Quote Originally Posted by revelations View Post
This quote is hidden because you are ignoring this member. Show Quote
You need a certain plenum width and CFM fan to push air through the coils. If its not sufficient, you can just upgrade the fan.
The house I bought a few years ago came with A/C, the whole install could be the subject of a book called "How not to install residential air conditioning". Needless to say, it just didn't work, it would run all day and lose ground, even when running at night when the temperature outside fell below setpoint inside, it still couldn't gain ground. We often just shut it off and opened the windows.
This year with the humidity, I finally got tired of it and put some effort into fixing it. I did the ECM Azure upgrade to the furnace and I bought the programmer so I could custom set the blower speeds. I ended up driving the motor to max which is 99% torque and 1400 rpm, added a pound of gas to it, then I went nuts with the duct sealer on all the visible ductwork to seal up the gaps. The problem with the ductwork and registers in the floors, is you need really good air volume and velocity to get the cold air up to make a dent in the room temperature, not enough and it just floats along the floor and then falls to the lowest point in the house it can. With the mods and tuning, it now can maintain 21c even when its over 30c outside. By conventional sizing my house needs almost 6 tons of cooling, I have enough airflow now it's cooling the entire house with just 3 tons on the upstairs furnace. I programmed the motor to run on heating at the same speed the old motor tach'd at.