TSSA G3 propane and natural gas practice questions (Module 3).

Five free practice questions on propane and natural gas: module 3 of the nine modules of Ontario's Gas Technician 3 (G3) certificate exam, which TSSA runs. The exam is all multiple choice; these have four options and one correct answer, as every question in our bank does. Answer each one before opening the explanation. No account, no email.

01The questions

1. Recall · Heating value and its units: MJ/m3 and Btu/ft3

   The gas distributor gives the gross heating value of its natural gas as about 38.9 MJ/m³ (about 1 044 Btu/ft³). What does this figure describe?

   1. The heat released by one cubic metre of the gas–air mixture at the burner
   2. The heat an appliance delivers to the home when it burns one cubic metre
   3. The heat released when one cubic metre of the gas burns completely
   4. The energy used to deliver one cubic metre of gas through the meter
   Show the answer

   C. Correct. Heating value is the heat from complete combustion of a unit volume of the fuel at standard conditions (the distributor's gross figure also counts the heat given up as the water vapour formed condenses). Gas flow times heating value gives an appliance's input.

   Why not the others

   A. The figure is per cubic metre of gas, not of mixture. A cubic metre of natural gas needs many times its own volume of air to burn, so a cubic metre of mixture releases far less.

   B. That is the appliance's output, which is its input less the heat lost up the vent. The heating value describes the fuel itself, before any appliance efficiency.

   D. Heating value is a property of the fuel's combustion; it has nothing to do with the energy used to move gas through pipes or a meter.

2. Procedural · Complete combustion: the reaction, products and theoretical air

   A water heater burns 35 ft³/h (0.99 m³/h) of natural gas, taken as pure methane, with complete combustion. If all the water formed stayed as vapour at the same temperature and pressure as the gas, what volume of water vapour would form each hour?

   1. 35 ft³/h (0.99 m³/h)
   2. 70 ft³/h (1.98 m³/h)
   3. 105 ft³/h (2.97 m³/h)
   4. 140 ft³/h (3.96 m³/h)
   Show the answer

   B. Correct. CH₄ + 2 O₂ → CO₂ + 2 H₂O: each volume of methane forms two volumes of water vapour, so 35 ft³/h × 2 = 70 ft³/h (1.98 m³/h).

   Why not the others

   A. This takes one volume of water vapour per volume of methane, as for the carbon dioxide. The four hydrogen atoms form two water molecules (CH₄ + 2 O₂ → CO₂ + 2 H₂O), so 35 × 2 = 70 ft³/h.

   C. 105 ft³/h counts all the products formed from the methane, 1 volume of CO₂ plus 2 of water: 35 × 3. The water vapour alone is 35 × 2 = 70 ft³/h.

   D. This uses propane's four volumes of water per volume (C₃H₈ + 5 O₂ → 3 CO₂ + 4 H₂O): 35 × 4 = 140. Methane forms two: 35 × 2 = 70 ft³/h.

3. Recall · Explosion and fire from accumulated gas

   Why does gas that has built up in a closed basement explode when it is ignited, rather than burning quietly at the leak?

   1. The mixture fills the room, so it all burns at once and builds pressure
   2. The gas is held under pressure in the room and bursts outward when lit
   3. A mixture below the lower limit detonates violently when it meets a spark
   4. The flame travels back into the gas piping and bursts the meter
   Show the answer

   A. Correct. Gas premixed with air within its flammable range throughout a room burns through the whole volume in moments, and the hot, expanding gases build pressure faster than the building can relieve it. Both SDSs warn the gases form explosive mixtures with air.

   Why not the others

   B. Leaked gas in a room is at about atmospheric pressure. The pressure of an explosion comes from the near-instant burning of the premixed gas and the expansion of the hot products.

   C. A mixture below the lower limit will not carry a flame at all. Only gas–air mixtures within the flammable range can burn or explode.

   D. A flame cannot burn back into piping full of gas, since there is no air inside to burn with. The explosion happens in the room, where the mixture is.

4. Procedural · Shutting off gas at the appliance, branch, meter or service valve; valve positions

   On a January call, you find a small leak at the flare nut where a water heater's connector joins its gas control, downstream of the water heater's manual shut-off valve. The furnace on the same piping must keep heating the house. Which valve do you close?

   1. The meter valve at the outside wall
   2. The water heater's manual shut-off valve
   3. The furnace's manual shut-off valve
   4. The water heater's gas control, set to off
   Show the answer

   B. Correct. That valve is upstream of the leaking flare nut and serves only the water heater, so closing it stops the leak and leaves the furnace running. The joint is then remade and leak-tested before the valve is reopened.

   Why not the others

   A. Closing the meter valve would stop the leak but also shut down the furnace in January. The water heater's own valve isolates just the leaking appliance.

   C. The furnace valve isolates the furnace, not the water heater; the leak would continue and the house would lose its heat.

   D. The leak is at the flare nut on the control's inlet, so turning the control off leaves that joint under pressure and still leaking. Close the manual valve upstream of it.

5. Recall · Transporting, storing and securing cylinders

   During a renovation, two spare 9 kg (20 lb) propane cylinders must be kept on site overnight. Where are they kept?

   1. In the closed trunk of a vehicle, locked away from the public
   2. In the basement beside the furnace, where they stay warm and dry
   3. Lying on their sides in the service van, so they cannot roll over
   4. Outdoors, upright and secured, in an airy spot away from heat
   Show the answer

   D. Correct. The propane SDS calls for storage in a well-ventilated place, protected from sunlight and away from heat and ignition sources, and cylinders are kept upright and secured so they cannot fall. Outdoors, a leak disperses instead of collecting.

   Why not the others

   A. A closed trunk is an unventilated space where any leak builds up, and the sun can heat the cylinders. Store them in a well-ventilated place.

   B. A basement is the worst place: leaked propane collects there, the furnace is an ignition source, and heat raises the cylinders' pressure.

   C. Cylinders are kept upright so the valve and relief device stay in the vapour space; on its side a cylinder can release liquid. A closed van also lets leaked vapour build up.

02Answer key

03What module 3 covers

What natural gas and propane are and how to handle them safely: their make-up and physical properties, where each one goes when it leaks, propane as a liquid held under its own vapour pressure, heating value, the odorant and the limits of relying on smell, combustion and its products, flammable limits and ignition temperature, primary, secondary and excess air, the hazards of carbon monoxide, asphyxiation, explosion and liquid propane, finding and responding to leaks, and handling propane cylinders and tanks.

TSSA names module 3 “Properties, Characteristics, Safe Handling of Propane and Natural Gas”. It gives its G3 curriculum only to accredited training providers, so the five objectives below are TicketPrep's own, built from public sources such as college course outlines and Ontario's regulations, not TSSA's. TSSA does not publish the G3 question count or how many exam questions come from each module: its prep guide gives no G3 count and no per-module count, and candidates who fail get a mastery report broken down by module. The right-hand column is how many questions our bank holds on each objective, Standard and Advanced together, not a count from the exam.

04What the questions turn on

Where the gas goes decides where to look

Natural gas, with a relative density of about 0.58, rises from a leak, gathers under the floor above and climbs stairways and the openings around pipes and ducts. Propane vapour, at about 1.52, lies along the floor and collects in basements and low ground, so it is cleared through low openings such as a door at floor level.

A propane gauge shows temperature, not contents

While any liquid remains, a propane container sits at the vapour pressure for the liquid's temperature, so a gauge cannot show how much is left; weighing the cylinder can. A heavy draw chills the liquid and its pressure falls, with frost on the wall the cold liquid wets. Below about −42 °C propane makes almost no pressure, however full the cylinder.

Percent of the lower limit

The lower explosive limit is the leanest mixture of gas in air that a flame can travel through. A reading in % LEL is the gas concentration divided by that limit, times 100: 0.6 % natural gas against a 5 % limit is 12 % LEL. A mixture too rich to burn is still a hazard, because airing it out carries it down through the flammable range.

Carbon monoxide gives little warning

Carbon monoxide has no smell, taste or colour and cuts the blood's ability to carry oxygen, so it can affect people before they notice it. Tiredness, headaches and flu-like symptoms on a winter call point to low-level exposure. A burner short of combustion air, such as a furnace in a closet with the door shut, burns incompletely and its CO climbs.

05The other modules

The G3 exam covers nine modules, 1 to 9. Each has its own page of free questions:

The TSSA G3 practice test page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.