Packaged unit sizing for Smart *** Garage

[NakeDiesel]

I'm starting to think about adding heating and cooling to the shop. I've been heating it in the winter for the last 4 years by running 2 radiating oil heaters and a little oscolating heater up on a bench and when it really dips down outside and I need the temp brought up, I fire up a propane tank heater for short term use.

I added 3 60" ceiling fans this past summer and they helped along with a big fan on a stand but it stays humid in there.

Shop is 40' x 80' with 14' walls. Walls and roof are insulated with 5" of open cell spray foam. I have 2 3' x 80" steel insulated doors (1 north side, 1 south side), 3 12'x12' steel insulated doors on the south side and 4 double pane windows, 2 on the south side and 2 on the north side.

I've been thinking I needed a 5 ton unit according to the AC guys I've talked to and gotten quotes from. But running manual J calculations myself, I've come up with needing 25k btu cooling and 49k btu heating.

So, from those numbers, a 2.5 ton packaged heat pump 16 seer unit would work for the cooling side, it's rated at 28k btu. The heating side is 27k btu, but can add in a heating element as well. Normal operations in winter would be keeping the shop between 50 and 60 degrees and may need to bring it up to 70 if I need to paint or something else that needs higher temps.

Summer, I need the humidity removed and would be nice to keep it under 80.

Am I smoking crack with these numbers I've come up with? I used 2 different calculators and came up with similar results on both of them.

 

[zmaxmotorsports]

Seems to be undersized to say the least to me,but it is 2 in the morning and my bran may not be fully engaged at this point.

 

[theoldwizard1]

PTACs are a lot less efficient than a mini-split heat pump. Some rely on resistance heat only or when the temp drops below 30-40F. Resistance heat is the most expensive way to heat a space.

With a shop that big I would install 2 units. You could always start with one 2.5 ton unit and add the second if needed.

 

[KiltLifter]

Resistance heat is the most expensive way to heat a space..

Mr Wizard, can you quantify this statement? I was leaning towards resistance because all of the energy used is released as heat, where with other sources, up to 20% goes up the flue.

 

[Zeke]

Mr Wizard, can you quantify this statement? I was leaning towards resistance because all of the energy used is released as heat, where with other sources, up to 20% goes up the flue.

It's all in the cost of BTU's so when you factor in the waste you then compare your utility rates for different sources of energy. Around here electricity is very expensive and we're on a tiered program for usage.

 

[matt_i]

Mr Wizard, can you quantify this statement? I was leaning towards resistance because all of the energy used is released as heat, where with other sources, up to 20% goes up the flue.

Usually when you equalize it all out, per standard unit of energy, the electric is more $.

Example, me, DTE Energy...

I paid $0.17 per kW-hr electric in last utility bill.

Natural gas was 100 ccf = 100 therms * (100,000 Btu/therm) = 10,000,000 Btu x 3.4 kW-hr/BTu = 34,000,000 kW-hr energy equivalence for $86.67, so $0.00003 per kW-hr energy equivalent (no typo)

So you can see the financial interest in "burning stuff" for heating over the long haul

Granted, the electricity can be used more efficiently driving heat pumps rather than dead-short resistance.

 

[NakeDiesel]

No access to natural gas here, even though it's being produced 300 yards from my house. I've tried to get a tap to my house from the pipeline company, but haven't gotten anywhere in 5+ years on that front.

I have propane for the house furnaces, 500 gallon tank we fill up each august and then buy 1 to 2 hundred more gallons depending on how hard of a winter we have in Feb. I can power the house furnaces off my generator if needed on big power outages to keep the house livable.

Electric is fairly cheap out here. Around .15 cents per kwh.

Propane, I've paid anywhere from 1.40 a gallon to 7.00+ a gallon depending on the time of year and shortages.

I'm looking at heat pump units, with the possibility of additional resistance heat module for it.

I'm open to a mini split setup as well. My biggest question is the sizing. I don't want it oversized. I'm not worried about rapid cooling or heat recovery, I open the big doors as little as possible during cold weather and summer.

 

[KiltLifter]

Wow, just did the math for Xcel here in CO.
$0.046/KWH electric
$0.084/Therm Gas (100,000BTU)

1 Therm =29.3 KWH Therefore an electric therm in CO costs $1.35!!

Granted you don't get the whole therm out of gas, but still 16X more for electric? (is my math right?)

New Plan! Gas pipe to garage!!

 

[KiltLifter]

p.s. Yeah, I know, kWh, not KWH.

 

[PWC Repair]

If I were going to spend money doing something like this I believe I woud go with a 3 ton AND a 2 ton mini split. you WILL need the extra btu's in a metal shop. And when you don't you can just run one system. you can install them yourself to save money too.

 

[NakeDiesel]

Putting a 3 ton and a 2 ton system in and maybe not using one seems to me to be a waste of money. I'm a cheap ***, I'd rather not install a system and continue sweating my *** off in the summer and running small heaters in the winter than just guessing at the solution. I've gotten Swag's from 2 different HVAC companies of 5 ton units, neither has ever installed in a spray foamed environment, and can guarantee they didn't do manual j calcs as they never asked me for detailed info. So I'm assuming they just used the 1960's rule of thumb of 1 ton per 500 sq ft of space.

Here is the report from one of the manual J's I filled out. If I messed up somewhere, let me know.

 

[PWC Repair]

For starters. a real manual J that takes every calculation into account is NOT available online for free. They typically run in the $500-1500 range for the software. You also need to take into account the effects of the direct radiant heat from sunlight on the metal. Also I see no mention of a ceiling so I would assume it's wide open straight up to the roof which is like having vaulted ceilings. Also unless the slab is insulated underneath and around the perimeter there will be heat loss there also. I would also have to guess the overhead doors are only R7-R9 and normally have quite a bit of leakage around them. I can just about guarantee on really hot, or cold and windy days, you WILL need 5 tons of capacity to stay comfortable. While on milder days 3 tons will likely do the job. Not to mention the superior efficiency of the mini-splits. I've worked behind the counter of the HVAC supply house for 15 years. I've heard all the horror stories about what HAS NOT worked, and had to be yanked and redone. I'm just trying to save you some aggrevation and money in the long run. I supplied the HVAC for a new local powersports shop about 5 years ago. The front section alone is 40x40 with 12ft sidewalls and open span webbed trusses. R30 open cell foam. Glass double entry doors in the shade all day due to an eave and being on the East side. And several large windows on the front of the building which are commercial grade insulated and gas filled. The front section has a 4 ton package unit that runs all day to hold 74 degrees when it's 95 outside. The rear shop area 40x40 has a 3 ton package they bought used to break the heat for the mechanics when it's that hot. It also has a 115,000 btu gas fired tube heater to keep everything warm with frequent overhead door openings. This should be a real world comparison for you.

 

[NakeDiesel]

Actually, there is a ceiling in the shop. Currently I don't have additional insulation on it, but plan on blowing in cellulose on it in the near future probably around 6" or more.

M 12x12 doors have an r value of 17.5 as they are thermacore doors and seal pretty good.

Within the next 2 years, the north and south sides of the shop will have a 12' overhang built on the south side and an 18' overhang built on the north side. Half of the north side will eventually be enclosed to house a paint bay and a wood working shop.

This is a layout of the shop, top is north on the pic. The rooms on the northwest corner have spray foam on the outside walls and fiberglass bat on the interior walls and ceiling. I plan on putting a minisplit in this room. It's long term use will be a commercial kitchen type area for me to do canning and honey processing, but think my son is going to move out there for the short term once I finish getting the drywall and FRP up and wiring finished.

Think I may start with something like this in the shop, it's a seer 19.5 45k btu 3 zone AC/Heat Pump and use the 9k head unit in the room and the two 18k's out in the main area of the shop. I'm not really looking for ideal cooling and heating on extreme days. I'm not out there every day and the temps will be set to minimum cooling or heating when I'm not working on projects. In the winter, unless I'm painting a car or doing something temp sensitive I'm fine with it being 50 degrees in there. Summer, I'm racing on the weekend and outside more than in the shop, so 80 degrees in the shop when it's 100 outside will feel great if the humidity is taken out.

http://www.thermospace.com/ductless_split/thermocore-t116d-h236-9+18+18.php

 

[PWC Repair]

Now that sounds like a good place to start. You'll be glad you went that route.