Geo

This week I should be signing the lease on a modest shop that will be shared by my business partner and me. I'll finally have some space to finish my car.

The shop is 30x24 with a 9.5x11.5 office with a 5,000 BTU AC unit which we will use as a shop room to house the compressor, bead blaster, drill press, and that sort of thing.

My real question is does anybody have any experience sizing a room air conditioner for such a shop?

I know the proper calculation is not btu/sqft, but I'm not going to calculate heat gain and all that other stuff. I've found a few calculators on-line and come up with anywhere from 12,000 btu to 20,000 btu with modifiers for amount of insulation and tall ceilings. The shop is open to the roof but has an insulated roof. The wall height is about 12' with about another 6' to the peak. We're going to lightly insualte the metal walls with R3 or R4 foam board, mainly to keep the walls from radiating the heat.

My best guess is we'd want 18,000-22,000 btu given the height and the light insulation. But, this group is diverse and there may be someone here who can give me an informed opinion from experience and/or training.

So, anybody have a recommendation? I know it's as bad to go too big as too small.

Sam Lin

Nothing wrong with too big in my experience, but I didn't do the sizing on my garage. I don't know how your summer humidity there is, but are evaporative coolers a viable option? I have a couple of them and they're MUCH cheaper to run than an A/C unit, they keep my (large) garage at 80 degrees in 110 degree weather. Most of the time I don't have to turn on the A/C.

Sam

Geo

Houston? Humidity? Humidity was invented here.

Perhaps we can consider one of these coolers later in the year.

From what I've read, a window unit must run more or less continuously to be really efficient. According to what I've read, buying too big can cause too much cycling of the unit and making it inefficient. Of course on the days that are less than the hottest it will probably cycle anyway.

kurt M

To size Heat Load you need R value and size of walls doors and windows, construction type, exposure and location.

A too big AC will not be able to strip the moisture from the air very well and humid feels less comfortable than dry temp for temp. As an example shop I have a 24 X 30 in VA where 90% humidity is common and cool it just fine with a dinky 5000 btu window shaker. The shop is 2x4 stud walls insulated with drywall inside and brick facing outside. The ceiling is also enclosed at 10 feet 6 in. It gets less than 100% sun exposure as I have a large tree that shades it during the high angle sun exposure times. The window unit runs close to 50% cycle which is great for humidity control and economy. A big unit will cost more to cool a given space than an almost too small unit. The smallest unit that will cover most of the worst case days will be the best overall for initial cost, run cost, and comfort.
I had a commercial building project that was having humidity and cooling problems. After looking into the existing systems an outside contractor the owner called in wanted $150K to “fix” the issues. We rebalanced the airflow slightly and added better controls for dehumidification. The building had TOO much cooling cap so I actually programmed the heat to come on at times in the summer to give the AC more time to strip the water from the air. Final result, fixed the problems for under $2K and the yearly operating costs dropped as we were able to run higher cooling temps and acheve good comfort levels.

What is the building made from? What is the exterior material and color? What is the R value? What is the location, What is the exposure? With this info you can get close enough.
It is hard to get a good split unit that is less than a ton and a half so you might start there if installing a central unit. Unless you have an un insulated metal box that sits fully exposed with a flat black roof this might be a start. For a window unit in Houston 10K to 15K unit might be ok depending on the building and inside heat load. Good insulation is the cheapest heat and cooling you can buy!

Geo

Kurt,

Awesome post. Thanks.

I tried to follow along some of the heat load calculations I found yesterday and my head just started swimming. I'm usually good with numbers and complex calculations, but not yesterday. Perhaps I'll go back and try to build a spreadsheet to help me out.

Here are the particulars:

1. Wall/window unit only for this application.

2. Building is 24x30, one small northerly facing window in the office area. Two doors (no windows) and one overhead door. The outside walls and roof are white and made of steel. The ceiling is open. Walls 10-12' with ceiling peak approx 16'.

3. Insulation: The roof is insulated, but I don't know the R value. Looks to be pretty serious insulation - probably 6-8" thick fiber insulation. We're planning on sticking R3 or R4 foam insulation on the walls mainly to cut out the radiated head from the walls. My partner wants nothing to do with putting up fiberglass insulation, but I may be willing.

4. Doors are not insulated.

5. Roof is always open to the sun. The eastern wall gets some sun protection early in the day from the building next door. Southern exposure is always in sun. Western wall is fully exposed so will take full brunt of the late afternoon, early evening sun.

6. Weather.... This is Houston which means 90% of the year the humidity is 90% or better. For 6 months of the year (or more) the temps are in the range that require air conditioning. Humidity control is vitally important.

Yesterday I had a migrain so my brain wasn't working as well as it usually does. I'll try to work the heat load equation. I'm really concerned about buying too much AC given all I've learned in the last two days.

Thanks again for the info. I'm amazed you can keep your building cool with a 5k unit. Boy, this really has me thinking.

kurt M

If you plan to use window units just get more than one and set them up to run as needed. For a partly or not well insulated metal building you could try 2 10K window units or a 15K and a 10K. Run them as needed or set the thermostat on one a little higher than the other and it will only kick in when the other one can't pull the temp down. At night turn one off to save $ and let the second unit pull the temps down in the a.m. Two units = enough capacity and variable sizing according to load. little load run the 10 K 10K can't hold temp go the the 15K and shut off the 10K Still cant hold temp? Run both. With good insulation you might be able to use a 10 and a 6.

I knew a guy that put a lawn sprinkler on the roof of his metal outbuilding tghat he converted into a shop of sorts. When the sun would cook him out of the place and the little AC was not enough he would just turn the sprinkler on and it would cool off the roof and the temps would drop right off. First time I stopped by and saw it running I laughted at and then after thinking about it a sec, with him. Made a cool sound on the roof too.


(As an edit to the previous post the 50% run time should read 80% for the warm days and 100% for the scorchers. My shop is built tight, has an insulated door and I don't run the temps into the low 70s)

Geo

Kurt, your help here is invaluable. Thank you. I hadn't thought about different sized units. The problem I have is that this is a leased building and I'm not certain I can cut into the walls. What I had planned to do is replace the rear door (no real use for it anyway) with one I can cut into and mount the unit in that. The shop has a 5k unit in the office (30" or so door to the shop). Perhaps that can be used to help in the scorchers?

I expect the building will only get about 10% usage weekly so it will also have the issues of bringing a warm (or hot) building down to temp because we will not run the AC other than when we plan to be there. The shop is about 5 minutes from my office so if we get a programable timer in the unit, I could stop by at lunch time and either start the unit(s) or program the new one to start mid-afternoon for evenings we plan to use it or program it to start early morning for weekend use.

Back to the sizing issue (I hope you don't mind me picking your brain), I looked on-line for more calculations for heat load and didn't find enough info (yet) to be able to make any calculations. All I've been able to find are simple calculations which yield a need from 12,000 btus to 24,000 btus.

My inclination (currently based upon what I have seen so far) is to get a 15,000 or 18,000 btu unit. But I'd like to get a better idea before I finalize any of this. I'll probably go to Lowe's at lunch to look at the insulation and get an idea what I'm looking at if we go with thicker fiber insulation.

I guess on the insulation front, another thing would could do is use the foam board and if we fine we undersized the AC unit we could add more foam. The only problem with that is I'm going to try to negotiate with the landlord to credit us for the insulation materials. I doubt he'd want to pay twice.

Thanks again. This is really helping me. This building is anything but tight and that has me thinking to err on the large side even though typically I'd be thinking to err on the smaller size. It also has a roof vent that will be great for shedding heat, but it will also lose some cooling as well.

Do you have a heat load formula or can you point me to one?

Geo

For reference, here are a couple of photos of the shop. The garage door faces directly north.

Geo

Photo 2, the inside.....

I'm also starting to think more like 15k btus now. I'm just worried about the humid hell that is Houston in the summer, and probably light insulation we'll have.

kurt M

Looks leaky, wind and temp dif will push unconditioned air(and dust) in and conditioned air out. It would be hard to calculate a usable load as you need true R values and infiltration #s. Get a good sized unit 230 volt 10 to 18 K and if it can't hack it, sell it and get a bigger one. Larger will pull down faster which would be nice for infrequent use. Even if the unit can't cope woth the worst of the worst days when it is 99 deg and 90% outside 80 deg and 40% inside still feels pretty good. I don't think you will have overcap problems unless you installed well planned insulation, weather stripped the roll door and worked to block air infiltration.

Bull

As was AC!

Geo

Kurt, thanks for all your input. I agree, it's pretty leaky. Since I don't own it, I don't want to put a bunch of money into it. I still have ownership in mind down the road, so getting by for now will work. I don't know how long we'd consider staying in this location.

I guess we'll go with 15k or 18k. I know the 5k in the office area won't contribute a lot, but hopefully it will be able to supplement the main unit when the weather turns unbearable. I dunno. For sure, whatever we have in there will be better than what I'm used to during the summer.

Bull, there are those who say Houston was built on oil. I say they're wrong. It was built on AC.