Something to consider when running a load calculation.

[Mike007]

When I sized the heater for my garage I sized it based on my years of HVAC experience. I installed a 20K net BTU heater. During the recent cold snap I was able to see the ultimate performance. It was 7* out and it ran continuously to maintain 62* inside.

I just got around to filing a permit. The building department wanted a manual J. Out of curiosity I punched the design conditions in 62* inside temp with a 7* outside temp. Since I insulated and finished the garage, I knew all the other variables were accurate. The program spit out a heat loss of 81,000 BTU's under those conditions. Obviously 80K would be a major oversize.

When I looked at where the BTU's were going, the program has 60,500 BTU loss for the 2 garage doors. I called the company who puts out the program. The guy I spoke with was aware of this and told me any program based on Manual J (most are) will spit out ridiculous numbers for insulated garage doors. He suggests putting insulated garage doors in the program as a wall.

So if you are running a manual J you may want to take a look at what the heat loss for the doors looks like.

 

[mygarageone]

When I sized the heater for my garage I sized it based on my years of HVAC experience. I installed a 20K net BTU heater. During the recent cold snap I was able to see the ultimate performance. It was 7* out and it ran continuously to maintain 62* inside.

I just got around to filing a permit. The building department wanted a manual J. Out of curiosity I punched the design conditions in 62* inside temp with a 7* outside temp. Since I insulated and finished the garage, I knew all the other variables were accurate. The program spit out a heat loss of 81,000 BTU's under those conditions. Obviously 80K would be a major oversize.

When I looked at where the BTU's were going, the program has 60,500 BTU loss for the 2 garage doors. I called the company who puts out the program. The guy I spoke with was aware of this and told me any program based on Manual J (most are) will spit out ridiculous numbers for insulated garage doors. He suggests putting insulated garage doors in the program as a wall.

So if you are running a manual J you may want to take a look at what the heat loss for the doors looks like.

I quit using J a long time ago.

 

[Mike007]

I quit using J a long time ago.

Ok....It's code here.

 

[anthony666]

that might explain the dude that asked about 3/4" pex in his slab

 

[mygarageone]

Ok....It's code here.

The only time we have to produce one for the state is if a client complains about the heating system , not heating the house.

 

[brewchief]

What software program?

Got specs on the garage? I'll punch it in mine and see what it comes up with.

 

[Jackfre]

Manual J also builds in an 18% adder for duct leakage. Worth considering when you use something that is "net to the space".

 

[meathooker]

Op, what program are you using?


In idaho some cities are requiring manual J on all jobs. Such a waste on a lot of projects. As a member or ASHRAE I'm annoyed at all things acca.

 

[hidollartoys]

Mike, Could you provide details on the building? Size, construction type, insulation, interior finish, etc.

 

[Mike007]

What software program?

Got specs on the garage? I'll punch it in mine and see what it comes up with.

Good idea. It doesn't need to be exact, just punch in four 24' walls, a ceiling, floor, and two 8 x 8 insulated garge doors. Same conditions I used, 7* out 62* in and post what the door heat loss comes up as. Thanks.

 

[brewchief]

My program doesn't have a specific entry for garage doors that I know of, it gives a choice of door materiel and construction so I used metal with polystyrene core.

24x24 with 9' ceilings with average construction( I used a building from my file and just adjusted size slightly), two 8x8 doors and 1 3x7 man door and I came up with 19k and change.

I simply removed the doors so it made them wall and it dropped about 2000 btu.

This is with Elite Rhvac.