Heating System Design and Sizing

Design and Sizing of Heating Systems

Introduction to Sizing

Sizing an oil heating system is not simply a matter of choosing a burner and a tank. It is a rigorous process that must comply with the requirements of the Canadian Electrical Code, Part I (CE Code) (for associated electrical installations), CSA B139 (installation and maintenance of oil burners), and CSA B149.1 (natural gas and propane code, applicable when the system is combined). Sizing determines the energy efficiency, safety, and longevity of the installation.

Sizing Objectives

A properly sized system must:

Maintain the indoor setpoint temperature (typically 21 °C to 22 °C) under the coldest winter design conditions;

Compensate for heat losses through transmission, infiltration, and ventilation;

Provide adequate domestic hot water (DHW) without compromising space heating;

Operate at a reasonable load factor (between 40% and 80% during peak periods) to avoid short cycling.

The heat load calculation is the first step. It is performed according to the standardized CSA F280 method (Determination of the heating and cooling load of buildings). This standard is the mandatory reference in Canada for sizing residential and small commercial heating equipment.

Heat Load Calculation (CSA F280)

Fundamental Principles

The total heat load (Q_total) is the sum of:

Q_transmission: losses through the building envelope (walls, roof, floor, windows, doors);

Q_infiltration: losses from uncontrolled air exchange (leaks, openings);

Q_ventilation: losses from controlled air exchange (mechanical ventilation system);

Q_DHW: additional load for domestic hot water (if the same appliance serves both functions).

The basic formula for transmission is:

Q = U × A × ΔT

Where:

Q = heat loss in watts (W);

U = thermal transmission coefficient (W/m²·°C);

A = area of the element (m²);

ΔT = temperature difference between inside and outside (°C).

Design Temperature

The outdoor design temperature is the minimum winter temperature for a given region. It is provided in the climatic tables of the National Building Code (NBC) or in the appendices of CSA F280. For example:

Montreal: -26 °C;

Toronto: -23 °C;

Vancouver: -9 °C;

Edmonton: -34 °C.

The indoor design temperature is set at 21 °C for living areas and 18 °C for bedrooms (according to the NBC). The ΔT is therefore calculated between these values and the outdoor design temperature.

Typical U-Values

The U-value is the inverse of the total thermal resistance (R): U = 1 / R_total. The following values serve as reference for exams:

Element	U (W/m²·°C)
Insulated exterior wall (R-20)	0.28
Uninsulated exterior wall	1.40
Insulated roof (R-40)	0.14
Double-glazed window	2.80
Triple-glazed window	1.80
Exterior wood door	2.20
Floor above a crawl space	0.85

Important: These values are approximations. The exam may provide exact values in the question. Always use the data provided.

Infiltration Calculation

Cold air infiltration often represents 20% to 35% of the total load. It is calculated using the air change rate method:

Q_infiltration = 0.34 × V × n × ΔT

Where:

0.34 = volumetric heat capacity of air (Wh/m³·°C);

V = heated interior volume (m³);

n = air change rate per hour (ach/h);

ΔT = temperature difference (°C).

Typical air change rates:

Airtight construction (new home with air barrier): 0.25 ach/h;

Average construction: 0.50 ach/h;

Older or leaky construction: 1.00 ach/h or more.

Complete Calculation Example

Problem: A house with 120 m² of floor area, ceiling height of 2.4 m, located in Quebec City (design temperature: -28 °C). Indoor temperature: 21 °C. Surfaces: exterior walls = 140 m² (U = 0.30), roof = 120 m² (U = 0.15), windows = 18 m² (U = 2.80), door = 2 m² (U = 2.20). Air change rate: 0.35 ach/h. Calculate the total heat load.

Step 1 — ΔT: 21 - (-28) = 49 °C.

Step 2 — Transmission:

Walls: 0.30 × 140 × 49 = 2,058 W;

Roof: 0.15 × 120 × 49 = 882 W;

Windows: 2.80 × 18 × 49 = 2,469 W;

Door: 2.20 × 2 × 49 = 216 W.

Total transmission = 2,058 + 882 + 2,469 + 216 = 5,625 W.

Step 3 — Infiltration:

Volume = 120 × 2.4 = 288 m³;

Q_infiltration = 0.34 × 288 × 0.35 × 49 = 1,679 W.

Step 4 — Total load: 5,625 + 1,679 = 7,304 W (≈ 7.3 kW).

Conversion to BTU/h: 1 W = 3.412 BTU/h. Therefore 7,304 W × 3.412 = 24,921 BTU/h.

Burner and Nozzle Sizing

Burner Capacity

The burner capacity must be selected to supply the calculated heat load, with a safety margin of 10% to 15% for exceptional cold periods and to compensate for distribution system losses. However, oversizing is a common mistake: an oversized burner causes short cycling, premature wear, soot buildup, and reduced efficiency.

The fuel flow rate is calculated as follows:

Flow rate (L/h) = Capacity (kW) / (LHV × η)

Where:

LHV = lower heating value of fuel oil (approximately 38.2 MJ/L or 10.6 kWh/L);

η = combustion efficiency (decimal, e.g., 0.82 for 82%).

Example: For a load of 7.3 kW with an efficiency of 80%:

Flow rate = 7.3 / (10.6 × 0.80) = 7.3 / 8.48 = 0.86 L/h.

Nozzle Selection

The nozzle determines the flow rate and spray pattern. Key parameters are:

Flow rate (in US gallons per hour, GPH): 0.50, 0.65, 0.75, 0.85, 1.00, 1.25, 1.50 GPH;

Spray angle: 45°, 60°, 70°, 80°;

Pattern: solid (S) or hollow (H).

Conversion: 1 GPH ≈ 3.785 L/h. For a flow rate of 0.86 L/h, the closest nozzle is 0.85 GPH (3.22 L/h) or 1.00 GPH (3.79 L/h). The choice depends on pump pressure (typically 100 psi or 690 kPa).

Rule of thumb: The nozzle should be selected so that the actual burner flow rate matches the calculated flow rate within ± 5%. A flow rate that is too high increases consumption and soot production; a flow rate that is too low reduces capacity and can cause difficult ignition.

Pump Pressure

Pump pressure affects the actual nozzle flow rate. The relationship is:

Actual flow rate = Rated flow rate × √(Actual pressure / Rated pressure)

If the nozzle rated pressure is 100 psi and the pump is set at 120 psi:

Factor = √(120 / 100) = √1.2 = 1.095;

A 0.85 GPH nozzle will deliver 0.85 × 1.095 = 0.93 GPH (3.52 L/h).

Exam trap: Never neglect the effect of pressure. Questions often include pressure variations to test your understanding of this relationship.

Fuel Oil Tank Sizing

Tank Types

Tanks are classified according to their location:

Indoor tanks: installed in the basement, garage, or a mechanical room;

Outdoor tanks: installed outside, often made of steel or fiberglass;

Underground tanks: regulated by strict environmental requirements (uncommon in new installations).

Capacity and Autonomy

The tank capacity must provide an autonomy of at least 30 days during peak periods. To calculate autonomy:

Autonomy (days) = Usable capacity (L) / Daily consumption (L/day)

Usable capacity is typically 90% of total capacity (the tank is never filled to 100%). Daily consumption is estimated from the burner flow rate and daily operating time.

Example: A burner consumes 0.86 L/h and operates an average of 6 hours per day in January. Daily consumption = 5.16 L/day. For 30 days of autonomy: usable capacity = 5.16 × 30 = 155 L. A 200 L tank (usable capacity 180 L) is suitable.

Regulatory Requirements (CSA B139)

CSA B139 imposes minimum distances for outdoor tanks:

Minimum distance from any building opening (door, window): 1.5 m;

Minimum distance from a catch basin or basement entrance: 1.5 m;

Minimum distance from a neighbouring property: 1.5 m;

The tank must rest on a stable, non-combustible base.

For indoor tanks, the maximum capacity is generally 1,000 L without special approval. Beyond that, additional requirements apply (containment, ventilation, etc.).

Piping and Accessories

Piping sizing depends on the system type:

Gravity system: oil flows by gravity from the tank to the burner. The piping must be sloped at least 1% (1 cm per metre) toward the burner;

One-pipe system: a single line connects the tank to the burner. Suitable when the tank is located above the burner;

Two-pipe system: a supply line and a return line. Required when the tank is located below the burner level (the burner must then be equipped with a pump with suction capability).

Maximum suction length: For a standard pump, the maximum suction lift is 3 m (10 feet) with 3/8 in. piping, and 6 m with 1/2 in. piping. Beyond that, a two-pipe system with a return line is necessary.

Warm Air Duct Sizing

Distribution Principles

In a forced warm air system, duct sizing must ensure:

Sufficient airflow for each room;

Acceptable air velocity (no noise, no drafts);

Static pressure within the fan's capacity limits.

Airflow Calculation

The airflow required for a room is calculated:

Flow rate (L/s) = Room heat load (W) / (1.2 × ΔT_air)

Where:

1.2 = volumetric heat capacity of air (kJ/m³·°C);

ΔT_air = temperature difference between supply air and return air (typically 20 °C to 25 °C).

Example: A room has a load of 1,500 W. With a ΔT_air of 22 °C:

Flow rate = 1,500 / (1.2 × 22) = 1,500 / 26.4 = 56.8 L/s.

Recommended Velocities

Duct type	Maximum velocity (m/s)
Main duct	5.0 to 7.5
Secondary duct	4.0 to 5.0
Branch duct	3.0 to 4.0
Diffuser grille	1.5 to 2.5

Rectangular Duct Sizing

The cross-section of a duct is calculated:

A (m²) = Flow rate (m³/s) / Velocity (m/s)

For a flow rate of 0.057 m³/s (56.8 L/s) and a velocity of 4 m/s:

A = 0.057 / 4 = 0.0143 m² = 143 cm².

A rectangular duct of 15 cm × 10 cm gives a cross-section of 150 cm², which is suitable.

Circular duct equivalence: The equivalent diameter of a rectangular duct is:

D_equivalent = 1.30 × ((a × b)^0.625) / ((a + b)^0.25)

For a = 15 cm and b = 10 cm:

(150)^0.625 = 22.4;

(25)^0.25 = 2.24;

D = 1.30 × 22.4 / 2.24 = 13 cm.

Domestic Hot Water System Sizing

DHW Load

The DHW load is estimated based on the number of occupants and consumption habits. Reference values:

Number of occupants	Daily consumption (L at 60 °C)
1	75
2	110
3	150
4	185
5	225

Power Required for DHW

The power required to heat a volume of water is:

P (kW) = Volume (L) × 4.18 × ΔT / Time (s)

Where:

4.18 = specific heat capacity of water (kJ/kg·°C);

ΔT = temperature rise (from 10 °C to 60 °C = 50 °C).

Example: Heating 150 L from 10 °C to 60 °C in 2 hours (7,200 s):

P = 150 × 4.18 × 50 / 7,200 = 31,350 / 7,200 = 4.35 kW.

This power is added to the heating load if the same appliance serves both functions. In this case, the sizing must consider the combined load: the heating load + the DHW load, with a diversity factor (often 0.5 to 0.7 depending on usage).

Energy Efficiency and Load Factor

Combustion Efficiency

Combustion efficiency (η) is measured by flue gas analysis. Typical values for a well-tuned system:

AFUE (Annual Fuel Utilization Efficiency): 80% to 87% for standard furnaces, 90% to 96% for condensing models;

Instantaneous combustion efficiency: measured with a combustion analyzer (CO₂, O₂, flue gas temperature).

The simplified formula:

η = 100 - (Flue gas losses + Radiation losses)

Flue gas losses are calculated:

Losses (%) = (T_flue - T_ambient) × (K / CO₂)

Where K is a constant depending on the fuel (for fuel oil, K ≈ 0.38).

Example: T_flue = 260 °C, T_ambient = 20 °C, CO₂ = 12%:

Losses = (260 - 20) × (0.38 / 12) = 240 × 0.0317 = 7.6%;

Efficiency = 100 - 7.6 = 92.4% (accounting for radiation losses, the actual efficiency will be approximately 88%).

Load Factor

The load factor is the ratio of burner operating time to total time. An optimal load factor is between 40% and 80% during peak periods.

Load factor (%) = Operating time / Total time × 100

A load factor below 30% indicates oversizing; above 90% indicates undersizing.

Applicable Canadian Electrical Code Rules

The Canadian Electrical Code, Part I (CE Code) (C22.1) applies to the electrical installations associated with the oil heating system. Relevant rules:

Rule 8-200: Calculation of electrical demand for heating appliances. The load of a burner motor is calculated at 125% of the motor's full-load current;

Rule 26-700: Requirements for fuel-fired heating appliances. The disconnecting means must be visible and accessible, and must interrupt all ungrounded conductors;

Rule 26-704: The disconnecting means must be installed within a maximum distance of 3 m from the appliance, unless the appliance is equipped with a locking device.

Exam trap: The burner disconnecting means must be lockable in the open position (hasp) to allow safe maintenance.

Pitfalls to Avoid

183.Confusing W and BTU/h: 1 W = 3.412 BTU/h. A 10% conversion error can lead to critical undersizing;

184.Forgetting the 10-15% margin: The load calculation gives the theoretical value. The burner must supply this value plus the margin, but not more than 20% beyond;

185.Neglecting the effect of pump pressure: Excessive pressure increases the actual nozzle flow rate. Always check the pressure before selecting the nozzle;

186.Using outdated U-values: Insulation requirements have changed. Use the values from CSA F280 or those provided in the question;

187.Ignoring the DHW load: In a combined system, the DHW load can represent 20% to 30% of the total load. Ignoring it leads to undersizing;

188.Confusing total capacity and usable capacity: The usable capacity of a tank is 90% of the total capacity. A 200 L tank only provides 180 L of usable oil;

189.Forgetting CSA B139 minimum distances: The 1.5 m distances for outdoor tanks are absolute minimum values;

190.Calculating infiltration with too low a rate: For older buildings, a rate of 0.5 ach/h is a minimum. A rate that is too low underestimates the load;

191.Not accounting for altitude: At high altitude, air density decreases, which affects airflow and combustion. The burner must be adjusted accordingly;

192.Confusing one-pipe and two-pipe systems: A one-pipe system cannot draw oil from a tank located below the burner. Check the configuration before sizing.

Summary

Sizing begins with the heat load calculation according to CSA F280, considering transmission, infiltration, and ventilation;

The basic formula is Q = U × A × ΔT for transmission, and Q = 0.34 × V × n × ΔT for infiltration;

The outdoor design temperature varies by region (from -9 °C in Vancouver to -34 °C in Edmonton);

The burner flow rate is calculated from the capacity and efficiency: Flow rate (L/h) = Capacity (kW) / (10.6 × η);

Nozzle selection depends on flow rate, angle, and spray pattern, and must be verified against the actual pump pressure;

The tank must provide a minimum autonomy of 30 days, with a usable capacity of 90% of the total capacity;

The CSA B139 minimum distances for outdoor tanks are 1.5 m from openings and property lines;

Duct airflow is calculated with Flow rate (L/s) = Load (W) / (1.2 × ΔT_air);

Combustion efficiency is measured by flue gas analysis; an efficiency above 85% is expected for a well-tuned system;

The Canadian Electrical Code, Part I, Rule 8-200, requires a demand calculation at 125% for burner motors;

The optimal load factor is between 40% and 80%; outside this range, the sizing is incorrect.

Exam Tips

Memorize the conversions: 1 W = 3.412 BTU/h; 1 GPH = 3.785 L/h; 1 in. of water column = 249 Pa;

Practice multi-step calculations: Sizing questions are often compound problems that require calculating the load, then the flow rate, then the nozzle;

Check your units: A result in watts cannot be directly compared to a flow rate in GPH. Convert systematically;

Learn the typical values: U-values and air change rates are often provided, but knowing them by heart saves you time;

Re-read the questions: Questions sometimes include extraneous information (distractors). Identify the relevant data before starting the calculations;

Use a systematic method: Write out each step, even if the final answer is wrong. Marks are often awarded for the reasoning.

Design and Sizing of Heating Systems

Introduction to Sizing

Sizing Objectives

Heat Load Calculation (CSA F280)

Fundamental Principles

Design Temperature

Typical U-Values

Infiltration Calculation

Complete Calculation Example

Burner and Nozzle Sizing

Burner Capacity

Nozzle Selection

Pump Pressure

Fuel Oil Tank Sizing

Tank Types

Capacity and Autonomy

Regulatory Requirements (CSA B139)

Piping and Accessories

Warm Air Duct Sizing

Distribution Principles

Airflow Calculation

Recommended Velocities

Rectangular Duct Sizing

Domestic Hot Water System Sizing

DHW Load

Power Required for DHW

Energy Efficiency and Load Factor

Combustion Efficiency

Load Factor

Applicable Canadian Electrical Code Rules

Pitfalls to Avoid

Summary

Exam Tips

Ready to test this chapter?