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Building Science

Understanding Manual J Occupant Loads

How many people? Where and when do they get counted? How much do they contribute?

People complicate things. If, say, you want to sell a motorcycle, you place an ad and interested potential buyers come check it out, maybe take it for a little ride, and then one of them buys it. But every once in a while, one of those potential buyers arrives with a small instability that gets amplified by some kind of cosmic resonance and next thing you know, you’re scraping your motorcycle off the driveway and letting the would-be buyer use your shower. And it just gets weirder from there.

But I’m not here to talk about that kind of complication. No, this is about a small but important aspect of Manual J heating and cooling load calculations: the effect of the people in a house on said loads. I’ve mentioned in previous articles about how some people inflate the loads by getting this part wrong, but I’ve never talked about some of the nuances associated with occupant loads and how to do this part of the load correction correctly. So, let’s tackle that today and see if we can avoid destructive resonances, cosmic or otherwise.

Heating, cooling, or both?

People inside a house add heat to the living space. If you count this in the winter, the heating load would be smaller than without occupants, meaning you may be able to get by with a smaller heating system. In summer, people increase the cooling load, requiring more air conditioning. Before I reveal the official answer from Manual J, let me point out that the peak heating load occurs at night, when people are asleep. That’s when it’s coldest outdoors. (OK, if you have teenagers, they may be playing video games at 3 in the morning, but teenagers are unreliable. Sometimes they will have snuck out of the house and be playing video games at a friend’s house.)

Peak cooling loads occur during the day, when the sun is shining and people are active. So the answer is that occupant loads (and other internal loads like refrigerators, too) add to the cooling load, but Manual J doesn’t let you take credit for them in the heating load.

How many people?

It’s a simple rule. The total number of people in the house, for the purpose of calculating the cooling load, is equal to the number of bedrooms plus one. It is not the actual number of people who live in the house (unless that happens to be the same as the rule). Nor is it the number of people you have over at your 4th of July party, the slideshow you host after your trip to Portugal, or the party that your teenager has when you’re out of town.

The Manual J load calculation should include 4 people for a 3 bedroom house, 5 people for 4 bedrooms, and 6 people for 5 bedrooms. It could hardly be simpler.

How much load per person?

Since occupant loads affect only the cooling loads, each person in the house has the potential to increase two types of load: the sensible cooling load (temperature) and the latent cooling load (humidity). And in fact we humans do affect both. Here’s what Manual J prescribes for the amount of cooling load added by each person:

230 BTU/hr — Sensible

200 BTU/hr — Latent

The sensible component is easy to understand. It’s the amount of heat our bodies give off as we attempt to regulate our internal body temperature. The latent component is due to breathing and evaporative cooling (sweating), of course, but also 45 of the 200 BTU/hr is from moisture-generating activities like showering, cooking, and water balloon fights. OK, OK!  Manual J doesn’t actually mention water balloon fights, so if that’s an important part of your indoor life, you’ll have to adjust for that manually.

When and where?

Peak cooling loads occur in the early evening, so they should be assigned based on where people are at about 6 pm. Normally, that’s kitchens, dining rooms, living rooms, family rooms, offices, and other common areas where people hang out. Each house is different, which is why Manual J says the room assignment should be “based on the judgment of the designer.”

Room assignment for the people in a Manual J load calculation
Room assignment for the people in a Manual J load calculation

In my house, for example, we have 4 bedrooms, so I put the 5 occupants in 4 different rooms. One is in the basement family room, and the others are shown in the floor plan above: 2 in the den and 1 each in the living and dining rooms.

What about parties?

One of the most common justifications for inflating the number of people in the load calculation is that “these homeowners entertain a lot.” OK. That’s fine. But guess what? Manual J discusses those kinds of temporary occupant loads. The solution is handle such temporary loads with either a supplemental cooling system (e.g., a ductless minisplit) or a system that can move excess capacity from unused parts of the house to the party zone.

To find the amount of temporary cooling load, estimate the number of guests and use these numbers for the sensible and latent components:

230 BTU/hr — Sensible

180 BTU/hr — Latent

The sensible is the same as the regular occupant load, but the latent is lower because your party guests don’t usually take showers at your house. If they do, they probably just crashed your motorcycle so handling that cosmic resonance will be more important than a little excess humidity. You’re trying to prevent your house from shaking itself to bits the way the Tacoma Narrows bridge did in 1940.

People are complicated but simple

The rules for handling the cooling loads from people in a house are straightforward and simple. Manual J explains it all in just three paragraphs. (Yet here I am writing the 14th paragraph of this article. Hmmmm.) Where it gets complicated is when people doing load calculations try to make up their own rules on the fly. Time travelers, they say, have sensible loads two to three times normal, and even higher if they’ve just returned from an apocalypse. Follow their logic and you may find yourself the basement, drinking a beer with a stranger who’s wearing a pair of your underwear.

No, no, no. Just follow the rules above. Understand how to read Manual J reports. And above all, watch out for destructive resonances.

_________________________________________________________________________

Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog.  You can follow him on Twitter at @EnergyVanguard. Photo courtesy of AmazonIllustration courtesy of the author.

 

8 Comments

  1. User avater
    Jon R | | #1

    > temporary occupant loads. The solution is handle such temporary loads with either a supplemental cooling system or ...

    Say there will be 10 guests, requiring an extra 4300 Btu/hr. I expect that a little up-sizing will be far more cost effective with almost no downside. But let's see numbers showing otherwise.

  2. user-6892763 | | #2

    In the GA 2020 energy code, we added an amendment to the IECC (R403.7) that deals with Manual J and S with the hopes of at least partially addressing the issue of temporary loads:
    “For automatically modulating capacity heating and cooling equipment, the system shall be deemed to comply with appropriate portions of Manual S provided the lowest output capacity of the equipment is less than the peak design load as determined by Manual J.”
    Under the (un-amended) code, you'd do a load calc, then fudge the inputs to make it so you could install whatever size equipment you wanted to install.
    Under this new approach, as long as your equipment can automatically throttle to less than your correctly calculated design load, you are still in compliance and still have the "extra" capacity that you think you need (but probably don't).
    It's certainly a challenge to help this industry but the fact that you can legally oversize equipment might help push more people to variable capacity (which is certainly where we want them to go).

    Cheers, mikeb

    1. User avater
      Jon R | | #3

      NEEP is even more lenient - minimum cooling output can be 115% of of load.

      https://neep.org/sites/default/files/Sizing%20%26%20Selecting%20ASHPs%20In%20Cold%20Climates.pdf

      1. User avater
        Peter Engle | | #5

        Be a little bit careful with that recommendation. That's in an article on sizing ASHP for cold climates. In most cold climates, the required capacity is driven by the heating design and the system will be oversized for cooling. NEEP recommends that, if it is oversized by more than 115% of the peak cooling load, you should change the design or select different equipment. That's different than Mike and Allison's designs. They are designing for cooling in Georgia. That NEEP article would not apply, probably not even up in the mountains.
        For my mid-Atlantic climate zone 4, with 2 story houses, I like separate ASHP's for the separate floors of the house. This allows use of a single system in the shoulder seasons for even lower total capacities (upstairs only for cooling and downstairs only for heating). That's one way to deal with minimum modulation issues.
        IMVHO, it is silly to design for the occasional party. Precooling and fans can make a world of difference. If that's not enough for the occasional blow-out, you can rent portable A/C equipment. For very large houses, it is not unheard of to have separate HVAC systems for the daily loads and for the party loads. Over the long term, this still saves $$ and provides better performance in these two very different load regimes.

        1. User avater
          Jon R | | #7

          Agreed, it is referring to "(IECC) climate zone 4 and higher". But don't lose track of the point that whatever the right percentage, it's of min output, not max. Does Manual J say otherwise? Ie, is a system that modulates from 5K to 15K complaint for a 10K design load?

          > separate HVAC systems for the daily loads and for the party loads...better

          In the case of something small like 4300 Btu/hr (resp #1), I disagree. A slightly larger inverter based system will handle the load with lower cost and no noticeable difference in performance (might even be better).

  3. User avater
    Peter Engle | | #4

    Mike,

    If the lowest capacity is just shy of the peak design load, that still leaves the equipment oversized for about 95% of the time. You're no better off than if you picked fixed-output equipment at the minimum modulation capacity and you've spent a whole lot more. I'm not sure this is an improvement.

  4. user-6892763 | | #6

    Peter - I agree that in that scenario you are barely better than a fixed speed system, but you'd certainly be better off than if you had fudged the load and put in a larger fixed speed unit (which is definitely what happens). The hope is that you would select a system that was able to throttle more than just a little lower than the peak design.
    Again, the incentive is to encourage variable speed/capacity and this is minimum code we're talking about. Also, with climate change effects, variable speed/capacity seems like a really good idea.

    Allison, forgot to mention that I enjoyed your post, as always. best, -mikeb

  5. Lance Peters | | #8

    We hosted a pre-wedding event last summer which saw about 30 people on the 600 sqft main floor of our townhouse. The 2-ton AC was running full blast and the mercury was rising, eventually getting to uncomfortable levels. I was not surprised to find the bedrooms on the second floor absolutely freezing cold when we hit the sac afterwards.

    True story, but also only a real scenario for one night every ten years. Were the standard forced-air furnace single-stage AC designed to handle that situation it would likely be 100% larger since only half of its cooling is routed to the main floor.

    A zoned system sounds much more practical.

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