Question - My utility bills are skyrocketing. Does it make sense to replace my noisy old heat pump, even though it still works, with a new super-efficient model? What comfort and efficiency features should I consider? - W. H.

Answer - Replacing your old heat pump with a new quiet model can lower your heating costs by 30 percent to 40 percent. New super-efficient heat pumps can produce up to $3 worth of heat for each $1 on your utility bills. The new designs are much more reliable and quiet than your old one.

Even if someone has a gas furnace, installing a heat pump in place of an old central air conditioner can lower both heating and cooling costs. In mild weather, a heat pump can heat cheaper than an old gas furnace.

The newest innovation in heat pumps is multiple- and totally-variable-speed compressors and fans. These heat pumps vary the heat (and cooling) output into your house as the outdoor temperature and weather conditions change.

The majority of the time, the heat pump is running at a slower speed. This reduces the amount of electricity used. During very cold or hot weather, the compressor automatically shifts to a higher speed. Variable-speed electric condenser fan motors are also more efficient.

By coupling this with a new variable-speed indoor blower, it fine tunes the heat output. At the long-running slower speed, it cycles on and off less often. This maintains a constant indoor temperature. Also, if you use a central air cleaner, its overall effectiveness is improved.

The most efficient single-speed heat pumps use a scroll compressor. Scroll compressors operate differently than standard compressors and have few moving parts. Without the pistons and hardware of standard compressors, scroll compressors are much quieter.

As these few scroll moving parts wear over years of operation, they actually seal better and operate smoother than when they were new. The basic design is very reliable and they should continue to operate at high efficiency-levels as they age.

When you compare efficiencies of various heat pumps, use the Heating Season Performance Factor (HSPF) for heating, and Seasonal Energy Efficiency Ratio (SEER) for cooling. Also compare the type of compressor and speeds, which effect comfort and noise level. The most expensive model is not always the best choice for your specific house.

Write for Utility Bills Update No. 786 showing a buyer's guide of 15 superefficient heat pumps, HSPF and SEER efficiency ratings, heat and cooling output capacities, compressor types and speeds, and an annual savings chart. Write to James Dulley, Deseret News, 6906 Royalgreen Dr., Cincinnati, OH 45244. Please include $2.00 handling fee - cash or check.

Question - I want to help reduce the greenhouse effect. About how much carbon dioxide (greenhouse gas) is produced per person each year? - H.K.

Answer - The majority of the carbon dioxide gas (CO2) produced is from the burning of fossil fuels to produce energy. On average, more than 40,000 pounds of CO2 is released into the air per year for each resident of the United States.

To run at typical average-efficiency refrigerator for one year, a coal-fired power plant produces more than 2,000 pounds of CO2. Switching to a new high-efficiency refrigerator can reduce this amount by about 600 pounds.

A general rule of thumb is that for each kilowatt-hour of electricity saved, 1.5 to 2 pounds of CO2 is prevented from entering the atmosphere.