HRAI Residential Air System Design (HRAI RAS) Overview
These study notes are designed to prepare candidates for the HRAI Residential Air System Design (RAS) exam. The notes cover the core subjects of residential heat loss/gain, air distribution, duct sizing, equipment selection, ventilation, and commissioning, based on official sources including HRAI, ACCA, ASHRAE, and the IMC/IECC. Candidates should verify specific exam details (e.g., pass mark, format) with HRAI.
For Technical Conquer practice planning, this module is tracked as 80 questions over about 120 minutes with a listed pass mark of 70%. Treat those numbers as practice baselines and verify the current official format before scheduling.
How This Guide Is Organized
The sections below turn the syllabus into studyable subject blocks. Read a subject first, explain the must-know ideas without notes, then use questions, flashcards, and mind maps to test whether the knowledge holds under field-style pressure.
- Residential Heat Loss and Heat Gain (CSA F280-12)
- Air Distribution System Fundamentals
- Duct Sizing and Friction Rate Calculations
- Equipment Selection and Performance Matching
- Ventilation System Design and IAQ
- System Commissioning and Air Balancing
Exam Snapshot and Readiness Target
Format: 80 questions, 120 minutes (practice baseline; verify with HRAI)
Candidate level: Entry-level to technician-level for residential HVAC design
Readiness target: 70% pass mark (practice baseline; verify with HRAI)
Most candidates should budget at least 36+ focused study hours, then adjust upward for unfamiliar equipment, code, regulatory, commissioning, controls, or calculation-heavy content.
Residential Heat Loss and Heat Gain (CSA F280-12)
Syllabus Focus
- CSA F280-12 methodology
- Heating and cooling load calculations
- Design conditions and indoor/outdoor temperatures
- Building envelope characteristics
Key Notes
- CSA F280-12 is the Canadian standard for residential heat loss and heat gain calculations, used to size heating and cooling equipment.
- Heat loss calculations consider conduction through walls, windows, doors, ceilings, floors, and infiltration; heat gain adds solar radiation and internal loads.
- Design outdoor temperature is based on the 2.5% winter design temperature and 1% summer design temperature from local climate data.
- Indoor design temperatures are typically 22°C for heating and 24°C for cooling, with 50% relative humidity.
- Infiltration rates are estimated using the air change method (ACH) or effective leakage area, per CSA F280.
- The calculation must account for all conditioned spaces, including basements and attics if conditioned.
- Equipment sizing must not exceed the calculated load by more than 15% (per HRAI guidelines).
Must Know
- How to perform a room-by-room heat loss/gain calculation using CSA F280-12.
- The difference between sensible and latent heat gains.
- How to select design temperatures from climate data.
- The impact of insulation, windows, and air sealing on load calculations.
Field and Exam Application
- Field: Measure actual room dimensions, window U-values, and insulation levels to input into load calculation software.
- Field: Verify that the calculated load matches the equipment capacity within the allowable oversizing margin.
- Field: Identify thermal bypasses (e.g., uninsulated ducts in attics) that increase load.
High-Yield Distinctions
- Heat loss is dominated by conduction and infiltration; heat gain adds solar and internal loads.
- CSA F280-12 uses a different methodology than ACCA Manual J; Canadian exams follow CSA F280.
- Latent heat gain is significant in humid climates; equipment must handle both sensible and latent loads.
- Basement heat loss is calculated differently (below-grade vs. above-grade walls).
Common Pitfalls
- Using incorrect design temperatures (e.g., using average instead of 2.5%/1% values).
- Forgetting to include infiltration in heat loss calculations.
- Oversizing equipment beyond the 15% allowance, leading to short cycling and poor humidity control.
- Ignoring internal heat gains (people, appliances, lighting) in cooling load calculations.
Review Tasks
- Practice a full heat loss calculation for a typical two-story house using CSA F280-12.
- Compare your calculated load to a manufacturer's equipment capacity chart.
- Identify the impact of upgrading windows from double-pane to triple-pane on heat loss.
- Calculate the latent load for a 2000 sq ft home with 4 occupants.
Air Distribution System Fundamentals
Syllabus Focus
- Principles of air distribution
- Supply and return air systems
- Ductwork materials and construction
- Airflow measurement and balancing
Key Notes
- Air distribution systems deliver conditioned air to rooms and return air to the equipment; proper design ensures comfort and efficiency.
- Supply air should be directed to exterior walls and windows to counteract heat loss; returns should be located to promote good air circulation.
- Duct materials include sheet metal, fiberglass duct board, and flexible duct; each has different friction and installation requirements.
- Duct sizing is based on friction rate (typically 0.1 in.w.c./100 ft for residential) and airflow (CFM).
- The total external static pressure (TESP) of the system must match the fan's capability; excessive static pressure reduces airflow.
- Return air pathways must be adequate to prevent negative pressure and ensure proper ventilation.
- Duct sealing is critical to prevent leakage; duct tape is not acceptable; mastic or metal tape is required.
Must Know
- How to calculate friction rate and use duct sizing charts (e.g., ACCA Manual D or HRAI duct sizing).
- The relationship between static pressure, airflow, and fan performance.
- The importance of return air sizing and location.
- How to measure airflow using a flow hood, anemometer, or pressure drop method.
Field and Exam Application
- Field: Measure TESP across the fan to verify it is within the manufacturer's range.
- Field: Use a duct leakage tester to quantify leakage and seal leaks with mastic.
- Field: Adjust balancing dampers to achieve design CFM to each room.
High-Yield Distinctions
- Supply ducts are typically smaller than return ducts due to lower velocity requirements for returns.
- Flexible duct has higher friction than sheet metal; must be installed straight and supported.
- Duct board provides insulation and sound attenuation but has higher friction than metal.
- The longest duct run (critical path) determines the fan's required static pressure.
Common Pitfalls
- Undersizing return ducts, causing high static pressure and reduced airflow.
- Using flexible duct with sharp bends or kinks, increasing friction.
- Failing to seal ducts, leading to energy loss and unbalanced airflow.
- Placing supply registers in dead zones (e.g., behind furniture) that block airflow.
Review Tasks
- Sketch a simple duct layout for a single-story home, labeling supply and return runs.
- Calculate the friction rate for a 100 ft duct run with 0.5 in.w.c. available static pressure.
- Measure the airflow from a supply register using an anemometer and compare to design CFM.
- Identify three common duct installation errors from a photo.
Duct Sizing and Friction Rate Calculations
Syllabus Focus
- Friction rate determination
- Duct sizing methods (equal friction, velocity reduction)
- Duct design tools and charts
- System effect factors
Key Notes
- Friction rate is the pressure loss per unit length of duct, typically expressed in in.w.c./100 ft.
- The equal friction method sizes ducts so that each run has the same friction rate; the velocity reduction method reduces velocity in branch runs.
- Duct sizing charts (e.g., ACCA Manual D) relate CFM, friction rate, and duct diameter for round ducts or equivalent diameter for rectangular ducts.
- System effect factors (e.g., elbows, transitions, dampers) add pressure loss that must be accounted for in the total static pressure.
- The total pressure loss of the duct system must be less than the fan's available static pressure at the design CFM.
- Duct sizing software can automate calculations but requires accurate input of duct lengths and fittings.
- For residential systems, a friction rate of 0.1 in.w.c./100 ft is a common starting point.
Must Know
- How to use a duct sizing chart to select duct diameter for a given CFM and friction rate.
- How to calculate equivalent length for fittings (elbows, tees, transitions).
- How to determine the total static pressure loss of a duct system.
- How to adjust duct sizes when the available static pressure is limited.
Field and Exam Application
- Field: Measure duct dimensions and calculate equivalent diameter for rectangular ducts.
- Field: Use a manometer to measure static pressure at various points to verify design assumptions.
- Field: Replace an undersized duct run with a larger diameter to reduce pressure loss.
High-Yield Distinctions
- Round ducts are more efficient than rectangular ducts for the same cross-sectional area (less friction).
- Flexible duct requires a higher friction rate (0.08-0.1 in.w.c./100 ft) than sheet metal (0.05-0.08).
- The critical path is the longest run with the highest pressure loss; it determines the fan's required static pressure.
- Duct sizing must consider both supply and return sides; return ducts are often larger to keep velocity low.
Common Pitfalls
- Using a friction rate that is too low, resulting in oversized ducts that are expensive and hard to fit.
- Ignoring the pressure loss of fittings, leading to underestimation of total static pressure.
- Sizing ducts based on velocity alone without considering friction rate.
- Not accounting for the pressure drop of filters, coils, and grilles in the total static pressure.
Review Tasks
- Size a supply duct for 200 CFM with a friction rate of 0.1 in.w.c./100 ft using a chart.
- Calculate the equivalent length of a 90-degree elbow (R/D=1.5) for a 10-inch round duct.
- Determine the total static pressure loss for a system with 80 ft of duct, 4 elbows, and a filter with 0.2 in.w.c. drop.
- Compare the duct diameter needed for 300 CFM at 0.08 vs. 0.12 in.w.c./100 ft.
Equipment Selection and Performance Matching
Syllabus Focus
- Equipment types (furnaces, A/C, heat pumps)
- Performance ratings (AFUE, SEER, HSPF)
- Matching equipment to load calculations
- Blower performance and static pressure
Key Notes
- Equipment must be selected to meet the calculated heating and cooling loads without excessive oversizing (max 15% oversizing per HRAI).
- Furnace efficiency is rated by AFUE (Annual Fuel Utilization Efficiency); minimum 90% for high-efficiency condensing furnaces.
- Air conditioner efficiency is rated by SEER (Seasonal Energy Efficiency Ratio); higher SEER means better efficiency.
- Heat pump efficiency is rated by HSPF (Heating Seasonal Performance Factor) for heating and SEER for cooling.
- The blower performance curve shows CFM vs. static pressure; the system's TESP must fall within the fan's operating range.
- Equipment must be matched with the correct coil and metering device (e.g., TXV) for optimal performance.
- Duct design must accommodate the equipment's airflow requirements (typically 350-450 CFM per ton for cooling).
Must Know
- How to read a blower performance table to determine CFM at a given static pressure.
- How to select a furnace or heat pump that matches the heat loss calculation.
- How to calculate the required CFM for cooling based on sensible heat gain.
- The impact of oversizing on efficiency, comfort, and equipment lifespan.
Field and Exam Application
- Field: Measure TESP and compare to the blower table to verify airflow.
- Field: Check that the evaporator coil is matched to the condenser (same manufacturer and SEER rating).
- Field: Adjust fan speed (if multi-speed) to achieve design CFM.
High-Yield Distinctions
- Condensing furnaces (90%+ AFUE) require stainless steel heat exchangers and PVC venting.
- Heat pumps provide both heating and cooling; backup heat may be needed in cold climates.
- Two-stage and variable-speed equipment improves comfort and efficiency but requires compatible controls.
- Equipment capacity is affected by altitude; derating may be required above 2000 ft.
Common Pitfalls
- Selecting equipment based on square footage alone without a load calculation.
- Oversizing equipment, leading to short cycling, poor humidity control, and higher energy bills.
- Ignoring the blower performance curve; a high-static system may deliver less CFM than needed.
- Mismatching indoor and outdoor units (e.g., 3-ton condenser with 2.5-ton coil) reduces efficiency and capacity.
Review Tasks
- Select a furnace for a home with a heat loss of 60,000 BTU/h; choose a model with AFUE ≥ 90%.
- Calculate the required CFM for a 3-ton A/C (sensible heat gain 36,000 BTU/h, ΔT=20°F).
- Plot a blower performance curve and determine CFM at 0.5 in.w.c. TESP.
- Explain why a 4-ton unit is not appropriate for a 3-ton load.
Ventilation System Design and IAQ
Syllabus Focus
- Ventilation requirements (CSA F326, ASHRAE 62.2)
- Mechanical ventilation systems (HRV/ERV)
- Indoor air quality (IAQ) considerations
- Exhaust and makeup air
Key Notes
- CSA F326 is the Canadian standard for residential mechanical ventilation; ASHRAE 62.2 is the US standard.
- Ventilation rates are based on floor area and number of bedrooms; e.g., ASHRAE 62.2 requires 7.5 CFM per person + 3 CFM per 100 sq ft.
- Heat recovery ventilators (HRV) and energy recovery ventilators (ERV) provide fresh air while recovering energy from exhaust air.
- HRVs transfer sensible heat; ERVs transfer both sensible and latent heat (moisture).
- Ventilation systems must be balanced: supply and exhaust flows should be within 10% of each other.
- IAQ issues include CO2, VOCs, radon, and moisture; ventilation helps dilute pollutants.
- Exhaust fans in bathrooms and kitchens must be vented to the outside, not into attics.
Must Know
- How to calculate the required ventilation rate for a given home size and occupancy.
- How to select and size an HRV/ERV based on ventilation needs and climate.
- The difference between HRV and ERV and when to use each.
- How to balance an HRV system using flow hoods or manometers.
Field and Exam Application
- Field: Measure outdoor airflow from an HRV using a flow hood and compare to design.
- Field: Check that bathroom exhaust fans are vented to the exterior and have backdraft dampers.
- Field: Test for radon and recommend mitigation if levels exceed 200 Bq/m3 (Canadian guideline).
High-Yield Distinctions
- HRVs are preferred in cold climates to avoid moisture issues; ERVs are better in humid climates.
- Ventilation systems must be interlocked with the HVAC system to ensure distribution.
- Makeup air is required for large exhaust systems (e.g., range hoods > 400 CFM) to prevent negative pressure.
- Passive ventilation (e.g., trickle vents) is not sufficient; mechanical ventilation is required by code.
Common Pitfalls
- Installing an HRV without balancing, leading to poor performance or frost buildup.
- Venting bathroom fans into attics, causing moisture damage.
- Oversizing ventilation, increasing energy costs and reducing humidity in winter.
- Forgetting to provide makeup air for high-capacity exhaust fans.
Review Tasks
- Calculate the ventilation rate for a 2500 sq ft home with 4 bedrooms using ASHRAE 62.2.
- Select an HRV for a home in Winnipeg (cold climate) with a ventilation requirement of 150 CFM.
- Balance an HRV by adjusting dampers to achieve equal supply and exhaust flows.
- Identify three IAQ pollutants and their sources in a typical home.
System Commissioning and Air Balancing
Syllabus Focus
- Commissioning process and documentation
- Air balancing procedures
- Measurement instruments and techniques
- Troubleshooting common issues
Key Notes
- Commissioning verifies that the HVAC system is installed, calibrated, and performs according to design specifications.
- Air balancing adjusts dampers and fan speeds to achieve design airflow to each room.
- Key measurements include static pressure, airflow (CFM), temperature rise, and refrigerant pressures.
- Instruments: manometer, flow hood, anemometer, thermometer, psychrometer, combustion analyzer.
- The total airflow should be within 10% of design; individual room flows within 20%.
- Documentation includes design reports, test results, and balancing reports.
- Common issues: low airflow due to dirty filters, undersized ducts, or fan speed too low.
Must Know
- How to perform a static pressure test and interpret results.
- How to use a flow hood to measure supply and return airflow.
- How to adjust balancing dampers to achieve target CFM.
- How to verify equipment performance (temperature rise, superheat, subcooling).
Field and Exam Application
- Field: Commission a new furnace installation by measuring temperature rise and comparing to nameplate.
- Field: Balance a duct system by measuring CFM at each register and adjusting dampers.
- Field: Troubleshoot a complaint of uneven temperatures by measuring airflow and static pressure.
High-Yield Distinctions
- Temperature rise across a furnace should be within the range specified on the nameplate (typically 40-70°F).
- Superheat and subcooling are used to verify refrigerant charge in A/C and heat pumps.
- A duct leakage test (e.g., duct blaster) is part of commissioning for energy efficiency.
- Commissioning should be done with all filters clean and all registers open.
Common Pitfalls
- Skipping the balancing step, leading to comfort complaints.
- Using a flow hood incorrectly (e.g., not sealing around the register).
- Adjusting dampers without measuring airflow, causing imbalance elsewhere.
- Failing to document test results, making it hard to verify performance later.
Review Tasks
- Perform a static pressure test on a residential system and identify if it is within limits.
- Balance a simple duct system with three branches to achieve 100 CFM each.
- Measure temperature rise on a gas furnace and compare to nameplate.
- Create a commissioning checklist for a new residential HVAC system.
How To Use These Notes With Practice Questions
Do not jump straight from reading to a full mock. Work by subject first: review the key notes, make a short recall sheet from memory, then answer a focused question set. After each miss, decide whether the problem was missing theory, weak code/source recall, poor measurement setup, calculation error, or a field sequence you did not visualize.
Technical Conquer's question bank, flashcards, mind maps, and spaced review tools are most useful after this instruction layer because they reveal which parts of the notes are not yet retrievable.
Final Review Checklist
- Review CSA F280-12 heat loss/gain calculation steps and practice with sample homes.
- Memorize duct sizing friction rate formula and practice using duct sizing charts.
- Understand equipment performance ratings (AFUE, SEER, HSPF) and how to match to loads.
- Know ventilation requirements per CSA F326 or ASHRAE 62.2 and HRV/ERV selection.
- Practice air balancing procedures and static pressure measurements.
- Review common commissioning steps and documentation requirements.
- Verify all exam details (format, pass mark, eligibility) with HRAI directly.
Official Sources and Further Reading
Use these sources as the final authority for format, eligibility, rules, regulatory limits, and exam updates. Study notes are a preparation layer, not a replacement for official candidate guidance.