By Kent Penning

San Jose, CA – Airflow is important for conditioning the air in a confined space.  I have seen and read many studies over the more than 20 years in the heating and air conditioning business, with the underlying theme of them being that the duct work (the air delivery system) for the cool or warm air for your office or home has close to a 70% likelihood of being under sized.

So what??? Well, what this means is that the forced air systems are not flowing the amount of air they need to be – for efficient operation of the system.

I was reminded of recently when I looked at a 50 year old home in the Willow Glen (San Jose, CA area). The new home owners wished to relocate and then replace the existing furnace as well as add air conditioning to the home. Upon inspection, I found that 4 inch ducts were connected to all the floor air outlets.

Four inch ducts are much too small for adequate air flow for a heating and air conditioning system. The picture is of 4, 6 and 8 inch ductwork, this helps most people see the difference in the 3.

Good duct design dictates that these duct need to be replaced with new ducts that can range in size from 6 to 8 at the “boots”.  The boots are the transition from the stacks (rectangular or oval ductwork in the walls). See picture for 90 degree perimeter boot.

The original boots in this home measure 2 ¼” X 14” – I intend to replace them with 4” X 14” or 6” X 14” boots.

It is not enough just knowing how to design a duct system and install the system; it needs to be tested too.

At the completion of a project such as this, our technicians have an “air side start up report” to fill out. The technicians use test equipment to verify that the static pressures are within the equipments design parameters and the air temperature rise / drop meets the equipment manufacturers specifications. This is in addition to a “furnace or air conditioning start up sheet”, which tests the equipment.

Failure to install correctly sized ductwork creates system problems.  The static pressures and temperature rise / drop will not be within the manufacturer’s specifications.

So in closing, size does matter.  The ductwork picture above shows the 4, 6 and 8 inch ductwork.  A future entry will explain how these differences affect airflow.

Disclaimer, the opinion are exclusively those of the author. Mr. Penning has been in HVAC industry for 30 years. The firm Cold Craft Inc. is focused on green HVAC products, reducing energy consumption, reducing energy costs and cleaning up the environment.

If you need help with temperature, contact Cold Craft, Inc.

408.374.7292 or [email protected]