HVAC Sizing questions

[Spencer Hochstetler]

I'm going to be installing a Mitsubishi mini split (ductless) heat pump in an insulated shop. The shop is 600 sq.ft. with 10' ceilings, insulated, with about 30 sq.ft. of double-pane low E glass windows. I plan on running HVAC all the time to keep the humidity in check. I'll probably set it at about 78F or so in the summer and about 60F in the winter. In the winter, I'll probably have a unit heater to kick the temperature up quickly and then let the heat pump maintain when I'm working as I'm only a part timer in the shop. I've contacted a few HVAC folks for bids and one bid specified the 16,000 BTU unit and another bid specified a 12,000 BTU unit. I'm in upper eastern TN.

Questions:

1. Anyone with HVAC sizing experience have any words of wisdom on which size to go with? Mitsubishi also makes a 15,000 BTU unit.

2. Now a more Mitsubishi-specific question. The room is 20' x 30' and I can mount the interior unit in the center of either the 20' or the 30' walls. Which would be better? i.e. does the unit throw air wider than farther? If so, then centering it high on the 30' wall makes more sense to me...

[Jeff Wright]

I have enjoyed my mini-split system for just over a year. My shop is a 2-car garage measureing 18.5 x 24 and a ceiling that peaks in the middle at about 12 feet sloping down to 8 feet. I have an insulated double garage door that is the highest hurricane rated door available (I live in St Petersburg Florida on the water). I have NO insulation in walls or ceilings. I intend to install closed cell foam panels between the rafters. The inside unit is mounted in the center of the longer wall with the outside compressor just outside that interior unit. I have only one small window (that is about 45 years old and rather drafty!). One shorter wall abuts the house and half of one of the longer walls does too. The garage door faces west, the east and southern sides are mostly buffered by the house walls. The north wall is exposed. The small window is on the north wall. I keep the remote set at about 73 degrees all the time. Anything lower makes it too cold in the shop. I have NO humidity problems. I have seen no rust form on my tools (other than from my sweaty hands!). The unit is so quiet I hardly notice it is on. It seems to circulate the air very well. I have had little or no sawdust collection on the unit's interior filters, but then I have a 3HP Oneida system hooked to all machines and use Festool tools with their efficient dust collection systems. I have noticed only a small increase in my electric bill. The unit does a fine job heating the garage in the winter months (yes, it can get cold here in Florida in January and February . . . in the low 30's at times). My unit is a one ton unit with 12,000 cooling BTUs and 13,000 heating BTUs. I am told one should size the system on the low side for cooling, otherwise the system cools too fast and does not run long enough to remove humidity. Perhaps with your shop being slightly larger than mine you would want to go with the 18,000 BTU. It cost me $1,650 to have the system installed. I already had my 220 wired in for my other machines. My installer said this unit was one left over from a job not completed; otherwise the pricing would be in the low $2,000 range. I am definitely glad I went the mini-split route and would encourage others to do so. They are popular in Europe and have been for over ten years. Hope this is a help.

[Spencer Hochstetler]

Thanks Jeff. I think I'll install the interior unit on the long wall... We have about the same general ceiling height and if you scale your BTU output by the increase in square footage, I should install the 16,000 BTU unit. BUT, you keep the temperature about 5 degrees cooler than I will and you aren't insulated, which would suggest 16,000 is a bit big. Hmm. Maybe I'll put the 15,000 unit in. :confused:

[Tom Leasure]

Spencer,
If you want to keep the humidity in check you might want to consider a dehumidifer plus the addition of the A/C syatem. My shop is 30 x 40 with a 9' ceiling, insulated quite well & I installed a 2 1/2 ton central A/C system. The main A/C will remove the humidity & keep the shop really cool but I don't like leaving it run all the time ( just cheap ) The dehumidifier however keeps the shop dry & doesnt cost 1/8 of what the A/C does to run.

Good Luck

[Jeff Kerr]

Sizing is very important.

If you buy to small it will run all the time and never get the temp correctly. If it is too large it will cycle on/off very rapidly and waste energy.

I am no HVAC guy. I believe there are some internet calculators that will help you figure out the math.

[Spencer Hochstetler]

I believe there are some internet calculators that will help you figure out the math.

That's the problem. Using 3 of them, I was able to range the BTU requirements from 12 to 18k BTU. Now it's time to make a decision. I was hoping that someone would see this and offer a voice of real-world experience. I was really hoping the contractors from whom I obtained bids would have sized things a little more closely than they did.

[Al Willits]

Problem lies without a heat loss its hard to figure what you'll need, old standard was 12,000 btu's for 600sq ft.
Not the best way to size a unit though.
Your going to keep it at a steady temp which will help, and like said, to much AC will give cool but damp air.

Can you add any insulation if needed?
Personally if this is possible I'd go with the 12,000 unit and see what happens, 78 is warm enough I think (and thinking is for people who don't know) that you should be ok.

The reason others in the field may not be replying is lack of information, sizing over the web is chancy and you'd hate to tell someone a wrong size and have them spend a lot of money for something that doesn't work.

just a thought....
Al

[David LaRue]

Spencer,

My experience with HVAC, you need to factor in Sun Load, and SQ Ft of glass. If you have sun loading, go bigger... you can't get cooler with a smaller unit, but you can add a de-humidifier.

Plus when I run my ceiling mounted dust collector, it generates quite a bit of heat. Add to that the other machines, lights (I have a bunch in my small shop). Plus add to to all of this that large door. If you open it how fast do you want the system to recover? .. just some thoughts..

[Spencer Hochstetler]

The reason others in the field may not be replying is lack of information, sizing over the web is chancy and you'd hate to tell someone a wrong size and have them spend a lot of money for something that doesn't work.

Agreed. I could provide more information to what I've already provided, but without someone asking me for specifics, I don't know what I don't know...

I think I'm going to go with the 15,000 BTU unit for the reason David suggested: you can't get cooler with an undersized unit, but you can add a dehumidifier if an oversized unit fails to adequately control humidity.

[Randy Denby]

Spencer, I am an HVAC engineer/tech...at least I play one during the day. I believe you are making the right decision.Without knowing all the specifics of the job and if it turns out you are oversized.You will not be oversized enough to make a big difference in humidity levels. Shop environment vs. living has different design parameters . More machinery, light load, usually more physical activity (which produces more latent load), and alot of shops will have a refrigerator too (heat producing )...keep cool :)

[Al Willits]

The flip flop to that is he's only gonna be in the shop part time, less of a heat load that way, like I said, with out a heat loss its just guess work, also personal choice, I'd prefer dry slightly warm air to moist cool air, others may not mind either way.

Al

[Randy Denby]

If you find that its alittle more humid than you like, run the fan on low speed. This , in effect, slows the air down thru the coil,which makes for a colder coil and will remove more humidity.
I doubt the 3000 (1/4 of a ton)BTUH difference of the 2 choices will make much of a difference in this case. But like All says, without building methods, materials used, insulation, windows, orientation, shade factor,roof color/type, etc...its just an educated guess. But even with a computer program, designing the system around all parameters to the nth degree. Most calculations rarely fall close to a manufactured size ...say it totals 39000BTU. They dont make a 39000 BTUH unit.... So the next step up is 42000BTUH.(3.5 ton)Also, every sizing program/ manual J? etc has at the bottom before the total, a safety factor. This just brings in an educated guess back into the equation. Nobody wants to be responsible for installing a unit that will not perform. Its better to be a little over than under.

[Spencer Hochstetler]

Thanks all. 15,000 BTU it is. I'm tired of wringing my hands; time to get the unit on order...

[Al Willits]

I think its not gonna make much difference either way and like Randy says 3,000 byu's isn't all that much difference.
Be nice to fine tune a system in, so it deals with humidity, but probably easier and cheaper to just get the larger unit and be done with it.

Al