What size unit should I go with?

[ka41]

I'm almost finished with my garage/shop. Ended up being 30x32. It has a 15' ceiling height. I have 6" of insulation in the roof and walls. I was considering a mini split system but unsure of the size. The room will only be occupied maybe 10 hrs a week. I have one 3x7 walk door and one 10x12 uninsulated roll up door. No windows. I live in south Louisiana so it gets hot and humid in the summer and winters are mild. I'm looking for heat and cool. Is 18,000 btu large enough?

 

[redsky49]

The answer is most likely no.

Which is the closest location to your garage:
Lake Charles
Baton Rouge
New Orleans
Lafayette
Or are you further north?

Desired indoor temps?

Is the roll-up door gasketed along the sides?

I am assuming that you have a flat ceiling at 15', insulation on top of ceiling. and that the attic space is adequately vented. Please confirm.

Amount and type of lighting?

Heat producing equipment - welders, refrigerators, compressors and any devices that run continuously?

Ventilation requirements. Do you run an exhaust fan?

I will give you a ballpark heating/cooling load calc with the above data.

As always, offered only as opinion

 

[ka41]

I'm near Baton Rouge
76-80 deg
Door will have a weatherstrip
The ceiling is vaulted, so no attic space
15 T8 4 foot fluor. bulbs 32watts ea
No heat producing equipment nor exhaust fans

Thanks for your help.

The answer is most likely no.

Which is the closest location to your garage:
Lake Charles
Baton Rouge
New Orleans
Lafayette
Or are you further north?

Desired indoor temps?

Is the roll-up door gasketed along the sides?

I am assuming that you have a flat ceiling at 15', insulation on top of ceiling. and that the attic space is adequately vented. Please confirm.

Amount and type of lighting?

Heat producing equipment - welders, refrigerators, compressors and any devices that run continuously?

Ventilation requirements. Do you run an exhaust fan?

I will give you a ballpark heating/cooling load calc with the above data.

As always, offered only as opinion

 

[tdkkart]

I'm near Baton Rouge

Heating???
Ha, what is your lowest overnight temp, maybe just below freezing??
I'd be surprised if you could get a dixie cup full of water to freeze in a building that holds any heat at all.

Put in a 15,000btu window/wall air unit and be done with it.

 

[nate379]

How mild is mild?

To me 20-30* is pretty mild.

 

[redsky49]

[QUOTE Put in a 15,000btu window/wall air unit and be done with it.[/QUOTE]

Very good guess!

The design conditions are:
Indoor - 76db/63wb degrees cooling, 70 degrees heating
Outdoor - 92db/77wb degrees summer, 30 degrees winter
These are for 99% historical weather averages.

Not having a complete set of drawings in front of me, it is possible that I could be off a slight amount (5%? I have had to estimate infiltration, etc.), but given the price you are being charged, this should work for you

Sensible cooling: 8699 btuh
Latent cooling: 6412 btuh
Total cooling: 15111 btuh

Heating: 15634 btuh

One of the 1-1/2 ton ductless split systems should be adequate, though if you are a getting reverse cycle heat pump, confirm that there is adequate heating capability. Given your wintertime temps, a heat pump would be a good choice. If your budget allows, I would recommend a split system over a window unit - better performance, including operating efficiencies, as well as longer service life.

Do your best to eliminate infiltration at the overhead door. A lot of leakage will defeat the best of cooling systems. Provide a sweep at the bottom of the door, and gaskets top and side.

Your lighting level looks very low, particularly if you plan to perform any detailed work. For fluorescent lighting, 1-1/2 watts to 2 watts per square foot is typical.

Hope this helps.

As always, offered only as opinion

 

[nate379]

Seems like more than enough lights to me. My garage isn't that much smaller and I have 4 housings, 2 4' bulbs each. Was thinking of maybe putting in another 2 but I haven't decided yet.

 

[redsky49]

Original poster states that he has 15 T-8 32 watt bulbs. If that is correct, the space is seriously underilluminated. You won't stumble into the station wagon, but performing anything else will be very difficult at the stated lighting levels. Of course, some people enjoy working with a flashlight.

 

[nate379]

Right and I have 8 and my garage is 6 feet smaller on both directions. Plenty bright in there.
Haven't needed to use a flashlight yet other than in spots that I'd need one if I was working outside even.

 

[ka41]

I'm sorry, I told you wrong on the lighting. I have (12) 4 foot fixtures with 2 t8 32 watt bulbs in each fixture and I have (3) 8 foot fixtures with 4 - 4 foot t8 32 watt bulbs.
Thats a total of (36) 4 foot t8 32 watt bulbs.
Do you think this will be enough lighting?
Back to my original question, will a 18,000 btu split ductless a/c heat pump work. Also what brand do you recommend. I was looking at a Frederick. Summer days here from May to October are 90-95 degrees and winter time temps do fall into the mid 30's and sometimes mid to upper 20's.
Thanks for the help.

 

[DHCrocks]

sorry to **** in....how much BTU should be added for cooling for typical appliances found in a garage like a standup freezer, water heater, compressor, watts of florecent lights.

 

[redsky49]

I'm sorry, I told you wrong on the lighting. I have (12) 4 foot fixtures with 2 t8 32 watt bulbs in each fixture and I have (3) 8 foot fixtures with 4 - 4 foot t8 32 watt bulbs.
Thats a total of (36) 4 foot t8 32 watt bulbs.
Do you think this will be enough lighting?

That's 1.2 watt per sq. ft. That's at the lower end of acceptable. My concern would be that you have little to no reflectance from the ceiling due to the vaulted construction. Again, it depends what you intend to do in the garage and what your expectations are. As I get older, a little additional seems to help me. I also try to avoid a dark space at the top of the ceiling and I use a little uplighting to avoid that. See how your current plan works out, and make any adjustments accordingly.

That also raises your total cooling load to 17,408 btuh, based on a 76 degree interior temperature setpoint. That's pretty much the maximum of a 1-1/2 ton unit, but I wouldn't go any larger in capacity. During excessively high temperatures your indoor temp may rise a bit. In my mind that is acceptable.


Back to my original question, will a 18,000 btu split ductless a/c heat pump work. Also what brand do you recommend. I was looking at a Frederick. Summer days here from May to October are 90-95 degrees and winter time temps do fall into the mid 30's and sometimes mid to upper 20's.
Thanks for the help.

Yes, 18,000 btu should be fine. As shown above, I used 92 degrees for summer design temp and 30 degrees for winter. This size unit will keep you comfortable for all but the record breakers or extreme stretches of exceptional weather. It doesn't pay to design for every conceivable situation.

Frederich are fine units, though my preference is for Mitsubishi or Sanyo only because they have a wider selection of units for commercial applications, and they have been on the market the longest. Go with whomever gives you the best local support - not just price. Try to find a unit that has 15,000 btuh capacity on the heating side for winter use. Many heat pumps will be a little short in this area. Try to physically locate the unit in the area you will mainly occupy. 30 feet is a long throw for any of these units so you will find that the furthest corner of your garage will not have much discernible airflow. For the occasional use you describe you should be happy with the unit. They also are pretty quiet, many have an oscillating airflow, freeze protection setting, etc. Lots of features and I like the remote controls.

I hope this answers your questions. Post some pics of the finished project.

As always, offered only as opinion

 

[redsky49]

Some of my response got buried in the quote above. You will have to sort that out by yourself. Sometimes computer challenged

 

[ka41]

Thanks Redsky. You've been a lot of help. Like I mentioned, I will not be occupying this space but maybe 10 hours or so a week. Do you think I will have a problem maintaining 77 or 78 degrees when the temp is 90+ outside. Is it best to leave the unit set at 80 degrees when not occupied.

 

[redsky49]

sorry to **** in....how much BTU should be added for cooling for typical appliances found in a garage like a standup freezer, water heater, compressor, watts of florecent lights.

Read the nameplate of your equipment for rated power. One watt equals about 3.42 btu. Depending on the device, anywhere from 30%-100% of the total wattage will be converted into heat and added to your cooling load.

A typical refrigerator for example, adds about 300 btuh for every 10 cub. ft. of storage capacity, or about 40% of the input wattage is heat to the space.

Make a list of all equipment that will normally run and add that to your calculations. Also include the occupants who are both a sensible as well as a latent load.

As always, offered only as opinion

 

[redsky49]

Thanks Redsky. You've been a lot of help. Like I mentioned, I will not be occupying this space but maybe 10 hours or so a week. Do you think I will have a problem maintaining 77 or 78 degrees when the temp is 90+ outside. Is it best to leave the unit set at 80 degrees when not occupied.

You will have to experiment. You will not have a lot of cooling muscle for rapid cool down or warm up of your space. 80 degrees seems a reasonable start point for "off hours". You may want to adjust this (down) for humidity control, or (up) for increased cost savings. The nice thing about most of these split systems is that they have sophisticated programming capabilities. You can set it for whatever you want ( but you have to read the Manual. Most of these units are international pictograms and not English as we know it).

If you can keep infiltration to a minimum, you will maintain your desired indoor temps. Pay attention to that overhead door. That is a lot of potential crack that can allow outside air to enter. Without any windows you don't have the solar load which is a major factor in most cooling loads. Just the reduction in humidity will be a welcome relief. As long as you don't add to the cooling load, you will be fine. I would rather see you slightly undersized than oversized.

If not convinced, have the vendor for the AC system run a load calc for you. See if we are all on the same page, or close to it.

 

[tdkkart]

Very good guess!

Well Shucks

All I know is that a 15k unit worked wonderfully for our first house which was similar sized.

I still question the need for heat at all, especially since he stated it's only gonna be used approx. 10hrs/wk, or a couple hrs/day.

If it's too chilly when you go out, pick a different couple hours.
His "cold" is a WHOLE lot different than mine, had a few days this winter when it was -20F +windchill when I went to the shop in the morning.

 

[redsky49]

Well Shucks

All I know is that a 15k unit worked wonderfully for our first house which was similar sized.

I still question the need for heat at all, especially since he stated it's only gonna be used approx. 10hrs/wk, or a couple hrs/day.

If it's too chilly when you go out, pick a different couple hours.
His "cold" is a WHOLE lot different than mine, had a few days this winter when it was -20F +windchill when I went to the shop in the morning.

In Saudi Arabia it may be 120 degrees during the day. When it drops to 60 degrees at night, you look for your down jackets! All depends what you are used to. These Louisiana folk want to stay warm. Can't blame them.

Adding reverse cycle heat to most cooling units is a small incremental cost, generally $100-$200 dollars, sometimes even less. For the slight cost increase it makes sense to add it. It also helps to avoid space heaters which burn down too many homes.

The final cooling load is now about 17,500 btuh, so you were a little bit off. Still, pretty close

 

[redsky49]

A final thought or two.

The height of the ceiling continues to bother me a bit on a few fronts. First, the typical indoor portion of a split system is installed on the upper wall, a few inches below the (normally horizontal) ceiling juncture. The supply air discharged from the unit uses the presence of the ceiling to help direct the airflow along the ceiling until other factors cause the air to begin to drop towards the floor. The return air then travels back across the floor before rising up beneath the indoor section where it is drawn into the unit, conditioned and then discharged back into the space. This sets up a "circulation loop" if you can picture this. The absence of a horizontal ceiling will affect this air flow to some extent. Without actually modeling the effect, my sense is that the airflow will tend to drop earlier, reducing the "throw" of the conditioned air. This isn't a deal killer in any sense, just the obsessive/compulsive side of my brain chiming in. There are some mind-numbing technical issues if any engineers want to jump in here offline (PM).

Second issue is that during heating, the warm air will rise to the peak of the ceiling, contributing nothing to the comfort of the occupants that live in the 6 foot space above the floor. Also, the presence of stagnant warm air at the ceiling peak may (that is just may) result in the formation of mold or mildew, particularly if dust and dirt accumulate in that area. Doesn't happen often, but I have seen it in atriums and other high ceiling situations.

My suggestion is to provide for a ceiling fan, either now or at a future date. A conductor run from a lighting circuit to the ceiling peak and terminated at a junction box would do just fine. Configured to blow downward, you may find the fan to be useful during both heating and cooling. I would also select a very small fan. You just want to encourage some "stirring" of the air.

Finally, I am curious how you are installing the light fixtures. Are you suspending the fixtures by aircraft cable from the ceiling to say 10' AFF? Are you providing any uplighting to reduce the dark area of the upper ceiling? How are you running the wiring from fixture to fixture? In conduit? I just think that the lighting should be given some thought to avoid any disappointments or unexpected expenses. If attached directly to the underside of the ceiling you may have some challenges to achieve satisfactory lighting.

Neither issue I would consider critical, just some tweaks to get the best possible installation.

In any case, good luck with your project.

As always, offered only as opinion

 

[tdkkart]

My suggestion is to provide for a ceiling fan, either now or at a future date.

Heh, I was going to suggest a ceiling fan also. I'm a firm believer in them, have 2 running over my head rght now.


While I am no means an HVAC expert, nor did I stay in a Motel last night, I echo your concerns with the potential issues caused by the vaulted ceiling.
Expert or not, the laws of thermal dynamics need to be taken into consideration, and a ceiling fan is one of the ways to deal with the problem, in both seasons IMO.

These concerns are why, IMO, the best palce for AC ducts are on the ceiling, and the best place for heat is in or near the floor.

Most residential HVAC folks seem to ignore at least the cooling portion of the formula.......

 

[redsky49]

Different horses for different courses. And of course economics.

A garage typically occupied for 10 hours a week (though possibly much more) doesn't call for the same level of attention that a primary residence does. In this particular situation, based on the OP's location, I feel that cooling would take precedence over heating. Therefore the presence of the vaulted ceiling actually has little direct impact other than the reduction of supply air throw that will result.

Typically, southern homes were all built, presuming you could afford it, with high ceilings to allow natural thermal currents to rise up away from the occupied floor area. This was figured out by the Greeks and most likely by their predecessors as well. On many homes you will see a little cupola or structure at the top of the roof to allow the rising warm air to exhaust from the building, creating a nice natural air current, similar to what is referred to as the "chimney effect".

Now if the final user had an unlimited budget, I would consider radiant heat (including heated towel bars, snow melt sidewalks, etc.) for the heating system, cooled forced air for the cooling system, and a separate constant flow ventilation system to provide tempered filtered outside air. Occasionally someone has the finances to do precisely that. Those are the fun projects.

As for the rest of us, well, we do what we can afford. And usually that is just fine.

 

[kvom]

My garage is slightly smaller (25x36x12). I have a 2-ton mini-split unit up on the end wall 1 foot below the ceiling, and it will make the garage pleasantly cool for working in a quite short time. I set the unit to 70 degrees and it maintains the internal temperature at around 75 when it's 90+ outside. My space is well insulated.

The cost difference between a 1.5 and a 2 ton unit isn't that great, so I'd go bigger.