Evaluating Heating Needs
In the past a heating contractor would guess a house’s needs and order a system with considerably more capacity than needed. Such seat-o f-the-pants engineering guarantees warmth but wastes energy. Now knowledgeable contractors calculate the exact heat load based on the house’s design. They measure total heat losses, then compute the load for the geographic region, using design temperatures. The result, expressed in Btus (British Thermal Units), provides a guide for sizing the heating plant. If, for example, your home has a total heat load of 93.000 Btus, you’ll need a furnace or boiler with perhaps up to 100.000 Btus. If you’re replacing a furnace, make sure that it wasn’t oversize to begin with. Properly sized units run almost all the time in cold weather; oversize units often cycle on and off. Insulation, storm windows, and weather stripping substantially reduce heat losses.
Calculating Heat Loss
Every part of a house loses heat to the outdoors, but at different rates, calculated as an R-value. To assess your home’s heating needs, a contractor computes the volume of air and its exchange rate, then the heat loss value for each exterior surface—each surface’s area divided by its R-value. The main areas of heat loss are the roof, chimney, walls, windows, doors, and foundation.
Shopping for Energy
What are the relative costs of heating with oil, natural gas, liquid propane (LP) gas, fuel oil, or electricity? To find out which would be the most economical, determine the unit cost of each form of energy from each utility, and then get out the calculator and do some math. Electricity is priced by the kilowatt hour (kwh); natural gas by 100 cubic feet (ccf); and oil and LP gas by gallons. Rating these might seem like comparing apples and oranges. But each unit yields a predictable number of Btus. It’s Btus—millions of them—that you need to heat your home. To figure the cost of each Btu generated by a new heating plant, divide the cost of each energy unit by its Btu yield. Energy forms are not equally efficient, so divide the cost per Btu you just obtained by the COP for the equipment you’re considering. Because a single Btu doesn’t amount to much, multiply by 1 million. Your final computation for each choice should look like this:
Cost per million Btu = Cost of the energy unit its Btu yield -h COP X 1 million.
Selecting a Heat Plant
Once your home’s heating load has been calculated, a knowledgeable contractor can go one step further. Using the energy-yield data and heating degree days from the U.S. Weather Service, a contractor can predict with surprising accuracy what you’d actually pay per year to operate a particular system. Make sure to get this estimate. It enables you to predict the payback period for equipment that might be costly to begin with, but economical in the long run. If, for example, heating unit A is priced at $500 more than unit B, but would save $100 a year in energy bills, the payback period for unit A would be five years.
Estimating payback periods calls for some guesswork, of course. Experts can’t say exactly what will happen to relative energy costs in the years to come. It’s also hard to make allowances for auxiliary equipment that adds to your comfort by cooling, humidifying, or cleaning the air, as well as heating it. Imprecise as they may be, payback calculations offer the best way to make sure you’re getting your money’s worth when you purchase a new heating unit.
The drawings show the different ways to install a forced-air furnace and its ductwork in a house that doesn’t already have them. At the bottom of the page, you’ll find information about three devices that improve the efficiency of a gas or oil furnace or boiler. The payback period is even more important when you investigate more sophisticated (and expensive) equipment, such as solar units and heat pumps. Payback periods for these units can be lengthy.
Selecting a furnace style. Upflow furances make sense for besement installations. Most ductwork is usually down there, where it’s easy to run.
A downflow furnace can be concealed in a closet. Ducts run through a crawlspace or in the floor.
Horizontal furnaces and their ductwork can be installed in either an open crawlspace or (if the climate is warm) in an attic.
■ Gas and oil furnaces lose about 20 percent of their heat up the flue. A motorized flue damper (shown at left) stops flue heat loss by closing the vent after a unit has cycled off.
■ Gas pilots that constantly burn are wasting fuel. Electric ignitions usually are coupled with the damper control. When heat is called for, the control opens a valve and lights the pilot, which ignites the burners.
■ A newer, tightly sealed home may benefit from installing a heat recovery ventilator (HRV). It provides ventilation without letting in cold air.