Heating, Ventilation, and Heat Recovery Systems

Introduction to the Chapter

This chapter covers the fundamental principles, components, calculations, and regulatory requirements related to heating, ventilation, and heat recovery systems, as assessed on the Red Seal exam for the Refrigeration and Air Conditioning Mechanic trade. You must master these concepts to diagnose, install, and maintain integrated systems that combine thermal comfort, indoor air quality, and energy efficiency.

Fundamental Principles of Heating

Thermal Loads and Energy Balances

Thermal load calculation is the foundation of any system sizing. The heating load represents the amount of heat that must be supplied to a space to maintain a design indoor temperature when the outdoor temperature is at its design minimum. The cooling load represents the heat that must be removed to maintain the same indoor temperature when the outdoor temperature is at its design maximum.

The basic formula for heat transmission through a wall is:

Q = U × A × ΔT

Where:

Q = heat flow in watts (W)

U = overall heat transfer coefficient in W/(m²·°C)

A = wall surface area in square metres (m²)

ΔT = temperature difference between indoors and outdoors in degrees Celsius (°C)

The total thermal resistance (R) is the inverse of the U-factor: R = 1/U. R-values of materials add in series. For example, a wall composed of brick (R = 0.35), insulation (R = 2.5), and gypsum board (R = 0.08) has a total resistance of 2.93 m²·°C/W, therefore U = 1/2.93 = 0.341 W/(m²·°C).

Sensible Heat and Latent Heat

Sensible heat causes a temperature change without a phase change. The formula is:

Q = m × c × ΔT

Where m is the mass in kg, c is the specific heat in kJ/(kg·°C), and ΔT is the temperature change in °C. For air, c ≈ 1.006 kJ/(kg·°C). For water, c = 4.186 kJ/(kg·°C).

Latent heat causes a phase change without a temperature change. For water, the latent heat of vaporization is approximately 2,257 kJ/kg at atmospheric pressure, and the latent heat of fusion is 334 kJ/kg.

Heating Degree-Days

Heating degree-days (HDD) are a measure used to estimate seasonal energy consumption. One degree-day is accumulated for each day where the average temperature is below 18 °C, with the difference being the number of degree-days. For example, a day with an average temperature of 5 °C accumulates 13 HDD. The simplified formula for estimating annual consumption is:

Annual consumption (kWh) = (HDD × U × A × 24) / 1000

Ventilation Systems

Ventilation Requirements According to the National Building Code

The National Building Code of Canada (NBC) specifies minimum ventilation rates for different occupancy types. For residential buildings, the minimum mechanical ventilation rate is 0.35 air changes per hour (ACH) or 7.5 L/s per person, whichever is greater. For commercial buildings, rates vary by use: 10 L/s per person for offices, 15 L/s for classrooms, and 20 L/s for restaurants.

The following table summarizes typical requirements:

Occupancy Type	Minimum Ventilation Rate (L/s per person)	Minimum Air Change Rate (ACH)
Residential	7.5	0.35
Offices	10	—
Classrooms	15	—
Restaurants	20	—
Hospitals (patient rooms)	25	6
Laboratories	20	8

Mechanical and Natural Ventilation

Mechanical ventilation uses fans to move air. Systems can be single-flow (exhaust-only or supply-only) or double-flow (combined exhaust and supply). Natural ventilation relies on pressure and temperature differences, but it is rarely sufficient for modern airtight buildings.

Balanced ventilation is essential to avoid negative or positive pressures in the building. Excessive negative pressure can cause backdrafting of combustion products from gas appliances, creating a risk of carbon monoxide poisoning. Excessive positive pressure can cause moisture infiltration into the building envelope.

Balancing Ventilation Systems

Balancing involves adjusting airflow rates in each branch of the duct network to meet design flow rates. The standard procedure includes:

33.Measure the total fan airflow using a manometer and Pitot tube or a flow hood.

34.Adjust fan speed if necessary to achieve the required total airflow.

35.Measure airflow at each diffuser or grille.

36.Adjust balancing dampers to balance the branches.

37.Verify that total flow rates match the sum of branch flow rates.

The relationship between static pressure (Ps) and airflow (Q) in a duct system is: Ps ∝ Q². If you double the airflow, the static pressure quadruples. This relationship is crucial for understanding the effect of fan speed changes.

Heat Recovery Systems

Principles of Heat Recovery

Heat recovery in ventilation systems involves transferring heat from the exhaust (stale) air to the incoming fresh air. This reduces the heating or cooling load of the building. The efficiency of a heat recovery unit is expressed by its sensible effectiveness:

η = (T₂ - T₁) / (T₃ - T₁) × 100 %

Where:

T₁ = temperature of fresh air entering the heat recovery unit

T₂ = temperature of fresh air leaving the heat recovery unit

T₃ = temperature of exhaust air entering the heat recovery unit

Types of Heat Recovery Units

Plate Heat Recovery Unit (Plate Heat Exchanger)

The plate heat recovery unit is the simplest type. Fresh air and exhaust air flows travel through adjacent channels separated by metal or plastic plates. Heat is transferred through the plates by conduction. Typical effectiveness is 50 to 70%. There is no moisture transfer, only sensible heat. Advantages: no moving parts, low maintenance. Disadvantages: risk of frost formation at low temperatures, requires a bypass for the summer season.

Rotary Heat Recovery Unit (Thermal Wheel)

The thermal wheel is a rotating cylinder composed of a metal or fibre matrix. The wheel rotates slowly (10 to 20 revolutions per minute) between the two air streams. The matrix absorbs heat from the exhaust air and releases it to the fresh air. Effectiveness can reach 75 to 85%. Some wheels are hygroscopic and also transfer moisture (latent heat). Advantages: high efficiency, possible moisture transfer. Disadvantages: moving parts, risk of cross-contamination (generally less than 5%), maintenance of motor and belts.

Heat Pipe Recovery Unit

The heat pipe is a sealed tube containing a refrigerant fluid. Heat from the exhaust air evaporates the fluid at the hot end of the tube. The vapour travels to the cold end where it condenses, releasing heat to the fresh air. The condensate returns by gravity or capillary action. Effectiveness is 50 to 70%. Advantages: no moving parts, no cross-contamination. Disadvantages: limited effectiveness, requires inclination for condensate return.

Glycol Run-Around Loop Recovery Unit

This system uses two coils (one in the exhaust air stream, the other in the fresh air stream) connected by a liquid loop (glycol-water solution). A pump circulates the liquid. Effectiveness is 40 to 60%. Advantages: coils can be located far apart from each other, no cross-contamination. Disadvantages: lower effectiveness, requires a pump and expansion tank.

Type of Heat Recovery Unit	Typical Sensible Effectiveness	Moisture Transfer	Moving Parts	Cross-Contamination
Plate	50-70%	No	No	Negligible
Rotary	75-85%	Possible (if hygroscopic)	Yes	1-5%
Heat pipe	50-70%	No	No	None
Glycol run-around loop	40-60%	No	Yes (pump)	None

Frost Formation in Heat Recovery Units

Frost formation occurs when the temperature of the exhaust air drops below the freezing point of water. Water vapour from the exhaust air freezes on the heat exchanger surfaces, obstructing airflow and reducing effectiveness. Defrost strategies include:

Bypass: fresh air bypasses the heat recovery unit during defrost.

Preheating: an electric or hydronic heater preheats the fresh air before the heat recovery unit.

Periodic shutdown: the fresh air fan stops periodically to allow frost to melt.

Recirculation defrost: exhaust air is recirculated through the heat recovery unit to defrost it.

The frost threshold depends on the relative humidity of the exhaust air. For a relative humidity of 30% at 21 °C, frost formation begins at approximately -5 °C outdoor temperature. For 50% relative humidity, frost formation begins at approximately 0 °C.

Heat Pumps and Hybrid Systems

Air-to-Air Heat Pumps

Air-to-air heat pumps are heating and cooling systems that transfer heat between indoor and outdoor air. The coefficient of performance (COP) is the ratio of heat delivered to electrical energy consumed. A COP of 3 means that for 1 kW of electricity consumed, 3 kW of heat is delivered.

COP decreases as outdoor temperature drops. At -25 °C, the COP of a conventional heat pump can drop to 1.5 or less. Cold climate heat pumps maintain a COP above 2 down to -25 °C.

Bivalent Systems

A bivalent system combines a heat pump with a backup heating system (electric, gas, or hydronic). The balance point is the outdoor temperature at which the heat pump can no longer supply the full heating load. Below this point, the backup heating takes over.

Calculating the balance point requires knowing the building heating load at the design temperature and the heat pump capacity at different temperatures. For example, if the heating load is 10 kW at -30 °C and the heat pump delivers 8 kW at -10 °C, the balance point is at -10 °C.

Canadian Electrical Code Requirements

The Canadian Electrical Code, Part I (CE Code) (C22.1) applies to all electrical installations, including heating and ventilation systems. Relevant rules include:

Rule 8-200: Calculation of demand for heating appliances. The minimum demand for fixed heating appliances is 100% of the total load.

Rule 26-700: Requirements for electric heating appliances. Electric heaters must be installed with a minimum clearance of 75 mm from combustible surfaces.

Rule 26-744: Control devices for heating appliances. Each appliance must have an accessible means of disconnection.

Rule 62-100: General requirements for ventilation installations. Fans must be disconnected from the power supply during maintenance.

For heat recovery systems, the code requires that fan motors be protected against overloads in accordance with Rule 28-308.

CSA B149.1 Code Requirements

CSA B149.1, Natural Gas and Propane Installation Code applies to gas-fired heating appliances. Relevant requirements include:

Clause 8.2.1: Venting of combustion products must comply with manufacturer specifications and the code.

Clause 8.10.1: Combustion air supply must be adequate. For appliances installed in a confined space, the ventilation opening must have a minimum free area of 1 cm² per 4.2 kW of input rating.

Clause 8.22.1: Gas appliances must be installed with a minimum clearance of 150 mm from combustible materials, unless otherwise specified by the manufacturer.

The interaction between mechanical ventilation and gas appliances is critical. Excessive exhaust ventilation can create a negative pressure that prevents the venting of combustion products. The code requires that ventilation systems be designed to maintain a slightly positive or neutral pressure in spaces containing gas appliances.

Installation and Commissioning Procedures

Installation of Heat Recovery Units

The installation of an air-to-air heat recovery ventilator (HRV) must follow these steps:

87.Location: Install the unit in an accessible location for maintenance, with a minimum clearance of 600 mm at the front for filter replacement.

88.Duct connections: Fresh air and exhaust air ducts must be insulated to prevent condensation. Exterior ducts must have a 3% slope toward the outside for condensate drainage.

89.Condensate drain: Install a drain with a trap of at least 75 mm to prevent air from being drawn through the drain.

90.Bypass: Install a motorized bypass for the summer season if the heat recovery unit is not equipped with one.

91.Filters: Install filters on both air streams, with a minimum efficiency of MERV 6 (or equivalent).

Commissioning and Verification

Commissioning includes:

94.Verify fresh air and exhaust air flow rates using an anemometer or manometer.

95.Measure the heat recovery unit effectiveness under actual conditions.

96.Verify duct airtightness (smoke test).

97.Verify the operation of controls and safety devices.

98.Document measurements and adjustments.

The duct leakage test is performed by pressurizing the duct system to 250 Pa and measuring the leakage. The maximum allowable leakage is 5% of the total airflow for sealed ducts, according to ASHRAE standards.

Diagnostics and Troubleshooting

Common Heat Recovery Unit Problems

Symptom	Probable Cause	Corrective Action
Excessive frost formation	Very low outdoor temperature, high indoor humidity	Activate defrost, reduce humidity
Insufficient airflow	Clogged filters, obstructed ducts, faulty fan	Replace filters, clean ducts, check fan
Reduced effectiveness	Dirty heat exchanger, bypass open	Clean heat exchanger, check bypass position
Abnormal noise	Worn fan bearing, wheel imbalance	Replace bearing, balance wheel
Excessive condensation	Clogged drain, insufficient trap	Clean drain, check trap

Measuring Effectiveness Under Actual Conditions

To measure the actual effectiveness of a heat recovery unit, you must measure temperatures at all four points (fresh air entering, fresh air leaving, exhaust air entering, exhaust air leaving) and apply the sensible effectiveness formula. Measurements must be taken at steady state, meaning after at least 15 minutes of stable operation.

Practical Calculations for the Exam

Example 1: Heating Load Calculation

A warehouse measuring 20 m × 15 m × 4 m has 200 mm concrete walls (U = 1.7 W/(m²·°C)) and an insulated roof (U = 0.4 W/(m²·°C)). The design indoor temperature is 18 °C and the design outdoor temperature is -30 °C. Calculate the transmission heating load.

Solution:

Wall area: 2 × (20 × 4) + 2 × (15 × 4) = 160 + 120 = 280 m²

Roof area: 20 × 15 = 300 m²

Q_walls = U × A × ΔT = 1.7 × 280 × (18 - (-30)) = 1.7 × 280 × 48 = 22,848 W

Q_roof = 0.4 × 300 × 48 = 5,760 W

Q_total = 22,848 + 5,760 = 28,608 W ≈ 28.6 kW

Example 2: Heat Recovery Unit Effectiveness

A plate heat recovery unit has the following temperatures: fresh air entering = -10 °C, fresh air leaving = 12 °C, exhaust air entering = 22 °C. Calculate the sensible effectiveness.

Solution:

η = (T₂ - T₁) / (T₃ - T₁) × 100 %

η = (12 - (-10)) / (22 - (-10)) × 100 %

η = 22 / 32 × 100 % = 68.75%

Example 3: Required Ventilation Airflow

A residential building has a floor area of 200 m² and a ceiling height of 2.5 m. Calculate the minimum required ventilation airflow.

Solution:

Volume = 200 × 2.5 = 500 m³

Air change rate: 0.35 ACH

Airflow = 500 × 0.35 / 3600 = 0.0486 m³/s = 48.6 L/s

The per-person rate (7.5 L/s) applies if the number of occupants is known. For 4 occupants: 4 × 7.5 = 30 L/s. The higher value (48.6 L/s) applies.

Pitfalls to Avoid

128.Confusing sensible heat and latent heat: Sensible heat changes temperature, latent heat changes phase. In load calculations, both must be considered separately.

129.Forgetting the factor of 24 in degree-day calculations: The annual consumption formula uses 24 hours per day. Forgetting this factor gives a result 24 times too small.

130.Reversing the numerator in the effectiveness formula: Effectiveness is always (fresh air leaving temperature - fresh air entering temperature) divided by (exhaust air entering temperature - fresh air entering temperature). Do not use exhaust air leaving temperatures.

131.Neglecting the Ps ∝ Q² relationship: If you reduce a fan speed by 20%, the airflow decreases by 20%, but the static pressure decreases by 36% (0.8² = 0.64). This can make the system unable to overcome duct pressure losses.

132.Ignoring Canadian Electrical Code clearance requirements: Electric heating appliances must have a minimum clearance of 75 mm from combustible surfaces. This distance is often tested on the exam.

133.Confusing residential and commercial ventilation requirements: The 0.35 ACH rate applies to residential, but commercial buildings use per-person rates that vary by occupancy.

134.Forgetting the trap on the condensate drain: Without a trap, air is drawn through the drain, which reduces effectiveness and can cause odours.

135.Not considering the ventilation-gas appliance interaction: Excessive exhaust ventilation can cause backdrafting of combustion products. CSA B149.1 requirements must be respected.

136.Using the design temperature instead of the average temperature for degree-days: Degree-days use the daily average temperature, not the design temperature.

137.Confusing COP and efficiency: COP is a unitless ratio (heat delivered / energy consumed), while efficiency is a percentage. A COP of 3 does not mean 300% efficiency in the thermodynamic sense, but 300% efficiency relative to electrical energy consumed.

Summary

The heating load is calculated using Q = U × A × ΔT, considering all building envelope surfaces.

Ventilation must meet NBC requirements: 0.35 ACH or 7.5 L/s per person for residential, with higher rates for commercial buildings.

Heat recovery units transfer heat from exhaust air to fresh air. The main types are plate, rotary, heat pipe, and glycol run-around loop.

Sensible effectiveness is calculated using the formula η = (T₂ - T₁) / (T₃ - T₁) × 100 %.

Frost formation in heat recovery units is a major issue in cold climates; defrost strategies are necessary.

The Canadian Electrical Code, Part I imposes specific rules for heating appliances (Rules 8-200, 26-700) and fans (Rule 62-100).

CSA B149.1 governs the installation of gas appliances and combustion air supply.

Commissioning includes verification of airflow rates, effectiveness, and duct airtightness.

Practical calculations for the exam include thermal loads, heat recovery unit effectiveness, and ventilation airflow rates.

Mastering these concepts will enable you to succeed on Red Seal exam questions covering heating, ventilation, and heat recovery systems. Pay particular attention to units, conversion factors, and regulatory requirements, as these elements are frequently tested.

Heating, Ventilation, and Heat Recovery Systems