Planning heating, cooling and ventilation

Sizing heating and cooling equipment is one of the few places where bigger is not better. An oversized air conditioner cools the room quickly and shuts off before it removes the moisture, leaving the air clammy. An undersized heater runs all day and never catches up. The right size comes from the room, the climate and the insulation, so fix the building envelope before you size the machine.

This planner starts with insulation, then sizes the equipment, and ends with running costs and humidity.

Step 1: Start with insulation and air sealing

The cheapest way to cut heating and cooling loads is to insulate the attic and seal the gaps. Check the depth of attic insulation and the ventilation, since both change the size of the equipment you need.

Step 2: Size the air conditioner and the furnace

Equipment is rated in BTU per hour, and cooling in tons of 12,000 BTU per hour. The room size, the ceiling height, the sun and the number of people all change the answer. For a whole house, a load calculation (Manual J) from a contractor is the most reliable.

Step 3: Size heaters for single rooms

Space heaters, baseboard heaters, electric floor heating and garage heaters are sized by the watt or the BTU for the room. Electric resistance heat turns one kilowatt-hour of electricity into one kilowatt-hour of heat, so it is the most expensive heat to run in most places, and a heat pump is much cheaper.

Step 4: Compare the running cost of the fuels

Natural gas, propane, oil, electricity, heat pumps and wood all cost different amounts per unit of heat. Compare them on the same basis, and consider the efficiency of each system.

Step 5: Ventilation and fans

Bathroom fans, range hoods and ceiling fans move air, and the airflow in CFM is what counts. Size a fan for the room volume and the number of air changes you need, and vent it outdoors.

Step 6: Control humidity and air quality

Aim for 30 to 50% relative humidity, and under 60% to avoid mold. A dehumidifier is sized by the pints of moisture it removes a day, and an air purifier by the clean air delivery rate.

Before you start

• The current state of the insulation and the ducts.
• Quotes that include a load calculation, not only a size guessed from the old unit.
• The electrical capacity for new equipment.
• Rebates and rules in your area for heat pumps and high-efficiency equipment.
• A licensed technician for gas, refrigerant and electrical work.

Mistakes that cost people money

• Choosing a unit by the size of the old one without checking what has changed.
• Oversizing the air conditioner, which cools quickly but leaves the air damp.
• Ignoring the insulation and the air leaks.
• Venting a bathroom fan into the attic instead of outdoors.
• Using resistance heat for whole-house heating where electricity is expensive.

Frequently asked questions