March 23, 2023
If you have questions or comments, join the discussion on our LinkedIn forum.
From the 2021 UPC Illustrated Training Manual, Chapter Chapter 5, WATER HEATERS
509.3.1 Appliance Draft Requirements. A venting system shall satisfy the draft requirements of the appliance in accordance with the manufacturer’s instructions. [NFPA 54:12.4.1]
This section provides an overall performance requirement for a natural draft vent system. The vent system must remove all the products of combustion from gas appliances from a building. The principle on which natural draft vents operate is simple. Reduced to fundamentals, heat is the power that operates a natural draft vent or chimney, as shown in Figure 509.3.1a. Combustion gases rise in the chimney or vent only because they are hotter and, therefore, lighter, than the surrounding air. The hotter the gases and the higher the vent, the more swiftly and powerfully the gases will rise. Conversely, the cooler the gases and the shorter the vent, the more sluggish the gases’ movement will be. The flue gases in the vent must remain hot enough over the length of the vent to provide a strong draft. If cooled enough, the upward motion of the gases stops altogether and combustion gases can spill into the building through the relief opening of the appliance draft hood.
When the combustion products and dilution air rise in the vent, this volume must be replaced by air from outside the building. This replacement air can be supplied through normal air infiltration (through small openings in the building walls), through outdoor openings purposely installed in outside walls or by a mechanical air system. Thus, to ensure proper vent operation, the building’s air tightness and other devices exhausting air from the building must be taken into account.
Fan-assisted combustion appliances cause some confusion because they are listed as Category I appliances, meaning that their vents operate by natural draft due to the heat of the vent gas. However, these appliances have a fan to assist the flow of the combustion product through the appliance. The fan is necessary because modern, higher-efficiency appliances have a higher pressure drop through their heat exchangers. The pressure provided by the induced draft blower is matched carefully to the resistance of the heat exchanger. Once the combustion products exit the appliance, the natural buoyancy takes over in the vent. If the vent is designed using the tables of Section 510.0, the pressure in the vent system will be negative.
Some furnaces are listed as Category I or III appliances, depending on the method used to vent the appliance. The fan in these furnaces is sized to result in negative vent pressure when connected to a properly sized vertical vent (Category I) or in a positive vent pressure when vented horizontally (Category III). Category III installation requires using venting materials recommended by the manufacturer’s installation instructions.
Blockage of a natural draft vent may cause flue gases to spill into the building through the relief opening of the draft hood. Spillage can also be caused if the pressure in the vicinity of the appliance is much lower than the pressure outside the building (depressurization). Depressurization may be caused by mechanical exhausts, fireplaces, wind — anything that removes air from the building. Consequently, periodically verifying the performance of appliance venting systems is wise. Verification can be done readily with a natural draft system, described as follows:
• Operate the appliance for at least 5 minutes to allow the flue gases to heat the vent.
• Move a lighted match across the entire width of the draft hood relief opening (see Figure 509.3.1b).
• If the match flame is blown downward or extinguished, have the venting system or chimney checked by a qualified agency.
• If the appliance is not equipped with a draft hood, the match test may not be applicable. In this case, the plumber should consult the appliance instruction manual.
Good air circulation in adequate amounts is also vital for good venting and effective appliance operation. Signs of improper operation include a yellow or wavering flame, discoloration around access doors, a pungent odor in the building, excessive humidity, condensation or mold, corrosion of the vent material and soot near the burner or vent area. If any of these conditions are observed, a qualified agency should be contacted to have the installation inspected.
Under extremely adverse conditions, carbon monoxide can be produced as a result of either improper venting of combustion products, insufficient fresh air to support the proper burning of gas, or improperly adjusted appliances. If symptoms of carbon monoxide poisoning are experienced — headache, yawning, ringing in ears, weariness, vomiting, heart fluttering or throbbing — fresh air is needed promptly.
Open windows or go outdoors. Any gas appliance suspected of operating improperly should be shut off and immediately checked by a qualified agency.
The 2021 Uniform Plumbing Code Illustrated Training Manual is available for purchase here.
(This is not to be considered the official position of IAPMO, nor is it an official interpretation of the Codes.)
Last modified: March 23, 2023