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Fundamentals of Venting and Ventilation

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Fundamentals of Venting and Ventilation Fundamentals of Venting and Ventilation • Vent System Operation • Sizing, Design and Applications • Installation and Assembly • Troubleshooting © American Standard Inc. 1993 Pub. No. 34-4010-02 Contents Subject Page • Motive force in vents ................................................................................................................................... 2 • Factors affecting operation ......................................................................................................................... 3 • Ventilation air requirements........................................................................................................................ 4 • Vent design considerations - ....................................................................................................................... 6 – Category I furnaces............................................................................................................................... 6 – Category II furnaces.............................................................................................................................. 6 – Category III furnaces ............................................................................................................................. 6 – Category IV furnaces ............................................................................................................................ 7 • Types of vents .............................................................................................................................................. 7 – Vent classifications ............................................................................................................................... 7 – Vent materials ....................................................................................................................................... 9 • Vent system design configurations ............................................................................................................ 9 – Single appliance natural draft .............................................................................................................. 9 – Multiple appliance – single story common venting............................................................................ 9 – Manifold venting .................................................................................................................................. 11 – Multi-story vent systems ...................................................................................................................... 11 • Sizing and Design .......................................................................................................................................... 13 – Category I furnaces • Single appliance w/masonry chimney ............................................................................................ 13 • Single appliance Type B vent .......................................................................................................... 14 • Multiple appliance single story vents- common venting .............................................................. 16 • Multi-Story Vent Systems - common venting ................................................................................ 17 • Vents in high altitutde locations ...................................................................................................... 19 • Power venting .................................................................................................................................. 19 – Category II furnaces ............................................................................................................................. 19 – Category III furnaces ............................................................................................................................ 20 – Category IV furnaces ............................................................................................................................ 21 • Combustion and Ventilation Air.................................................................................................................. 22 – Confined space ..................................................................................................................................... 22 – Unconfined space ................................................................................................................................. 23 • General Installation ..................................................................................................................................... 24 • Safeties and Accessories ............................................................................................................................ 27 • Prohibited Installations ............................................................................................................................... 28 • Troubleshooting .......................................................................................................................................... 30 • Appendix – Vent System Design Tables ..................................................................................................... 32 • Glossary of Terms........................................................................................................................................ 37 Portions of the text and illustrations in this book are used by permission of the copyright holder, American Gas Association. 1 Venting and Ventilation When man first began to burn fuel for warmth, he soon learned that the smoke, created during combustion, caused problems when it was allowed to collect at the source. His early caves were selected and later lodges were constructed with an escape for these unpleasant fumes. As he began to employ natural gas as a fuel, he continued to use what he had learned. Today, we still find that proper vent system operation is vital to safe and efficient furnace operation. In this manual, we will discover the things which are needed to provide this vent system. We will also learn that a gas furnace requires a substantial amount of fresh air to operate properly. This is called ventilation air. This air is used in combustion and as a re- placement for the air that a modern home loses through outward flow due to mechanical ventilation. Although we cannot control the loss of air, we can properly diagnose Figure 1 it if it becomes a problem and suggest cures. The following topics are covered in this manual: Heat drives the flue gasses out of the furnace because heat • How a natural draft (gravity) vent works. always moves from warmer areas to cooler areas. The hot • Fan assisted vent systems. flue-gases would much rather travel up the relatively cool • Power venting. flue pipe than to “back-track” to the much hotter main burner • Sizing and designing vent systems. flame. • Supplying adequate amounts of fresh air for The final force, which results from the heat which is created, ventilation. is the pressure difference found between the inside of the • Vent system installation. vent and the atmosphere around it. The combustion air entering the furnace is at atmospheric pressure which, at • Troubleshooting. sea level, is about 14.7 PSIA (Pounds to the Square Inch Using the guidelines in this book can help to ensure safe Absolute pressure). The hot flue-gases rising out of the and reliable vent performance for both natural draft and furnace are less dense, creating a pressure drop within fan assisted furnaces. the heat exchanger. This “gap” is continually being filled by the higher pressure entering air/gas mixture. So, in effect, Note: No two installations are alike! Take the time the gasses entering the furnace are helping to “push” the to study each application. Plan ahead, use good flue gasses out of the furnace. See Figure 2. judgment, and always comply with national, state, and local codes. Never sacrifice safety for efficiency or ease VENT of installation! GASES Motive Force in Vents What causes a natural draft vent to operate? Heat! As we all know, heat rises. This is because heated molecules have a tendency to become agitated and to move about. Be- ATMOSPHERIC PRESSURE cause of all this motion, the molecules “bump” into each other creating more space in between them. See Figure HEAT 1. This makes the substance less dense - lighter. Referring RISES COMBUSTION AND to Figure 1: As heat is applied to the air inside the balloon EXCESS AIR it expands and becomes lighter. When enough heat is applied, GAS AND PRIMARY AIR the balloon will become lighter than the air around it and begin to rise. For this reason, gas furnaces are designed with the heat exchanger outlet located above the inlet. This allows the heated products of combustion to travel in a natural, upward path forming a natural draft! To come back down, the balloonist in Figure 1 merely has to stop applying heat. Soon, the air in the balloon will begin to cool and become heavier. The balloon will then start Figure 2 dropping back to earth. 2 Factors Affecting Vent System Design Vent Height and Operation Where vent height is concerned, taller is better. As an illustration There are many variables dictating how well a vent of this principle, refer to Figure 3. In 3A, the air leaving the system will operate. They are: 1. flue-gas temperature heat source has only a short distance to travel before 2. heat loss in the vent 3. vent height 4. vent system capacity encountering
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