Seclecting an Air Conditioning System for Your Historic House | Wisconsin Historical Society

General Information

Selecting an Air Conditioning System for Your Historic House

Seclecting an Air Conditioning System for Your Historic House | Wisconsin Historical Society
EnlargeMini split system

Brown County. Installing the mini split system exterior unit at the rear of the house is essential to retaining the home's historic appearance. Source: WHS - State Historic Preservation Office.

Historic houses were never designed for air-conditioning systems, so it can be challenging — but not impossible — to retrofit your house for air conditioning. Your decision about the type of air conditioning system will be critical to your budget as well as your comfort. You have two basic options to get air conditioning into your house during the hottest months of the year:

  • Window air conditioners
  • Split-unit air conditioning systems

Window Air Conditioners

For years, historic houses have used window air conditioners to cool individual rooms and spaces. Although window units are less efficient than modern central air conditioning systems, modern window units offer three benefits:

  • Window units are usually cheaper to run than central air systems.
  • Window units cool only the rooms you need to cool.
  • Window units have no ductwork.

Window air conditioners are rated for efficiency based on their SEER (seasonal energy efficiency ratio) rating. In the last 30 years, the efficiency of window units has improved so much that a modern unit will cut your electrical bill for air conditioning in half over a 30-year old unit. Window units from the 1970s and 1980s were rated in the 5 to 6 SEER range. A modern unit should have a SEER rating at 10 or above.

Window units are sized based on BTU's per hour. You can determine how many BTU's you'll need from the square footage of your floor space and ceiling height. To decide on a unit size, use the rule of thumb of 20 BTU's per square foot of floor space. It's important to get the size right. A smaller unit sized properly for the room you need to cool will run longer but will cool the room and remove humidity more efficiently than an over-sized unit.

Window air conditioners have three downsides:

  • They must hang out of a window and therefore are unattractive.
  • The humidity they remove from the room must be released, so you must make sure the water doesn't drip on important architectural elements below the unit.
  • They are too noisy for many people.

Split-Unit Air Conditioning Systems

All split-unit air conditioning systems have an outside condenser/compressor and an inside air handling unit with a copper evaporator coil inside. The minimum SEER efficiency rating for a new split-unit air conditioning unit is 13. You should have a professional size the system properly for your house. A system that is too large can't remove humidity as well as a properly sized unit, and a unit that is too small will run constantly and use too much electricity.

Split-unit air conditioning systems are available in three types:

  • Conventional central air conditioning systems
  • Ductless, mini air conditioning systems
  • High-pressure, high-velocity central air conditioning systems

Each type has a unique set of features that may adapt better to your historic house.

Conventional Central Air Conditioning Systems

Conventional central air conditioning systems require standard ductwork to supply the cool air as well as ductwork for the return air. These ductwork needs can present a problem in a historic house that has radiators and a boiler system. Your decision about this type of system may depend on the type of house you have:

  • If you already have a forced-air heating system with ductwork, this system will be the most cost effective to install.
  • If you have a mid-century, one-story ranch house, you can run all the ductwork in your basement.
  • If you have a two-story historic house, you may need to create two zones in your house with two systems of conventional ductwork. A zoned system has two separate conventional split-unit air conditioning systems with separate thermostats. If you have over 2,500 square feet of living space, you should probably have two systems anyway. If you install a zoned, two-system set-up, you can run the second-floor ductwork into the second floor from the attic and the first floor ductwork from the basement. Be sure that every foot of ductwork is insulated to a minimum of R-11.

Ductless, Mini Air Conditioning Systems

EnlargeMini split system

Brown County. Here is the mini split system interior unit installed within a stair well. Source: WHS - State Historic Preservation Office.

Ductless, mini air conditioning systems can offer a solution for some historic houses with problematic spaces where conventional ductwork cannot be installed. These systems are small units attached to the interior wall of a room. They have outside condenser/compressor units but no ductwork. The refrigerant, electrical and condensation lines are run to and from each room unit. These systems can be very efficient if sized properly. They run quietly and can be much less intrusive to the historic fabric of your house than a conventional system. Some higher-quality mini air conditioners include small, minimal ductwork used to cover a second room or space. Some units can be mounted on the ceiling.

High-Pressure, High-Velocity Central Air Conditioning Systems

High-pressure, high-velocity central air conditioning systems are outstanding for historic homes with boiler heat, but they can also be set up to provide heat. They adapt to geothermal systems as well. However, these systems are also the most expensive system to install. The interior air-handling units for these systems push the heated and cooled air through small ducts that are the size of a vacuum hose. The high-pressure, high-velocity air handler pushes air into the room in such a manner that the air is virtually the same temperature from floor to ceiling. These systems also have a very high efficiency rating. The ductwork for these systems is so small in diameter that the ducts can be run through 2 x 4 -inch wall cavities and closet and floor joists without harming your original plaster and trim. The vents are round and look like a porthole in a speaker.