Pressure Tests & Air Delivery. The Devil is in the Details.

One of the heating and cooling industry’s standards is to design duct work for 1/10 of an inch pressure. (Actually, one authority recently reduced that to .08”.) Let’s take a moment and picture how little pressure that is.  Imagine sipping soda through a straw but only hard enough to pull the soda 1/10 of an inch up the straw. That is almost nothing!

Here is the rub: when we assemble a furnace and a cooling coil and a good filter and a supply and return air duct system, we used to design for .5" pressure leaving 1/10” for the duct system. The problem is that is really hard to do. Rarely have I seen a system with overall static pressure (resistance to air flow) below .5”. I commonly see .7” and higher. So, what does this do to the performance of the system?

Before starting Compton’s HVAC, I worked for another HVAC company.  I was called out to do preventive maintenance on a pretty new and high-end heat pump system. As soon as I saw the system I could just tell it was struggling. So, I did a pressure test. Technicians in the field rarely do these on systems. The idea is that there is little chance to improve the system if it is poor so why bother finding out how bad it is. This system had a pressure of .9”. I approached the home owner and suggested that he consider adding some return air to his system. His reply was pretty tough to take, “Your company put this system in from scratch.” Ouch. So, I approached my boss. He was incredulous. He told me the worker who designed the system was very competent and he thought I may have made a mistake. I assured him that I had not and, after a cool down period, he gave me the green light to fix the problem. Our men came and added more return air to this customers home. The next time I was there for preventive maintenance, the homeowner took me aside and told me that the electric company had come out and inspected his meter to see if it was broken, because his usage had fallen so much.

Heat pumps need air. If they don’t get it, they don’t work well. Furnaces need air too, but they are more flexible. If they don’t get quite the amount they want, they just make that air hotter. That is a good way to know if your filter needs changing by the way. If your air seems too hot it probably is.

Fixing air delivery after the house is finished can be expensive, but it often pays for itself in comfort and savings. We have known for years and years how to design duct systems to work right, but builders are always looking for the lowest bid not the most competent designer. Building codes don’t care if something works or not, only if it has been installed or not. As the expression goes, “The devil is in the details.”

If you want your equipment to work well, have us check it out. We do our best to find things that aren’t working as well as they should be and make them better.