Ontario G3 Gas Technician exam preparation
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The four categories under B149.1, what separates them, and why the category decides everything else about the vent.
| Category | Vent pressure | Flue loss | Typical appliance |
|---|---|---|---|
| I | Non-positive | Below 17% | Standard furnace or boiler |
| II | Non-positive | 17% or more | Condensing, uncommon in practice |
| III | Positive | Below 17% | Power-vented furnace |
| IV | Positive | 17% or more | High-efficiency condensing |
Two questions, asked in order. Is the vent under positive or non-positive pressure? Is the flue loss below 17%, or 17% and above? Those two answers give you the category, and the category tells you what the vent has to be.
Odd numbers — I and III — are the lower flue loss. Even numbers — II and IV — are the higher. Positive pressure is the back half, III and IV.
A positive-pressure appliance pushes combustion products into the vent, so the vent must be sealed and rated for it. Fitting the wrong vent to a Category IV appliance is a serious safety failure, which is why the exam keeps returning to it.
Category I has non-positive vent pressure and flue loss below 17%. Category II is non-positive with flue loss of 17% or more. Category III is positive pressure with flue loss below 17%. Category IV is positive pressure with flue loss of 17% or more.
One that vents under positive pressure with a flue loss of 17% or more — typically a high-efficiency condensing appliance.
One that vents under non-positive pressure with a flue loss below 17% — a standard furnace or boiler.