AC Size and Loadcalc.net Accuracy

[mvh]

Has anyone used loadcalc.net site to size AC units? I'm curious on it's accuracy. I'm getting just under 11k cooling Btu's for 1,080 SF w/ 11' ceilings. I've tried to double check this with the ACCA Manual J excel spreadsheet and am get similar results. However, I fumbled my way through that so who knows if the two being similar is correct or just dumb luck. To me they seem a bit low.

Here is what I have in the 36'x30'x11' room:
- Wall is R19 batts with 1" R5 rigid insulation tucked in between the wall purlins.
- Ceiling is R38 batts with R13 batts laid perpendicular on top.
- Only one double pane window- 4x3 (north), man door(south). No overhangs.
- 10x10 overhead metal door(south facing). 1" rigid foam attached to the inside to provide some minimal insulation.
- Uninsulated slab on grade.
- Location central Iowa.

Mini split sellers have told me that I need a 36k unit, but that seems like complete overkill. Initially I was thinking 24k Btu's. However if the calcs are accurate, I'd go the 18k route. I don't store vehicles in there so no large objects radiating heat and opening OH door is very minimal. Plan is to keep it 75 degrees. Do the calcs seem accurate? Anyone have similar sized shop in midwest with an 18k or 24k AC?

 

[ericm]

I found that it's easy to miss entering something in that form and that it won't tell you. There's a Manual S to size the units based on the Manual J output. For me it didn't change aything but I was calculating for both heating and cooling and need more btus for heating, so it just told me the cooling was oversized.

I could not find an existing location in loadcalc.net's manual J that matches my actual climat. We are at a markedly different elevation than the local cities/airports. So I had to put in my own climate numbers.

 

[rantapawan]

You can try the load calculator from servicetitan.com. It seems more legit.

 

[toyotadriver]

18k sounds like a pretty reasonable estimate to me.

 

[PWilks]

Sounds pretty good. I imagine they want to sell you the most expensive unit they can, that or they were sizing for heat.

 

[yeldogt]

When I build my places in the mid-atlantic w/ foam I'm always over 1k per ton ... often 1200sf depending on windows.

People use 450sf per ton as a guide and round up .... it makes for oversized equipment. In a humid area you want the unit to run all the time (low) ... this is where a VS unit like a mini is great. Unless that door leaks like crazy the 18 will be fine .... tight and running all the time it's going to have no trouble even at 12k.

Sizing correctly and running longer is the best bang for the buck. A big system running 1/2 the time and not being able to remove humidity costs the same as a smaller running 2x longer and getting the water out of the air.

 

[jjrbus]

When I find that I am unsure of calculations on anything, like you I ask. For something like minis that are going to be expensive not only to install but to operate I would find an online company to do the manual J for me. Should be under $200.

 

[pingsb]

Send us your information to [email protected] (NOT to be confused with loadcalc). ACCA Certified Designers, hundreds of designs, over 5 years experience - we can help you out!!

 

[pingsb]

When I find that I am unsure of calculations on anything, like you I ask. For something like minis that are going to be expensive not only to install but to operate I would find an online company to do the manual J for me. Should be under $200.

Absolutely agree!!

 

[jjrbus]

Absolutely agree!!

When doing your calculating make sure to include my finders fee