Understanding the Manual J from loadcalc.com

[Dave-H]

Hi all -

Things are moving along in the garage and it's time to order a mini-split unit. I am having a bit of trouble understanding how to size the unit, and it pains me to pay $250 for someone to come out and give me a pro opinion

All of the installers have told me to go with an 18k unit, but in some cases I can see that they don't have anything lower than 18k and I think they are just throwing it out there. Another installer who I trust and wasn't selling anything said the 12k would be more than enough!

I have attached the manual j as prepared online - obviously that's not best, but I gave it a shot. I'm trying to understand how to translate this into a decision about how big of a unit to buy. Can anyone shed some light on this???

Any assistance much appreciated.

thanks, Dave

 

[mygarageone]

Hi all -

Things are moving along in the garage and it's time to order a mini-split unit. I am having a bit of trouble understanding how to size the unit, and it pains me to pay $250 for someone to come out and give me a pro opinion

All of the installers have told me to go with an 18k unit, but in some cases I can see that they don't have anything lower than 18k and I think they are just throwing it out there. Another installer who I trust and wasn't selling anything said the 12k would be more than enough!

I have attached the manual j as prepared online - obviously that's not best, but I gave it a shot. I'm trying to understand how to translate this into a decision about how big of a unit to buy. Can anyone shed some light on this???



Any assistance much appreciated.

thanks, Dave

Most if not all contractors will do a guesstimate because why would you do a proper heat loss/. Load calc if the home owner hasn't hired them.
I do this all the time and if I get the job , I will do the proper load calc's.
We don't have time to waste on people fishing and most of the time our guesstimate is very close.

If you trust the guy that said to use a 12,000 unit , why not use him.

 

[Dave-H]

Most if not all contractors will do a guesstimate because why would you do a proper heat loss/. Load calc if the home owner hasn't hired them.
I do this all the time and if I get the job , I will do the proper load calc's.
We don't have time to waste on people fishing and most of the time our guesstimate is very close.

Thanks, I guess, for that. I don't really understand how someone would price out an 18k unit, then if I agree to do the job they would come out and do a proper sizing.

But I'm just trying to do a little DIY and asking advice here on the forum.

 

[Fixin'Stuff]

Keep in mind that mini splits can operate just fine well below their rated BTU/hr. However, what they can not do is operate above their rated BTU/hr. So you don't have to be as precise with the calcs as you would with a typical full-on/full-off system. On those weekends when it is -15 degrees outside, just a little extra capacity might be nice, and the 18K unit can still ramp down to somewhere in the 6K BTU's/hr range and keep the garage comfy on those days that it's 45 outside.

 

[Dave-H]

I do understand that, but the reason I started to get interested in accurately sizing the unit is that the lower limit of the 18k unit was somewhere in the neighborhood of 6k, like you say, but the manual J seems to be suggesting that I might not even want that much!

Also, when the spray foam truck was here they packed my rafter bays with more than I paid for - I now have 2x4s and 2x6s that are essentially full of closed-cell foam, so this little garage is going to be super tight and well insulated!

I spend a lot of time in SE Asia and I have seen how, when the AC is oversized, it keeps the room temp down but it never really has a chance to reduce the humidity much. And, since I love to try to understand complex things and get it dialed-in, I'm hoping to make the best choice on this

 

[Jackfre]

Understand that the manual J is based upon "design condition". Design condition in Denver is 4*F. That means that if you match the equipment to the load at 4* the equipment will operate continuously but keep the place at 68-70. Design condition typically happens about 1/2 of 1% of the time, on a heating hour basis. Given that you are super insulated, go with your experience in SE Asia and do not oversize.

 

[Dave-H]

Understand that the manual J is based upon "design condition". Design condition in Denver is 4*F. That means that if you match the equipment to the load at 4* the equipment will operate continuously but keep the place at 68-70. Design condition typically happens about 1/2 of 1% of the time, on a heating hour basis. Given that you are super insulated, go with your experience in SE Asia and do not oversize.

Very helpful, thanks..

 

[MushCreek]

All HVAC systems are oversized at some point, such as when you barely need heat or A/C. We used Mitsubishi HyperHeat units in our house, and they are amazing. They also have a dehumidify mode for when it is damp, but too cool for A/C. I used a $50 program and entered the info very carefully and honestly. The resulting system has performed flawlessly and economically. In the summer, the A/C only adds $20 a month, and that's in hot, humid SC.