Setback System On Heat Pump Might Actually Be Energy Waster

HOME CLINIC: Your Dec. 2 article on home weatherizing contained one item that I would like explained further.

I have heard much about thermostat setback at night to save energy. When we had a gas furnace we bought a thermostat that automatically lowered the temperature setting at night and raised it back up in the morning. Our new house has a heat pump. I asked the installer about operation, including setback. He stated that the setback method was an energy waster. He said the amount of energy needed to reheat the house and stabilize the temperature was more than the savings. He said this was due to setting the temperature back during the night, and that the most efficient operation was to set the temperature and leave it alone.

So where does the truth lie? Is setback an energy saver or waster? Turning up the temperature on a heat-pump system kicks in the backup electric system and that really makes the meter dial spin.

- M.S., Kent

DEAR M.S.: Unfortunately, the installer's statement was misleading. Thermostat setback is generally an excellent energy-saving method. Setting your thermostat back 10 degrees at night - or during periods when you are away from home for more than a few hours - can save about 10 percent on your energy bill.

But the installer was correct about heat pumps. Regular thermostat setback on heat pumps can actually be an energy waster.

Here's why.

Heat pumps are very efficient: They can heat your home more economically than standard electrical resistance-type heaters. Therefore, it is always best to run your heat pump instead of electric resistance heating systems.

But, there is a trade-off for this efficiency - the air supplied from heat pumps is generally cooler than air supplied from standard electric resistance, gas, or oil heaters. This means it takes longer for a heat pump to heat up your house after a setback period.

For example, on a cold morning, after a standard night setback, your heat pump will come on to warm the house. But since the temperature of the supply air is fairly low, it will take longer for the house to warm up (this is known as ``recovery time''). Your thermostat will sense this ``delay'' and automatically turn on the electric resistance ``secondary (auxiliary or backup) heat'' to warm the house more rapidly.

This use of the secondary heat system cancels out any energy savings that you had gained by the night setback.

Therefore, by allowing the heat pump to chug along all night you can help prevent the electric resistance heater from coming on in the morning and actually save energy and money in the long run.

There is a setback option for heat pumps. You can install a special setback thermostat made especially for heat pumps. These units have an internal microprocessor that actually ``learns'' how long it takes for your house to recover from various setback conditions (it takes about two weeks after installation for the unit to completely understand how your house works).

These units automatically adjust the amount of setback - appropriate for your house - in order to maximize efficiency. For example, during very cold weather the thermostat may not setback the temperature at all because it ``knows'' that the more expensive secondary heat will be required in the morning.

Estimated savings with these devices is in the 8 percent to 12 percent range. Make sure you use only special heat-pump setback thermostats.

Oil Help Hotline

By calling the Oil Help Hotline, 1-800-544-8958 you can receive written information on tips for reducing your use of gas- and home-heating oil. Titles include ``Watching Where Your Gas Goes'' and ``Saving Oil At Home.'' A separate packet is available for employers. The hotline is operated by the Washington State Energy Office.

Home Clinic answers questions about home maintenance, repair and energy conservation. It is prepared by the Energy Extension Service, a division of the Washington State Energy Office. It appears Sundays in the Home/Real Estate section of The Times.