Special Notice

ADDING AIR CONDITIONING to EXISTING DUCTWORK

and a NEW or EXISTING FURNACE

If the forced warm air furnace in your house supplies heat to more than one floor/level in your house you need to be aware of the cooling limitations on added air conditioning. If your house is a one level house, i.e. a ranch style what is said here is still true; however, the air in your house will not be subject to as much stratification as in a multistory house.

GET THE FACTS... 

• Fact. . . Warm air rises to the highest level it can find.
• 
  Fact. . . Cool air falls to the lowest level it can find.
• 
  Fact. . . Ductwork in your house has friction losses related to duct material, size and length. Rooms connected to your furnace via long duct runs may not receive the cooled air needed to reduce the temperature to a desired comfort level.

• Fact. . . Your house duet system was (very probably) designed for heating . . . NOT COOLING.
• 
  Fact. . . Adding a higher cooling capacity (expressed in tons) compressor/condenser unit than your ductwork is designed to handle is wasteful. A five ton unit won’t give you five tons of cooling if the duet system is not capable of moving the cool air to the parts of the house needing cooling.

• Fact. . . Water condensed from the air on a forced warm air system evaporator coil/cooling coil will freeze at a temperature close to 32°F so you can’t over cool the air moving through the evaporator/cooling coil or you will make ice which plugs up the system thus stopping air flow.
• 
  Fact. . . Due to laws of physics and engineering more air flow through the duet system is required for air
  conditioning (cooling) than for heating.
• 
  Fact. . . In addition to needing more air flow for air conditioning than for heating we live where the air density due to altitude is only about 80% of the air density at sea level. This means that we need more air flow (quantity of air) at our altitude to do the same work as a given air flow (quantity) would accomplish at sea level. At sea level 400 Cu Ft per minute of air flow per ton of air conditioning performs well. To get the same performance we, at a 5,000+ Ft altitude, need about 450 Cu Ft of air flow per minute per toa of air conditioning.
• 
  Fact. . . With any given duet system increasing blower speed to push the additional quantity of air needed for air conditioning through the duct system increases air velocity. Air moving at too high a velocity in ductwork may be considered NOISY and may even whistle wherever there is a small hole in the ductwork and/or register opening.
• 
  Fact. . . Production homebuilders don’t spend any more money providing a heating system than absolutely necessary.
• 
  Fact. . . Home buyers seldom themselves or with the help of a professional heating and air conditioning contractor or eagineer test the heating and/or cooling system BEFORE buying the house.
• 
  Fact. . . Dirty furnace filters and/or decorative supply registers, some curtains and furniture obstruct air flow so the efficiency and effectiveness of the forced air system is reduced.
• 
  Fact . . . Since air movement is so important related to air conditioning you can help by running your furnace blower/fan continuously. Turn the “Fan” switch on your thermostat to “On”. A modern furnace variable speed blower motor may only consume 65 watts of power so it can be compared to leaving a 65 watt light bulb turned on all the time. The use of ceiling (paddle), portable or other fans will help to move air around in your house.
• 
  Fact. . . Adding air conditioning to an existing furnace is usually much more work than most people realize. If the furnace installer did his/her job with the idea that air conditioning might be added later the addition of air conditioning may be straight forward. On the other hand and more often true than not, the furnace was installed with little or no regard to the fact that air conditioning might be added later. In that case the ductwork may need to be modified or adapted. Natural gas and/or electrical lines may need to be moved etc. Doing this detail work takes considerable time and skill.
  
  
• Fact. . .your house has a whole house attic fan it can he used in combination with air conditioning to accomplish what in the commercial air conditioning world is called an “economizer system”. Use the whole house attic fan ONLY when the temperature is lower outside than inside. Use the whole house fan early in the morning when even during hot spells the outdoor temperature may 68°F or less until 8:00 a.m. or so.

After cooling your house down with morning fresh air close it up, close the windows, pull the curtains and blinds (especially on the south and west sides) and verify that the air conditioning is on with the thermostat set to 73°F to 75°F or whatever temperature you are comfortable with. Then leave the thermostat alone to do its job. . . don’t play with it. You may again use the whole house attic fan in the late evening when the outdoor temperature is equal to or lower than the indoor temperature.