Get lots of inquiries about air-conditioning especially is regards to sizing
specifically what is the correct size unit I need for my home or business.
One would wonder why there are so many different sizes of air conditioning
units when one big enough to do the job should be enough. the fact is that
unlike a heating unit that only needs to add heat to the room or building
the air conditioner must remove moisture as well as cool.
is that a typical air conditioner will remove a certain amount of moisture
over a period of time in relation to how much it lowers the temperature
of the air. In humid climates like here on the east coast the main reason
for air-conditioning is to remove moisture from the air more than cool
it (this is why a small window unit seems to "cool the whole house"). To
do this we must size the unit so it runs most of the time on the hottest
days. This is why heat pumps never seem to work in the winter because they
are usually sized for cooling. A two speed heat
pump would solve this problem and greatly improve efficiency. Large
commercial systems have compressors that can unload cylinders either by
electrical controls or suction pressure.
solution is to make chilled water in a tank and distribute the chilled
water to fan coil units or one central unit and slow the fan down to reduce
the cooling capacity but still maintain dehumidification. With a conventional
cooling system you need the capacity of the duct work and air flow to be
at least the capacity of the compressor.
There are many
ways to determine the size of the unit you need. For example there are
room charts that have a map of the country and the room square footage
and that will tell you what size in BTUs or Tons that you need. There are
other methods that take into account the insulation in the walls, ceilings,
and floors. As well as the local temperature, humidity and other factors.
The most accurate method that I know of is to use what is known as the
"ACCA Manual J" this takes into account everything including the electrical
appliances that add heat to the room. I believe that there is a program
that you can download that uses this protocol called "Quickoads". When
I find the website for this product I will post it here.
When you do
a complete calculation you will find that in most cases you will require
a smaller system than you may have thought. Including for heating too.
The only exception to this would be for a church, school or office where
the room or building is used intermittently throughout the week and the
temperature must reach the "occupied setpoint" in a reasonable amount of
time and you don't have the luxury of waiting for a "properly sized" system
to heat or cool it.
Back in the
early days of heating it was thought that for health reasons you needed
to keep the windows open in the wintertime. Therefore heating systems were
sized to keep the house at 70 degrees on the coldest days with the wind
blowing. This sizing school of thought is finally disappearing although
most gas and oil furnaces are still too big today.
We service and repair the following brands:
American Standard, Amana, Arco, Arco-Aire, Bryant, Carrier, Coleman
Evcon, Comfortmaker, Day/Night/Payne, Dunham-Bush, Fedders, Fredrich, Goodman,
General Electric, Hotpoint, Heil, Intertherm, Janitrol, Kenmore, Lennox (Armstrong, Johnson
Air-Ease), Miller, Modine, Nordyne, Rheem/Ruud, Sears, Stewart Warner, Trane,
Williams, White-Westinghouse, Whirlpool, Weil Mclain, York, (Frasier Johnson/Borg
Warner) and others.
1. Blower: usually a squirrel cage centrifugal air moving device.
Will move large volumes of
air relatively quiet. Will use less energy with
more back pressure.
2. Fan: a paddle type air moving device used where noise is
not a major consideration. Will
use more energy with more back pressure.
3. Combustion blower: A blower used on high efficiency gas furnaces
or oil burners to move
combustion air. usually 1/20 to 1/6 horsepower.
4. OEM: Original equipment manufacturer.
5. Horsepower: 746 watts
6. RPM: revolutions per minute.
7. Service factor: the extent to which a motor can be safely
overloaded beyond its name plate
ratting without over heating.
8. Air over horsepower: The rating of a motor assuming air flow
through the windings usually
as a result of the air moving device.
9. SAE: Society of Automotive Engineers.
10. KW Kilowatt (1000 watts) or 3400 BTUs per hour .
11. High efficiency furnace: Furnace that uses over 85% of the
energy in the gas.
12. Condensing furnace: Gas furnace that uses over 92% of the
energy in the gas and condenses
the gas into liquid condensate and hot air. If your
furnace has PVC pipe venting it then you
have a condensing furnace.
13. Evaporator. The part of an air conditioner or refrigeration
system that gets cold due to
14. Condenser. The part of an air conditioner or refrigeration
system that gives off heat by
condensing the refrigerant.
15. Indoor coil or unit. On a heat pump unit we call the inside
section "indoor" so as not to
confuse it with the evaporator on an air conditioning
Outdoor coil or unit. On a heat pump we call the
outside section "outdoor" so as not to
confuse it with the condenser on an air conditioner.
16. Ton 12,000 BTUs per hour of heating or cooling
Written By: Scott
Meenen N3SJH of: G&S MECHANICAL SERVICES. Specializing in Mechanical, Controls
and Electrical Modifications Of
Heating, Air-conditioning, Refrigeration,
Ice Production and Food preservation.
Anything having to do with Heat
Serving MD, DC, and Northern
VA. Contact us by pager: 1-877-467-2914