Proper HVAC Air Delivery is Good Business

Central air conditioning systems should be designed to deliver conditioned air to each room in proportion to the room’s heating and cooling load.  A room located on an outside wall with a window will need an air duct delivery system capable of delivering much more air than required by a room the same size with no windows or interior walls.

The goal in designing the air duct delivery system is to size duct with a proportional balanced friction loss.  Larger ducts produce less friction.  We want as little friction as is reasonable to keep the friction pressure loss in the duct low.  Higher pressures require more energy to power the fan.  Leaking duct is also much worse under higher pressure.  In the real world a balanced system is not realistic.  Dampers are installed in the ducts delivering too much air to induce artificial friction resistance to force the air where we need it.  Dampers increase the pressure in the entire duct system and should be avoided if possible.

In the Air Conditioning Business equipment is designed to deliver 400 cubic feet of air per minute (CFM), per ton of capacity.  If the duct system is too small the pressure goes up and the air flow (CFM) goes down.  Less than 400 CFM can mechanically damage the compressor and reduces system energy efficiency.

When your contractor or engineer sizes the AC System, he should calculate the heat gain/loss for each room.  This data is needed to determine the size duct needed to deliver the proper amount of air as determined by the number and size of windows, walls etc.

Upon completion of the AC installation, the contractor should submit for your approval an “Air Balance Report” listing the room name, design CFM and actual CFM.  The sum of all supply outlets should equal 400 CFM per ton, as an example, a 5 ton system should deliver 2000 CFM total.

A tool called an air flow hood is used to quickly measure air supplied at each supply air grill.

The return air duct system is even more critical to system performance than the supply air duct system and is cover in another article.

HVAC Business