Both my gas hot water heater and forced-air furnace are going to need to be replaced soon. Are there any types of energy-efficient combination systems that can heat my house and the hot water? M.F.

A - There are several combination gas (natural or bottled) that can heat your house and hot water. Some of the large systems are at the super-high-efficiency levels - about 94 percent. Several of the smaller output systems are at the high-efficiency levels - about 85 percent.

The larger 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 flows 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 additional hot water, a pump circulates the very hot water through a heat exchanger in the water heater tank. The heating capacity of these systems is over 90,000 Btu per hour.

The smaller, high-efficiency systems use a more-standard hot 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 to heat the air. It is still about 120 degrees when it returns to the water heater. This is still hot enough for most household hot water needs.

These smaller systems are a good fit with an existing heat pump. During the very mild weather, the heat pump is very efficient. When it gets colder, the gas water heater provides the backup heat instead of electricity-guzzling strip elements.

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. Write to James Dulley, The Deseret News, 6906 Royal Green Drive, Cincinnati, OH 45244. Please include $1 and a self-addressed STAMPED BUSINESS-SIZE envelope.

Q - My house is constructed with brick veneer and insulated 2x4 studded walls. I would like to increase the wall R-value to at least R-19. Is it worthwhile to add additional insulation to the indoor wall surface? J.E.

A - Generally, unless you are going to stay in your house for a very long time, it would probably not be feasible to add foam insulation board indoors. If you are involved in a remodeling job that requires the removal of much of the interior wall surface, then it may be feasible.

Another option is to add insulation to the exterior surface of your walls. This is the most efficient location for insulation. It blocks many of the air leaks into your house and your existing wall provides thermal mass. Several companies make special complete exterior insulation systems.