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Venting Gas Appliances - Continued

Common venting. Now let's add a complication: two appliances common-vented on a single flue (Figure 4). The table now has to consider "vent connector rise," and it matters whether we've got two "nat" appliances, two "fan" appliances, or one of each.

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Figure 4.When two units are commonvented, each vent connector must be sized independently, and vent connector capacity dictates appliance size ranges. Again, single-wall vent connectors sharply restrict the choice, and for practical purposes fan-assisted units will nearly always require B vent. Given a 10-foot vent height and a 3-foot connector rise (circled in table), a B vent connector could handle a fan-assisted unit between 7,000 and 92,000 Btu/hr (for 4-inch-diameter pipe) or between 69,000 and 220,000 Btu/hr (for 6-inch-diameter pipe). If single-wall connectors were used, the appliance on the 4-inch connector would have to fall between 89,000 and 91,000 Btu/hr — essentially no choice at all. Increasing connector diameter boosts both the maximum and the minimum, leaving the allowable range quite narrow. Many units would be too large for a 4-inch connector and too small for a 6-inch connector.

Of course we still have all the various vent types to choose from: B vent or masonry chimney, B vent connector or single-wall. As you see, chimneys don't give you much choice compared to the B vent options.

Improving the venting arrangement can widen your equipment options (Figure 5). If the table won't permit the appliance you need, try to increase total vent height or connector rise by repositioning the appliance or rerouting the vent. Consider the options carefully: Common-venting two appliances with a single vent often lets a vent handle more total Btus and conserves material, but if it takes too many elbows and horizontal runs to accomplish, you may be better off giving each unit its own straight vent. That way, neither unit will ever have to operate alone on a flue that is sized for two.

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Figure 5.Reconfiguring the vent to gain total height or connector height can improve the performance of a common vent. Venting each unit separately can also improve reliability, because it right-sizes each vent, whereas the common vent is oversized whenever either appliance operates alone (and requires lateral runs and elbows that degrade performance).

You can see why a lot of hvac contractors don't use these tables. It's a real pain in the neck. But with software available to automate the process, there's no excuse for a professional to ignore the rules. And most cases aren't that complicated -- once you get the hang of the system, you can home in on working solutions without too much trouble.

However, it's important to realize that the tables don't account for every possibility. They don't include the effect of elbows in the system, for example: Each table assumes no more than two 90-degree bends, and vent capacity is reduced by 10% for each additional 90. And all the main table numbers are based on vents that run inside the house: Vents placed outside the walls are more prone to failure. (There are some special tables for masonry chimneys on exterior walls, but the allowable unit minimums are quite large; it's always better to reline the chimney with a listed metal liner instead.)

With experience, people learn how to make adjustments when the tables limit their choices. Reconfiguring the vent to increase total vent height or connector rise is one way to increase a vent's capacity when a large appliance is called for. This may be more economical than using a larger-diameter vent.

Do's and Don'ts of Venting

The rules are based on some fairly simple ideas: Hot air rises, and flue gases will condense if they cool; a strong draft is good, and condensation is bad. The important ideas are to supply plenty of combustion air, to keep the flue warm and dry, and not to restrict the flow of exhaust in the flue. If you try to maximize all the good influences and minimize all the bad influences, most systems will fall well within the permissible range in the tables. A few basic do's and don'ts will take you a long way toward getting good results:

* Never use unlined chimneys. Avoid masonry chimneys even with tile liners. Reline chimneys with listed liners or B vent.

* Avoid single-wall vent connectors if possible. If you must use a single-wall connector, choose a corrosion-resistant brand. Single-wall vent pipe cannot pass through unconditioned space (attic or crawlspace) and cannot run through concealed spaces. It must be readily accessible for inspection, cleaning, and replacement.

* Keep the vent or chimney within the conditioned envelope.

* Keep appliance locations warm.

* Maximize total vent height (in all cases provide at least 5 feet of vent above the highest draft hood or flue collar).

* Maximize vertical connector rise off the appliance (Figure 6).

* Avoid long horizontal runs.

* Maintain correct pitch in horizontal connectors (no sag). Support vent connectors properly.

* Avoid elbows.

* Use 45s and 60s instead of 90s -- two 45s equals one 90 (Figure 7).

* Use wyes rather than tees.

* Consider independent venting for separate appliances.

* Avoid oversizing equipment.

* Plug any unused openings in the chimney or vent.

* Observe required clearances.

* Use an approved firestop system wherever B vent passes through a wall or floor. Never run single-wall vent pipe through any wall or floor.

* Don't share flues with oil, coal, or wood. Mark all gas vent systems clearly to prevent their use by any other type of appliance.

* Do not allow two vent connectors to enter a main vent or chimney opposite each other. Instead, offset the connections.

* Terminate vents well away from walls and well above steep sloped roofs. Use UL-listed vent caps, not locally made or DIY caps.

* Never connect positive-pressure vent systems to a negative-pressure vent system.

* Isolate appliances from house indoor pressures and provide an air supply from outdoors.

* Always test combustion products and draft after installation.

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Figure 6.When space allows, always maximize the rise of the connector directly off the appliance — this helps to develop a strong draft. A minimum of 1 foot is required, but more is better.

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Figure 7.When elbows are needed, 45-degree or 60-degree bends are preferable to 90-degree bends. The sizing tables assume two 90-degree elbows per appliance, and each additional 90-degree elbow reduces vent capacity by 10%. The two extra 90-degree bends in the left-hand example reduce the vent’s capacity by 20%; if 45-degree bends are used instead, the capacity reduction is only 10%.