What size air conditioner do I need for my house?
What is the recommended indoor humidity level for homes?
My co-workers and I frequently argue about where to set the thermostat in our office. What is the recommended temperature for offices?
What is a Btu?
What is a ton?
What does EER mean? Should I spend more money for a higher EER unit?
What does SEER mean? Should I spend more money for a higher SEER unit?
What does HSPF mean?
What does AFUE mean?

Disclaimer: ASHRAE Technical Services Staff responds to technical inquiries and has compiled answers to the following consumer frequently asked questions (FAQs). Answers to consumer FAQs are provided as a service specifically for homeowners and the general public. While every effort has been made to ensure their accuracy and reliability, they are advisory and provided for informational purposes only, and in many cases represent only one person's view. They are not intended and should not be relied on as an official statement of ASHRAE.

Sizing a residential HVAC unit is a multi-variable problem that depends on such variables as geographic location, orientation to the sun, construction details, insulation values, window area and type, etc. Under sizing a system may lead to warmer or cooler temperatures than desired by some occupants. Over sizing the system may lead humidity or moisture control problems in the space.

The industry standard procedure for sizing residential systems is the Air Conditioning Contractors of America (ACCA) Manual J procedure. You can learn more about the procedure and ACCA by visiting www.acca.org or calling 703-575-4477. ASHRAE has provided information in the past to ACCA for use in this procedure. The Society also is conducting research with support from ACCA that will develop information to be used in a future edition of Manual J.

A good range is between 30% and 60% relative humidity. You can determine humidity levels with a relative humidity sensor typically referred to as a hygrometer or psychrometer. This level of humidity minimizes the indoor growth of allergenic or pathogenic organisms such as dust mites and molds.¹

A wider range of humidity levels, from a low of 25% to a high of about 80%, can be acceptable in terms of thermal comfort (your comfort level) depending on the type of clothing worn and the level of physical activity.²

ASHRAE specifies the combinations of indoor space environment and personal factors that will produce thermal comfort conditions acceptable to 80% or more of the occupants within the space.² to be specific, temperatures in the winter should range from 68-74° F and 73-79° F in the summer.³

BTU stands for the British Thermal Unit (Btu). It is a unit of heat energy in the inch-pound unit system, which is common today in the United States. The Btu is defined as the amount of heat necessary to raise the temperature of one pound of water one degree Fahrenheit from 58.5 to 59.5.

A Btu is commonly used to indicate the heating and cooling capacity of a system, heat losses, and heat gains. To give an example of system capacity, a 10,000 Btu window air conditioner is capable of removing 10,000 Btu's of heat per hour. To give an example in terms of heat gains, the typical heat gain added to a room by a person at rest is about 230 Btu/hour.

A ton is the unit of measurement for air-conditioning system capacity. One ton of air conditioning removes 12,000 Btu's of heat energy per hour from a home. Central air conditioners are sized in tons. Residential units usually range from 1 to 5 tons.

EER stands for energy efficiency ratio. The higher the EER rating, the more energy efficient the equipment is. This can result in lower energy costs. The following US Dept. of Energy Web site can show how to calculate potential energy cost savings of a more efficient unit - www.eere.energy.gov/femp/procurement/pdfs/cac.pdf. The Air-Conditioning and Refrigeration Institute (ARI.org) posts a Certified Ratings directory, which lists EER ratings of various air- conditioning equipment.

SEER stands for seasonal energy efficiency ratio. The higher the SEER rating, the more energy efficient the equipment is. A higher SEER can result in lower energy costs. This U.S. Dept. of Energy Web site, www.eere.energy.gov/femp/procurement/pdfs/cac.pdf, can show how to calculate potential energy cost savings of a more efficient unit. The Air-Conditioning and Refrigeration Institute (ARI.org) posts a Certified Ratings directory, which lists SEER ratings of various air-conditioning equipment.

HSPF stands for heating system performance factor. The higher the HSPF rating, the more energy efficient the equipment is. A higher HSPF can result in lower energy costs. The Air-Conditioning and Refrigeration Institute (ARI.org) posts a certified ratings directory, which lists HSPF ratings of various air-conditioning equipment.

AFUE stands for annual fuel utilization efficiency. The higher the AFUE rating, the more energy efficient the equipment is. A higher AFUE can result in lower energy costs. The Gas Appliance Manufacturers Association, www.gamanet.org, publishes a directory of certified AFUE ratings.

¹ ASHRAE Standard 62-2001, Ventilation for Acceptable Indoor Air Quality
² ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy
³ Addendum 55a, ASHRAE Standard 55, Thermal Environmental Conditions for Human Occupancy