Air Handling and Ventilation Systems

Chapter Introduction

This chapter covers the fundamental principles, components, calculations, and regulatory requirements related to air handling and ventilation systems. As a sheet metal worker, you will be called upon to fabricate, install, balance, and troubleshoot these systems. The Red Seal exam assesses your ability to apply this knowledge in practical contexts. Mastering this chapter is essential, as ventilation questions represent a significant portion of the exam.

Fundamental Principles of Airflow

Static Pressure, Velocity Pressure, and Total Pressure

Understanding pressures is the foundation of all air handling work. Three types of pressure coexist in a duct:

Static pressure (Ps): The pressure exerted perpendicular to the duct walls, in all directions. It is measured in pascals (Pa) or inches of water column (in H₂O). Static pressure can be positive (discharge) or negative (suction).

Velocity pressure (Pv): The pressure associated with the velocity of air. It is always positive and can only be measured when air is in motion. Formula: Pv = 0.602 × V² (with V in m/s, result in Pa).

Total pressure (Pt): The algebraic sum of static pressure and velocity pressure: Pt = Ps + Pv.

Relationship with velocity: The velocity of air (V) in a duct is calculated from velocity pressure: V = 1.29 × √Pv (V in m/s, Pv in Pa). This relationship is fundamental for measurements during balancing.

Continuity Law and Airflow Rate

Airflow rate (Q) is the product of velocity (V) and the cross-sectional area (A) of the duct: Q = V × A. Common units are m³/s, L/s, or ft³/min (CFM). The continuity law states that in a closed system without leaks, the flow rate is constant: Q₁ = Q₂ = Q₃.

Parameter	Formula	Common Units
Flow rate	Q = V × A	m³/s, L/s, CFM
Velocity	V = Q / A	m/s, ft/min
Area (round duct)	A = π × D² / 4	m², ft²
Area (rectangular duct)	A = W × H	m², ft²

Calculation example: A circular duct 400 mm in diameter carries air at 8 m/s. The flow rate is: A = π × 0.4² / 4 = 0.1257 m². Q = 8 × 0.1257 = 1.005 m³/s = 1005 L/s.

Pressure Losses in Ducts

Air flowing through a duct experiences pressure losses (friction) that manifest as a drop in static pressure. These losses are classified into two categories:

17.Friction losses: due to air contact with the duct walls. They depend on the roughness of the material, duct length, air velocity, and cross-section shape.

18.Dynamic losses: due to changes in direction (elbows), changes in cross-section (reductions, enlargements), and accessories (dampers, grilles, etc.). They are expressed as a loss coefficient (C) multiplied by the velocity pressure: ΔP = C × Pv.

Rule of thumb: Friction loss in a straight duct is generally estimated between 0.8 and 1.2 Pa/m for low-pressure systems. High-pressure systems can tolerate higher losses (up to 3 Pa/m).

Components of Air Handling Systems

Fans

The fan is the heart of the system. It provides the energy needed to move air and overcome pressure losses. There are two main families:

Type	Characteristics	Typical Applications
Centrifugal fan (scroll housing)	Forward-curved, backward-curved, or radial blades	Ducted systems, high pressure
Axial fan	Propellers within a cylinder	Low pressure, high volume (rooftop, wall-mounted)

Fan laws: These laws allow you to predict fan behavior when the rotational speed changes:

Flow rate varies proportionally to speed: Q₂ = Q₁ × (N₂/N₁)

Pressure varies as the square of speed: P₂ = P₁ × (N₂/N₁)²

Power varies as the cube of speed: W₂ = W₁ × (N₂/N₁)³

Example: A fan runs at 800 rpm, flow 5000 L/s, pressure 500 Pa, power 4 kW. If the speed increases to 1000 rpm: Q₂ = 5000 × (1000/800) = 6250 L/s. P₂ = 500 × (1000/800)² = 781 Pa. W₂ = 4 × (1000/800)³ = 7.8 kW.

Dampers and Vanes

Dampers control airflow. They include:

Volume dampers (balancing): installed in branch ducts to balance the system. They are equipped with a position indicator and sometimes a locking device.

Fire dampers: installed at fire-rated wall and floor penetrations. They close automatically in case of fire (fusible link at 72 °C or 165 °F typically). They must be accessible for inspection and testing.

Smoke dampers: activated by smoke detection, often combined with fire dampers in fire control systems.

Air Filters

Filters protect equipment and improve indoor air quality. They are classified according to ASHRAE 52.2 (MERV) or EN 779 (G, M, F) standards. Filters are characterized by:

Efficiency: the ability to capture particles of a given size.

Initial and final pressure drop: pressure drop increases with loading. Replacement is typically done at a final pressure drop of 250 Pa (2.5 in H₂O) for pleated filters.

Dust holding capacity: the amount of dust retained before replacement.

Trap to avoid: Never confuse initial efficiency with average efficiency. A clean filter's efficiency is often lower than its average in-service efficiency.

Heat Exchangers and Recovery Units

Modern ventilation systems often incorporate heat recovery. The main types are:

Plate heat exchanger (air-to-air): transfers sensible heat between exhaust air and fresh air without mixing the streams.

Rotary heat exchanger (thermal wheel): a wheel rotates between the two air streams, transferring heat and humidity (efficiency of 70 to 85%).

Heat pipes: sealed tubes containing a refrigerant fluid that vaporizes and condenses to transfer heat.

Recovery efficiency: η = (T_supply_out − T_supply_in) / (T_exhaust_in − T_supply_in) × 100%. An efficiency of 70% means that 70% of the energy from the exhaust air is transferred to the fresh air.

Duct Design and Sizing

Sizing Methods

Three main methods are used to size ducts:

50.Constant velocity method: a maximum velocity is set in the main ducts (e.g., 8 m/s) and sections are sized accordingly. Simple but can create imbalances.

51.Constant pressure drop method (constant friction): a pressure drop per meter is set (e.g., 1 Pa/m) and all ducts are sized for this same drop. This is the most common method for low-pressure systems.

52.Static regain method: ducts are sized so that static pressure remains constant throughout the system. Used for high-pressure systems and large networks.

Equivalent Diameter

For rectangular ducts, the concept of equivalent diameter is used to apply the same charts as for round ducts. The Huebscher formula is the most widely used:

De = 1.30 × [(a × b)⁵ / (a + b)²]^(1/8)

Where a and b are the dimensions of the rectangular duct in mm, and De is the equivalent diameter in mm.

Example: A duct of 600 mm × 400 mm: De = 1.30 × [(600 × 400)⁵ / (600 + 400)²]^(1/8) = 1.30 × [(240000)⁵ / 1000000]^(1/8) ≈ 530 mm. A round duct 530 mm in diameter would have the same pressure drop for the same flow rate.

Recommended Velocities

Application	Main Duct	Secondary Duct	Branch Duct
Residential (low pressure)	4 to 6 m/s	3 to 5 m/s	2 to 4 m/s
Commercial (low pressure)	6 to 9 m/s	5 to 8 m/s	4 to 6 m/s
Industrial (high pressure)	10 to 15 m/s	8 to 12 m/s	6 to 10 m/s
Exhaust air (kitchen)	8 to 12 m/s	7 to 10 m/s	5 to 8 m/s

Note: Velocities that are too high generate noise (above 10 m/s in ducts, air noise becomes noticeable). Velocities that are too low (< 2 m/s) can lead to stratification problems and poor distribution.

Duct Fabrication and Installation

Fabrication Standards

Sheet steel ducts are fabricated according to SMACNA (Sheet Metal and Air Conditioning Contractors' National Association) standards and the requirements of the National Building Code of Canada (NBC) . Minimum sheet metal thicknesses depend on duct size and service pressure:

Largest Side Dimension (mm)	Minimum Thickness (mm) — Low Pressure	Minimum Thickness (mm) — High Pressure
≤ 300	0.51 (26 ga)	0.64 (24 ga)
301 to 750	0.64 (24 ga)	0.79 (22 ga)
751 to 1350	0.79 (22 ga)	1.00 (20 ga)
1351 to 2200	1.00 (20 ga)	1.30 (18 ga)
> 2200	1.30 (18 ga)	1.60 (16 ga)

Reinforcements: Large ducts require reinforcements (angles, formed sheet metal) to withstand internal pressure and prevent deformation. Reinforcement spacing is determined by service pressure and sheet metal thickness.

Joints and Sealing

Ducts are assembled using:

Slip joints: for rectangular ducts, with or without a gasket.

Flanged joints: for large ducts or connections to equipment. Flanges can be formed sheet metal or aluminum profiles.

Sleeves: for round ducts, with tightening collars.

Seal classifications (SMACNA):

Class	Maximum Leakage (L/s per m² of surface)	Application
A	0.5	High-pressure systems, clean rooms
B	1.5	Standard commercial systems
C	3.0	Residential systems, low pressure

Important rule: Ducts located outdoors or in unconditioned spaces must be insulated and fitted with a vapour barrier on the warm side (interior side in cold climates). The vapour barrier must be continuous and sealed at all joints to prevent condensation.

Supports and Hangers

Ducts must be supported according to NBC requirements and SMACNA recommendations:

Largest Side Dimension (mm)	Maximum Support Spacing (m)
≤ 750	2.4
751 to 1500	2.0
> 1500	1.5

Supports must be galvanized steel or corrosion-protected. Suspension rods must have a minimum diameter of 6 mm (1/4 in) for small ducts and 10 mm (3/8 in) for large ducts. Supports must never be attached to the duct itself by spot welding on the flat surface — they must be attached to reinforcements or flanges.

Balancing and Commissioning

Balancing Principles

Balancing is the process of adjusting airflow rates in each branch of the system to achieve design flow rates. It is done in several steps:

81.Preliminary check: all dampers open, filters clean, fans running.

82.Flow measurement: at diffusers, return grilles, and in main ducts.

83.Damper adjustment: start with the branches farthest from the fan, then work progressively back toward the fan.

84.Final verification: measure fan static pressure and compare with design values.

Rule of thumb: If the measured flow is more than 10% below the design flow, first check the filters, fan belts, and dampers before modifying the system.

Measuring Instruments

Pitot tube: measures velocity pressure and static pressure. It must be inserted perpendicular to the flow, with the opening facing into the air stream. Measurement is taken at multiple points (traverse method).

Vane anemometer: measures velocity directly. Used for grilles and diffusers.

Manometer: displays pressures in Pa or in H₂O. Digital manometers are the most common.

Balometer (hood): measures flow directly at diffusers and grilles. It must be properly sealed around the grille to prevent leaks.

Traverse method: For a round duct, divide the cross-section into concentric rings of equal area. For a rectangular duct, divide the cross-section into equal rectangles (at least 16 points for ducts larger than 300 mm). The average velocity is the arithmetic mean of the measurements.

Balancing Tolerances

Parameter	Acceptable Tolerance
Total system flow rate	± 5% of design flow
Flow per diffuser	± 10% of design flow
Fan static pressure	± 10% of design value
Velocity in ducts	± 10% of design velocity

Applicable Codes and Standards

National Building Code of Canada (NBC)

The NBC, published by the National Research Council of Canada (NRC), is the primary reference for the design and installation of ventilation systems. Relevant sections include:

Section 6: Heating, Ventilating, and Air-Conditioning. This section covers ventilation requirements, minimum flow rates, fire protection, and duct construction requirements.

Section 3: Fire Protection. Requirements for fire dampers, fire-rated wall penetrations, and fire-resistant ducts.

Section 9: Housing. Requirements for residential ventilation systems.

Key requirement (NBC 6.2.2.1): Ducts must be constructed, installed, and supported so as to withstand the anticipated internal pressures and external loads, and not present a fire hazard.

Canadian Electrical Code, Part I

The Canadian Electrical Code, Part I (C22.1-21) applies to the electrical installations of HVAC systems. Relevant rules include:

Rule 8-200: Calculation of current demands for motor loads. The sizing of conductors and protection must account for the rated current of fan motors.

Rule 28-100: Motor overload protection. Each motor must be protected by an overload protection device.

Rule 28-600: Disconnecting means and controls. A disconnecting means must be installed within sight of each motor.

Important note: The sheet metal worker is not responsible for the electrical installation, but you must know where the disconnecting means are located and how to cut off power in an emergency during maintenance work.

CSA B149.1 — Natural Gas and Propane Code

CSA B149.1 applies to ventilation systems that exhaust combustion products from gas-fired appliances. Relevant clauses include:

Clause 8.10: Venting of combustion products. Venting ducts must be sized according to the code tables and installed with appropriate slopes.

Clause 8.14: Vent connections. Vents must be supported at maximum intervals of 1.2 m (4 ft) and held in place by approved supports.

Clause 8.22: Condensing appliances. Vents for high-efficiency appliances must be made of corrosion-resistant materials (PVC, polypropylene) and must not be connected to masonry chimneys.

Trap to avoid: Never connect a combustion appliance to a mechanical ventilation system without ensuring that the room pressure is maintained at a safe level (maximum negative pressure of 5 Pa in rooms containing combustion appliances).

ASHRAE and SMACNA Standards

ASHRAE 62.1: Ventilation for Acceptable Indoor Air Quality. Defines minimum ventilation rates for commercial buildings.

ASHRAE 62.2: Ventilation and Indoor Air Quality in Low-Rise Residential Buildings. Defines minimum rates for residences.

SMACNA HVAC Duct Construction Standards: Duct construction standards, including sheet metal thicknesses, reinforcements, and joints.

SMACNA HVAC Systems Testing, Adjusting and Balancing: Balancing and commissioning procedures.

Practical Installation Considerations

Connections to Equipment

Connections to equipment (fans, air handling units, etc.) must be made with flexible connections (canvas or neoprene) to:

Isolate equipment vibrations from the ducts.

Allow for slight misalignment.

Facilitate disassembly for maintenance.

The flexible connection must be installed taut, without wrinkles, and with a minimum overlap of 50 mm on each connecting surface. It must be secured with flanges and screws, and sealed to prevent leaks.

Fire-Rated Wall and Floor Penetrations

When a duct passes through a wall or floor with a fire-resistance rating, it must be protected by:

127.A fire damper rated for the same fire-resistance rating as the assembly being penetrated.

128.A sleeve of sheet steel with a minimum thickness of 1.3 mm (18 ga) surrounding the duct.

129.Intumescent caulking or an approved fire-stop material between the duct and the sleeve.

NBC Rule 3.1.8.4: Fire dampers must be installed in accordance with the manufacturer's instructions and be accessible for inspection and maintenance. They must be provided with an inspection window or access door.

Insulation and Vapour Barriers

Duct insulation serves to:

Reduce heat losses or gains.

Prevent condensation on cold surfaces.

Reduce noise transmission.

Types of insulation:

Type	Conductivity (W/m·K)	Maximum Temperature	Application
Fiberglass	0.032 to 0.040	450 °C	Air ducts, high temperature
Mineral wool (rock)	0.035 to 0.042	650 °C	High temperature, fire protection
Polyethylene foam	0.035 to 0.040	100 °C	Cold ducts, condensation
Elastomeric (NBR)	0.032 to 0.038	105 °C	Cold ducts, flexibility

Vapour barrier: In cold climates, the vapour barrier must be placed on the warm side of the insulation (interior side of the building). It must be continuous, sealed at all joints and penetrations, and protected against mechanical perforation.

Diffuser and Grille Connections

Diffusers and grilles are connected to ducts by distribution boxes (plenums) or flexible ducts. Flexible ducts must be:

Installed with a minimum bend radius of 1.5 times the diameter.

Stretched without kinks or pinches.

Supported at maximum intervals of 1.5 m.

Cut to the required length — never left with excess coiled up.

Trap to avoid: A flexible duct that is too long or kinked can reduce flow by 50% or more. The maximum recommended length is 1.5 m for a flexible branch connection, and each bend adds a pressure drop equivalent to 1 m of straight duct.

Advanced Calculations and Applications

Fan Power Calculation

The power absorbed by a fan is calculated by:

P = (Q × ΔP) / (η_fan × η_motor)

Where:

P = power in watts

Q = flow rate in m³/s

ΔP = total fan pressure in Pa

η_fan = fan efficiency (0.50 to 0.75 for centrifugal fans)

η_motor = motor efficiency (0.85 to 0.95)

Example: Q = 2 m³/s, ΔP = 800 Pa, η_fan = 0.60, η_motor = 0.90.

P = (2 × 800) / (0.60 × 0.90) = 1600 / 0.54 = 2963 W ≈ 3 kW.

Duct Cross-Section Calculation

To determine the cross-section of a duct for a given flow rate and maximum velocity:

A = Q / V

Example: Flow rate of 1500 L/s (1.5 m³/s), maximum velocity of 6 m/s.

A = 1.5 / 6 = 0.25 m². For a round duct: D = √(4 × A / π) = √(4 × 0.25 / 3.1416) = √(0.318) = 0.564 m = 564 mm. You would choose a duct of 560 mm or 600 mm diameter.

Unit Conversions

Unit	Equivalent
1 in H₂O	249 Pa
1 Pa	0.004 in H₂O
1 CFM	0.472 L/s
1 L/s	2.119 CFM
1 m/s	196.85 ft/min
1 ft/min	0.00508 m/s

Conversion example: A pressure of 1.5 in H₂O = 1.5 × 249 = 373.5 Pa. A flow rate of 1200 CFM = 1200 × 0.472 = 566 L/s.

Traps to Avoid

167.Confusing static pressure and total pressure: Total pressure is the sum of static and velocity pressure. When measuring fan pressure, you are generally measuring total pressure (the difference between total pressure at the discharge and total pressure at the inlet).

168.Forgetting the fan laws: If the rotational speed changes, pressure changes by the square and power by the cube. A 10% change in speed results in a 21% change in pressure and a 33% change in power.

169.Neglecting the equivalent diameter: Using the arithmetic mean of a rectangular duct's dimensions to estimate the equivalent diameter gives erroneous results. You must use the Huebscher formula.

170.Ignoring fire damper requirements: Every fire-rated wall or floor penetration requires a rated fire damper. Forgetting this requirement is a violation of the NBC and a real danger.

171.Measuring velocity at the centre of the duct: The velocity at the centre of a duct is the maximum velocity, not the average velocity. For turbulent flow, the average velocity is approximately 80% of the centre velocity.

172.Confusing pressure scales: A manometer can display pressures in Pa, in H₂O, or mbar. Always check the displayed unit before recording a measurement.

173.Installing a vapour barrier on the wrong side: In a cold climate, the vapour barrier must be on the warm side (interior) of the insulation. If it is on the cold side, condensation will form within the insulation and destroy it.

174.Forgetting balancing tolerances: Tolerances are ± 5% for total flow and ± 10% for individual flows. A system that exceeds these tolerances must be rebalanced.

175.Using flexible ducts that are too long: Each additional metre of flexible duct adds significant pressure drop. The maximum recommended length is 1.5 m.

176.Neglecting duct protection on site: Ducts must be protected from dust and debris during construction. Open ends must be capped until diffusers and grilles are installed.

Summary

Total pressure is the sum of static pressure and velocity pressure. Velocity pressure is related to air velocity by the relationship Pv = 0.602 × V².

Flow rate is the product of velocity and cross-sectional area: Q = V × A. The continuity law requires a constant flow rate in a leak-free system.

Pressure losses include friction losses (linear) and dynamic losses (fittings). They are measured in Pa or in H₂O.

Fan laws relate flow, pressure, and power to rotational speed: Q ∝ N, P ∝ N², W ∝ N³.

The equivalent diameter of a rectangular duct is calculated using the Huebscher formula: De = 1.30 × [(a × b)⁵ / (a + b)²]^(1/8).

Minimum sheet metal thicknesses and support spacings are defined by SMACNA standards and the NBC.

Balancing is done using the damper method, starting with the branches farthest from the fan. Tolerances are ± 5% for total flow and ± 10% for individual flows.

Applicable codes include the NBC (Section 6), the Canadian Electrical Code, Part I (Rules 8-200, 28-100, 28-600), and CSA B149.1 (Clause 8.10).

Fire dampers are mandatory at fire-rated wall and floor penetrations. They must be accessible and rated for the same fire-resistance rating as the assembly.

Flexible ducts must be installed with a minimum bend radius of 1.5 times the diameter and a maximum length of 1.5 m.

This chapter has presented you with the essential knowledge to succeed on exam questions about air handling and ventilation systems. Review the formulas, data tables, and regulatory requirements. Practice solving problems involving flow calculations, sizing, and application of the fan laws. Good luck with your Red Seal exam preparation!

Air Handling and Ventilation Systems