Heat Load Calculation: Manual J Made Easy

Doing a Manual J doesn't have to be difficult. Travis Farnum, Senior HVAC Tech with Williams Plumbing and Heating, walks through the steps for getting a heat load calculation for your home.
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[@comfort-calc6402]

I tried this on my own home and it was way off using building codes for 2000. I performed a manual J and compared the two just for the heck of it. The heat load was off by 18K btu's and the cooling load was off by 11K btu's. Both numbers are higher than the currently installed equipment. FYI - I have performed hundreds of manual J calc as I have been in the HVC trades for over 25 years.

[@briannatarelli430]

Have you used any heat load calculation software that is accurate?

[@Mystbilly]

We added closed cell foam to the outside of our above ground basement walls and open cell under the roof deck. Is there a place in this system to record and account for that change? Our efficiency is considerably better than the building codes of the 50s.

[@britestart1]

Hey great, to the point video.

[@ericf9414]

My question is I am working on a heat load on a log cabin. I am wondering if there is a section to let you know for log cabins

[@YTZack408]

I’m getting “null value in column area violates not-null constraint” error when tracing the home dimensions….

[@boedillard8807]

Just tried it, unfortunately it doesn't let me calculation with R9 or R21 for exterior insulation. couldn't put in 10" concrete basement, couldn't put in R20 basement insulation, it doesn't seem to make clear when itemizing - external vs. internal door.

[@biblebloopers946]

Wait at the beginning you said if you're going to do a mini split you can use this program. This is only for mini splits? This isn't for like central air conditioning

[@shaunhuggins9860]

My thing how do you know which room is which when you trace it

[@bradleythompson6338]

What about windows, doors, insulation type, attic type? What is this?

[@tangamandapio9013]

gracias

[@Anunakipower]

Where to purchase the software? Thanks!

[@cliffwillever4816]

I want to add two mini split units to a top floor of my house. Each room is 15 x 16 with a knee wall that is 5 foot high, the angle is 4 foot(directly under roof and the ceiling is 9.5 foot across. The entire top of hose has R19 closed cell spray insulation. The two rooms have R5 rigid foam board on walls and ceiling. The floor under is heated and cooled. How do I adapt this program.

[@fabbritechnology]

I’d suggest using the average ceiling height, assuming you place your mini split indoor units in an optimal spot. E.g. the average between 5 and 9 is 7. A little geometry will show you this yields the correct volume. You may want to add 10% to the height to account for convection, so 7.7’ in this example.

[@juliorodriguez8863]

Thanks for the tutorial, where did you click for the year?

[@ljv2094]

Just Google the address Zillow usually pops up first or it's competitors but you'll find the year and compare at least 2 to make sure year is around the same

[@dansouthwick1952]

The house that I am working on is a new construction and google maps does not have any data on it yet. How would I use the trace function from a blueprint? Thanks!

[@jaredking8690]

Don’t use this for that

[@madayc604]

you make the lines according to your construction plan

[@mahlince]

In one of the steps, there is a part where is Data Source. Select the text mode where you can enter the size of the rooms individually.

[@Mudo94]

What if the construction profile is 3/4 siding but the front of the house is brick…

[@tomjcarty]

Site crashes when at HVAC. :-/

[@manuelsosa4037]

What if is a new home project?

[@danpmatz]

WHAT IF ITS NEW CONSTRUCTION?

[@zelllers]

3:26 - Ah there it is... the magic sauce is it assumes the house was built to minimum building standards and nothing more.

[@privatename3621]

course tracing a zillow picture of the home to get square footage?? Good Lord that is so inaccurate. What if part of the roof line is covering a outdoor porch which you can't see from outside? What about a 0, 6, or 12 inch roof overhang? Sites like Zillow and RedFin report the square footage of the home. Note that some of that may or may not be under air conditioning, which matters when sizing a unit. With Manual J being so absurdly complex and specific, I don't know how such a simplistic program can come up with the same calculations.

[@quantaengineeringllc]

Manual J calcs use ceiling area, not roof area and R-value of insulation. It also uses wall area, overall sqft area, window and door areas, etc.

[@pjc_deleon7290]

Licensed Mechanical/HVAC Engineer and Master Plumber here, worked 10 years for Carrier Corp before, How can the software provide an answer? Simple. The developers used reasonable assumptions plus built-in safety factors within the program. You are correct Manual J is complex and specific although not as complex and specific as an ASHRAE cooling load calculation which an hvac design ngineer does. Because the purpose of the software is to provide quick easy to use results that are nevertheless reliable enough. The expert developers of the app simplify the process by selecting reasonable , "typical" values for the various inputs. For example, thermal resistance values for light weight concrete walls in strictly speaking vary depending on specific construction, but the values should range around X to Y, based on properly done, industry practices. The same for other inputs. Based on occupancy density from code or manual input, you can also estimate average heat load from a human occupant, around 300-400 watts per person. Another critical factor they coded in for example is the temperature and humidity data for the location. However , based on extensive ASHRAE research, there is an "average climactic condition per location", ashrae usually updates every 4 year cycle. With all those reasonable estimates, educated guests nonetheless, there is bound to be errors to the software output. So this is where technical studies comparing the results from various cooling load calculation methodologies are used, from the simplest to the most detailed across various building types. In one study, we actually done in-house, Equivalent Temperature Differential (ETD) Method which is the method used in Manual J underestimates the Results if you use the most accurate method, Radiant Time Series (RTS) by 30% and other hand say, an actual test model house has 10% higher cooling load than RTS. So let say, the software selects a 40% safety factor. Now you have a more reliable but simplified estimate for the Software User without going thru hell for a simple residential project. Hope that helps. I forgot to add, peak peak peak cooling load over an 8760hrs in a year, happens only 1% of the time in the year or 88 hrs annually, based on Climate Data, So Manual J results + Fixed Speed Systems will get you an AC system that definitely meets the cooling load assuming you input the "right data" but is nevertheless less efficient than using an variable speed /inverter system that can match the load with capacity. But hey, one step at a time.

[@Ephbaum]

The ridiculousness of it all...

[@pvgangster1]

Is this shit free? Thx

[@tycobb8621]

Shit, nah nuckle

[@JackPowell-vl4ss]

I did all the steps just to have it ask for payment. Smh