ROANOKE TIMES
Copyright (c) 1995, Roanoke Times
DATE: Sunday, December 24, 1995 TAG: 9601180002
SECTION: HOMES PAGE: E2 EDITION: METRO
COLUMN: Hints for Homeowners
SOURCE: STEVE ELDER
Uh oh! That ominous blue light is glowing on the thermostat again. This means either the phantom thermostat fiddler has struck again, or it's colder than Santa's home outside, or you didn't do any of the maintenance stuff we talked about in my ``heat pump'' article. And now the backup heat is running.
Most heat pumps have a backup heat source. The control system is designed to activate the backup source when the outside temperature drops below about 20 degrees (10 degrees for newer high-efficiency units) or when someone has cranked up the thermostat more than two or three degrees beyond the existing setting. If you see the blue light on at any other time, it usually indicates a system or maintenance problem. Most heat pump thermostats also have an ``emergency heat'' setting. This lets the backup source operate by itself without the heat pump. This setting is for use only when the heat pump is out of order.
In most cases the backup heat source is straight electric resistance heat, normally the most expensive heat you can buy. However, not to worry. There are alternatives, including use of the heat pump in conjunction with either a gas or an oil furnace. This alternative, called a dual-fuel system, goes to the head of the class as far as overall system efficiency is concerned.
Basically, it uses a heat pump in its most efficient range and a fossil fuel furnace at its optimum capacity. The new high-efficiency heat pumps will provide heat without need of back-up assistance on down to 10 degrees, but their most efficient range is between 35 and 60 degrees. On the other hand, fossil fuel furnaces operate most efficiently below 35 degrees because the cycling losses are less at lower temperatures. The ideal mating of the two is the dual-fuel system.
Each heat pump manufacturer makes a fossil fuel kit for adding a heat pump to an existing gas or oil furnace. The kits have different brand names, but they all consist of a standard heat pump thermostat, an outdoor sensing thermostat, a plenum switch and a control box. When the indoor thermostat is set on ``heat'' and the outdoor thermostat tells it that outdoor air temperature is above 35 degrees, the heat pump will operate.
If the outdoor temperature drops below 35 degrees, then the heat pump will shut off and the fossil-fuel furnace will turn on. The better control systems have an easily adjustable switch to allow the homeowner to change the switch-over point to suit personal preference. The standard setting is 35 degrees, but some like to set the point lower or higher. Just make sure that this switch is easily accessible; occasionally contractors have been known to put it on a duct at the rear of the crawl space.
A dual-fuel system can be installed at any time, but the most cost-effective times to do it are in new construction or when your air conditioner is either dead or very old with a low SEER (Seasonal Energy Efficiency Ratio). Most AC units made before 1987 have a SEER of 6 to 8. The new units range between 10 and 12. Enough said.
Nowadays, all compressors are manufactured as heat-pump compressors, so you may as well replace the old AC unit with a heat pump. If they don't add a reversing valve at the factory, then you get a standard AC unit. Whether you're replacing that old AC with a heat pump or a standard AC, be sure that your contractor checks the condition of the A-coils (the evaporator coils) over the furnace. It is usually advisable to replace them, particularly on old systems where they won't be able to handle the capacity of the new compressor unit. Also make sure that the ductwork is adequate for a heat pump and the furnace blower motor is up to the job.
Besides neighborhood bragging rights, the main reason to install a dual-fuel system is money. It may cost an additional $1,000 to $1,500 for the dual-fuel kit, but crunching the numbers will show that it's usually a good idea. Based on an electricity cost of 6.5 cents per kilowatt hour and a natural gas cost of 70 cents per therm, you will realize an annual savings of $256 if you have an 80 percent efficent natural gas furnace and a two-ton heat pump with a SEER of 10.
If your gas furnace uses propane at $1.10 a gallon, then you save $378 per heating season. Similar savings result if you have a 70 percent efficient oil furnace and are paying 90 cents per gallon of heating oil. Divide your annual savings into the extra cost for the dual-fuel kit to figure out how many years it will take for the system to pay for itself. And, of course, remember that actual costs depend on the varying heat requirements of the house and its occupants.
Steve Elder is a Roanoke home inspector. Questions and comments may be sent to him in care of The Roanoke Times, P.O. Box 2491, Roanoke, Va. 24010-2491.
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