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Richard D. Malin & Associates: Technical Library


Many of us are interested in becoming more "green" by responsibly using natural resources. Additionally, we all want systems in our home that are reliable yet cost efficient. The term high-efficiency on any appliance is popular because the idea is that they will save fuel and save you money on your utility bills. What exactly is an heat efficient furnace, and what is meant by furnace efficiency? Actually, there are two ways to measure it - steady state and seasonal.


Steady state efficiency refers to how much usable heat is created when a furnace is running. Conventional gas and oil furnaces have steady state efficiencies of roughly eighty percent. When the furnace is on, twenty percent of the heat that is generated goes up the chimney while the remaining eighty percent is transferred through a component called a heat exchanger and stays in the house. This is the heat that travels through the ductwork and ends up coming out of the registers in each room.


Even in the dead of winter, furnaces are not on all of the time. They cycle "ON" as the thermostat calls for heat and "OFF" when the thermostat is satisfied. During this start-up and cool-down, the furnace is not operating as efficiently. If you add these off-cycle losses to the steady state losses, you end up with the seasonal efficiency. Seasonal efficiencies for conventional gas and oil furnaces are about sixty to sixty-five percent. How does this translate into dollars and cents? If you buy a thousand dollars worth of gas or oil over the course of a winter and you have a conventional furnace, only about $600 to $650 worth of the fuel will be used to heat your house, and the rest will be wasted. OK, so that's what happens in a conventional furnace. So what about mid and high-efficiency furnaces?


Mid-efficiency furnaces have a season-

al efficiency of about eighty percent.

They achieve this by cutting off-cycle

losses. Mid-efficiency gas furnaces do

not have a continuously running pilot.

It is shut off when the furnace is idle.

Additionally, to prevent heat from es-

caping up the chimney when the furn-

ace is not on, manufacturers design

their furnaces so that the exhaust flue

closes during idle periods.


High-efficiency furnaces employ similar

techniques to reduce off-cycle loss, but

go further to improve the steady state

efficiency. Instead of having one heat

exchanger, most have two or three to

extract more heat from the burning fuel. A high-efficiency system can be 90-95 percent efficient. Because high-efficiency sys-

tems are more complex, they cost more

initially at the time of purchase.

However if you look at fuel costs, the house that cost $1,000 to heat with a conventional furnace should cost only $650 or so to heat with a high-efficiency system, yielding an

estimated savings of a $350 or so a year. Theoretically, in a few years the system should pay for itself. However, keep in mind some models of high-efficiency furnaces have had reliability problems. Additionally, like expensive foreign sports cars, most high-efficiency systems require more costly maintenance which can eat into the savings. Our recommendation if you are considering a high-efficiency system to replace your present system or as an option on a new home, is to speak with a reliable heating contractor to discuss the pros and cons of various models and the estimated increase in maintenance costs.

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