Need some A/C advice

[HowardH]

We are in the process of buying another house which will have a 3 car garage. The single garage door side will be my shop area and I want to climate control it, the whole thing, since we live in Dallas where the summers are notoriously long and hot. I will have an outside wall where I can put one of those LG or Mitsubishi wall mounted inverter style units. The other house A/C compressor units are along the same outside wall so it will fit right in. My plan is to insulate the garage doors and tightly weather seal the outside edges of the doors and buy a 24000 btu unit. If I can get the shop under 80 during the summer I will be happy and able to work in the "shop". It's about a 600 sq ft space so will this plan be adequate? They are about $1700 so I don't want to buy something and then not have it work correctly. If not this type of system, what else?

 

[Holzhacker]

Since this will be a shop I would tend to say going with a wall mounted split system is probably a better option than some other things. The split systems work pretty well. For around here a 2 ton unit would be a bit big for that square footage. I know you guys are hotter down there. Based on experience I would suggest a 1.5 ton unit but the 2 ton might be better for your area. It's better to undersize a bit rather than oversize. You want the unit to run rather than cycling on/off constantly. Too much oversizing can also lead to humidity control issues. A 1/2 ton doesn't normally make a big difference usually but that is a small space.
You could get a couple estimates from HVAC guys and ask them for a Manual J load calc.
If heat build-up is a big issue, you may want to also consider either installing a vented skylight or tube vent through the roof with a controllable damper. Allowing the heat build-up on the ceiling to escape out the roof can make a huge difference. All you need to do is open the skylight a couple inches for it to act like a chimney. Opening it all the way is however counter productive.
If you wanted to save money you could by one of those freestanding units that roll around. The good ones are actually really good and you can hook up a drain hose to the condensate pan. The cheap ones work but aren't all that great. Since floor space is probably a premium you might need to hang it up off the floor on a shelf along the wall. (Did that for a client)

 

[stvrowe]

We are in the process of buying another house which will have a 3 car garage. The single garage door side will be my shop area and I want to climate control it, the whole thing, since we live in Dallas where the summers are notoriously long and hot. I will have an outside wall where I can put one of those LG or Mitsubishi wall mounted inverter style units. The other house A/C compressor units are along the same outside wall so it will fit right in. My plan is to insulate the garage doors and tightly weather seal the outside edges of the doors and buy a 24000 btu unit. If I can get the shop under 80 during the summer I will be happy and able to work in the "shop". It's about a 600 sq ft space so will this plan be adequate? They are about $1700 so I don't want to buy something and then not have it work correctly. If not this type of system, what else?

I am in Arkansas and have a similar climate to Dallas area except maybe a few degrees cooler.  Humidity is about the same.  I have a 936 sq ft detached 3 car garage shop that I installed a 2-1/2 ton heat pump in several years ago.  I had 12" of cellulose insulation blown in the attic and used 1" foam foilboard to insulate the garage doors.  I had several HVAC guys look at my situation and I wanted to go with one of the mini-split units you describe.  Without fail, everyone of them discouraged going that route for the shop.  It wasn't that they didn't like them, it just wasn't good for how I wanted to operate it.  In the winter, I keep the heat setting at about 58F and summer about 86F when I am not in the shop.  When I go to the shop, I will crank the heat up to about 62F or cool down to 78F.  The high efficiency units do not like this type of operation so they discouraged me from using them in my application.  Had I just set the temperature at one setting and left it there, it would have been a good fit.

My experience with my system has been that in the heat of summer, late afternoon, with the sun beating on the west facing garage doors, the A/C runs continuously.  The temperature will creep up to about 1 degree above the thermostat setting and just stay until the sun goes down.  I have absolutely no regrets about installing the system I chose as working in the summertime before A/C was just absolute misery and something to be avoided unless absolutely necessary.

 

[HowardH]

I think I'll have an A/C guy come out to the house after we close and make some recommendations.  I don't want spend $3000 to $4000 on a conventional A/C system for one and I also won't have anywhere to put the blower and condenser.  No attic space to speak of.  That's why I was thinking of a split system.  I also hate giving up four months of the year because it's dangerously hot in the garage. 

 

[peter halle]

Paul Marcel put a split system in his garage this past year.  You might want to check with him.  His location (Phoenix) might be a little hotter but my guess is that you could always adjust the thermostat.

Peter

 

[joraft]

... My plan is to insulate the garage doors and tightly weather seal the outside edges of the doors and buy a 24000 btu unit. If I can get the shop under 80 during the summer I will be happy and able to work in the "shop". It's about a 600 sq ft space so will this plan be adequate? They are about $1700 so I don't want to buy something and then not have it work correctly. If not this type of system, what else?

Howard, as it happens I'm in the process of doing the exact same thing as you. I'm insulating my 640 sq. ft. garage, replacing the doors with insulated roll-ups, and having a/c installed.

I had an HVAC contractor out here last week, and he recommended the following unit:

Fujitsu 24RLXFW

He says that here in Southern California it should be perfect for either maintaining a desired temperature, or getting it to almost any temp within 15 minutes.

This guy owns one of the largest HVAC companies in SoCal and he uses these units to cool some of his offices and his service department. He invited me over to see them in action. I swear, you can stand right next to the unit and not hear it running.

He also pointed out out to me that since I have both a furnace and water heater in the garage I cannot completely seal it. I'll need to vent in enough oxygen for proper combustion. I still have to figure out how to do that and still control the temperature in the garage.

 

[HowardH]

Thanks John.  That's very interesting.  Let us know how it turns out. 

 

[joraft]

Thanks John.  That's very interesting.  Let us know how it turns out. 

Howard, one thing that may be a concern is how fast the outer filter will clog up. Even with using Festool equipment there will be some dust. The outer filter is washable, but how often will that have to be done?

I won't be doing any woodworking in my garage, so it won't be an issue for me.

 

[Dane]

I have a mini-split that conditions my entire house.  The issue of filter cleaning is no joke.  It is stunning how the efficiency drops off if those filters aren't vacuumed out every 2 weeks or so.  It's certainly not hard to do, but in a dusty environment, I'm not sure how it would work out.  That said, I love my system.  On the sizing issue, if you go with an inverter system, you can way oversize it and not sacrifice any of the efficiency due to the system's ability to modulate its output to the load.  I bought my system online (mitsubishi Mr. slim) and did the rough install myself.  Then I just paid an hvac guy to connect the lines.  Saved me about half of what I was quoted for the HVAC guy to do the whole thing.

The other thing you might look at is one of the Kuhl systems- https://www.friedrich.com/products/residential/window/kuhl

They are also heat pumps, but install like a window or through the wall unit.  You can do that install yourself and save some serious money.  Probably easier to clean the filters too....I am putting one in my rental unit.

 

[rdesigns]

You'll probably be happier with a 1-1/2-ton unit (18,000 BTU nominal) because you'll get better humidity control, and the unit will last longer because it won't cycle on and off as frequently. Compressors are happiest when they run constantly on the hottest days.

Of course, the wise thing to do is to do a load calculation to better estimate the cooling load of the space. In your case, the load calc can be done using one of the free online programs, and you will be accurate within a very acceptable range. The space is all one room, so that greatly simplifies the load calc.

Be sure to use the outdoor summer temperture for your location that is recommended in ACCA Manual J, which, for Dallas, is 98 degrees. Don't be tempted to use a higher design temperture just because you know there are times when the temperature gets higher than 98. That design temperature means that only 1% of the time is the temperature higher than 98, and you will likely find that even a 1-1/2-ton unit will exceed the cooling load enough to handle higher tempertures.

Other needed inputs specific to Dallas:

Coincident wet bulb temp. = 74 degrees
Design grains of moisture = 25 (this means the difference in amount of moisture  betweeen inside and out with 50% RH inside the shop)