This particular article might get a little more technical than some previous articles but a recent visit to California prompted me to put this together.
During a meeting with a consulting engineer I was told of a problem that he was having with a direct fired make-up air unit…not one of ours but the principles are the same. The issue was a burner that would shut down for “unknown reasons” shortly after coming on-line. After some discussion the engineer stated that his estimate of external static pressure was wrong because the unit was not installed as he had originally designed. The duct system attached to the unit had several more bends in it than he designed and thus the external static pressure was higher than he estimated and higher than the manufacturer was told at the time of production. This was the clue we needed to help the engineer understand what was going on.
Almost 10 years ago the safety standards for direct fired equipment were changed. As a result most certified direct fired equipment provides heated air that is extremely safe from carbon monoxide, carbon dioxide, and other gaseous contaminants. That is because the direct fired product is designed to meet a specific set of operating conditions, by code, that assure a velocity across the burner that is roughly 2,800 to 3,200 feet per minute. When a direct fired product operates in that range then the combustion is “clean” and harmful byproducts are not produced in any significant amount.
In order to assure the user that the direct fired burner is operating in the proper range the manufacturer uses the total static pressure that is calculated for the unit (including the external static pressure) to size the airflow opening across the burner to stay within the target range. Most manufacturers will size the opening to land in the middle of the range so that there is some room for minor errors in the estimates.
In order to make this even more fool-proof the safety standard required manufacturers to add pressure switches to the units that would shut down the burner if the velocity across the burner was either too high or too low. In the case that this engineer was describing the overly high external static pressure was causing the velocity across the burner to be too low and the burner would shut down.
Because the code and testing standards required manufacturers to be very specific you will find that the acceptable range between high and low is unique to each model and size in a product line. Some of these ranges are very narrow…less than 0.25 inches of static from the high set point to the low set point. This gives the equipment only 0.125 inches of static from the midpoint to the trip point in either direction. Too many bends in the ductwork can easily add that much static to the system and cause the premature shutdown of the system.