering but the building can hold about 10F below outside even with no insulation over the 672 sq/ft part. the 60 gallon compressor is isolated from the room and vented to pull air only when the compressor comes on. Long side faces south with an insulated 16' x 7' door. The roof is vented well too. The building is red and FWIW, the HardiPanel siding on the west end gets to around 130F inside on sunny afternoons. The insulation cuts that to about mid 80s on the paper backing. Part of te building's ability to hold temp is the mass of the floor - figure it's around 61,000 lbs of concrete (16 yards). Leaving the windows open on cool nights "charges" the slab.

So it kinda depends on how the building is constructed, exposure, ventilation, etc. I go at least a 24K unit in 1000 sq/ft open. You might also hit up some local AC companies for a 2.5 ton take out unit from an older house. Who gives a rip if it's 10 SEER or 13 SEER since it's occasional use. They get all the cash for the install and are out zero for the equipment, except for scrap value. An old vertical unit would be an easy setup. Or consider using a 15K or 18K unit in both ends.

05-18-2012, 10:44 PM
My heated part of the hanger where the tools will be soon is 12x54 & I have a 18,000 btu I plan to build in. I have 6" of iso foam in the outside wall & R26 in the ceiling & other walls. When it's hot out I have to walk thru there to get to the main hanger & I bet it's 30 degrees colder in there so it will heat & cool well. Honestly we only us air in the house 2-3 weeks a year anyway. I wouldn't bother but I have it already.

05-19-2012, 01:24 AM
I'd get a good industrial licenced electrical and air-conditioning and ventilation contractors to both assess the adequacy of the existing supply to the house/shop to see what really is required in terms air-conditioning and ventilation and perhaps the service/main supply as well.

If it were me (I don't need A/C) I'd get it done professionally and get it done right first time.

"Charts" and "rules of thumb" are a big no-no for me in regards to these items.

Humidity never seems to be a problem here as I get very little rust (or condensation) in the shop and the house is cooled with a really good ducted evaporative cooling system. It really works well.

The house is heated with three wall-mounted natural gas wall furnaces with only two needed and mostly set on the lowest setting.

Its very rarelty that the shop needs heating so it doesn't get it. Warmer clothes and good gloves do the job if required. No cooling or A/C either as our heat rarely exceeds 100 deg F and himidity is quite low mostly - opening and closing the roller door and louvred windows - perhaps with a strategically placed industrial fan does the job as does opening or closing the roller door and working on the car-port slab.

I'd hate to have shop energy bills for some of the shops I see on here.

05-20-2012, 09:49 PM
I just researched a lot of this a few months ago.

A standard central air conditioning setup must have an SEER (energy efficiency) of 12 to meet DOE specs. A typical window unit has an SEER between 5 and 7 SEER.

I had planned to go with window units until I found how low their efficiencies were. Then I found out about the mini-splits. They have SEERs of 16 to a bit over 21.

Window units are by far the cheapest for installation cost. Central AC units require duct fabrication and installation, and in some areas every step requires permits, or licensed installers, or both. The mini-splits cost twice what the basic central AC hardware costs, though the difference is less if you can do the ducting for the central AC yourself.

I just went for a 3 x 9000 BTU LG mini-split at $3,000. That's one compressor, three evaporators, sort of like what you see in inexpensive motel rooms, except they go near the ceiling instead of on the floor. The mini-split uses a variable-speed motor and runs all the time, like late model car AC. SEER is 21.7. Oh, and they're heat pumps too, not that that's really useful in my climate.

$3,000 hurts, but our electric rates and high and I don't see them dropping, and the bill comes in every month. The break-even period between the mini-split and regular AC should be around 3 years.

05-21-2012, 02:05 AM
Heat is far more needed thats why I cut the heated part to only 12 x 54. Tools down both sides & a full 12' door on 1 end & a double 3' between heated shop & hanger high enough to get the forklift in & 1 ext man door. Now I have electric base board,gas,wood & haste oil for het Almost ready to start setting tools. 120v & 240V every 8' on the walls with 3 phase drops at the ceiling at every tool Welding in another bay 12 x 24 with full open sliding doors & a huge blower unit all steel lined. Can't wait as it's been a long haul. Now I want to use an extra 14 x 24 area upstairs in the house as a mini shop. That's another story.

05-22-2012, 10:39 AM
I had Tuff Shed build me a 25 X 30 shop on concrete slab, 1 door, 1 roll up door, one small window. I finished the inside with 4" insulation in the walls, and the thickest stuff I could get (8"?) in the ceiling. I covered the walls and ceiling with 7/16" OSB. I put 2 layers of foil backed foam on the roll up door. I don't recall the size of the window unit, but it's about the biggest one you can get that plugs into a standard 120V outlet. The building gets very little shade.

I live in Southwest Louisiana, very high temps, very high humidity. I was worried that I would have to leave the unit running all day to avoid a lengthy 'catch-up' time when I got home in the afternoon.

It's not near as hot in there when I get home as I thought it would be. I think good insulation and the big ground temperature concrete slab have a lot to do with it. I do want to add a couple of gable vents to the attic, I think that will help too.

I get home about 5 in the afternoon, turn the unit on, about 1/2 an hour later it's workable, another half hour after that it's down right comfortable. I don't have trouble with the a/c not keeping up with the heat generated by machinery, but then I'm not working in a production environment.

Also, the unit is through the back wall up pretty high.

Personal experience, YMMV.

05-24-2012, 08:36 AM
I have a 24x36 pole barn and I installed the local Home Depot's biggest window A/C (24k BTU) rated to cool 1700sq ft.

When we were insulating the barn, it was bare steel and almost 100 deg outside... The A/C would keep it about 80 inside. Now that it's insulated it really cools it fast.

This is Colorado, so cooling needs aren't that high but we do see 100 in the summer fairly regularly.

My typical mode is come home from work and turn on the a/c. I'm really glad it's so oversized as it immediatley feels cool especially in the draft of the a/c.

The new window units work far better than my last experience with them (20 years ago). The a/c cools my shop much faster than the electric heater warms it.

If you are planning to run the a/c for hours on end, maybe a smaller unit would be better due to less cycling but for my use the overkill unit works great.