Go with the flow
Since I’m in the business, friends and neighbors often come to me for advice before purchasing a new HVAC system. And what they usually want to know is which brand is the best to buy?
My advice to them is to find the most competent contractor (not the cheapest) and buy whichever brand that company suggests, because the quality of the installation is much more important than the equipment brand name. A botched job will negatively affect the system efficiency, the system capacity and the equipment life.
Here in Florida, it is hard to find contractors who know anything about proper airflow and distribution, because good sheet metal (or fiberboard) duct designing is a dying art here, as it is in many other places. As a result, far too few systems (both heating and cooling) actually achieve the manufacturers’ efficiency ratings.
What are many contractors doing wrong?
Low airflowFirst, recognize that, in order to reach the manufacturer- rated efficiency: the duct system must be able to deliver at least the rated airflow. But, with almost all residential flex-duct systems (which we see so much of here), few can actually do the correct engineering, because no one can calculate the losses from all the bends, turns and loops. Also, to save money, many are not designing their duct systems to meet the manufacturers’ recommended maximum static-pressure drops. And some of the air filters that they sell, while very efficient at removing particulates, have static losses that are far too great for the designs. As a result, too many systems are going in with insufficient air. What does this do?
Well, recognize that the warmer the air is across a furnace heat exchanger, or the cooler the air is across an evaporator (due to low airflow), the lower the heatexchange efficiency. And at the same time, this raises the thermal losses through the ductwork.
Improperly designed plenumsThe problem that I’ve noted with many contractors is that few (locally) have ever taken HVAC engineering courses, and fewer still have ever served in an HVACR apprenticeship program. As a result, they just don’t understand how to properly move conditioned air. Many feel that all they have to do is tie ducts somewhere into a plenum and then run them any way that they wish to distribute or return the air. However, proper plenum designs and ductwork connections that allow for equal air distribution are critical to reaching peak system efficiency.
Things that I see repeatedly are:
- Plenums that are too short.
- Takeoffs that are too high in the plenum.
- Takeoffs that send the bulk of the air in one direction, or return it from one direction.
Understand that when most of the air enters a furnace or air handler from one side in a return, or leaves from one side of the discharge plenum, the airflow through the equipment is unequal. So half is being starved, while the other side is getting all the air. And as a result, part of the heating surfaces are losing their efficiency up the vent pipe, and part of the evaporator is running far too cool, which reduces refrigerant flow through the coil, cutting both capacity and efficiency.
Of course, not everyone at an HVACR supply house is expected to have attended an engineering course or to have participated in an HVACR apprenticeship program. So, what I’m doing here is just pointing out some common-sense things to look for whenever customers complain about not getting the promised efficiency or capacity from their new systems.