Inputs on Heating/Cooling Sizing Please

[Angelfire]

Hiya.
I'm thinking down the road I will probably go with a min-split system in my garage. The garage isn't complete but when it is, will be well insulated and separated into two areas, a wood shop and an auto/mech shop. The wall between them will also be insulated so I won't have to heat/cool the entire building when I'm only working in one area. Rough dimensions are 32x40 and roughly split in half for each "shop". Ceiling height in one side is ~10' and ~12' in the other. Anyway, I contacted ecomfort.com about getting an estimate on sizing. They performed a Man. J calculation for me based on the specs I gave them regarding windows, insulation, etc.... Here's a brief synopsis of what they calculated:

Wood Shop: Area=544SF, Htg Load (BTUh)= 6375, Clg Load (BTUh)=3843
Auto Shop: Area =589SF, Htg Load (BTUh)= 8733, Clg Load (BTUh)=4499
Total Load: Heating=15108 BTUh, Cooling=8358 BTUh

All's well to this point. However, they then sent me a quote for a mini-split system to supposedly match their calculations. They quoted me for a 24k BTU outside unit with 1 indoor unit rated at 15k BTU and another indoor unit at 9k BTU. So I'm a little confused as to why the calculations show a ~15k BTUh heating load and they are quoting me a 24k unit. Are mini-splits generally sized at twice the actual values or something like that or are they just trying to sell me a unit that is bigger than necessary? I'd rather have the unit work a little harder to keep up vs. cycling. They quoted a Mitsubishi MXZ-3B24NA-1 as the outdoor unit which appears to be an inverter unit but I haven't researched enough to know if it is a variable speed or not.

So bottom line, are they quoting me the right size unit based on the calcs above or is it too large?

Thanks much,
cc

 

[Milton Shaw]

You also need to consider how long you are going to run the units when you look at size. Running 24/7 will take a lot smaller unit than going into shop turning it on and expecting to be hot/cold in a reasonable amount of time. For a shop used every day most of the day without opening garage doors the 24/7 option size might be right but opening shop doors, pulling car in, running fans etc requires larger unit. Plan now instead of buying twice.

 

[Angelfire]

I'm a weekend warrior so don't need full time heat/cool. However, there are times, particularly in the wood side of things where I will want to maintain temps over the course of a few days or so to allow for finishes/glues to cure. So with that in mind, is the 24k unit the right one or should I be looking at an 18K unit (or even smaller)? As stated before, I'd rather slightly undersize vs. oversize.
Cheers,
cc

 

[pseudorealityx]

What part of New Mexico are you in? The 24k BTU rating is at 95 degrees. If it's 105 degrees outside, that capacity drops in a hurry.

There are also losses associated with refrigerant line length.

 

[Angelfire]

This is for my garage in ABQ. 105 occurs but very infrequently. I need to understand the mini split specs better as the cooling btu load is 8300 or so and the heating is 15k. Seems to me, perhaps incorrectly, is that the sizing should be based on the larger of the two. Is my logic flawed? Thanks much

 

[pseudorealityx]

Where are you getting these specs?

The cooling capacity at 95 degrees will be approximately the same as the heating capacity at 47 degrees.

 

[Angelfire]

I haven't quoted any specs, only numbers from a load analysis done on my place (man J). Check a few posts up for details but essentially my heat load came in at approx 15k btuh and the cooling at around 8300btuh. Seems to me a unit in the region of 15-18k capacity should suffice yet they quoted me a 24k unit which is where my confusion begins

 

[Falcon67]

Call me suspicious but I don't think 15K of heat is going to get it done for you in an AlbuQ-Q winter. I'm 4000' lower in altitude and way farther south with 17.5K heat in 960 sq/ft insulated. The 17.5 works but it takes quite a while to get there. It will keep the 288 sq/ft work room nice on a very low setting but turn it loose on the whole building and you're talking a few hours from dead cold - which would be 30s here. I think the cooling for your climate will be plenty. If you're in the sand and not in the neighborhoods where the non-natives try to grow grass in the desert, the low RH will make it easier on the AC.

 

[Angelfire]

Thanks. It's actually amazing to me that people do the Man J calls and come up with such a disparity of results. I've spoken to a number of manufacturers and gotten all sorts of recommendations relating to my heat loss that I don't know who to believe any more. I have the enginner's email from the last calc so I'll take it up with him to see what he says
Cheers

 

[Falcon67]

I didn't do any Man J's on mine, I just did a SWA-G