SYSTEM DESIGN:
The size of a heating & cooling system is dependent on
- Size of house, construction materials, and integrity of house envelope.
- Direction house faces
- Amount of insulation
- Number and type of windows
- Number and activity level of occupants
- Location (average high and low temperatures and humidity levels)
NOTE: design conditions for this area are 91F summer (30 grains moisture
removal for 50% inside humidity) and 18F winter. Near these conditions a
properly designed system will run nearly 100% of the time!
Rules of Thumb:
- A typical house requires about 1 ton of air-conditioning for each 600 square foot of area.
- One ton of air-conditioning requires 400 cubic feet/minute of air-flow.
- A typical return vent requires 200 square inches/ ton of air-conditioning.
- A typical supply vent provides 80-100 cfm of air. Therefore a house with a 2 ton heat pump would
typically have 8 or 9 supply vents and a 20"x 20" return grill. FAILURE TO PROVIDE
ADEQUATE AIR FLOW FOR A UNIT WILL DECREASE ITS OVERALL EFFICIENCY. CLEAN OR
CHANGE FILTERS REGULARLY!
- BIGGER IS NOT BETTER-- Over-sizing an air conditioner will result in short cycling, short unit life
and poor dehumidification.
COST COMPARISON:
This comparison is based on today's (5/1/2008) energy prices; they are as follows:
- Oil - $3.64/gal., Natural gas - $1.58/Therm (100,000 BTU),
- LP gas - $2.75/gal, and electricity - $0.085/Kw. For this example I have assumed a house requires
50,000 BTU/ hour to maintain a comfortable temperature (this would be a fairly large house on a cold
day).
- Electric heat -- $1.18/ Hour
- Natural gas (95% efficient furnace) -- $0.83/ Hour
- LP Gas furnace (95% efficient) -- $1.61/Hour
- Oil furnace ( 84% efficient) -- $1.54/Hour
- Heat pump (13 seer-8.4 HSPF) at 17F outside temp. -- $.75/Hour (electric back-up)
- Heat pump (13 seer-8.4 HSPF) at 47F outside temp. -- $0.39/Hour
- HEAT PUMP (16 SEER-9.5 HSPF) AT 17F OUTSIDE TEMP --$.68/HOUR (ELEC. BACK-UP)
- HEAT PUMP (16 SEER-9.5 HSPF) AT 47F OUTSIDE TEMP -- $.29/HOUR
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