What is a load calculation

All structures either lose heat in the winter time or gain heat in the summer time. This heat loss or heat gain is caused by the fact that the transfer of heat, even in a super insulated house, cannot be completely stopped.

If we know how much heat is being transferred through its walls, ceilings, floors, windows and doors, ducts and through infiltration (air leakage) on an hourly basis, then we could calculate the precise size heater or air conditioner the house would need to maintain a comfortable temperature. This calculation is called a load calculation.  The load is measured in BTUH’s

Why perform a load calculation?

The obvious reason is to prevent installing a system that is too small to do the job. However, if this were the only reason, why wouldn’t we just put a 5-ton air conditioner and a 140,000 BTUH furnace in a 1200 sq. ft. house and never worry about it again? The real reason for a load calculation is to size the equipment in order to assure comfort, economy, and good indoor air quality.

When heating, it is important to size the system as close to the heat loss calculation as possible to prevent (1) drafts, (2) hot and cold spots, and (3) short cycling of equipment. When a furnace is grossly oversized, the unit will constantly shut off and on. It may satisfy the thermostat but leave other parts of the home either over or under heated thus, leaving the occupants uncomfortable. A correctly sized unit runs longer, resulting in a better distribution of air and reduced short cycling. Short cycling also leads to higher energy costs. Each time a furnace fires up, it must heat up the heat exchanger before the indoor fan comes on. This heat is wasted up the chimney. Short cycling (short on-off periods) increases the amount of heat wasted up the chimney. In addition, if the occupant is cold in the area he is sitting in (a cold spot), he will turn up the thermostat, which wastes fuel as other areas are over heated.

Sizing rules for heating (ACCA):

• Fossil fuel furnaces – Do not exceed 125% load calculation (may be twice the size required).
• Electric resistance heat – Do not exceed 110% of load calculation.
• Heat pumps (used for heating and cooling) – Do not exceed 125% of cooling load
• Heat pumps (used for heating only) – Do not exceed 115% of heating
• Auxiliary heat (electric resistance) – Install only enough KW to make up for the heat pump’s deficit. If more heat is desired, the additional heat must be controlled to remain off during normal heat pump operation.

Sizing an air conditioner correctly is even more important than sizing heat. Aside from causing hot and cold spots, over-sizing an air conditioner can result in causing high humidity and the problems associated with it. When an air conditioner runs, it is not only cooling, it is also dehumidifying. An oversized air conditioner will cool the house but, will not run long enough to dehumidify. High relative humidity can have two detrimental effects: (1) higher energy bills because higher humidity requires lower thermostat settings to remain comfortable, and (2) promote mold, mildew, moisture, and possibly health related problems.

NOTE: Even a correctly sized air conditioner is oversized most of the time. For example, a load calculation may call for a three-ton unit at 95-degree outdoor temperature, however, 97% of the time it is less than 95 degrees outdoors. A practical solution is to slightly undersize the unit but, talk this over with the owner.

Sizing rules for air conditioners (ACCA):

• Air conditioner – May be sized up to 115% of calculation.
• Heat pump – May be sized up to 125% of cooling calculation if needed to supply extra heating capacity.