Reply by clin

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Posted on Pondering a mini-split AC

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1056 posts in 1474 days

#1 posted 07-24-2019 07:33 PM

Mini-splits are great. I use one in my shop. 2 tons of AC is almost certainly too much for a 1250 sq-ft shop. Though I wasn’t 100% sure how this storage room figured into what you wanted to heat and cool. Even if this is added in at for a total of 1,675 sq-ft, 2 tons is probably more than you need.

Using square footage to estimate HVAC is close to useless. Heat load, has some relationship to square footage, due to the roof/ceiling area. But walls and windows matter a lot.

How the space is constructed, how much exterior wall area there is. Window size and location. Size of eaves and overhangs, if any. Type of roof and attic. And of course your local climate makes a huge difference in what your needs are.

Almost universally an HVAC installer will quote an oversized unit. Not because they are trying to trick you, but becasue they either don’t know how or don’t want to take the time to figure out what you really need. They don’t get callbacks on oversized systems. The fact you paid more for a larger system, that it runs less efficiently (costing you more in electricity), and perhaps is not as comfortable as it could be. These are all things that won’t cause a callback. So they have no reason to worry about over-sizing.

The above is based on how you cool a typical space. On the other hand, if you want to leave the space unconditioned through the week, then walk in Saturday afternoon and have the shop cooled in minutes, then bigger is better. And probably not a mini-split.

Mini-splits are extremely efficient, but they are like a car on the highway. Keep them cruising at a constant speed and they sip electricity. This is why they would typically be running 24/7. Or if you’re a weekend warrior, perhaps turn it on Friday night or early Saturday morning, to have everything perfect later.

Mine, I just turn the thermostat back for the days I know I won’t be in there. That way the shop never gets super hot, or cold in the winter. And that’s good for the glues and paints stored in there anyway.

Unless you do the calculations, you’re guessing. The “official” way of doing this is to run a “Manual-J” calculation. This is based on the ACCA Manual J for residential heating and cooling load calculations.

It’s fundamentally easy to do, but the manual is an inch thick and full of tables to cover all the different construction methods, insulation types and amounts, types of windows, even color of the roof factors in. The ACCA provides a nice Excel spread sheet to fill out, but without the tables and the information on how to use them, they’re of no value.

There’s even software you can get. One program I used was HVAC-calc:

For a one-off job you can buy a license for $49 good for 2-months. It is based on Manual J, but is easier to use and you don’t need to know all the Manual J stuff to use it. I know it seems like an unnecessary expense, but you’re looking at spending maybe a few thousand dollars, so $49 to make sure you get the correct unit is money well spent. Might even save you outright if you need a smaller and less expensive unit. Plus they have a demo you can check out. So no harm in looking at it.

As always, first step is to insulate the shop if not already.

I also agree that they are relatively easy to install. I did NOT do my own because it was part of a larger installation of a total of three units. But it still took 2 guys about two days to do that. And that was me providing the electrical connection. But every install is different. Most complex thing is running the drain line for the condensation. But even that is trivial if the head (inside part) is mounted on an exterior wall.

Anyway, mini-splits are great. I’m sure you’ll love it.

-- Clin

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