CSA ANSI Z21.12-1990 (R2020): Draft Hoods

Technician repairing a draft hood in a gas furnace that adheres to safety procedure in CSA ANSI Z21.12-1990 (R2020).

A draft hood is a necessary part of any gas burning appliance. For a gas furnace, a home heating appliance that runs off natural gas, it ensures steady air flow to the burners to avoid flares or the pilot light being put out by fluctuation in temperature and air flow. CSA ANSI Z21.12-1990 (R2020): Draft Hoods applies to the safe operation, substantial, and durable construction, and acceptable performance of draft hoods for gas appliances.

What Is a Draft Hood?

A draft hood is a component of gas-fired appliances designed to maintain air flow to the chimney. This device, which is placed above the upper most part of the appliance, is designed to assure the ready escape of all the products of combustion. The draft hood mixes secondary air with the combustion gases thereby enabling a smooth, continuous relief of gases up the vent pipe. It makes it possible to draw more or less air via the chimney as necessary to create a constant flow, making it possible for the burner to enjoy consistent air flow without any wind gusts or sudden temperature spikes or drops.

What Is the Purpose of a Draft Hood?

CSA ANSI Z21.12-1990 (R2020) details that a draft hood is “a device built into an appliance or made a part of the vent connector from an appliance, which is designed to:

  1. Provide for ready escape of flue gases from the appliance in the event of no draft, backdraft, or stoppage beyond the draft hood;
  2. Prevent a backdraft from entering the appliance; and
  3. Neutralize the effect of stack action of the chimney or gas vent upon the operation of the appliance.”

What Is CSA ANSI Z21.12?

CSA ANSI Z21.12-1990 (R2020) covers separately constructed draft hoods for installation on appliances required to be vented. This American National Standard applies to newly produced draft hoods constructed entirely of new, unused parts and materials:

  • For use with natural, manufactured, mixed, and liquefied petroleum gases, and LP gas-air mixtures
  • For use with natural, manufactured, and mixed gases 

What Is the Stack (Chimney) Effect?

The stack effect or chimney effect is the movement of air into and out of buildings through unsealed openings, chimneys, flue-gas stacks, or other containers, caused by thermal differences. The stack effect results from air buoyancy, which occurs due to a difference in indoor-to-outdoor air density. Hot air is less dense than cool air, so it rises and in so doing, it creates a pressure difference with lower pressure below and higher pressure above.

Why Is a Draft Hood Important?

A draft is created in fuel-fired furnaces when high temperature gases are discharged at a level higher than the furnace openings. Hot air, if not put through a draft hood, would create a strong air flow through the burners. A draft hood cools the air since it is released by the burners from 500 degrees F to between 300 degrees F and 350 degrees F. The cooling needs to be carefully calibrated to avoid condensation build up in the chimney. This problem occurs when the temperature gets too low.

CSA ANSI Z21.12-1990 (R2020): Draft Hoods is available on the ANSI Webstore.

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