8.4 Manual CS: Commercial Applications, Systems & Equipment

Manual CS: Commercial Applications, Systems & Equipment

The division of labour among ACCA manuals: Manual N tells you how big the load is. Manual Q tells you how to distribute the air. Manual CS tells you what kind of system to install in the first place, and it is on the reference list for both Heating Group 1 Class I and Heating Group 2.

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1. The Zoning Question Comes First

A thermostat controls one zone. A single-zone constant-volume unit therefore satisfies exactly one set of conditions, and every square foot it serves inherits that zone's schedule, orientation and internal gain.

Spaces belong in separate zones when any of the following differ materially:

• Exposure — a west-facing conference room and an interior file room peak six hours apart.
• Internal gain — a server closet or a display-lit sales floor runs a cooling load in January.
• Occupancy schedule — a sanctuary used twice a week versus offices used daily.
• Ceiling height and stratification — a high-bay area behaves differently from an adjacent 9-foot office.
• Required conditions — a walk-in vestibule, a computer room, or a space with humidity limits.

Perimeter vs. Core

Core zones have no exterior exposure and are cooling-only year round; their load is people, lights and equipment. Perimeter zones swing with the weather and need heating in winter and cooling in summer, sometimes on the same day. Any commercial system that ignores this split delivers a building where the interior is warm and the north perimeter is cold.

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2. System Families

Terminal Reheat and Why the Energy Code Restricts It

A constant-volume reheat system cools all supply air to satisfy the coldest zone, then adds heat back at each terminal. Comfort is excellent and control is trivial, which is why it survives in laboratories and process spaces. Energetically it is the worst common choice, so the energy code responds with supply-air-temperature reset, VAV turndown to a minimum primary airflow before reheat is allowed, and limits on simultaneous heating and cooling.

The Water-Source Heat Pump Loop

Each zone has a small water-to-air reversible heat pump. All units share one condenser-water loop maintained roughly between 60°F and 90°F:

• A cooling tower or fluid cooler rejects heat when the loop rises toward the upper limit.
• A boiler adds heat when the loop falls toward the lower limit.
• In the shoulder seasons, units in cooling mode reject heat into the loop and units in heating mode absorb it — the building moves its own heat from core to perimeter, and neither tower nor boiler runs.

The loop is the entire design problem: pump head, loop volume, freeze protection on the tower side, and the water treatment that keeps the small coaxial condensers clean.

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3. Equipment Selection After the Load

Selection is a psychrometric problem, not a nameplate problem.

1. Match total capacity at design conditions, not at the AHRI rating point. Capacity falls as outdoor temperature rises and as entering wet-bulb changes.
2. Match the sensible heat ratio. A restaurant at SHR 0.70 needs the latent capacity to match; a data closet at SHR 0.98 needs almost none, and a unit selected for the restaurant will short-cycle in the closet.
3. Check the supply air temperature difference. Commercial cooling generally runs a 18°F to 22°F supply-to-room temperature difference; the resulting airflow is $\text{CFM} = q_{\text{sensible}} / (1.08 \times \Delta T)$.
4. Verify the available external static pressure against the Manual Q calculation, including system effect.
5. Confirm the ventilation share. The outdoor air quantity computed in Manual N must actually reach the coil at part load, which is a control-sequence question in VAV systems.

Worked Example — Airflow and Reheat Penalty

A 5-zone office has a block sensible load of 96,000 BTU/h at a 20°F supply temperature difference:

$\text{CFM} = \frac{96{,}000}{1.08 \times 20} = 4{,}444\text{ CFM}$

If the coldest zone requires 55°F supply while an interior zone needs only enough cooling to hold 75°F, a constant-volume reheat design reheats that interior zone's full share of air. Reheating 900 CFM from 55°F back to 68°F costs

$q_{\text{reheat}} = 1.08 \times 900 \times (68 - 55) = 12{,}636\text{ BTU/h}$

of heating, paid for on top of the cooling already spent to reach 55°F. VAV throttles that zone to its minimum primary airflow instead, so only the minimum flow is reheated.

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4. Building Pressurization, Exhaust and Relief

Every commercial system is also an air-balance problem:

$\text{Outdoor Air In} = \text{Exhaust Out} + \text{Relief} \pm \text{Intended Pressurization}$

• Slightly positive in offices, retail and lobbies — keeps humid outdoor air, dust and insects from being pulled through the envelope, and keeps entrance doors manageable.
• Negative in restrooms, janitor closets, locker rooms and commercial kitchens — so odour and grease-laden vapour are contained.
• Relief or return fans must remove the outdoor air the economizer introduces; an economizer that opens to 100% outdoor air with no relief path will pressurize the building until the doors will not close and the unit's fan rides up its curve.

A hood exhausting 3,000 CFM with a makeup air unit supplying only 1,500 CFM pulls the balance through the dining room, the door undercuts and, in the worst case, backwards down an atmospherically vented flue — a combustion-safety failure, not merely a comfort complaint.

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5. Applying Manual CS Judgement

License check: as soon as the selected equipment serving interconnected space exceeds 15 tons in aggregate, the work is Heating Group 2 under 21 NCAC 50 .0501(a) — the Manual CS decision changes who may legally sign the contract.