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Heating Capacity

Most people think of storage heaters in terms of tank and burner size — as in a 40-gallon 40,000-Btu heater (see chart). The Btu rating is a measure of fuel input to the burner; output is measured in gallons of water heated per hour to a particular temperature rise. The condensing heaters we install have an input range from 76,000 Btu all the way to 199,000 Btu. (By comparison, the typical tankless model we install is rated at 199,000 Btu.)


Recovery. The recovery rate tells how fast the heater can replenish hot water as it is drawn from the tank. Recovery is measured in gallons per hour at a 90°F temperature rise; it’s a function of the burner size (Btu input) and heat-transfer efficiency.

First-hour-rating. The number that we look at when sizing a storage heater is the first-hour-rating (FHR) — the amount of water it can provide in one hour at a 90°F temperature rise. FHR is a function of the size of the heater’s tank and the recovery rate. The tank factors in because it’s a reservoir of heated water, most of which is considered to be available for immediate use. The FHR is equal to the recovery rate plus 70 percent of the tank size.

Installation

Condensing heaters have the same footprint as conventional water heaters, so they work well for replacement jobs. The units cost more than tankless models, but because they’re easier to install in existing construction, the higher equipment cost is often offset by lower labor figures. A typical tankless heater wholesales for about $950, and a 90-percent-efficient condensing storage model for about $1,700.

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This 50-gallon condensing water heater will be installed in the space previously occupied by a 40-gallon conventional storage model.

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Many models can be connected to an existing 1/2-inch gas line, though some of the larger units require a 3/4-inch line. All condensing heaters require a 120-volt electrical circuit to run the fan and electronics.

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Condensing heaters can often connect to an existing 1/2-inch gas line. The plumber extends the line to reach the inlet at the top of the heater.

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The air intake — the PVC fitting with the screen inside — is connected to a draft-inducing fan.

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Combustion gas and condensate exit through a fitting near the bottom of the tank. The elbow connects to the flue and the condensate hose runs to a drain.

Flue. The existing flue can’t be reused, but a new plastic flue is inexpensive and easy to install. The draft is fan-induced, so flue runs can be long — up to 128 equivalent feet, depending on the heater and whether the vent is 2-, 3-, or 4-inch-diameter pipe. Makeup air can be drawn from the room or piped directly to the heater from the exterior. We try to terminate the flue at an inconspicuous location on the outside, because it may emit a visible plume of vapor and the fan may be audible there.

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A plumber installs a section of flue pipe, taking care to slope it so that condensate drains back to the heater.

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With the particular heater shown here, the flue can be ABS or PVC; in this case, it’s a combination of the two because the plumbers ran out of ABS.

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The flue passes out through the wall and terminates at a screened fitting.

Condensate. The water that condenses in the heat exchanger and flue drains to a condensate trap and is fed through a plastic hose to the nearest plumbing drain. The condensate is acidic enough to erode concrete and metal, so it has to be neutralized before discharge; this is done by running it through a cartridge filled with crushed limestone or marble. The cartridge needs to be checked yearly and the stones topped off or replaced if they’ve dissolved.

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Condensate is acidic and should be neutralized, which is done by running it through a cartridge filled with marble chips or limestone. On this job, the condensate is pumped into an air-gap fitting on a drain line. At right, the condensate from a pair of heaters flows into a floor drain (both heaters have neutralizers, though only one is visible).

Base Models

Condensing heaters have been used commercially for about 15 years; they’re a proven technology. Because of the size of their burners, the heaters from the four manufacturers targeting the residential market are technically commercial units. Most of them have electronic controls and diagnostic sensors that can be accessed by a digital screen.

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In this installation, the heater and condensate pump are plugged into a nearby electrical outlet.

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A digital readout allows the homeowner to adjust the water temperature and the plumber to diagnose problems.

Vertex and Premier. The least expensive condensing heater is A.O. Smith’s Vertex. It’s sold in two versions, both with 50-gallon tanks: a 76,000-Btu 90-percent-TE unit and a 100,000-Btu 96-percent-TE unit. Both have glass-lined tanks and taps that allow them to be used for combination space-heating and water-heating applications. The same heaters are also sold by State Water Heaters under the Premier brand.

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The Vertex (A.O. Smith) and Premier Power-Vent (State Water Heaters) heaters are the same units sold under different labels. Shown here is a 96-percent-efficient Vertex and a 90-percent-efficient Premier Power-Vent model.

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The Vertex (A.O. Smith) and Premier Power-Vent (State Water Heaters) heaters are the same units sold under different labels. Shown here is a 96-percent-efficient Vertex and a 90-percent-efficient Premier Power-Vent model.

We like these heaters for retrofits because they’re easy to install. They can often use existing gas lines, so field-supplied materials are limited to piping and fittings near the heater, the neutralizer cartridge, and flue pipe. The heater itself is prewired; all we have to do is plug it into an adjacent outlet. As a replacement unit, the installed cost of one of these heaters is frequently less than the installed cost of a comparable tankless model.

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The Polaris has a stainless steel tank and a simple interface — indicator lights and a dial for setting temperature.

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This Phoenix heater provides hot-water storage and boiler backup for a solar thermal space-heating (radiant) and domestic hot-water system.

Premium Units

The next step up is to a Polaris, made by American Water Heaters, an A.O. Smith company, or a Phoenix, manufactured by Heat Transfer Products. These heaters have long-lasting stainless steel tanks and come in a variety of tank and burner sizes, with inputs up to 199,000 Btu. Both brands include taps that allow them to be used for combined space and water heating. Several of the Phoenix models also contain heat exchangers that can be connected to solar collectors for heating or preheating the water. Although we have installed both brands, we have more experience with the Phoenix because it’s readily available and better supported in our area.

Jim Lunt co-owns The Lunt Marymor Co. in Emeryville, Calif., with Leigh Marymor.