Manual J Load Calculations for Heating & Cooling

Now that Corbett has been doing HVAC load calcs for almost a decade, he's ready to make this very long (and also much too short) video tutorial about the what, why, how, when, and who of heat load calcs for sizing your HVAC equipment.

[johnscott]

This video was a point of discussion at our Austin, TX BS+beer meeting this week. Thanks for taking the time to put this out there. It’s having a tangible effect on improving our country’s housing stock.

[Texas_Tee]

I’ve been a hvac tech for 6 years...just this past year 2020 I got moved into the office and I’ve been doing heat loads ever since. I love it!

[tommcmillan]

Thanks Corbin, very good! I like how you explain why placement of the house on the lot, windows, design day, construction quality, etc, all must be factored as you plan and draw the house. I admit in you're area of expertise I am only scratching the surface as we plan out our final home in northern MI, no doubt I need a professional that is objective. Not sure how I found this channel but glad I did!

[cemprotecta]

Wow, what a great explanation for such important concepts. Greetings from a builiding biologist based in Spain! Thanks a lot!

[babbitteb]

This is an awesome video, Corbett! After watching your video, I gained the confidence to pull the Manual J that was run for my new townhome. We moved in 6 months ago and I was very disappointed with how my cooling system was performing. It seemed to always struggle. I reviewed the Manual J for my home and without surprise, it said my system was sized accordingly (cooling power at 102% of load). I thought I hit a dead-end after seeing this, but I then found out that in my new townhome complex, the townhome building built right next to me that was finished just a couple months after my building (which also happened to be the exact same floorplan and orientation to the sun) got bigger equipment than I did. This made me scratch my head wondering how they could be sized for bigger equipment when our homes are apples to identical apples. I had one of my neighbors in this building pull his Manual J that was done by the same consulting firm. When you compare the reports side by side, things are pretty much identical. However, one big thing that tips the scale is a line on our reports under the "Sensible Cooling Equipment Load Sizing" Section calling out each home's blower motor. I see the BTU loads of the structures, ducts, and central vent for outside air. In my neighbors report, his calculations add 1707 btuh for his blower. However, when you look at the same line for my blower, it says 0. So I guess my million dollar questions are a) is it possible for my neighbor to have 1707 btuh of heat gain from his blower but 0 for me? and b) if I were to generalize my question, is it possible for any blower motor to have absolutely no heat gain working against the AC system?

[dosadoodle]

Per the comment at 26:18 regarding tradeoffs (add insulation, require smaller and less costly equipment), I've heard a high performance builder approach is to group the HVAC costs with the sealing and insulation costs into one large "performance" line item, which makes it much easier to talk to buyers about tradeoffs. Like in the small example noted in the video, it might be that notable improvements in sealing or insulation at a cost of $25, 000 may offset HVAC costs of $13, 000 (e.g. if can move to mini splits with no ducting), so that one category of costs is really just +$12, 000 to spend $25, 000 in high performance.

That eases conversations around around tradeoffs. For example, if the +$12, 000 in the example is only offset by energy savings of $7, 000 in 10 years, the homeowner is really just paying $5, 000 to get a home with $25, 000 of additional materials and work on windows, insulation, and air sealing, which makes the home more comfortable to live in. It's a much easier sell.

[slengoslengaw]

This is fantastic! Amazing work and very clear and well explained. How did I not come across you before?

[AC-SuperCool]

Hey Corbertt, I understand and perform load calcs myself, I used Elite as well. However when doing load calcs on new construction it is impossible to perform a blower door to determine the homes leak rate. Therefore you must assume the leak rate.  

Doing blower door on every retrofit is simply not feasible for most homeowners. Many home owners struggle to pay for the system replacement, much less the addition cost of the blower door test.  

I admire the work you do & appreciate the changes your bring to home building. I am now persuing my GC license & I will be reaching out as transition from 20+ years in the HVAC industry to start building clean homes.

[alexleach]

Thanks Man! Been down the rabbit hole with this one.

[jamestente]

Great video Corbett, clearly breaks down the topic into understandable concepts!

[miltonadams]

A couple of questions from the homeowner. We have built an off-grid 2-story ICF home in West TN and have lived in it for 4 years (Yes, still finishing the insides :) It takes a candle to heat it in the winter (actually, we use a cookstove). When 100 degrees outside, it is 85 degrees inside (without AC) during summer. But our challenge is, you guessed it, humidity. Now ready to add a min-split. Question 1) Where can I find someone to do a Manual J load test? They want to use their "own" load test system and claim years of experience with results ranging from 12K - 42k cooling. Question 2) is CoolCalc reliable as a DIY approach?

[jl]

Amazing video. I am halfway through and learning so much. For example i knew about latent and sensible heat load but i didn't really understand why latent heat was called that but you explained it perfectly. I didn't know where the term ton came from but you explained that. Thanks.

One thing i have to disagree a little with is that an undersized ac will run 24 hours on the hottest day- not quite- Temperatures will drop at night and two things will happen
1. The delta T is smaller resulting in less heat gain, and two the AC efficiency will increase as acs are most efficient when delta T is smallest.
So a mildly undersized ac will turn off sometime during the night and then only come on as needed.

[josephshaw]

Thanks. Great video! I am a home owner working through a Manual J room by room load calculation on an existing two story house using the MJ8ae speed sheet from the ACCA website. I have a copy of Manual J8 v2.50. One question I have not been able to find guidance on is whether to include the thickness of interior and exterior walls in the room dimensions when calculating wall, floor, and ceiling areas, and slab perimeter length. Including wall thickness increases the envelope area by about 5%. I have the same question re whether to include the thickness of the second level floor when calculating the above grade volume.

[EricLangenmayr]

Hi Corbett, just listened to your really excellent Man J load calc video. I see that for your forever home you use Alpen windows. Did you use the ones with suspended film or thin glass? Operable or fixed? I watch your Home Diagnosis show on TV which is also excellent and am going through all your other videos - they are just wonderful. Explanations are really clear and it's obvious you both are really enthusiastic about your work. Thanks!

[dennmillsch]

Around 32:30 you state that having less than 3 ACH doesn't make much difference. How so? Seems to me if the air changes air 3 times in an hour my equipment needs to work harder to heat or cool the replacement air, compared to only 1 ACH. Could you explain? Or are you saying that the air exchange becomes a negligibly small percentage of the excessive heat gain (or cooling load), that the air change losses are dwarfed by all the other factors?

[thesoulblesser]

Does Manual J works on ISHRAE standards too ?

[TJ-]

Terrific video! Very informative! At the 21:50 mark, you mentioned that an air conditioner needs about, on average, 10 minutes to start drying out the air. My single stage AC unit runs for about 6–9 minutes. The humidity reading on the smart thermostat is mid 60s. However, the humidity reading on my hygrometer is mid 50s. Which is likely to be more accurate? If the humidity readings are in the 50s, would the 6-9 minute run time do you something of concern?

[HANDYMANDAN]

Great video, have you thought about a modulating AC for your new home so that you can save energy.

[fromtheburbstothetetons]

What does it mean to have your front door facing south? It the assumption most of your windows are on the front of the house? What if you have a beach house, lake house, or mountain house with a window wall. How does that play into the "front door" orientation?

[danpmatz]

if your ac runs for 24 hours straight when its 92 you are going to be hot when its 95 and above also the load is never the same at night soo 24 hours strait?