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Mechanical room venting
From the 1950s through the early 1980s, most furnaces had AFUE efficiency ratings of about 65 percent with
35 percent of the heat the furnace produced being lost up the fluepipe.
Currently, national standards require that furnaces yield a minimum of 78-percent efficiency.
All forced-air furnaces need to mix air with the fuel they burn to combust that fuel properly.
Lower-efficiency furnaces draw combustion air from the room in which they are installed; high-efficiency
furnaces draw combustion air directly from outside the house.
Every time the burner on a standard-efficiency furnace starts up, it draws air into the combustion chamber.
That air is burned along with the fuel and sent up the fluepipe. The air comes from within the house -- and
therefore has been previously heated by the furnace. And as the furnace draws in air to burn, new air has
to come from somewhere to replace that which is being burned up and vented outside. Additional air is
drawn into the house through cracks and gaps in the exterior walls and ceiling. This creates dry conditions
(exterior air is extremely dry in the winter), drafts, and inefficiency, because the incoming cold air has to be
heated by the furnace.
When the combustion air supply is closed off, the fire starts to smoke as the air supply is exhausted.
Incomplete combustion occurs and carbon monoxide is generated. The fire then goes out, but often before
the flame detection system can act to close the fuel safety shutoff valve(s). The accumulation of fuel is
re-ignited as oxygen seeps in through cracks and crevices; a furnace explosion frequently occurs with
disastrous effects on personnel and property. An adequate combustion air supply is therefore a
requirement to minimize the possibility of a furnace explosion.
From the 1950s through the early 1980s, most furnaces had AFUE efficiency ratings of about 65 percent with
35 percent of the heat the furnace produced being lost up the fluepipe.
Currently, national standards require that furnaces yield a minimum of 78-percent efficiency.
All forced-air furnaces need to mix air with the fuel they burn to combust that fuel properly.
Lower-efficiency furnaces draw combustion air from the room in which they are installed; high-efficiency
furnaces draw combustion air directly from outside the house.
Every time the burner on a standard-efficiency furnace starts up, it draws air into the combustion chamber.
That air is burned along with the fuel and sent up the fluepipe. The air comes from within the house -- and
therefore has been previously heated by the furnace. And as the furnace draws in air to burn, new air has
to come from somewhere to replace that which is being burned up and vented outside. Additional air is
drawn into the house through cracks and gaps in the exterior walls and ceiling. This creates dry conditions
(exterior air is extremely dry in the winter), drafts, and inefficiency, because the incoming cold air has to be
heated by the furnace.
When the combustion air supply is closed off, the fire starts to smoke as the air supply is exhausted.
Incomplete combustion occurs and carbon monoxide is generated. The fire then goes out, but often before
the flame detection system can act to close the fuel safety shutoff valve(s). The accumulation of fuel is
re-ignited as oxygen seeps in through cracks and crevices; a furnace explosion frequently occurs with
disastrous effects on personnel and property. An adequate combustion air supply is therefore a
requirement to minimize the possibility of a furnace explosion.
Example: With a furnace of 80,000 Btu input, there are 40 units of 2,000 BTUs. 40 X 1 inch = 40 square
inches. A grille 10 inches X 4 inches = 40 square inches. But since only the free area of the grille, and not
the slats, is counted, two grilles (an inlet and an outlet) that measure 10 inches X 6 inches each, are used for
this furnace.
inches. A grille 10 inches X 4 inches = 40 square inches. But since only the free area of the grille, and not
the slats, is counted, two grilles (an inlet and an outlet) that measure 10 inches X 6 inches each, are used for
this furnace.
Ventilation air must be supplied to the furnace area
through two openings to the main house area located
through two openings to the main house area located
- A ventilation air outlet grille located in a wall or
door, at a level higher (C) than the draft hood
opening. The area of the grille must provide 1 square
inch of free area for every 2,000 BTUs per hour of
natural gas input to the gas equipment in the area. - A ventilation air inlet grille located in the wall or
door, level with or below (D) the combustion air
inlet to the lowest burner. The area of the grille
must provide 1 square inch for every 2,000
BTUs per hour of natural gas input to gas
equipment in the area.
By comparison, a super-high-efficiency, or "sealed-combustion," furnace draws its combustion air from
outside the house via a PVC plastic pipe. Since the combustion air is coming directly into the firebox from
outside, room air is not being burned up and vented out the flue. Because there is no demand for
additional air drawn from within the house, there is no continual influx of cold, dry air from outside. The
result is much greater efficiency, fewer drafts, not as many problems with dry air, and a warmer, more
comfortable house.
A high-efficiency furnace has an annual fuel utilization efficiency (AFUE) rating of 90 to 97 per cent. In other
words, a furnace at the high end of this range will convert 97 per cent of the combusted natural gas to
usable energy, with the remaining three per cent exhausted to the outside.
Most high-efficiency furnaces use a condensing process. Two heat exchangers draw enough heat from
combustion gases that moisture in the gas condenses, releasing additional usable heat. The condensate is
piped to a floor drain and exhaust gases are cool enough to be vented out a sidewall with a four-inch piece
of plastic pipe, thus eliminating the need for a chimney. Because the combustion in these furnaces is
sealed, there is also no risk of carbon monoxide leaks.
As such, the requirement for high / low vents is eliminated.
outside the house via a PVC plastic pipe. Since the combustion air is coming directly into the firebox from
outside, room air is not being burned up and vented out the flue. Because there is no demand for
additional air drawn from within the house, there is no continual influx of cold, dry air from outside. The
result is much greater efficiency, fewer drafts, not as many problems with dry air, and a warmer, more
comfortable house.
A high-efficiency furnace has an annual fuel utilization efficiency (AFUE) rating of 90 to 97 per cent. In other
words, a furnace at the high end of this range will convert 97 per cent of the combusted natural gas to
usable energy, with the remaining three per cent exhausted to the outside.
Most high-efficiency furnaces use a condensing process. Two heat exchangers draw enough heat from
combustion gases that moisture in the gas condenses, releasing additional usable heat. The condensate is
piped to a floor drain and exhaust gases are cool enough to be vented out a sidewall with a four-inch piece
of plastic pipe, thus eliminating the need for a chimney. Because the combustion in these furnaces is
sealed, there is also no risk of carbon monoxide leaks.
As such, the requirement for high / low vents is eliminated.