Manual J Load Calculations for Heating & Cooling
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- čas přidán 10. 07. 2024
- 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.
A list of approved softwares is at: www.acca.org/standards/approv...
Learn a lot more about Home Performance Diagnostics in general at: BuildingPerformanceWorkshop.com - Věda a technologie
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.
Hey, THANKS John! Very nice to hear!
Great video Corbett, clearly breaks down the topic into understandable concepts!
Hey Jim, thanks man! Good to hear from you.
Thanks Man! Been down the rabbit hole with this one.
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?
Excellent energy and thoughtfulness, Eric! Just FYI, I've been asked before to build a Man J that would match the equipment that HAD ALREADY BEEN INSTALLED.
Just that amount of btu from blower is not enough to up the cooling tonnage. 9 out of ten times is improper duct design. sometimes you can overcome this with a more powerful blower. You need to have a hvac expert come to your home with a flow hood and test your total cfm. they might have gave your neighbor a bigger system when he just needed more cfm. lots of contractors do that.
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!
Hey, that’s great to hear Tom! We’ll be happy to help if you need any 1-on-1 support.
Wow, what a great explanation for such important concepts. Greetings from a builiding biologist based in Spain! Thanks a lot!
Hola amigo! Muchas gracias!
@@HomePerformance De nada. Un saludo!
Wow this was excellent!
So glad you enjoyed it, Dave
Thank you for sharing this video.
This is fantastic! Amazing work and very clear and well explained. How did I not come across you before?
Hey, thanks @Slengo Slengaw! Happy you found us!
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!
GOOD TO HEAR BROTHER. The world needs you.
Great video thank you.
You’re welcome- thanks for following
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.
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.
Thanks for following and commenting, O
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.
Interesting budget org trick! Love it.
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!
Hey thanks for watching Eric- the windows are mostly fixed, a few casements, with film.
Great video, have you thought about a modulating AC for your new home so that you can save energy.
Yes, both of our systems are variable capacity. Great idea.
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?
MILTON! Don’t put in any A/C yet! You’ll need a dehumidifier for your home if you’re using ductless minisplits, AND you might not even decide you need them once you feel how comfortable dry air is. I recommend an Aprilaire or Santa Fe unit. Also, I recommend you be very careful with that cookstove and make sure your outdoor air ventilation is tuned correctly. If you need further help with details, I’d be delighted to work with you:
buildingperformanceworkshop.com/video-consulting
Thank you very much, you explain things very easy and slow for us to understand. But I have one question, what if the house is 800sq feet total but has two or three level ?
Aha! Then stack effect will play a much greater part in forced accidental air leakage out of, and into, the house.
Does Manual J works on ISHRAE standards too ?
Thanks Corbett! So I have to get the software, is there one you'd recommend?
I think Wrightsoft may be more powerful at this point
You don't have to get software. It's extremely easy to calculate the heat or cooling load for your envelope. Then you calculate the internal loads and you have your load calc.
Example: you have a wall that's 30 ft long and 10 ft high, so 300 SQ ft. Your design temp is 100 degrees and you keep your house at 75 degrees, so difference is 25 degrees. The wall is r20
So 300sqft x 25 degrees difference/r20
(300x25)/20
=375 btu per hour of heat gain or loss
Then you do it for all surfaces add the heat gain for leakage, internal heat gains, and you are done.
Or you can do it for free at load-calc.com :)
I tried using Elite Software Rhvac software to do the Manual J and D calculations so I can engage with a HVAC contractor with a new home build. In general, the software contained many options however in too many cases I had to compromise because I had to settle for selections that were the closest with my build. Even though I am working to minimize air infiltration, use low-e dual pane glass, increase insulation, and use nominal ventilation the load still showed roughly 1 ton per 500 sqft. Any thoughts?
Assumptions must be made in existing homes, but you can make custom components within the software if you don’t see your exact assemblies. Also, blower door test inputs, not defaults!
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.
Keep up the good work, Supercool. On new construction, any builder who’s been BD tested before should know what ACH50 they build, and that’s the best assumption for the HVAC load. On retrofits, as long as the offer of a test is made and the importance is explained, it’s the homeowner’s fault if they opt out. Bet you $1000 the offer and explanation doesn’t even happen even 1% of the time.
How do I find someone to do a blower test and perform these calcs? Have a 50’s home in the West Texas desert that we’re spray foaming and creating a very tight home. Every A/C contractor in 100 miles of me has never heard of a manual J, some have told me that I need to upgrade my 5 ton to two 3.5 tons, some have said my 5 ton is sufficient and just replace it in kind (the ac is currently out) and some have said I can reduce to a 4 ton and still be ok. Some have asked about sqft and some have not. Kinda frustrating. Thanks @Home Performance!
Sorry to hear, Texas. Closest people I have to you are Austin and Santa Fe. Probably not, right? Anyway, you should be able to find someone to run a blower door test, it's state law in Texas that all new homes get one, and has been for years. Try this:
homediagnosis.tv/pro
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?
That’s just how you define the direction in heat load calc software. Yes, it’s all about where the bulk of your windows face (west being generally bad).
Hi Corbett, I’m currently struggling to understand what to buy for our HVAC remodel. I’ve paid for a load calculation and manual d along with some other manuals all bunched together in a residential software. I live in Florida so humidity control is the goal. What type of unit do you recommend, variable stage or single stage with stand alone dehumidifier? I’m trying to make the right choice for our house. Required unit is a 3 ton and house is built in 2003 but I do plan to air seal and insulate better
Dehu in either case, all else really depends on particulars Ryan. Sorry can’t be more helpful without more detail- feel free to book a consult.
@@HomePerformance Will do but first I’m going to get a blower door test conducted as I think that’ll aid in the conversation
Do ducted versions of minisplits also have the 95% / 5% split between cooling and dehumidifying?
I believe they do, but they aren’t quite the maintenance headache as high wall units.
the comment about the quirk with the ceiling height might explain how they are getting some designs pass codes here. i've seen a few "award winning designs" that have 70% or more window aera. there is basically almost no wall insulation used.
could you elaborate on why houses smaller than 1000ft act differently to normal houses please.
Hey Tweake- just getting around to this comment, sorry. Small houses react much faster to upticks in humidity, contaminants, fumes, occupancy pollution, etc. Also, the enclosure-area-to-volume ratio is massively different. The heating and cooling loads may hang out for extended periods of time in the 1-1000 Btuh range, which is below the capability of any heat pumps on the market (and way out of range of any other technology). Because you have to use a ductless unit due to size constraints and super low load, you need supplemental dehumidification, which adds heat back into the space, and so on and so forth. Very complicated. Can be done right, as we know, but complicated.
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?
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?
The latter- and it DOES make a big difference for air quality, what I meant is it makes little difference for energy savings.
@@HomePerformance I'm still a bit confused. I hired a company to do a Manual J for our super-insulated house (R40 walls, R60 ceiling, very tight with an ERV) and their numbers came back with our 204 sqft, 1800 cuft Master Bedrm heating load at 1369 Btuh. I can't tell if they are including any infiltration in their calcs. Regardless, I calculate that it takes about .018 BTU to heat 1 cuft of air 1'F. So for the MstrBedr 1800 cuft and a design TD of 56'F I calculate 0.018 x 1800 x 56 and get about 1800 Btuh to heat the replacement air at 1 ACH. That 1800 BTUs is larger than the heating load calculated by the Manual J so the heat loss at 1 ACH is significant for my house. For 3 ACH it would be a whopping 5400 BTUs which overshadows the 1369 for heat loss through the walls etc.
But that's with my super-insulated house. If the house had half the insulation, that would double the wall/ceiling heat loss to over 2700 Btuh. The added 1800 BTU loss at 1 ACH is still 40% of the heat loss -- which I would consider signifcant -- let alone a 5400 BTU loss at 3 ACH. That's how I'm understanding this.
Or maybe these numbers I state are true at 1 ACH per hour but in reality the infiltration rate is much lower because it's not usually that breezy???
But other than my questions here, it was a great video, explained lots of good stuff. Appreciate it.
Question about when you say that if your furnace/air conditioner are not running 100% of the time, that they are oversized. Are you only referring to on the days that reach the hottest/coldest, or are you saying the system should be running constantly all the time regardless of the temp outside?
Just on the ‘design day’ for 99% hottest and coldest
@@HomePerformance ahh gotcha. That make sense. Thanks!
what software should be used? I get different answers for load depending on the software used even though the input info is the same.
What! if they’re both Manual J approved, they must calculate the same load, that’s the rule. I bet there’s a default in there somewhere sneaking past you. Try Elite or Wrightsoft, they’re the biggest.
what is the diffrence between convection and radiation?
What are you using to get that readout for component load?
REM/Rate or Elitesoft RHVAC
How can I get certified to perform load calculations for money? What do I have to do how long does it take I am in South Florida. Is there a demand for this kink of work?
Kink is right, Chris- this is mostly an unregulated, unlicensed activity. You just take a training and start doing it. Sounds crazy, right? That said, you DO need to have legit experience or cred to back you up when you get into arguments.
What do you charge for the Manual J? Do you also do design work?
Depends on the home plan, but between $0.12-0.30 per sq ft.
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?
I wouldn’t trust either- trust a $130 datalogger like a HOBO MX1101. The RH readings in my home are 5% high and 5% low for the two conventional readouts you mention.
@@HomePerformance Thank you, I will order one. Can’t wait! The house is kept cool and comfortable. If the readings are at a safe level, such as the 50s, should I be concerned about having only a 6-9 minute run time?
If the weather is cooler and humid, that’s when you’ll have more of an issue, TJ
@@HomePerformance Thank you - good to know. More of an issue with feeling sticky and humid, more of an issue with possible mold and mildew, or more of an issue with where and tear on the compressor/system? Or all of the above?
Only the first two
I know you are installing evr plus make up air for the house. how much of the total heat load is purely for heating .. you did not really break it out in the chart at 37.09 btuh
Aha- all of it is for heating. The software accounts for heat recovery in the ventilation.
Do you offer services to do load calculations?
Yes, Jonathan, I may be sble to help you. Get in touch via BuildingPerformanceWorkshop.com please.
What is the software that you are using?
I use Elitesoft
Sir i have a question! What about HAP hourly anylasis for load calculation for home please reply
Sorry, havrn’t heard of it, but a Man J calc does give hourly load profiles per room.
@@HomePerformance thanks
Can or should a manual j be done on an existing home before purchasing a new ac unit?
Yes, ideally- it was never done in the first place, so you’d be repeating the mistakes of the past (best case)
@@HomePerformance thanks. Now since it’s an existing home, is it possible to check the heat/cool load calculation without doing a manual j by using historical data from a smart thermostat ?
whats the name of the company thats building the prefab hobbit houses?
Compass Green.
czcams.com/video/IIw7QKug4ho/video.html
What did you desgin you house in?
Trimble Sketchup
I see a Cybertruck in your future...
🔼
⚫️⚫️
Would you be interested in doing a load calculation on a 3500 square-foot 1907-1910 victorian
Sure, David- happy to
Hi. I would like to talk to you. My dad. Owns a company called ENERGY INNOVATIONS BY HARRY BOODY INC
is there a way to engage a conversation I have about what my dad is doing in relationship to what your offering. I need some advice. ?
Hi John- I know Harry and his work, and I may know your dad too. If you’d like a chunk of coaching/consulting, go to:
buildingperformanceworkshop.com/training
If it’s a quick question, just try to catch me by calling.
Ty. I will
*house gets punched in the face*
The house: OMG it’s so hot in here 😂
21:41