The Comfort Zone
No. 15
by Maury Tiernan
Geary Pacific Corporation
January, 1998
Page 1 of 2
The Comfort Zone column appears regularly in the Modular Building Institute’s Commercial Modular Construction Magazine
SENSIBLE, LATENT, AND TOTAL BTUH . . . HUH ?
Our last visit to the Comfort Zone talked about the ? People Load ? , and how that load varies by each person's
metabolism, and activity level. A chart was provided showing a variety of activity levels with some category
headings you may not have recognized. The chart is reprinted below.
Application
Homes and theaters
Offices and department stores
Restaurants
Light factory work
Moderate factory work
Heavy factory work
HEAT GENERATED BY PEOPLE
Sensible
Latent
Total Btuh
195
155
350
200
250
450
220
330
550
220
530
750
300
700
1000
465
985
1450
So what are sensible, latent, and total btuhs, and how do they impact us ?
Sensible heat is that which can be measured or felt, and always causes a temperature change. This can be
demonstrated by putting a thermometer in a beaker of ice and watching the mercury fall, or by removing it from the
beaker, touching the bulb end with your finger tips, and watching as the mercury rises.
Latent heat is that which produces a change of state without a change in temperature. Remember your 9th grade
science class experiments ? Imagine ice cubes placed in a beaker, heat applied with a Bunsen burner, and reading
the temperature as the ice cubes melt. The water temperature reads 32 degrees F until the ice was completely
gone, then it starts to rise. The change of state, from ice to water, requires heat, yet the water temperature did not
change. That was latent heat causing the change of state. When the temperature started to rise, after the ice
melted, that was sensible heat causing the temperature change.
“Yeah, so what,” you may be asking yourself. “Why the science lesson?” Well, it will help you to do more
accurate calculations and make better decisions when you bid a job. Here’s how these terms relate: the sensible
cooling capacity plus the latent cooling capacity equal the total cooling capacity of an HVAC unit. An air
conditioner in the cooling mode absorbs heat through the indoor coil, and rejects it outside the building via the
Geary Pacific Corporation, 1908 N. Enterprise Street, Orange, CA 92865-4102. (800) 444-3279, Fax: (714) 279-2940
The Comfort Zone
by Maury Tiernan
Geary Pacific Corporation
No. 15
January, 1998
Page 2 of 2
outdoor coil. The approximate percentage of sensible heat that an Air Conditioner can absorb is 72-78%, with
the remaining absorption capacity being latent heat.
If you are bidding a telecommunication or equipment shelter you must recognize that electrical equipment loads are
totally sensible heat, (no humidity or moisture come from electrical equipment). So, if your project has a zone with
a 3 Ton (36,000 Btuh) sensible load, and you remember that an air conditioning unit can only remove
approximately 72-78% in sensible heat, then when you look at the unit’s specification sheet, you will select a 4
Ton model at standard conditions.
In contrast, if your bid is on a health fitness center, the load is mostly latent heat because of the occupants’ activity
level, perspiration rates, the showers, pool, and saunas. You will need to address the latent heat load first, then
adjust for the sensible.
Another familiar example of latent heat is humidity. Most of us know that 100 degrees in New Orleans, with it's
90% humidity, "feels" hotter than 100 degrees in Phoenix at 10% humidity. While the thermometer reads the
same temperature in both places at 100 degrees, the humidity level is drastically different. The moisture (humidity)
in the air is storing energy (heat). Mechanical Engineers in New Orleans must address the latent heat first, then
their sensible heat loads. Sizing an Air Conditioner too small in New Orleans will make it feel sticky inside a
building. Over-sizing can actually cause walls to drip with condensation. On standard residential/office
applications, Mechanical Engineers in Phoenix usually deal only with the sensible heat load, because the latent heat
loads never exceed the unit’s capacity to remove it.
So ask about the application of your building, and any internal heat loads created by people, computers,
equipment, or activities that may not be specifically called out in the bid. Take care of yourselves until the next
time we meet . . . in . . . The Comfort Zone.
Geary Pacific Corporation, 1908 N. Enterprise Street, Orange, CA 92865-4102. (800) 444-3279, Fax: (714) 279-2940