Many homes are heated with a furnace. This energy system transfers
heat created by burning gas, oil, coal, o r by heating a resistance
element with electricity to air that is circulated thought out the
house. The furnace itself is designed to be trouble free and requires
very little care but the gas or oil burner that creates the heat does
require maintenance to run safely and efficiently. Here are some
maintenance tips to keep a furnace operating efficiently:
Annual Inspection. Service a furnace in the fall before the
heating season begins. It is best to use a heating professional to
perform this annual maintenance. This professional will probably follow
these following inspection and maintenance steps:
The outside of the furnace will be inspected with careful attention
given to the flue pipe leading from the furnace to the chimney. He will
check for loose connections wherever two pipes join, at all elbows, and
where the pipe joins the chimney. If there are loose sections, they may
be reattached. Also, the furnace will be checked for large rust spots
especially on the bottom of the pipes. Condensation may cause rusting
and this is a sure sign of a maladjusted burner. If there is loose or
missing cement surrounding the pipe, it will be replaced or repaired.
Air Filter. Most furnaces have an air filter located in the
return air duct system, usually at the bottom of the furnace where the
large duct enters the furnace. This filter requires regular replacement
if it is not a permanent foam type filter.
Blowers. A furnace's blower that forces the air through the
heating system. Some blowers have a V belt drive that should be serviced
every year. Some newer furnaces have direct drive blowers and are not
belt driven. Both systems require cleaning and lubrication
Humidifier. If the furnace is equipped with a humidifier, it
will require at least yearly maintenance.
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How unsafe is a failed heat exchanger in your forced-air system? The
main safety concern with warm air furnaces, sometimes called "hot air
heat," is keeping the products of combustion from mixing with the air in
the home and negatively affecting the health of the occupants.
When fuel is burned, three products are produced: (1) heat, (2)
carbon dioxide (C02), and water (H20). This example assumes complete
combustion. If there is incomplete combustion, other products will also
be present. These may include the compounds such as carbon monoxide
(C0), formaldehyde (HCH0)and numerous other aldahydes, nitrogen dioxide
(N02), and sulfur dioxide (S02). The technicians who set up furnaces try
to keep the C0 to less than 100 parts per million (ppm) in the exhaust.
Problems develop when there is a blocked or partially blocked chimney
and/or a failed heat exchanger. A blocked chimney can fill the area
where the heater is located and the first floor with toxic C0 gases in a
few hours, depending on how much air flow there is in the house. In most
situations, a blocked chimney is relatively easy to clear.
A failed heat exchanger is much more difficult to determine, but, in
almost all cases, is much less dangerous than a blocked chimney. In
fact, when the furnace's fan is running, the heat exchanger is
pressurized from the house air side. In almost all cases, this pressure
will not allow dangerous gases to accumulate in the house air. The path
of least resistance for these exhaust gases is up the chimney. This may
not be the opinion of most gas utilities in the country, which is
somewhat understandable based on the liability exposure.
The pressure on the heat exchanger has a significant effect on the
tendency of flue gases to pass from one side of the heat exchanger to
the other. If the fan is off, the pressure from the burner will cause
the burner side to be positive and the C0 or C02 gas can pass to the
house side. The amount of gas passing from one side to the other is
based on the size and location of the failure in the heat exchanger.
However, it is rare this amount would exceed the amount of C0 or C02
gases emitted from a kitchen as range flame.
When the fan comes on, the house air side of the heat exchanger, in
almost every case, is positive. The positive pressure from the house air
or fan side would cause the house air to be pushed into the exhaust
side, not vice versa. The only exception may be some power burners which
would maintain positive pressure on the burner side while the fan was on
or a heat exchanger failure which was large enough to get your fist
into. The main thing to remember is that high pressure will always move
to a low pressure. There are a few other factors which must be added to
be totally accurate. These would include the location of the failure and
the design of the heat exchanger.
This is not to say that failed heat exchangers are safe, but you
should know they are rarely as much of a concern as we hear from most
information sources.
One last item: According to the American National Standards, it is
almost impossible to construct a heat exchanger that is entirely air
tight. Therefore, any test method developed to detect flue gas leakage
needs to have quantitative aspects. It would not be desirable to
identify as unacceptable any heat exchanger leakage that meets the
requirements/standards for heat exchanger joints. This standard says the
leak should not be more than 2% of the flue gases with the internal
pressure raised to .1 water column (WC) static pressure.
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This silent killer claims about 1,500 lives each year in the U.S.
Carbon monoxide (CO) is one of the toxins that remains from
incomplete combustion of fossil fuels. Fossil fuels include oil, gas,
and coal. Small amounts of CO, such as those emitted from the kitchen
range, will usually be found in the air in the home. These amounts pose
no health concerns for the occupants.
However, health problems can develop if one is exposed to CO in large
amounts, such as those emitted for many hours from a blocked chimney. In
extreme cases, the presence of CO can be lethal.
BLOCKED CHIMNEYS, NOT HEAT
EXCHANGERS, ARE THE REAL CULPRIT
Much has been written about heating systems causing dangerous levels
of CO gas in homes. The heating furnace itself will not cause CO amounts
of any concern to be emitted into the home.
If the heat exchanger fails (the heat exchanger is the part of the
furnace that keeps burned fuels separate from the air in the living
space) CO is rarely emitted in the air. If CO is emitted, the amount
released is not significant. Here's why:
The typical furnace has a fan that circulates the indoor house air to
and from the heating system and living space. This fan creates
approximately 18 times more pressure on the house side air than the
typical pressure created by atmospheric burners. In the event of a
failure, this pressure causes the air from the living space to pass to
the exhaust side of the unit and up the chimney.
This is not to say that a failed heat exchanger is acceptable. It is
not. However, the likelihood of significant CO gas being delivered to
the living space has been grossly overstated. A chimney that is blocked
for many hours or days is the only item that would deliver dangerous
amounts of CO gas to a dwelling.
THE REASONS WHY CO LEVELS VARY IN DIFFERENT
HOMES
Carbon monoxide in homes is difficult to research due to numerous
variables, including:
- The size and air volume of a home. The more air in the home, the
easier gases will dissipate.
- The number of air changes per hour in modern homes that have thick
insulation, etc.
- The type of construction. Various types of frame and masonry
construction will have an effect on the air changes and air
infiltration.
- The type of heating system. Combustion air requirements and
efficiencies have some effect on air movement and changes.
- Operating fans and exhaust systems. When on, these systems
dissipate all the air in the house in minutes. The size of the systems
and of the house will determine how effectively this is done.
According to an American National Standard's study on heat
exchangers, leakage of waste gases is acceptable as long as the
combustion chamber and vent do not leak more than 2% of flue gases.
(Testing parameters are .1" water column static pressure on the interior
of the heat exchanger.)
HOW CO KILLS
CO poisoning kills about 1,500 people a year. CO reduces the ability
of the hemoglobin in the blood to carry oxygen to the brain and body.
This is akin to not breathing. The blood recovers quickly if the
exposure is not continuous. Typical symptoms include headaches, fatigue,
insomnia, nausea, and heart palpitations.
The presence of CO in a home can cause physiological effects at any
level. However, the following parts per million (ppm) indicate when it
is a serious concern:
| 50 ppm |
Allowable for up to 8 hours of exposure. |
| 500 ppm |
Can be inhaled for one hour without appreciable
effect. |
| 700 ppm |
Unpleasant, but not dangerous, effects after
one hour of exposure. |
| 2,000 ppm |
Dangerous effects after one hour of exposure. |
| 4,000 ppm |
Fatal in less than one hour |
The most desirable condition would be a zero level of CO. To achieve
low and safe levels, use a CO monitor. It will detect when levels
surpass 10 ppm.
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Putting an addition on your home, such as a bedroom or kitchen, is
very exciting. It often affects your attitude and comfort level and can
truly renew your spirit. One consideration when adding to your home or
when creating a new living space from a previously unused area, such as
a porch, basement or garage, is the extra heating that will be
required.
Your current heating system is probably sized for your current living
situation, whether the unit is original or a replacement. There has
probably not been any consideration of a future addition.
The first thing you need to do is assess your current heating
situation. A general rule of thumb for heating requirements is that 40
to 50 BTU’s (British Thermal Units) are required for every square foot
of living space. So, determine the square footage of your current living
space (before the addition) and divide it by the 40 to 50 BTUs. This, of
course, will depend on the type of construction and geographical
location.
To figure out how much more you would need (assuming what you have is
enough) simply add the square footage of the new living space to your
current number. When you have this total, you can figure out the amount
of BTU’s you’ll need for the addition.
You may be able to use the same input size heater if you buy a more
efficient one. If you replace a typical heater that is 60 percent to 70
percent efficient with a heater that wastes just 5 percent to 10 percent
of its heat/fuel, and if it includes an outside air supply for
combustion, you could buy a heater sized at approximately 25 percent to
35 percent fewer BTUs for every square foot of living space. More
effective energy improvements may allow you to reduce the size even
more.
Heater efficiency is based on burner efficiency, transmission of
losses to the heater exchanger or boiler and flue or chimney losses. The
quantity of heat lost up the chimney is rarely discussed by utility
companies or fuel suppliers. However, it is significant. Approximately
one-third of all heat generated by a gas-fired unit goes up the chimney.
Oil-fired appliances have 5 to 15 percent more waste. However, oil costs
less than gas to purchase.
When the distribution of air is from an existing situation,
additional ductwork may be needed. The farther you travel from the
source, the smaller the ductwork should be to increase/maintain adequate
air velocity. Additional fuel and ductwork for equitable distribution.
Before you begin a new addition, consider the changes that will take
place. Don’t get discouraged; these calculations are not difficult. The
half-hour you may put in will be well worth the yeard of enjoyment you
will get from the new living space.
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By learning how the systems in your home operate, you
can often troubleshoot problems when they develop. At the very least,
you will be able to talk intelligently with a contractor who is brought
in to correct a problem. Showing that you "know your stuff" can gain you
respect and possibly better service.
A heat pump serves two functions: to heat and cool
your home. A central air system simply cools the house. Air conditioning
units operate the same way the heat pump's cooling side works. Here's
how your heat pump operates:
With your heat pump unit in the air conditioning
mode, the compressor compresses Freon gas. (It does this in both the
heating and cooling mode.) When you compress gas, as with anything, you
develop heat. You get a high pressure gas that is very hot -- typically
between 190-240 degrees. The hot gas gets pushed into the outside coil.
The outside coil's fan, which is on, is drawing the outside air across
the coil. The 90 degree outside air is significantly cooler than the gas
inside the coil. This is why you feel warm air coming off an air
conditioning unit.
The cooling of the gas causes a change in state. The
hot gas converts into a warm liquid, one of the unique properties of
Freon. The warm liquid continues through the final portion of the coil
and moves inside to the evaporator, which creates space for the liquid
to expand. The liquid expands into a low-pressure gas, which is now
cold. The cold gas goes through the "A coil," which is inside the plenum
of the heating system ductwork. The cold gas makes the coil cold.
The cooling into the house occurs from drawing the
warm house air across the coil. The velocity and volume of air across
the coil dictates the temperature of the air on the other side of the
coil. The technician will set the fan speed up so you get a 15-17 degree
differential between the supply and return air.
The cold gas moves through the coil and is pushed
back into the suction line to the outside unit, which draws the cold gas
back to the compressor where the whole process starts over.
In the heating mode, the compressor does the same
thing. However, instead of pushing the hot gas to the outside coil, it
pushes it to the inside coil. Once into the inside coil, a similar
process happens. Hot gas moves around the coil. The air from the house
takes the heat off the coil, which is desirable, but while it does this
it drops the temperature of the gas. When it does this enough, the high
pressure gas changes into a liquid. This continues through the coil,
then through the expansion valve which allows the liquid to change to a
low-pressure cold gas. This cold gas is taken to the outside coil -- the
opposite of the AC mode -- but simply by moving gas/liquid the other
way. Once it goes through the outside coil, it ends up back at the
compressor and then starts the process over again. In either mode, the
compressor simply compresses the Freon gas.
There is one more important point when the system is
in the heating mode and the cold gas comes to the outside coil. If the
temperature is low enough, you may start to freeze the outside coil.
When this happens, you will develop ice. When you start to form ice, the
unit will automatically change to a defrost cycle. An outside thermostat
controls this function. The defrost cycle will reverse or change the
function back to the AC mode and defrost that outside coil.
Note: While defrosting the outside coil, you will
have back-up heat engaged on the inside. The back-up heat could be
electric or a furnace operation. This will be dictated by the electric
and fossil-fuel costs.
When the temperature drops too low and too frequently, this situation
becomes economically impractical and the compressor will shut down and
will heat with the back-up process.
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