Q: My old central air conditioner costs lot to operate. What type of replacement unit will be cheapest to run?

A: There are many new super-high efficiency central air conditioners available and many use Earth-friendly (ozone-safe) R410A refrigerant.

Assuming your old air conditioner is of typical efficiency, installing a new one with the highest seasonal energy efficiency ratio of 21 will consume less than 50 percent as much electricity as your old one. Also, the newer outdoor compressor/condenser unit operates quieter than old ones.

The payback period from the utility bills savings depends upon your climate and the efficiency of your house. Particularly if your house is not energy efficient (old windows, little insulation, leaky ducts, etc.), the 50 percent savings can provide and excellent return on your investment. It also helps the environment.

Because you want a new central air conditioner with the lowest operating cost, have your heating/cooling contractor provide a quote on a two-stage model with a variable-speed ECM blower motor and matching thermostat.

This setup can cost several thousand dollars more than the cheapest single-stage model.

If you go this route, your contractor will likely recommend you also install a new two-stage or modulating furnace at the same time. This is because the furnace and central air conditioner use the same controls, thermostat and blower motor. Even if you do not install a new furnace, you will have to install a new indoor cooling coil to attain the high efficiency.

A two-stage model operates at a low-cooling-output level about 80 percent of the time. This uses less electricity and provides more even cooling. With the newest communicating thermostats, the indoor air temperature varies less than one-half degree from the setting.

This allows you to set the thermostat a degree or two higher for more savings without sacrificing comfort. With a telephone or system access module, you can change the setting remotely.

Even though it costs a little more, install a variable-speed blower motor. This allows the cooling system to constantly vary the air flow for maximum cooling or maximum dehumidification or any combination you like.

This also lowers the indoor noise level from the air flowing through the ducts. With the soft-start motor speed, it is barely noticeable when it comes on each time.

Write for (or download at www.dulley.com) Update Bulletin No. 921 -- buyer's guide of the most efficient 2008 central air conditioners listing stages, output, SEER, refrigerant, sound level, warranty and a payback chart. Include $3 and a business-size self-addressed, stamped envelope.