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Dual-purpose water heater

Question: I need to replace my water heater soon. Are there any types of energy-efficient water heaters that can also heat my house through the central air-conditioner forced air system? _ M.F. Answer: There are several natural or bottled gas combination systems that can heat your house and hot water. These are an ideal fit with the milder heating needs of your climate.

Several of the smaller output systems are at the high-efficiency levels _ about 85 percent. Some of the large systems are at the super-high-efficiency levels _ about 94 percent.

The super-high-efficiency systems are of a condensing heat exchanger design. The cool exhaust flue gases are vented outdoors through a plastic pipe. Therefore, you do not need a chimney. This makes them an excellent replacement for an electric system where no chimney now exists.

One of these systems is basically a large heavily insulated water heater. It circulates the hot water through a heat exchanger coil in the furnace air blower section. When your wall thermostat calls for heat, a small pump pushes the hot water through the heat exchanger.

The other super-high-efficiency system is basically a boiler with a heat exchanger coil in the blower.

When you need hot water, a pump circulates the very hot water through a heat exchanger in the water heater tank.

The smaller high-efficiency systems use a more-standard water heater. It is connected to a heat exchanger coil in the furnace blower. The water temperature of about 140 degrees circulates to the heat exchanger, which heats the air.

It is still about 120 degrees when it returns to the water heater. This is hot enough for most household hot water needs.

These smaller systems are a good fit with an existing heat pump. During mild weather, the heat pump is very efficient. When it gets a little colder, the gas water heater provides less expensive heat.

One high-efficiency combination unit has the blower and duct connections built into the top of the water heater. All you have to do is attach the forced air ducts. This saves floor space and is easy to install.

With any of these combination systems, a central air conditioner is used as usual. The evaporator coil from the outdoor condenser unit is located in the air handler blower section.

During the summer, the water heater heats only water, and the water is not circulated to theblower.

You can write to me for Utility Bills Update No. 012 showing a list of manufacturers and output specifications for the large super-high-efficiency and the smaller high-efficiency combination water/space heating systems.

Please include $1 and a self-addressed stampeD business-size envelope to the address listed below.

James Dulley is an engineer whose column on cutting utility bills appears twice a month. Questions can be addressed to James Dulley, Cut Your Utility Bills, St. Petersburg Times, 6906 Royalgreen Drive, Cincinnati, Ohio 45244.

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