Question - We don't have enough hot water for morning showers, and my heating bills are outrageous. How effective and efficient are the new high output water heaters that replace the furnace for space heating too? - T. F.

AnswerRecent innovations in super-efficient and high-output water heaters have made combination space/water heating feasible. These systems provide unlimited hot water, enough for many simultaneous showers, dishwashing and clothes washing while heating your entire house.

A combination system offers many advantages over using a conventional separate water heater and furnace. Overall efficiency is higher. There is only one appliance to maintain. It is more adaptable to comfortable zone heating - thermostats for different temperatures in different rooms.

The highest efficiency systems use condensing heat exchangers that do not require a chimney. Combined efficiencies are above 90 percent. This is much higher than overall figures for typical separate furnaces and water heaters.

These systems vent outdoors horizontally through a small plastic pipe. This is ideal for converting from costly electric heat to natural or bottled gas. A sealed combustion design pipes outdoor air directly into the burner. This improves efficiency, and minimizes drafts and the possibility of CO poisoning.

A combination system basically consists of a high output water heater and a heater exchanger coil in furnace blower cabinet. When your thermostat calls for heat, a small pump circulates hot water through the blower coil.

As the coil quickly heats up, the blower starts. This circulates warm air through your existing ducts. As the water heater tank cools, its burner comes on. The central air-conditioner still works the same as before.

The temperature of the water drops only about 20 degrees as it passes through the heater exchanger coil to heat your house. With the water heater set at 140 degrees, the return water is still adequately hot, 120 degrees.

Integrated (in one cabinet) combination systems are available to replace both your existing units. These use stainless steel tanks and components for a long corrosion-free life. The first hour hot water rating for one model, Complete Heat, is 200 gallons, more than you will ever need.

If your existing water heater is new, you can install just a heat exchanger/blower unit to use it as the heat source. The quiet blower unit can be located up to 30 feet from your water heater. Some blowers are small enough to be recessed inside the wall for heating just one chilly room.

Write for Utility Bills Update No. 969 listing 10 manufacturers of complete combination systems, heat exchanger/blower units, high efficiency water heaters, prices, size estimating chart, and specifications on no-chimney condensing units. Please include $2.00 and a self-addressed envelope. Write to James Dulley, Deseret News, 6906 Royalgreen Dr., Cincinnati, OH 45244.

Question - We are in the design stage of adding a bedroom to our house. Is it necessary to locate the hot air register under a window? We would like to place a sofa under the larger window. - E.D.

Answer - - It is not necessary to locate registers under windows. They are normally located under windows to minimize chilly drafts in the winter. Room air near the cold window cools, becomes denser and drops to the floor. This often creates a draft if you sit on the floor.

Another reason is to minimize window condensation in very cold weather. If you locate the register elsewhere and condensation forms, you may have to lower the indoor humidity level to an uncomfortable level to stop it.