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HVAC Load Calculation - Cooling Loads

Calculating how much cooling you need. A few decades ago, residential air conditioning was rare in colder areas of the U.S., and cooling load calculations were often unnecessary. Now it’s a different story, with most new U.S. homes routinely including air conditioning equipment so most U.S. builders are faced with the need to calculate cooling loads.

HVAC Load Calculation

HVAC Load Calculation

Rule-of-thumb sizing

Although most building codes require load calculations for heating and cooling equipment installed in new homes, the requirement is widely ignored and rarely enforced. Most HVAC contractors never perform such calculations; instead, they size air conditioners by rules of thumb.

The age-old rule of thumb used by most contractors was one ton of cooling equipment for every 400 square feet (sf) of conditioned space. In a concession to recent improvements in insulation levels and window specifications, some HVAC contractors have adjusted their rule of thumb (R.O.T.) sizing to one ton per 600 sf.

Because these R.O.T.s almost always result in gross oversizing of cooling equipment, most energy experts have been battling R.O.T.sizing for years. However, R.O.T.s have their place.

Using a R.O.T. is not really the problem; the problem is that HVAC contractors are using a bad R.O.T.

At least two well-known energy consultants — Michael Blasnik and Allison Bailes — have proposed a new R.O.T. for sizing air conditioners in homes with insulation that meets minimum code requirements: namely, one ton of cooling per 1,000 sf. According to Blasnik,“Sizing an air conditioner using tons per square foot actually works pretty well, as long as you choose the right R.O.T.”

A good R.O.T. has many uses; for example, it can be used by builders to get a general idea of whether their HVAC contractor’s sizing method is in the right ballpark or is totally nuts about HVAC Load Calculation.

Of course, using an R.O.T. to size an air conditioner isn’t a substitute for performing a room-by-room cooling load calculation. Room-by-room calculations are necessary for many reasons: to properly size ductwork, for example, and to address unusual architectural features like rooms with large west-facing windows. Moreover, as air-conditioning guru John Proctor points out:

“Rule-of-thumb sizing does not account for the orientation of the walls and windows, the difference in surface area between a one-story and a two-story home of the same floor area, the differences in insulation and air leakage between different buildings, HVAC Load Calculation, and the number of occupants, and many other factors.”

You know that your air conditioner is sized correctly if it runs 100% of the time on the hottest afternoon of the year. Since most air conditioners are oversized, however, they tend to short-cycle — even on very hot days.

How heat gets into buildings

To understand the theory behind cooling load calculations, it’s useful to understand all of the ways that a building gains heat.

Some heat comes from outside (these are external loads), while some heat is generated inside (these are internal loads).

•Whenever the outdoor air is warmer than the indoor temperature and increases the building envelope’s heat transfer

•When solar radiation raises the temperature of roofing or siding above the indoor temperature

•When the sun shines through the windows, causing heat to enter

•Warm infiltrating air entering through cracks in a building envelope can cause heat to enter as well

•Mechanical ventilation system pulling in warm exterior air causes heat to enter a building too.

•Pets, people, lighting, electrical appliances, and combustion appliances, like kitchen ranges and water heaters emit body heat and moisture, generating internal heat gains. All of these are located inside a building’s thermal envelope and the HVAC Load Calculation.

Sensible and Latent Gain

Two of these sources can cause both sensible and latent gain—air infiltration and internal gains. Latent gain is when dehumidifying air needs to be removed from a building; each pound of water has a latent heat capacity of about 1,000 BTU. This means that dehumidification puts additional strain on an AC unit since it adds heat to any given amount of air by way of phase change (water vapor → liquid water).

Rater Allison Bailes explains “The sensible load is how much cooling you need to do to bring the temperature down, and the latent load is how much cooling you have to do to bring the humidity down.” Calculating cooling loads means including psychrometrics because they consider a structure’s latent load (unlike its heating counterpart). Cooling calculations require designers not only to know specifics about what type of hot and humid outdoor conditions they’re dealing with for them to remove moisture from infiltrating air, but there also must be knowledge of how much moisture humans generate too.

Of course, all relevant information about any given structure’s specific situation needs to be considered when doing anything with it. Assistants like this one can’t reach everyone at once so it’s best practice for readers who are interested in learning more to do extra research about the HVAC Load Calculation.

• Climate matters, of course.

• Orientation matters. Because windows aren’t equally distributed on all four orientations, a 90-degree rotation in building design can change the cooling load.

• Latitude matters (because the sun angle changes with latitude).

• The roof overhang width matters, as does the distance from the top of the window to the soffit.

• The presence or absence of insect screens on windows matter, because they affect solar heat gain. (Because window screens are removable, most calculation methods assume that windows have no insect screens.)

• The presence or absence of curtains or blinds matters.

• The building’s air leakage rate matters.

• The mechanical ventilation rate matters. (Software programs usually assume that older leaky homes have no mechanical ventilation system, only infiltration and exfiltration through envelope cracks. For newer homes, most programs require users to input information on the ventilation rate or assume the existence of a mechanical ventilation system that complies with ASHRAE 62.2.)

• The number of occupants matters. (Most calculation methods assume that the number of occupants equals the number of bedrooms plus one.)

• The lighting and appliance specifications matter. (Energy-efficient appliances and lighting produce less waste heat than inefficient appliances and lighting.)

Calculating hidden loads

The main contributors to latent loads are infiltration and exhalation via occupants, perspiration and cooking, laundry, and bathing.

Some sources imply that a home’s latent load amounts to 30% of the total load, but this is highly variable. A home’s infiltration rate, the climate, plus occupants’ moisture production all impact latent load. For example, leaky homes in hot, humid states like Louisiana can have higher than 30% latent loads of total loads. On the other hand, homes located in arid states west of the Rocky Mountains usually have latent loads that are much less than 30% of their total load.

Sensible divided by total (including latent) heat is defined as sensible heat ratio (SHR) or sensible heat factor (SHF). Manual J, which is the most widely used calculation method for cooling loads assumes a default value of 0.75 for SHR although software programs allow users to enter a different number if they prefer. Most air conditioning equipment is designed to operate at an SHR between .70 and .75.

According to ASHRAE Fundamentals, “A latent factor (LF = 1/SHF) of 1.3 or a sensible heat factor (SHF = sensible load/total load) of 0.77 matches the performance of a typical residential

vapor compression cooling system. Homes in almost all other regions of North America have cooling loads with an SHF greater than 0.77 and latent factors less than 1.3.”

Internal Loads

Heat gain in commercial buildings is dominated by internal loads (for example, computers, lighting, and body heat). Compared to commercial buildings, most single-family homes have fewer electrical devices and occupants per square foot, and have a building envelope with a bigger surface area per square foot of conditioned space; as a result, internal loads play less of a factor in homes than in offices or schools.

Nevertheless, internal loads can be significant in homes and must be calculated. The sensible heat gain (body heat) emitted by one occupant is usually assumed to be 230 Btuh (67 watts). The default assumption for a home’s appliances is usually 1,600 Btu/h (469 watts); lighting is assumed to add another 1,600 Btu/h (469 watts).

These default assumptions can be adjusted up or down if necessary. If a house has an unusual number of appliances or equipment — for example, a home business that includes a computer server room — internal loads will be higher than these default assumptions.

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