Venting Gas Fireplaces - Continued

Water heaters have slightly less stringent efficiency standards, so most of them still use the old draft-hood technology and rely on natural convection to pull air into the combustion chamber. The draft hoods let dilution air enter the flue, which provides some drying (but at the cost of lower total efficiency).

One cubic foot of natural gas supplies about 1,000 Btus of energy. Fan-assisted units need about 15 cu.ft. of air to fully combust that 1 cu.ft. of fuel, producing about 16 cu.ft. of cool, damp exhaust. Draft-hood units require an additional 14 or 15 cu.ft. of dilution air for each 1 cu.ft. of fuel consumed, producing roughly 30 cu.ft. of relatively drier exhaust products for the flue to handle. (The ratios are slightly different for propane, but the principle is the same.)

This difference between fan-assist and draft-hood performance has an effect on venting requirements. Because a draft-hood appliance creates a large volume of exhaust, it needs a taller or larger-diameter vent than a fan-assisted unit of the same Btu rating. Oversizing, however, isn't a problem: A vent serving a draft-hood unit can only be too small, not too large.

A vent that serves a fan-assisted appliance, on the other hand, can be too small or too large. Because its exhaust gas is relatively cool and damp, the fan-assisted unit won't warm the vent quickly enough to prevent damaging condensation (and with fan-assisted equipment, there's no drying airflow through the system when the unit's not running). So for each vent configuration listed (height, lateral run, and diameter), the tables show both a minimum and maximum allowable Btu rating for fan-assisted equipment ("fan min." and "fan max."). For draft-hood equipment, the tables show only the largest equipment permitted on the vent ("nat max.") -- there is no minimum. For two appliances "common-vented" on the same flue, the tables provide a complex range of options depending on whether the setup involves two "fan" appliances, two "nat" appliances, or one of each.

Vent materials. The sizing tables also must account for the different behavior of different vent materials. Masonry chimneys take a long time to heat up and are prone to condensation, so they can serve only a very restricted range of appliance sizes (and seldom if ever can handle a fan-assisted unit by itself). Chimneys on outside walls are especially problematic and should generally be relined with a listed metal liner for use with gas. B vent (the standardized, listed double-wall insulated vent pipe material) is by far preferable to masonry and permits a much wider range of equipment sizes. B vents and chimneys each get their own sizing table in the code.

Single-wall metal vent connectors, though still in wide use as the least expensive choice, also limit both the minimum and maximum sizes of equipment permitted on a given size of vent. They lose heat more quickly than B vent connectors, weakening the draft and raising the risk of condensation wherever they're used. They're also less fire safe: They require a 6-inch clearance from combustibles, compared to 1 inch for B vent, and they can't run in a concealed space or through an unheated attic or crawlspace. In general, single-wall vent connectors are best avoided, in spite of their lower cost. Still, sizing tables are provided for single-wall vent connectors, either with masonry chimneys or with B vents.

Using the Sizing Tables

In matching equipment to vents, the pitfalls are many. Too small a vent diameter, and you won't have enough draft; too big, and you might get condensation. Too long a horizontal connector, too short a vertical vent, too low a connection to the vent, too many elbows, too small a vent diameter, or all of the above, and the vent won't draw. A setup that will work with B vent might not work with a masonry chimney; one that works with double-wall insulated vent connectors might not work with single-wall vent connectors.

The sizing tables reflect all of that complexity, with more than ten different tables that apply to different situations. We're not going to teach everybody how to use the sizing tables like a pro with this article. That takes training and practice, and even experienced professionals find it easier to use a computer. But we'll give you a few examples just to clarify the idea.

A single appliance. Let's start with the simplest case: a single appliance connected directly to a vent or chimney. You could connect it to a B vent with a B vent connector, to a B vent with a single-wall connector, to a masonry chimney with a B vent connector, or to a masonry chimney with a single-wall connector. The tables show you how big an appliance you can use in each case, for various heights, horizontal runs, and diameters of vents.

Figure 3 shows just a small section cut from the appropriate sizing table for each of those four options. We're showing what the table tells you for either a 10-foot or a 15-foot vent rise, with a range of horizontal runs. If the rise falls between 10 and 15, you're allowed to interpolate -- but let's not go there.