Thursday, May 7, 2009
Furnaces
You go to the thermostat and turn the
temperature setting up. Your cold and fully expect the house to warm
to your desired comfort level.
You go back to what you were doing and forget about the heat. A half
hour later you start to feel really cold and go back to the thermostat.
For most people that is the extent of their knowledge about their
heating or cooling system. The thermostat. Besides changing the filter
every once in a while and storing your things next to the old beast
in the closet, that is the only time we ever see or think about the
furnace. And now you are getting cold and your concern is rising.
Whether you are mechanically inclined or not, the best advice we can
offer at this point is to pick up a phone and call a professional.
Okay, so you are the fix-it-myself-type. Here are the things
you need to check first:
- Check the thermostat and make sure it is in the on position and
turned up. - Check the filter. A dirty filter can and will cause your furnace
to malfunction. - Check the main power. Start at the fuse or circuit
breaker box and then go to the emergency cutoff switch. These switches
usually
look like a regular wall switch except the cover is usually red (but
not always, sometimes they have white cover plates). So you check
and everything is good, all the switches are on and no fuses
blown. - Make sure there is gas. Did you forget to pay the gas bill? If
this happens the gas company will put a lock on the meter. Check
the meter and make sure this hasn't happened to you. Some gas companies will lock your meter if there is a suspected gas leak. They'll leave
a red tag and a lock on the meter. If the meter is okay go to the
furnace. There should be a valve on the gas line next to the furnace.
Make sure the valve is on. This valve is called a gas cock and the
position of the knob should be parallel to the line. This will indicate
that it is on. If the gas cock is on the next step is to check the
gas valve itself. This is located inside the panel where the burners
are. Look at the top of the valve. Is it in the on position? If it
is, move on to the next step.
Warning: There are some furnaces with pilots that are electronic ignition.
These are called intermittent pilots. Do not attempt to light an intermittent pilot. If you have intermittent pilot and it doesn't
work, call a professional. To tell if you have an intermittent pilot
look at the gas valve. It will not have a pilot position on the knob.
Now that you're inside the panel check to see if the pilot is lit.
If it is, and your furnace still won't start, then the best thing
to do is call a professional. Your problems are most likely beyond
the scope of your ability and really needs the attention of a person
trained in the repair of gas furnaces. If you decide to continue,
proceed at your own risk. If the pilot is not lit, you'll need some
matches or a lighter. Preferably matches or a lighter with a long
reach. Once you have obtained a source of fire turn the knob to the
pilot position. Make sure there is not a gas smell present. Remember
natural gas is lighter than air and will rise. Always
read the manufacturers safety and operating instructions.
Propane or manufactured gases are heavier than air and tend to accumulate
in lower spaces. Many eyebrows, beards, mustaches, and hairdos have
been singed because the person lighting the pilot forgot this fact.
If you do smell gas, call a professional after you turn all knobs
supplying gas to your furnace to the off position. Okay back to lighting
the pilot. Turn the knob to the pilot position and press down on the knob. Light the match or lighter and place the flame at the pilot.
The pilot should light. Continue holding the knob down for about
a minute. After the minute goes by release the knob. The pilot should
remain lit. If it doesn't, try lighting it again. If after several
attempts at lighting the pilot and it refuses to stay lit, call a
professional. You most likely need a new thermocouple or thermopile
depending on whether your pilot system is powered by either a thermopile
or thermocouple.
In the flash presentation above, a flame is burning continuously on
the tip of a thermocouple. As long as the flame is present on the tip
of a properly functioning thermocouple, the primary valve (inside the
gas valve) will remain open. If the flame goes out or the thermocouple
malfunctions, the primary valve will close off rendering the valve
inoperable and preventing any gas from escaping.
Modern Electronic Furnaces
The next step up from the standing pilot furnace is the electronically
controlled furnaces. These furnaces are more efficient than the standing
pilot furnaces but more complex in design. There are two types of electronic
furnaces: the intermittent pilot and the hot surface ignition furnaces.
While the two are different they both offer higher efficiencies and
are both controlled by solid-state circuit boards. Many of these furnaces
also have higher efficient heat exchangers. When the intermittent pilot
or hot surface ignition controls are combined with improved and more
efficient heat exchangers, the result is a furnace that can be 80 percent
or more efficient versus the older standing pilot furnaces which average
only 60 to 70 percent efficient.
Intermittent Pilot
The intermittent pilot furnace, upon a call for heat
(when the thermostat closes), will go through a trial for ignition.
(Note that the spark generated in this process can exceed 10,000 volts).
When a pilot flame is proven (normally through
flame rectification) the electronic module or circuit board sends
a signal to the main valve, in the gas valve, to open. The pilot
lights the main burner and it burns until the thermostat is satisfied.
When the thermostat is satisfied, the electronic module or circuit
board stops all ignition processes including the pilot.
Hot Surface Ignition
The Hot Surface Ignition Furnace has no pilot light.
It lights the burners direct. Therefore it is also referred to at times as Direct
Ignition. The igniter is made of silicon-carbon and glows red hot
when power is applied to it. The igniter is positioned above the
burner. Upon a call for heat, the furnace enters a sequence of operation.
Normally, after the purge cycle, power is applied to the igniter.
It glows red hot the gas valve opens allowing gas to flow. When the
gas makes contact with the igniter, ignition occurs. Once flame is
proven (usually with flame rectification), power to the igniter
is discontinued. The burners continue to burn until the thermostat
satisfies when the furnace goes into shutdown mode and shuts itself
down until the next call for heat.
The percentage of efficiency as described here means that if a furnace
is 80 percent efficient then 80 percent of the heat it produces is
usable heat beneficial to the home and the other 20 percent is vented
into the atmosphere through the flue with the noxious gases that are
a result of the combustion process. Thus, the higher the efficiency
of a furnace the lower the temperature of the flue gases simply because
much of the usable heat produced by the furnace is used in the home
instead of vented into the atmosphere. This causes two separate functions
that must be overcome by mechanical means.
Number one, since the temperature of the vent gases is negligible
and gas typically contains approximately five percent moisture content,
the moisture is condensed through the process of combustion and needs
a place to drain. Normally, this drain is tied into the same drain
that drains the Air Conditioning condensation but could be drained
in another location.
Number two, because so much heat is removed and used in the home,
the pressure or force needed to exhaust the flue gases is too small
to force the noxious gases outside. This is the main reason why all
high efficiency furnaces have induced draft motors attached to them.
The inducer motor forces a draft through the entire system from the
beginning of the combustion process to the end where the noxious gases
are exhausted.
Modern electronic furnaces are more efficient because
you don’t
need to keep a pilot light burning 24/7 and there has been major changes
made in heat exchangers. Changing your current furnace from a standing
pilot furnace to a modern electronic furnace can make a difference
in your comfort and the amount you pay on your fuel bill next winter.
The Flame that your furnace or boiler produces can effect the performance
and efficiency if it is not properly burning the fuel. A professional
HVAC Technician will have high tech tools so that your furnace can
be tuned to where the mixture, fuel and air, is proper ensuring the
furnace is getting the maximum amount of efficiency possible. This
is super important considering that the cost of fuels, whether it is
natural gas, propane, or oil, is continually rising.
The first test that can be performed is simple and requires no tools,
but does require a knowledge of what a proper flame should look like.
Natural Gas should have blue flame with a very minimum amount of yellow
tips on the top of the flame. Too much yellow tips would mean there
is not enough primary air or a combination of some other problems where
a simple adjustment can be made to correct the problem and get the
most out of the combustion problem. Problems other than yellow tipping
are rollout, flashback, lifting flames, and floating flames. All these
problems should be corrected to get the most out of your furnace or
boiler.
The next test that can be performed is the combustion analyzer test.
An expensive tool must be used for this test. An electronic device
that is capable of reading and analyzing several processes of combustion
can give the technician clues about any potential problems so that
some sort of action can be taken to correct it. A combustion analyzer
will tell the technician the stack temperature, carbon monoxide level,
carbon dioxide level, and oxygen levels in the exhaust gases leaving
the furnace. This test should also be done in conjunction with a
draft pressure test. It is important that all the byproducts of the
combustion process are exhausted outside the home. A draft test can
determine this will happen. A visual test using a mirror can also
be done to ensure there are no blockages of the chimney or the flue.
All the byproducts of the combustion process are analyzed and measures
taken to correct any abnormal readings by making adjustments according
to manufacturers recommendations.
Finally, the last test that can performed is a gas pressure test.
These pressures are different from one furnace or boiler to another
and from natural gas to propane to oil. Propane and Natural Gas are
measured in water column inches using a manometer. It is important
that the proper fuel pressures are present in the furnace or boiler
not only for efficiency purposes but also for complete combustion. Annual combustion testing is especially important for oil burners.
All these tests can be performed by a qualified HVAC technician and can
make a big difference in the amount you pay for the heating fuel you
use over the colder months.
Subscribe to:
Post Comments (Atom)
No comments:
Post a Comment