5.4 ACCA Manual N Commercial Load Calculation

ACCA Manual N Commercial Load Calculation

Why it is on the exam: the Board's reference book list places Manual N — Commercial Load Calculation for Small Commercial Buildings, 5th Edition on the H1 Class I, H2 and H3 Class I lists. Heating Group 2 candidates do not carry Manual J at all — for them, Manual N is the load calculation manual.

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1. What Changes When the Building Stops Being a House

A residence is envelope-dominated: skin loads and infiltration drive the peak, occupancy is sparse, and outdoor air is small. A small commercial building inverts that relationship.

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2. Internal Gains

People

Manual N tabulates sensible and latent gain per person by activity. Representative values used throughout the manual and ASHRAE Fundamentals:

$q_{\text{people, sensible}} = N_{\text{people}} \times \text{sensible gain per person}$ $q_{\text{people, latent}} = N_{\text{people}} \times \text{latent gain per person}$

Occupant count comes from the actual design occupancy where known, otherwise from the default occupant density in the ventilation table.

Lighting and Equipment

Every watt dissipated inside the envelope becomes sensible heat:

$1\text{ watt} = 3.412\text{ BTU/h}$

$q_{\text{lighting}} = 3.412 \times W_{\text{installed}} \times BF \times UF$

where $BF$ is the ballast/driver factor (1.20 for older magnetic-ballast fluorescent, ≈1.0 for LED drivers) and $UF$ is the use factor (fraction actually energized at the design hour).

$q_{\text{equipment}} = 3.412 \times W_{\text{connected}} \times \text{diversity factor}$

Hooded commercial cooking appliances transfer a reduced fraction to the space because the hood captures the rest; unhooded appliances contribute both sensible and latent gain.

Worked Example — Internal Gains for a 2,400 sq ft Retail Suite

• Lighting: 2.0 W/sq ft LED, use factor 1.0 → $3.412 \times (2.0 \times 2{,}400) \times 1.0 = 16{,}378\text{ BTU/h}$ sensible
• Equipment (registers, displays, back-office): 4,500 W connected, diversity 0.75 → $3.412 \times 4{,}500 \times 0.75 = 11{,}516\text{ BTU/h}$ sensible
• People: retail density 15 per 1,000 sq ft → 36 people at 250 sensible / 200 latent → 9,000 BTU/h sensible and 7,200 BTU/h latent

$q_{\text{internal, sensible}} = 16{,}378 + 11{,}516 + 9{,}000 = 36{,}894\text{ BTU/h}$ $q_{\text{internal, latent}} = 7{,}200\text{ BTU/h}$

At roughly 3.1 tons of internal gain alone, the lights and people in this suite outweigh a well-insulated envelope — which is exactly why a residential rule of thumb collapses on commercial work.

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3. The Ventilation Load

Outdoor air is introduced deliberately, so its load lands on the coil, not on the room. The three standard-air constants are the ones to memorize:

$q_{\text{sensible}} = 1.08 \times \text{CFM} \times \Delta T\ (^\circ\text{F})$ $q_{\text{latent}} = 0.68 \times \text{CFM} \times \Delta W\ (\text{grains/lb})$ $q_{\text{total}} = 4.5 \times \text{CFM} \times \Delta h\ (\text{BTU/lb})$

The constants derive from standard air at $0.075\text{ lb/ft}^3$: $1.08 = 60 \times 0.075 \times 0.24$; $0.68 = 60 \times 0.075 \times 1{,}061 \div 7{,}000$; $4.5 = 60 \times 0.075$.

Determining the Outdoor Air Rate

The North Carolina Mechanical Code Table 403.3 ventilation rates govern; Manual N uses the same values. The rate is the sum of a people-based term and an area-based term:

$\text{OA CFM} = (R_p \times P_z) + (R_a \times A_z)$

Worked Example — Ventilation Load, Same 2,400 sq ft Retail Suite

$\text{OA CFM} = (7.5 \times 36) + (0.12 \times 2{,}400) = 270 + 288 = 558\text{ CFM}$

Cooling design at Raleigh: outdoor $92^\circ\text{F}$ db / $110$ grains; indoor $75^\circ\text{F}$ db / $65$ grains.

$q_{\text{vent, sensible}} = 1.08 \times 558 \times (92 - 75) = 10{,}245\text{ BTU/h}$ $q_{\text{vent, latent}} = 0.68 \times 558 \times (110 - 65) = 17{,}075\text{ BTU/h}$

Heating design at $20^\circ\text{F}$ outdoor, $70^\circ\text{F}$ indoor:

$q_{\text{vent, heating}} = 1.08 \times 558 \times (70 - 20) = 30{,}132\text{ BTU/h}$

The ventilation latent load alone (17,075 BTU/h) exceeds the entire people latent load, and the ventilation heating load is larger than many small commercial envelope losses. Miss it and the equipment is undersized on both seasons.

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4. Block Load, Zone Load and the Peak Hour

• Room load sizes the outlet CFM for that room.
• Zone load sizes the branch or terminal unit serving that thermostat zone.
• Block load — the simultaneous peak for everything on one piece of equipment — sizes the equipment.

Because east glass peaks in the morning and west glass in the late afternoon, the block load is always less than the sum of the room peaks. Manual N therefore requires the designer to evaluate the load at more than one hour: an office with heavy west glazing frequently peaks at 4:00 or 5:00 p.m., when the solar gain is high and the occupancy and lighting are still fully on, not at solar noon.

$\text{SHR} = \frac{q_{\text{sensible}}}{q_{\text{sensible}} + q_{\text{latent}}}$

Commercial SHRs run lower than residential — a restaurant or classroom can fall to 0.65 – 0.75 — which forces selection of equipment with the latent capability to match, not simply the nominal tonnage.

Putting the Retail Suite Together

$q_{\text{total}} = 75{,}539 + 27{,}375 = 102{,}914\text{ BTU/h} = 8.58\text{ tons}$ $\text{SHR} = \frac{75{,}539}{102{,}914} = 0.734$

At 8.58 tons this suite stays inside the Heating Group 3 fifteen-ton ceiling — but if the same tenant space is one of four interconnected suites on a common system, 21 NCAC 50 .0501(a) aggregates them and the project becomes Heating Group 2 work.