Manual J for garage?

[WayneH]

Hello: I'm in the process of building a new garage. My existing shop is about 900 SF with 12' high ceilings. I heat and cool it with a 3 ton Mitsubishi Split. It works great and I could not be happier.

The new shop is 1350 SF with 12 foot high ceilings. All the HVAC manual J stuff I have seen does not address high ceilings. Can I just multiply by 1.5 to get the needed BTU for this shop? The walls and ceiling are insulated with R19, and we're in Maryland, so it is not too cold, but it gets warm and humid.

I'm afraid the largest split units are only 3 ton, and that will not quite be enough.

thank you

Wayne

 

[Rhsty]

Up here in new york state we usually figure 600 sqft per ton but i understand with 12 foot ceilings you might need more. We also use 95 degree & zero degree outside design temp. I would think a 3 ton unit would work here, again dont know your
Design delta T. Hope this helps

 

[Full Throttle]

A garage is considered Commercial. We use Manual N load calculations for this application.

Manual J should be fine for a simple garage though. You are doing it wrong. It asks for length and height for each wall, so you have a 30ft x 12ft wall theoretically speaking. It asks for the square footage of wall L x H, then glass and doors in each wall and referance of direction to the sun, shade factor and construction value

Do not use these rule of thumb of ton per sq, ft

 

[pseudorealityx]

Mitsubishi offers a 3.5 ton unit in their ceiling cassette type, and their high wall application.

http://www.mehvac.com/UploadedFiles/Resource/MrSlim_catalog.pdf

pages 35 and 37.


That said, unless you're doing something crazy, I see no reason why you would need more than 3 tons in a 1350 sq ft garage.

 

[WayneH]

. You are doing it wrong.

Do not use these rule of thumb of ton per sq, ft

Well... thanks for the help.... Although I have not done anything wrong yet since I am just asking questions now.

This can't be that complex though. It is not a house. If it does get down to 15 degrees outside, which it will a few days each year, the garage does not have to be 72 degree... And if it ever got up to 105 degrees outside, I'd be happy if it was 80 inside!

Thanks for the advice though.

Wayne

 

[Highbeam]

So have you thought of using more than one? No reason that you can't have two minis working together. These are inverter systems so they can be oversized with less drawbacks compared to a normal non-inverter ac.

 

[WayneH]

Highbeam:

Yes, I am thinking about getting two of the splits. The one I have now in the existing garage is just great. And, you can't hear it run outside. That may be the way do go with two two ton units. I'd just like to get a better feel for the right size, so I may get some estimates and see what they tell me. I installed the current split, and all I had to do was get somebody to pull a vacuum on the line and add about a pound of freon.

thanks

Wayne

 

[Falcon67]

I found a link to a Manual J calculator at hvac-talk. It just took a few minutes to get a result. http://www.loadcalc.net/

Takes into account all the things you mentioned. I have 960 sq/ft with part partitioned off, so I use two units. I have a total of 16500 btu in there and have no problem making it through the summer. The electric heater is 17K BTU and has done fine so far (however, we've been in shorts and t-shirts since December 1) but I have another 14K btu of propane to back that up, should it decide to finally get cold.

 

[pseudorealityx]

I really don't think you need 4 tons of cooling for 1350 sq. ft.

 

[CNGsaves]

Curious what these split Mitsubishi cost for say 2 ton or 3 ton? Do they have any ducting to better spread heat/cool throughout garage? Or does unit itself have enough airflow itself for entire garage?

Also what about cost of operation? I'm leaning more towards hanging NG heater, then add a 220v window A/C cut into wall permanently.

 

[WayneH]

I really don't think you need 4 tons of cooling for 1350 sq. ft.

For a house with 8 or 9' ceilings I would agree, but with the 12' ceilings that sounds to me to be 50% volume.

If I just wanted heat I would go with the Modine, but the AC is nice in the summer!

Wayne

 

[pseudorealityx]

For a house with 8 or 9' ceilings I would agree, but with the 12' ceilings that sounds to me to be 50% volume.

If I just wanted heat I would go with the Modine, but the AC is nice in the summer!

Wayne

I agree it's more volume, but a couple of thoughts...or my assumptions, so if they're wrong, then I'm off base.

1. Typically, you don't need the same temperature in a garage/shop than in a house. In my house, in summer, I like to keep it at about 73/74. In the garage, even with A/C, I'd be happy with 78-80 degrees. In the winter, same thing... house at 68, garage would be happy at 62 or 60.

2. In the cooling months, when you're running the a/c, if the air above ~8' is 'warmer' than your T-stat, who cares? You're not up there. Let it be warmer up there. You're more interested in the temperature in the occupied zone.

 

[pseudorealityx]

Curious what these split Mitsubishi cost for say 2 ton or 3 ton? Do they have any ducting to better spread heat/cool throughout garage? Or does unit itself have enough airflow itself for entire garage?

Also what about cost of operation? I'm leaning more towards hanging NG heater, then add a 220v window A/C cut into wall permanently.

Check the link I provided in an earlier post in this thread. That's their basic catalog, and includes the various styles of un-ducted versions, and shows some of their ducted versions. They are somewhat limited as far as airflow, so which is best depends on what you're trying to do.

I can't speak to cost, but they're more efficient than standard air conditioning units, and they're very quiet.

 

[RKA]

Running costs will depend on a bunch of factors, but here's something to give you an idea. 600 sq ft insulated garage (12 ft ceiling), 2 ton mini-split. For heat, outside temp is 30F, inside temp is 65F and that will run about $5-7 to keep is going for 24 hours (ceiling fan is also used to force the heat down). I might be able to bring that down a bit by installing some brush seals around the garage doors. For cooling, outside temp 95F, inside temp 75F and that will run $1.50 for a 24hr period. On the heat side what kills you is temps below 25F and trying to warm a cold garage. The heat pumps lose heat output when ambient temps drop too low. And obviously you're not just warming the air, you're warming everything inside the garage that's at 40F, so it can take 1-4 hours to bring a garage up to temp depending on how cold it is inside and outside the garage. All that translates into $$ as mine gobbles $0.45/hr running full tilt and about $0.15-0.20/hr once it's up to temp and just maintaining the temp.

A forced air NG option will raise the temp faster and have cheaper operating costs. The mini split might be more efficient on the AC side, but it's a whole lot more expensive at the outset, and recovering that in summer electric bills would likely take a very long time. If you want the mini-split, you want it because it's a cleaner install and it's quieter year round. You're not getting it to save $.

 

[pseudorealityx]

I don't think the OP stated a location, but he said his minimum temps were 15, and that it might get to 100+. I'm guessing he's somewhere that he's cooling a fair bit more than heating.

 

[RKA]

He said Maryland.