The Comfort Zone by Maury Tiernan Geary Pacific Corporation
No. 14 November, 1997 Page 1 of 2
The Comfort Zone column appears regularly in the Modular Building Institute’s Commercial Modular Construction Magazine
THE “PEOPLE” LOAD
One of the important factors in heat load calculation is the “people” load. As a rule of thumb, we can use 400 Btuh/person for internal heat gains. A family of three sitting sitting in a living room would then generate 1200 Btuh of heat, and 30 students sitting in a classroom would need one ton of cooling (12,000 Btuh) . . . or maybe more. Let’s take a look at factors that would affect the people load. While our rule of thumb is a people load of about 400 Btuh/person, this is only an average value for an adult male sitting at rest. Females average average 340 Btuh and children average less (300 Btuh). Further, while while an adult male at rest rest generates 400 Btuh of heat, heat, a child a play may create as much much as a 1500 Btuh load. Note the ballpark figures in the chart below.
Application Homes and theaters Offices and department stores Classrooms Restaurants Light factory work Moderate factory work Heavy factory work
HEAT GENERATED BY PEOPLE Sensible Latent Total Btuh 195 155 350 200 250 450 250 250 500 220 330 550 220 530 750 300 700 1000 465 985 1450
A person sleeping generates about 260 Btuh, when walking about 800 Btuh, and for heavy activity, such as swimming, more than 2000 Btuh. Thus, a person’s size, weight, age and sex (noun not verb), plus the degree of physical activity, all affect the heat generation (metabolic) rate. As a result, all these facts should be considered in order to choose appropriate heat gain values in HVAC design work. Earlier we stated that 30 people could impose approximately a one ton cooling load. If those same people were dancing they could generate a two ton ton load. How do HVAC systems handle that load load ? They remove the heat as people generate it. Let’s now take a look at that process to better understand the impact of the “people load” on the entire HVAC system and heat load calculations. Have you ever thought about how heat naturally flows? Heat flows from a higher to a lower temperature. The normal body temperature is 98.6 degrees deg rees F, and our normal inside room temperature is less than that, (maybe 75 degrees F.) Thus heat flows from the person to the surroundings. surroundings. A heating system does not warm a person, but rather, it warms the room and allows the occupant to lose heat at a slower, more comfortable rate. We feel warmer when our bodies loose heat at a slower rate. Geary Pacific Corporation, Corporation, 1908 N. Enterprise Street, Orange, Orange, CA 92865-4102. (800) 444-3279, Fax: (714) 279-2940
The Comfort Zone by Maury Tiernan Geary Pacific Corporation
No. 14 November, 1997 Page 2 of 2
In air conditioning as well, the environment is maintained so that body heat can be rejected at a comfortable rate. Have you ever known of two people in the same room, one is hot, one is cold ? It’s the difference in their metabolisms that causes them to feel differently in the same room. Do you remember when your mother told you to close the door in the summer because you were letting all the cold air out ? Not so. You were actually letting the heat in. Once the heat (energy) has infiltrated into the house, how do we get rid of it ? We use the air conditioner (expansion and compression of refrigerant) to absorb the heat via the indoor coil and reject it to the outside. That is why hot air blows off of the outdoor coils in the summer. Comfort really means: letting people lose heat in proportion to the rate at which they are generating it. The other day I was asked to do a load calculation on a large church. Seating was fixed at 1000 occupants. The church is located in the Phoenix Arizona desert. Besides collecting all the other load calculation data, I asked about the religious denomination. Kind of an odd question, huh? Well, not really. Some denominations are more active than others. Some pray sitting, and some pray at a higher activity level. You may laugh, but if you do not ask some of these types of questions, it may be your church that gets shorted 17 tons of cooling capacity. Let’s look at some further considerations and you’ll understand what I mean. In addition to the activity of the occupants, we also need to consider how many times the doors will open. What if the denomination has 4-5 services on Sunday mornings starting at 7AM, each lasting 1 hour, with a half-hour between ? That’s 1000 people in and out. The people load in this church differs from the denomination with only one service in the evening at 7 PM. Both have the same 1000 people in and out. The first example takes 65 tons of cooling, while the second requires 48 tons. The 17-ton difference is all due to the people “factor”. Here’s how we figure it. The First example has 2000 door openings every hour, 4-5 times, into the hottest part of the day (120 degrees F) in Phoenix. The second example has 2000 door openings, once, at maybe 95 degrees F outside temperature. The 17-ton difference between these two examples shows how important it is to learn as much as we can about the occupants and their activity before calculating heat loads. In relocatable classrooms, consider: the age and size of the students, the number students per class, how much activity they will be involved in while inside the classroom, what time they go outside for recess and PE (during the hot or cool hours of the day ?), how many door-openings and closings and the outside temperatures, and how much time the c lassroom is empty. All of this and more can impact the Heat Load of the classroom. Plan for the worst case so the system can keep up. Finally, some of you noticed I had “sensible”, “latent”, and “total Btuh” figures in the chart above. Curious about why that is? For an explanation, you will have to wait till we meet next time in . . . The Comfort Zone.
Geary Pacific Corporation, 1908 N. Enterprise Street, Orange, CA 92865-4102. (800) 444-3279, Fax: (714) 279-2940
