Calculating Heating Syst
Calculating Heating Syst
Calculating Heating Syst
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Nov 1, 2006
Rob Falke
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As winter settles in across the country, weve had an increasing number of tech support
calls asking for a quick and simple way to calculate airflow in forced air heating systems.
We take this as an indication that more and more of you are measuring system airflow
and understand its critical role in delivering a top-performing system.
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There are a number of methods that have been used over the years. But with the
introduction of new furnaces, any method would need to consider the type of furnace and
respond accordingly. The method we teach to calculate heating airflow differs from one
type of furnace to another, so lets first take a look at the basic formula for gas fired heat.
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Multiply that figure by the factor form the table below depending on the type of furnace to
find required system airflow.
Natural Draft Furnaces 100 CFM per 10,000 BTU of rated BTU input
Induced Draft Furnaces 130 CFM per 10,000 BTU of rated BTU input
Condensing Furnaces 150 CFM per 10,000 BTU of rated BTU input
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Divide the BTU input of 80,000 by 10,000 to find a multiplier of eight. Since you have an
induced draft furnace, multiply it by 130 to find the required system airflow of 1040 Cubic
Feet per Minute. Thats it.
Okay, one more. Youre servicing a condensing furnace, you can tell because the flue is
made of PVC Pipe. The nameplate reads the equipment is rated at a BTU input of
120,000.
Divide the input of 120,000 by 10,000 to find a multiplier of 12. Since you have a
condensing furnace, from the table above, multiply the 12 by 150 to find the required
system airflow of 1800 CFM. It sounds like a lot of airflow for a furnace, but thats what
the furnace requires to operate at maximum efficiency.
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This method of calculating heating system airflow works on just about every system Ive
ever applied it to. To be certain for yourself, pull some engineering data from the office on
several gas furnaces and run the numbers. Youll be impressed with how consistent the
simple formula applies across the board.
Heat Pumps
It may sound weird if youve never checked, but heat pumps require the same airflow in
heating mode, as they do in cooling mode. The magic number is 400 CFM per rated
nominal ton. 450 Per ton is often discussed, but thats to assure youll get at least the
required airflow of 400 CFM per ton. I questioned this number myself until I began to
measure the delivered BTU of heat pumps years ago.
As our customers would complain of cool supply air temperatures, our service techs
occasionally would reduce fan speed to heat up the temperature. This solved one
problem, but created a bigger one a few weeks later when the temperature fell and the
building failed to heat. This happened because the required airflow over the coil wasnt
available to remove the heat generated by the heat pump, so total BTUs took a nosedive.
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Design for, test, and verify the required airflow as recommended in this article. Then, and
only then, if an element is lacking, you may consider lowering airflow to increase supply
discharge temperatures. Once the desired temperature is obtained, then retest the
system to verify you havent lowered the total heat delivered by the system to a point
where the equipment cannot produce the rated BTU. Or you may be left with a far worse
predicament.
Check your Engineering Data
Equipment manufacturers are continually finding additional ways to squeeze a little extra
heat out of their systems in the name of energy efficiency. Some of these new methods
may require more or less airflow than recommended in this article. So take time to review
the published engineering data from time to time. Check the equipments required airflow
and verify it continues to follow these simple rules.
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Engineering data contains endless interesting facts and figures that may change some
assumptions we have held tight to for years. A fellow at a seminar I taught this week was
describing a series of functions designed into a new condensing furnace. I was amazed
at the research he must have done to discover how this equipment operated in the field.
The intent of the design was great, but in his higher elevation, the equipment continued to
fail under certain operating conditions that occurred about once a week since September.
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On the other hand, another contractor found some new functions available through
manipulation of dip switches that increase the performance of one of his furnaces nearly
15% under the weather conditions found in his area of the country. Knowledge of how to
maximize system-performance is rarely easy to come by, but the best contractors and
technicians regularly spend time with their faces buried in their manufacturers
engineering publications.
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Keep on Testing
The primary reason to calculate required airflow is to interpret what your airflow and
system performance readings are telling you. Weather you are verifying system airflow by
interpreting total external static pressure using the manufacturers fan performance
tables, or reading airflow using an airflow traverse or an air balancing hood, the
knowledge of required airflow is essential to verifying live installed system performance.
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Rob Doc Falke serves the industry as president of National Comfort Institute, a
training company specializing in measuring, rating, improving, and verifying HVAC
system performance. If youre an HVAC contractor or technician interested in a field
procedure to help you determining required system airflow, contact Doc at
robf@nationalcomfortinstitute.comor call 800/633-7058. Visit NCIs website at
www.nationalcomfortinstitute.com for free information, technical articles, and downloads.
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