Ontario G3 Gas Technician Exam: What to Expect and How to Prepare

A practical guide to the G3 certification exam — what it covers, what to study, and the facts worth memorising before you sit it.

The topics

The question bank in this app is organised into the 9 areas the exam draws on:

Topic	Questions
Appliances & Heat Transfer	136
Controls & Ignition	57
Electricity & Controls	178
Gas Code & Piping	177
Gas Properties & Combustion	318
Regulations & TSSA	76
Safety & Workplace	107
Tools & Measurements	58
Venting & Combustion	77

Facts worth memorising

These come up repeatedly and are quick marks if you know them cold.

Property	Natural gas	Propane
Heat content	~1,000 Btu/ft³	~2,520 Btu/ft³
Specific gravity	~0.55 (lighter than air)	~1.52 (heavier than air)
Flammability limits	4% – 14%	2.4% – 9.5%
Boiling point	−162 °C / −260 °F	−42 °C / −44 °F
Combustion air	~10 ft³ per ft³ gas	~25 ft³ per ft³ gas
Maximum CO₂	11.9%	13.7%

Conversions

• 1 PSI = 27.68" w.c.
• 1" w.c. = 0.0361 PSI
• 1 kPa = 4.015" w.c.
• °F = (°C × 9/5) + 32
• 1 Boiler HP ≈ 34,000 Btu/h

Fire extinguisher classes

• Class A — ordinary combustibles: paper, wood, cardboard, cloth
• Class B — flammable liquids: gasoline, kerosene, grease, oil
• Class C — electrical equipment: appliances, wiring, panels
• Class D — combustible metals: magnesium, titanium, sodium
• Class K — cooking oils and fats

Appliance categories under B149.1

Category	Vent pressure	Flue loss	Typical
I	Non-positive	< 17%	Standard furnace, boiler
II	Non-positive	≥ 17%	Condensing (unusual)
III	Positive	< 17%	Power-vented furnace
IV	Positive	≥ 17%	High-efficiency condensing

Pipe sizing: the four steps

Pipe sizing questions look intimidating and are mechanical once you know the order. It is the same four steps every time.

1. Find the LMR — the Longest Measured Run, from the meter to the farthest appliance.
2. Choose a CZ at or above the LMR, always rounding up.
3. Add the section load — the total BTU/h of every appliance fed by that section.
4. Read the pipe size — in your CZ column, the first row whose capacity meets or exceeds the load. Round up again.

Two rules never change: round the CZ up, and size the pipe up. Between two values, take the larger. Natural gas uses Annex A, propane uses Annex B.

Common questions

What is the Ontario G3 gas technician certification?

G3 is the entry-level gas technician certification in Ontario, administered by TSSA. It allows work on gas appliances rated up to 400,000 BTU/h under supervision, and is the first step toward G2 and G1.

What score do you need to pass the G3 exam?

The G3 exam is commonly cited as requiring 75%. Aiming for 80% in practice gives you a buffer for exam-day nerves and unfamiliar wording. Confirm the current requirement with TSSA or your training provider.

What topics does the G3 exam cover?

The syllabus spans 9 areas: Appliances & Heat Transfer, Controls & Ignition, Electricity & Controls, Gas Code & Piping, Gas Properties & Combustion, Regulations & TSSA, Safety & Workplace, Tools & Measurements, Venting & Combustion.

What is the best way to study for the G3 exam?

Work topic by topic rather than reading cover to cover, take full-length timed practice tests to find gaps, and re-drill every question you get wrong until you understand why the answer is what it is. Most people underestimate the code sections.

Which codes does the G3 exam draw on?

CSA B149.1 (Natural Gas and Propane Installation Code) is the main one, with B149.2 for propane storage and handling. The Technical Standards and Safety Act and the Occupational Health and Safety Act also appear.

Practising

G3 HVAC PrepBOX has 1,184 questions across those 9 topics, four full 160-question practice tests, and a timed Master Exam that runs 165 questions over 3 hours 30 minutes under real conditions. The 20-question mock exam is free and needs no account.

Try the free mock exam →   See sample questions with answers