Manual J Deep Dive (and Selecting HVAC Equipment with Manual S)- Part 1 of 2

Alex Meaney is my trainer when I level up on HVAC design calculations, and he came to visit us and share some of his expertise. Alex reveals what the J stands for and why you should never round up the tonnage from your Manual J software, and gives two rules for Manual S:
1. Never use AHRI data
2. Meet the latent load while not going too far over the sensible load (more in part 2)

Watch Matt Risinger's video on the Build Show about my Manual J report tour:

[edlauren]

I am a licensed hvac/refrigeration contractor. Great video, thank you!
Since I’ve passed my license exams I’ve re-done air ducts at dozens of residential and commercial properties. Dozens and dozens!!!!
Here is the typical story… Customer says that his convenience store/gas station don’t have enough A/C during July/August. So, the customer wants one more 5 ton RTU installed because people behind the counter literally sweat badly as soon as outdoor temperature goes above 90F.
So, I re-do the entire air duct set-up and…. There is no need for additional A/C anymore, the electrical bill drops 30-40%, no sweating any more…
So, with the ducts done correctly, old bitten up A/C becomes surprisingly efficient.
P.S. I’ve tried to use the manual J for my air duct designs …. and I gave up on using that Manual J because it more confusing than helping. :)
P.S. #2. I now have a line of small shops, stores, restaurants that are willing to get the air duct see-done.

[TECTubefilms]

Meaney and Corbett in one video! Couple of our favorite instructors.

[EGT]

Crazy stuff. Being a homeowner I have read enough to be dangerous. I asked my contractor to downsize my equipment when he replaced it and he didn't put up a fight or do any calculations. He just did it. I had my reasons and I think it's gonna be ok. But holy crap what a gamble I took without knowing what the hell I was doing. And still don't. I find this stuff fascinating and disappointing at the same time because I would probably venture to guess a lot of HVAC professionals don't even get this deep.

[paulmurray]

I love that you back up your comments with actual data. Thanks for the info.

[timdestasiohvac]

I love listening to Alex Meaney talk design! I'm glad to hear experts like him acknowledge there is still a level of estimation in HVAC design. We can do a lot of things to reduce the ductwork such as inserting the blower door ACH instead of picking "Average" or "semi tight" infiltration, but things like duct gain are just multipliers- SWAG at best.

[davec]

Alex Meaney talks and has mannerisms just like my brother-in-law who is fascinating to listen to just like Alex. Of course, Alex is talking about subjects my brother-in-law has no clue on, but are extremely interesting to me. I was in HVAC service briefly decades ago between my Army time and college. I grew up in a house with a father who believed only in window units and radiator heat and thought central air was the devil so I was slow to the game. Since working in apartment maintenance in my youth and spending a year learning how to repair all the gas furnace central air systems on the property I have always held an interest in moving heat around, but never went that route for a career.

One thing i definitely learned is just because a guy shows up in a van with a state license # on it, there is no guarantee he has much of an idea of what he is doing. This was clear when i was a young apt maintenance worker bugging the HVAC contractor we called with questions and he couldn't even tell me what the difference between PSIA and PSIG were. He also had no idea how a capacitor worked. I wasn't being a jerk. I didn't know either and was honestly curious. I found the answers on my own that night, but that service tech likely still doesn't have them.

I pretty confident Alex could answer any questions i could come up with and answer questions I didn't even know i had !

A ton is not 12, 000 Btu's and J is for 'joule'... marvelous.

Great video.

[ryanhoger]

The link to the ASHRAE weather data just saved me a ton of time instead of looking it up in the actual ASHRAE Handbook each time. I have been an ASHRAE member for 20+ years and never knew about the site!

[TJ-]

Two very knowledgeable and talented individuals. Excellent video. The video mentioned it’s not a good idea to set a thermostat below 72°, because on a peak outdoor humidity day, the outdoor dewpoint could reach 72° or above.

If that happens, when the indoor air and outdoor air meet, condensation will form indoors, usually in areas that are hidden, where you can’t see it.

Therefore, if we keep the house too cold, we risk condensation forming inside the ducts, on the equipment, in the walls, the ceilings, and hidden in places that we can’t see.

In the southeastern US, it’s not uncommon to have Dewpoint temperatures at 75°.

On the other hand, if we keep the house too warm, we risk the dewpoint rising. During the time that the AC is off, humidity from the outdoors is infiltrating our home. Which could cause condensation to form on the AC vents, potential mold growth, in addition to possible structural issues.

The longer the air conditioner pushes cold air out of the ducts, in an environment where the surrounding dewpoint is higher, The more sweat that will form on the surface of it.

It can be a challenge to find that perfect balance.

[adrianwright]

Thanks very helpful and informative. The new hvac curriculum has began.

[justinherman]

Wonderful video, can't wait for pt2.

[frankgall]

I’ve watched it 4 times already lol a lot to take in, great job as usual 🎉

[MattPetrowsky]

Way cool stuff. I'm actually glad my personal build has been delayed because of inflation, trade access, etc. I keep learning more cool stuff like this.

[davidhoover]

I need to do this or have it done by someone when we replace our A/C. We have a 2, 000 sq ft house with a 2 stage 4 ton A/C installed by an A/C contractor chosen by my builder. We literally never need anything except stage 1 at 2.8 tons even on unusually hot days over design temperature of 91. If it's 96 out and we're cooking, and we're running exhaust, and maybe also drying clothes then it will run 100% of those times still maintaining 74 on stage 1. It's a huge A/C for the house and it doesn't run a lot. I have installed a ventilating dehumidifier and dialed in the fresh air to keep the VOC's and C02 at a good average and it keeps us dry when the A/C isn't running. Our oversized A/C plus the dehumidifier now works well together.

[aldoogie]

Are you literally reading this comment and haven't hit the like button, literally the easiest thing you can do for the channel.

[Crusader]

Alex is one of the most knowledgeable people in this field, and his training is really valuable. I can't help but think, though, that he and all who are listening to him could benefit if he would modify his delivery. Maybe it's a Boston thing, but the consistent snarkiness makes it difficult to listen to him.

[marvingarcia]

Great information but one thing they forgot to mention is that all manufacturers are doing inverter compressors and at that point your 3 ton could perform as 2 tons if outdoor and indoor temp requires that, and yes will have to have a good duct design but the conversation here was manual j and equipment selection. As a contractor I have much respect for engineers. Thank you for the information.

[mzzm]

So glad I'm learning this now before I try and figure out our geothermal project. I'm getting so much bad information from folks in CT after doing some basic research. I'm pretty sure we will be reaching out for further building science reco's.

[Joe-qwil]

This is good stuff, I’m getting ready to re-design my ducting system in my attic, contractor who built my home did a horrible design job!!!

[tweake]

i absolutely love that you guys mention "ignore what temp the customer says". the human factors side drives me nuts a bit.

[microcolonel]

To nitpick: a joule is a unit of energy, but your load is quoted as power (ton/watts) since it is over time.
E.g, a Joule being one watt-second (energy), a ton being 12000btuh (power) or about 3.5kW

Good note on the equipment, and how conditions completely change the performance of heat pumps.