Manual J calcs came in much lower than expected.

[GroundLoop]

Just had manual J calcs done for adding minisplits to our garage/recroom. The garage is 45' x 30' (1350 sq ft) and the recroom is 28' x 23' (644 sq ft). Ceilings are 9.5'. Calcs came in at a 14,000 btus for the garage and 8,000 btus for the recroom. I wanted to put a dual system in both of them. Did quick cals using one of the online calculators and got 36,000 btu for the garage and 18,000 btu for the recroom. I was shocked I needed calcs done for a non living space. But here by the beaches you need a permit to wipe your a**. So I am having the lady who did the calcs redo them without insulation being added in the walls. I figured that would bump the numbers up. If I go by the calcs she did I would not even be able to put a dual system in either place. Am I correct that the calcs she got are very low?

 

[sands35]

Undersize it and it won't cool during a hot day. Oversize it and it won't pull down humidity.

I'd get a 2nd (or 3rd) opinion.

Why no wall insulation? You have none or you think it will work better without it?

 

[Highbeam]

You are in Florida, perhaps she assumed some odd numbers for the temperatures.

More important is that with a minisplit you can oversize with much better results than if you oversized a single stage heatpump. Since they are variable speed and inverter driven they can ramp output up or down to maintain temperature and keep running.

 

[GroundLoop]

Undersize it and it won't cool during a hot day. Oversize it and it won't pull down humidity.

I'd get a 2nd (or 3rd) opinion.

Why no wall insulation? You have none or you think it will work better without it?

I would still put insulation in. If she calculates the numbers without insulation it will raise the btu size. The city here will not let me oversize. Whatever the calculated size says is what I can buy. So with the current calcs the biggest split I could buy would be a 14000 btu single air handler system. In a 45' x 30' garage that will not work... at all. I know I can get a 2nd or 3rd opinion but that is just more wasted time and money.

 

[brewchief]

What are they using for the indoor design temp? Some programs will default it to 78 degrees or higher.

How much air infiltration are they figuring in? No matter what you do a garage door is going to be leaky and that needs to be accounted for.

How much info did they gather? A heatload is only as accurate as the info inputted into it. The software I use can get very detailed, even things like how much of a window has screens and is the ground outside the windows grass or something that reflects heat like rocks or concrete can be factored in.

The numbers do sound a little low but remember with a minisplit you don't have any ductloss in an attic like a conventional system.

 

[JakeKohl]

I used HVAC-Calc and it said 11,500 BTU was sufficient to cool my garage and 9k btu was sufficient for my 2nd story (24x36 - reasonably well insulated). I had bought 12k units but an insulation guy scared the **** out of me that I had undersized things so I sold those and went with 18kbtu units and have been running them for about a year. Upstairs, I think the 12k unit would have been sufficient but in the garage, it's nice to have the ability to change temperature in there rather quickly...and on a really warm or cold day, neither unit drops completely out of cycle (which they are intended to do).

 

[GroundLoop]

She "re-worked" the numbers and came up with 24,000 btu for the garage and 12,000 btu for the recroom. At least now I can get a dual system for the garage but still locked into a single system for the recroom. I guess I will have to look into getting a 2nd set up calcs done.

 

[JakeKohl]

She "re-worked" the numbers and came up with 24,000 btu for the garage and 12,000 btu for the recroom. At least now I can get a dual system for the garage but still locked into a single system for the recroom. I guess I will have to look into getting a 2nd set up calcs done.

That actually sounds about right.

 

[Riley]

So if my math is correct, she was 58% off on the 14K to 24K revision, and 66% off on the 8K to 12K revision?

This passes as a reasonable local program that constrains what you can purchase?

Amazing....

 

[GroundLoop]

Brought the calcs to the city inspector and he looked at them and from the floor plans I gave him determined that she was off on the overall size of the recroom - her calcs had the room smaller than it was by about 100 sq ft. Talked to him about wanting the dual system due to the size and he gave me the go ahead for the dual 18k split. So all is well.