Study HRAI SCA material by practicing design decisions, not by rereading notes: scope the plan, run heating and cooling calculations separately, size ducts from a computed static-pressure budget, treat ventilation air as a design input, and document every assumption. HRAI, the Canadian HVACR trade association that issues this credential, publishes administrative details at hrai.ca; this guide teaches the underlying design reasoning.
Scope the floor plan before calculating: where small commercial diverges from house habits
Start every design by marking zones, orientations, occupancy, and equipment on the plan. Small commercial units mix house-scale equipment with commercial internal gains and ventilation, so scoping errors propagate into every later number.
Trace a strip-mall unit as an example: a glass storefront facing south, a sales floor with high occupant density, a washroom, and a stock room with little glazing. The perimeter responds strongly to weather and glazing, while the interior core responds to people, lighting, and plug loads. Recognizing which rooms are perimeter-driven and which are gain-driven tells you where heating and where cooling will govern the design.
Build a scoping habit you can repeat under time pressure: annotate the plan with orientation, ceiling heights, door locations, and any confirmed equipment nameplates, then write a short assumptions list for everything unknown. When a later calculation looks wrong, you can trace it back to a named assumption instead of redoing the whole design. This annotation also becomes the first page of your design documentation.
- Mark each room as perimeter-driven or gain-driven before any arithmetic
- Record orientation, glazing areas, and ceiling heights directly on the plan
- Write an assumptions list for every value you estimated, so it can be revised in one place
Separate heating design from cooling design: two calculations, two different answers
Heating and cooling design answer different questions under different outdoor conditions and different dominant loads. Never size equipment or airflow from one calculation alone; check that the selection satisfies both modes at the same supply conditions.
Worked scenario 1: a designer sizes cooling for a 140 m² retail unit using a floor-area rule of thumb and picks a nominal unit without checking loads. The unit ignores the stock room's refrigeration equipment, twenty occupants at peak, and ventilation outdoor air. A room-by-room cooling calculation that lists occupants, lighting, plug loads, and ventilation per zone points to a different capacity and, more importantly, a different sensible heat ratio. The mistake matters because nominal capacity tells you nothing about whether the delivered sensible cooling matches the load.
Compare the two calculations deliberately rather than sequentially. Heating design is driven by envelope loss through walls, glazing, roof, and infiltration at the winter design condition, and it sizes heat delivery and often supply air temperature. Cooling design is driven by solar gain, occupants, equipment, and ventilation at the summer design condition, and it sizes sensible cooling capacity and airflow. The table below makes the contrast explicit; practice stating, for any given room, which column dominates and why.
| Decision point | Heating design | Cooling design |
|---|---|---|
| Condition examined | Winter design outdoor temperature | Summer design outdoor condition |
| Dominant inputs | Envelope loss, infiltration, ventilation heat | Solar gain, occupants, equipment, ventilation cooling |
| What it sizes | Heating capacity, heat delivery per room | Sensible cooling capacity, coil and airflow choice |
| Shortcut trap | Carrying a house-scale rule into commercial glazing and ventilation | Using nominal capacity instead of load-matched selection |
Supply air temperature and sensible heat ratio: choosing airflow that works in both modes
The sensible heat ratio of the cooling load sets the supply air condition the equipment must deliver. Choose airflow from the equipment's blower table at actual conditions instead of assuming a fixed airflow-per-capacity habit.
Work the link in one direction first: given a load with a high sensible fraction, a cooler supply air temperature at higher airflow can meet it; a load with a large latent fraction needs the coil to remove more moisture, which changes the sensible cooling delivered at a given airflow. Practicing this reasoning on paper scenarios builds the habit of asking what the supply air must look like before asking what equipment to pick.
Then work the practical constraint: a single constant-airflow system must live with one supply condition through both seasons. Check the heating mode at the same airflow you selected for cooling, and confirm the supply temperature stays within the equipment's acceptable range. If heating needs warmer air than the cooling selection provides, that conflict is a design decision to resolve by zoning, reheat reasoning on paper, or a different equipment arrangement, not something to ignore in the notes.
Derive the friction rate from a static-pressure budget, not a copied chart value
Compute available static pressure from the equipment's rated external static minus the losses of filters, coils, dampers, and terminals, then derive the friction rate for the design duct run from that budget.
Worked scenario 2: a designer sizes the whole system from a single friction-rate value taken from a chart default. The longest run serves a far meeting room through several elbows and a long trunk, and its actual pressure drop exceeds the budget the blower can support at the required airflow. The room ends up starved. The better decision is to identify the design run, the longest or most restricted path, convert fittings to equivalent lengths, subtract known component losses from the available external static, and compute the friction rate that budget actually allows, then resize the trunk or far branch.
Make the why explicit in your notes: friction rate is an output of the pressure budget, not an input you memorize. Two systems with identical airflows can justify different friction rates because their equipment, filtration, and fittings differ. Practicing this derivation on paper scenarios, with the arithmetic labeled at each step, trains you to notice when a default value is silently overcommitting the available static pressure.
- Identify the design run: the longest or most restricted supply path
- Sum component losses (filter, coil, dampers, terminals) before assigning duct losses
- Convert fittings to equivalent length, then compute the friction rate the budget supports
Ventilation air is a design input: outdoor air changes both load and delivery
Outdoor air for ventilation adds sensible and latent load and frequently shifts the cooling requirement more than weather does. Account for it at zone and system level, and document how it will be delivered and mixed.
In a densely occupied small commercial space, the ventilation load can be comparable to the entire envelope cooling load. Practice separating the calculation: first the zone loads from people and envelope, then the outdoor air introduced at the system, then how that outdoor air appears in the mixed-air condition at the equipment. Skipping the second and third steps makes the selected capacity and coil condition wrong even when the room-by-room arithmetic is flawless.
Delivery is the second half of the decision. On paper, compare bringing ventilation air directly to the unit versus ducting it to specific zones: the two arrangements produce different mixed-air temperatures, different zone balance, and different documentation obligations. A plausible mistake is designing a sound load calculation and then leaving the ventilation path undrawn, so the installed system cannot deliver the outdoor air the calculation assumed. Your design sheet should show the path, the quantity, and where it joins the airstream.
Selecting equipment against published data: blower tables and the paper trail
Verify candidate equipment against the manufacturer's blower tables and published performance data at the actual airflow and static pressure, not against nominal capacity. Record model, conditions, and accessories so the design can be reviewed.
Nominal capacity labels describe a rating condition, not your building. Practice reading blower tables the way you will use them: find the row for your computed external static pressure and airflow, confirm the blower can deliver the design airflow there, and note how added accessories move the operating point. Two candidate units with the same nominal rating can behave differently at your static, which is exactly the comparison the selection step exists to make.
Documentation is the professional layer over selection. A one-page design summary should state the design conditions, the calculated loads per zone, the selected airflow, the friction rate and its derivation, the ventilation quantity and path, and the equipment model with the conditions used for selection. This is what lets another designer, an inspector, or your future self verify the reasoning. Practicing this summary for every paper scenario is direct preparation, because it forces each number to have a traceable origin.
A two-zone practice build: exercise, self-check rubric, and a six-week sequence
Practice on a simple two-zone plan: one perimeter office zone and one interior zone. Calculate both loads, choose airflow and supply condition, size one trunk, and grade your own work against the rubric before moving on.
The exercise: sketch or obtain a plan for a small unit with a south-facing office zone and an interior zone holding several occupants and plug equipment. Run a room-by-room heating and cooling calculation, include a stated ventilation quantity with a drawn delivery path, derive a friction rate from a stated static-pressure budget, and size the trunk serving both zones. Expected observations: the interior zone's cooling load is mostly gain-driven and insensitive to orientation changes, the perimeter zone's heating load moves sharply when you change its glazing assumption, and your derived friction rate differs from any chart default once component losses are counted.
A realistic sequence: weeks one and two, room-by-room heating and cooling calculations on three different small plans; weeks three and four, psychrometric reasoning and airflow selection against blower tables; week five, duct sizing from static-pressure budgets plus ventilation delivery layouts; week six, two full timed practice builds with the design summary. Readiness checks you can score honestly: you can complete a two-zone load calculation with all assumptions listed in one sitting; you can state why your friction rate is what it is; you can show that one airflow satisfies both heating and cooling modes; and your summary page lets a peer reproduce your capacity choice. Treat these as learning milestones, not predictions of any exam result.
- Rubric: heating and cooling calculated separately, per room, with design conditions stated
- Rubric: internal gains and ventilation listed per zone with sources named
- Rubric: friction rate derived from a documented static-pressure budget
- Rubric: airflow checked in both modes; ventilation path drawn; assumptions revisable in one list
References and further reading
Use these references to explore the concepts and check the latest information from the relevant organizations.