Need advice for air conditioning my workshop

Discussion in 'Physics' started by strantor, Apr 8, 2018.

  1. strantor

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    Oct 3, 2010
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    @#12 batsignal

    I had this metal building built a couple of years ago as a workshop but only had enough money for the building itself, not enough to get electricity hooked up or a driveway or anything else. So it has turned into a really expensive storage shed. I'm now approaching a financial state where I will be able to finish what I started. I plan to air condition the whole thing. I don't plan to keep it a comfy 70 degress year-round but I do want to keep the humidity down so my tools and machines and machine tools don't rust in place. And it would be nice to have the ability to adjust to a "comfy" (75-80f?) temp from time-to-time while I'm working inside.

    I've used online calculators to determine an approximate BTU requirement for the given space, but I don't know what the assumptions are behind those things. I assume that they assume a house-type environment with an attic and framed walls, sealed windows, etc. and my situation is different.

    My building is 30x30ft with 11ft eaves and 14ft peak. Concrete slab, no windows. The only openings are an insulated man door which seems relatively well sealed and a 10ftx10ft insulated rolling door which seems relatively poorly sealed. I got "polar white" exterior which has a reflective paint advertised to be more energy efficient. I opted for roll insulation intalled first before the metal went on. It's about 2" thick where it isn't compressed between the inner and outer metal.

    I am looking for hints/tips, mainly what size A/C to install, but also things like:
    1. Do I need to do something about the sealing around the rolling door, and if so, what?
    2. What type of A/C should I be looking at? Huge window unit? DIY inside/outside kit? Full-blown commercial unit with ducts, professionally installed?
    3. Is this type of insulation effective at all? Or should I be considering building insulated walls inside?
    4. What am I not asking that I should be asking? I don't know what I don't know.

    My results with the online calculators and equations suggest maybe a 35000 BTU solution is sufficient. I can get a window unit rated as such for a few hundred $$. Is that the way to go?

    20180406_170227.jpg 20180406_170139.jpg 20180406_170124.jpg 20180406_170101.jpg 20180406_170204.jpg

    See all the sunlight coming in around the roll up door? I' afraid that will render any solution useless. Is that true? Any suggestions for improving that? The only thing I can think of is to build an insulated box around it which would be a PITA to build and to move out of the way each time I need to open the door. Or maybe a second set of insulated doors on the outside? I don't even know.

    P.S. the "insulated" rolling door appears to be just a normal rolling door with 1/4" thick "radiant barrier" type foil-covered-foam taped to the inside of it.
     
    Last edited: Apr 8, 2018
  2. #12

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    First, the things I expect you already know.
    The basic equation is BTU = area x conductivity x difference in temperature. BTU=AUdT
    For each of the walls, exterior temperature - interior target temperature =dT
    The exterior temperature is going to be pretty bad on the sunny side.
    Conductivity is 1/R value of the insulation because sheet steel has about zero R value.
    Air leakage is a little more complicated. It's about the enthalpy difference and cubic feet per hour.
    Air leaks are bad.
    The floor is irrelevant because it's at ground temperature which is 70F in Florida.

    Now for some opinion.
    You pay for insulation once. You pay for air conditioning forever.
    There is no such thing as a window unit that can deliver good air flow across 900 ft^2
    There is no window unit I ever saw that could throw air 30 feet.
    You can install a window unit, but you better plan on attaching some duct work.

    An air zone is self containing. However far you throw the air, it will come right back to the intake from there.
    The cold air will not wander all over the place. You have to deliver it.
    A residence that size would work on 21,600 BTU. Can you improve the insulation that much?
    I used some 2.5 inch thick foam boards I bought at a salvage yard.
    Fire code requires 5/8 inch of drywall on top of the flammable foam boards.
    If they catch on fire, the gasses are wildly toxic. I'm talking about cyanide.
    I've seen some good fitting garage doors, but I don't know anything about tightening up a bad fitting door.
    I would think about some kind of flexible weather stripping. Start by figuring out where you can find a straight line which the door does not violate as it opens and closes. That looks like a segmented door which would work with an exterior flap method around the perimeter.

    Am I helping?
     
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  3. strantor

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    I assume by Area you mean L X W X H (Volume)? My figures were very low when I just used L X W.
    30ft X 30ft X 12.5ft = 11,250 cubic ft

    I'm not positive, but per the internet, an insulation the same thickness as what I have, is about R8.
    1/8 = 0.125

    Highest temp I have seen here is 115f. Worst case, on that day, I want it to be 75f inside:
    115-75 = 40f

    So in your equation:
    11,250 * 0.125 * 40 = 56,250 BTU.
    That's around what I am getting for online calculators. This one is particularly close. And that one has an selection for Insulation Quality.
    When I choose "Many Leakages or Windows," it tells me 90kBTU

    Good point. When I use a value of R-19 in your formula, I get 23,640BTU. The calculator agrees, telling me 25,950BTU with "very good" insulation.
    So I estimate a requirement of 75,000 BTU if I do noting, and 25,000BTU if I seal & insulate the heck out of it.
    Using the math here, and the specs of this A/C for example (one unit vs 3 units); I have the option to pay $86/mo, $260/mo, or somewhere in between, depending on how much effort I go to.
    That's > $2,000/yr difference.

    I think that the cost of building out the interior with good insulation will be cancelled out by the energy savings probably in the first 2 years max.
    I have the obvious portion of the answer now. Thank you

    Yes, you are helping. Thank you.
    Based on this I think I need to:
    1. build insulated walls inside my already (meagerly) insulated walls and ceiling. Probably talking about a few thousand $$ but very much worth it.
    2. Find some way to seal off that big door. Maybe some kind of zipper flap if I can find/make something like that.
    3. Install some ducting. Or a bunch of little window units. Is one more efficient than the other: one big AC vs a bunch of little ones (total BTU equal to big one)?
     
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  4. gramps

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    Dec 8, 2014
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    It's hard to tell from your photos, but the roll up door looks to be a panel type door that rides in tracks, one track on each side. If so, it is installed incorrectly, there should not be a gap at the top when the door is closed. My suggestion would be replace that door with one that has a smooth outer surface AND is installed correctly. The smooth outer surface will allow a vinyl outer seal to completely seal the door.

    As already stated, for a space that size, ducting is essential. I would contact a local heating and A/C contractor in your area and have them look at your building. They can give you recommendations. Doesn't mean that you have to go with them, but that will give you some ideas.
     
  5. wayneh

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    Sep 9, 2010
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    He meant surface area, the area across which the heat flows in response to a delta T.

    My only other comment is to get a ceiling fan. Moving air will make you feel much cooler even when the A/C cycles off. Otherwise the dead air will feel much more oppressive at a given warm temp.
     
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  6. BR-549

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    Sep 22, 2013
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    I agree with your thinking. Insulation first. I would consider a heat pump and a single panel door for good seal.

    A fan is always a good idea. Winter and summer.
     
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  7. nsaspook

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  8. strantor

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    I'm not sure if you're describing my type of door or some other type of door. And I don't know it's installed correctly, only that it was installed "professionally."

    It's basically a one-piece sheet of corrugated steel, with the corrugated axis oriented such that it is free to bend around the spool. These web images should give a better idea:

    [​IMG]
    [​IMG]

    The light is coming in by the "B" in the next image:

    [​IMG]



    So are we talking about the same thing? Do you think it's installed incorrectly?
     
  9. strantor

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  10. strantor

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  11. cmartinez

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    They seem to be rather effective. One of the pages states that "When sealed on all four sides, nothing gets through; Insects, rodents and even air are forever blocked from entering your building."

    In fact, I'd say they're exactly what you need. And they don't look like a PITA to install either.
     
  12. #12

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    No. Area is square footies of the floor or any wall or any ceiling.
    Think, "boundaries of the system" as in Calculus. The volume doesn't mean anything because the volume does not produce heat energy, the boundaries do.
    The external temperature is not (temperature, ambient) except on a shaded wall. All walls in sunlight will heat up above Ta and that is the number you use. Dt/R x area = BTU
    A concrete block structure has a thermal time lag of 9 hours. A sheet metal wall has a thermal time lag of about a minute.

    Pick as many of the following as practical:
    Insulate the crap out of it. Add a shade tree. Overhang wide enough to shade the south wall completely on June 22 (Summer Solstice) (highest angle of the sun).

    Now, about the duct work. You always blow air away from the A/C. The jet of air coming out the vent entrains the surrounding air to make a larger, not so cold stream of moving air. Mount one air conditioner someplace. It doesn't really matter how high. You can put it at ankle level and pipe the output up a wall and then across the room. How high is the main duct? How much head room do you need? How high are you willing to climb to jerk the A/C out of its shell for cleaning and repairs? Extend a single duct across the building and place delivery grilles to blow air laterally such that the jets arrive above head height at the farthest wall. How do you find that? From a book of grilles. The diameter of the length of duct feeding a delivery grille determines the CFM and the grille size determines the, "throw" distance. But first you have to nail down the BTUs which nails down the CFM which nails down the duct size which nails down the grille sizes. Every grille has, "characteristics. Now...where did I leave that grille book?

    I recommend against a heat pump because they contain rustable steel Freon filters which...rust. Besides, how many days a year are you going to use heat compared to the several hundred dollar price of a Freon leak and repair every 7 to 10 years? Or you can insulate the steel cans so they can not condense any atmospheric humidity, but I don't do heat pumps in Florida. There is no net profit in paying more for a machine that is guaranteed to leak when you can get one that is all copper and might last 20 or 30 years with no problems. My shed has an A/C that a customer discarded when I installed his central A/C. It was old when I got it and it's been running in my shed for at least 25 years. The fan switch failed, so I just moved the high speed wire to the medium speed terminal and changed the label on the front panel.:D
     
    Last edited: Apr 9, 2018
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  13. #12

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    Yes, R8 is about right for the original specs.
    I would do a 2x4 wall with R13 because R13 + R8 = R21
    Or is it called R15 nowadays? I dunno. Read the label.
    Besides, 2x4 framing allows for electrical service and structure to nail things to.
    The roof counts, too. Want a ceiling? Make up your mind.
    I buried a 6-3-g cable to my shed with a dual 60 breaker. A local breaker box cost me about $20 for 16 spaces.
    Strips of hanging plastic are used inside truck bay doors. Lots of vertical strips about 6 inches wide allow semi's to intrude and the strips almost make a seal around the semi. I think a perimeter seal (weather stripping) on the door might be cheaper.

    You can't just, "go long" with the main duct and drop the air at the far end because it will fall to the floor and most of the volume will not be involved in air movement. You must deliver velocity everywhere.

    Keep thinking and ask questions.
     
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  14. #12

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    This is the wrong spec sheet, but learn how to read it for CFM and throw.
     
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  15. gramps

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    No, I was thinking of a different door style. I'm still surprised that a "professionally" installed door has so much space at the edges and top.

    I still think I'd get a heating and A/C contractor out to give you a professional idea and perhaps a quote as to what you need to accomplish your goal. That's a fairly large building and errors could be costly. They could also give you ideas to seal the roll up door.
     
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  16. #12

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    Why do I keep banging on about ducts? Because I think you're going to end up at about 2 tons, which is 24000 BTUs, which is 800 CFM.
    6000 BTU's is the smallest window air that is reliable, so you could use 4 of them. Each one would need to service a 15 foot square. When have you ever seen a 6000 window shaker throw air 15 feet? Not gonna happen. A 24,000 wall unit can't throw air 30 feet.

    So we get to ducts and grilles. One fan motor, about a quarter HP can put enough energy into an air flow to throw a velocity stream 15 feet in 4 places. Cheapest configuration is a center supply duct with 4 grilles. You're looking in the engineering and performance specs for 200 CFM and 15 feet.

    http://www.airmate.com/residential-grds/sidewall-registers-and-grilles
    http://www.airmate.com/submittals-performance-data

    Look at this Series 180 at 8" x 14"
    http://www.airmate.com/files/assets/files/1428946255_series-180.pdf
    That could work.

    This is not the last word in delivery grilles. There are always 6 ways to do it and 1 of them is the cheapest effective method.
    For instance, you could do (2) sideways grilles and (3) on the end of the duct pipe. Suddenly the static pressure loss at the grilles doesn't match and you just caused a balancing act. Four equal grilles on a single duct means the pressure in the duct is the same at all places and therefore you don't need to add sections of lateral duct to control the CFM at each grille.

    Now I'm going to teach you about wedge effect. At the end of a long run, if you put one or two sideways grilles right at the end, the dead air at the end of the main duct will form a dead wedge and the air will come out the grille angled along the axis of the main duct. If you want the air to come out at 90 degrees to the main duct, you have to continue the main duct at least 2 diameters past the last grille opening. In this case, that's not awfully important.

    Right now, I'm looking at 23 feet from the fan to the last grille in (at least) 12" by 11" square duct or a 12" round duct. Fiberglass duct board costs $36.60 (as of today) for a 4' x 10' board and this job is going to cost 4 sheets if you can get (2) four foot sections out of a sheet. (Yes, the end cap and the transition to the air handler use up duct board, too.) Some places refuse to sell to just anybody so, phone a friend, hire a contractor, look on the Interwebs, or kiss some butt to obtain the materials. While you're at it, get an estimate for a 2 ton split system (installed) or a 2 ton package unit (which is designed to fit duct board and contain enough nichrome to use up most of that 6 ga. power feed). I run my house on 5KW of heat and your shed is gong to be nicely insulated, so that only uses up 21 amps of your 250 VAC power feed.

    Round duct is cheaper and it comes in 25' lengths, but every attachment requires a box +(2) "start" collars + giant zip-ties and skills. A square duct is just cut a hole and make a sheet metal collar that goes inside the hole and accepts screws to attach the grille. You are a sheet metal worker, aren't you?

    OK. Ima gotta go up the ladder and sweat for a while. Post if you want to.
     
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  17. eduardoromero

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    Jul 6, 2018
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    If it's still helpful... Go for heat pumps! They are really convenient nowadays, and especially to workshops like yours. You can have them in many different forms (Ground source, air source, air to air, air to water...) and the investment is definitely worthwhile in the long run.
    If you need advice on which suppliers to choose, just read this page on why is convenient to get heat pumps.
    And if you are facing problems please contact me :)
    Have a nice day!
     
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  18. strantor

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    Thank you
     
  19. strantor

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    I've been sweating my butt off in this shop too long now. I've been pulled in so many directions the past few months that I didn't have time to attend to my own comfort. Time to make time.

    I spent a while looking at the Mr. Cool DIY ductless mini split systems. They have good reviews and special pricing through Costco, but I just can't convince myself that a little wall mounted unit mounted on any one wall will get cold air to any other wall inside a 30x30 space. I have the words of @#12 bouncing around in my head as I try to sleep... "ducts... grilles... ducts..."

    I'm looking at this unit, which is a self-contained (ducted) setup. Do we approve of this?

    On to the ducts... since the ducts will be inside the climate controlled space as opposed to in a crawl space or an attic, is there any point to insulating them or using fancy (proper) duct material? What if I just made my own square ducting out of plywood that I already have? That would cost me $0/ft, whereas ducting price seems to be around $4-$10/ft according to a brief search.
     
  20. shortbus

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    I can't speak to the unit but can to both the company selling it and the company making it. Alpine has been my families go to place for HVAC. We've bought 3 units over the last 5 years. The salesmen there are usually pretty well up on what will do the job and help pick the correct unit for your application.
     
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