In one of our previous pages we described the importance of return air filters. In this post we are going to talk about what happens when things go wrong and people do not use the correct size filters or maintain their systems properly.

Data plate of equipment with a rated static pressure of .20 inches water column.
Rated Static Pressure .20 inches water column

Every unit has a required air flow and external static pressures. For those that do not know the external static pressure is the outward “standing” or not moving pressure from the air in the return plenum and the supply plenum, added together (without the signs – or the absolute values). You can measure these with the digital tools.

Measured static Pressure .47″ WC

Once you have these pressures you need to compare to the label. In this case we have a rating of .20″ WC (inches of Water Column), and we have a measured static pressure of .47″ WC (inches of water column). This means that the system is trying to pull pressures a little over twice it’s rating. This system is not going to move enough air. The blower was also running at full speed trying to maintain air flow.

Picture of filter access – you can see edge of filter

Step 1 in diagnosing this problem is to check the filter. You can see in the picture that the system has a filter installed. You have to pull it out and look.

Dirty Filter

In the case of the above filter it was pulling up into the cabinet because of the amount of dirt on the filter. After removing the filter we still had extremely high static pressures, it only dropped .05″ WC from the prior reading. The next step is to look up from the filter onto the bottom of the coil and see if the coil is dirty. The next picture shows the bottom of the coil.

Looking up at the bottom of the coil

Looking up at the bottom of the coil there was another filter that had sucked up at some point from the filter rails and is blocking over half the coil. The coil is also dirty. This is the cause of the pressure and airflow issues. Between the dirty primary filter, the dirty and out of place secondary “surprise” filter, and the dirty coil, there is no way this system will run properly.

Solution? Perform routine maintenance on your HVAC equipment, use the correct size filters and change them frequently. This problem will never happen to you.