Testing Duct Static Pressure
We will cover how to check static pressure of the duct system, the pressure drop across the A/C coil and the air filter. The first topic will be static pressure of the duct system. The discharge side will be positive and the return will be negative. The use of a manometer is a good way to perform this test. You could also use a mini-healic gauge. There are single and double reading testers. If you use a single reading gauge you will have to add them together. If you use a double reading gauge it will automatically add them together. If you have to add them together do not being concerned that one is negative and the other as positive. For example -0.2 plus +0.3 = 0.5. Just disregard the + and - signs.
Doing a total static check a common mistake is the first drawing below. You will not get an accurate reading this way as you do not see the static of the coil and filter. Without seeing the pressure for the coil and filter will make it look better than it actually is. The 0.5 is normally with a/c and the lower number 0.2 is heat only.
The probes used for these checks should be at a 90º angle with holes drilled in the sides. You do not want the air to blow directly into the sensing holes. Insert the tube as per the drawing so sensing tube is parallel to the duct pointing away from the airflow.
The first test we are going to show is a ductwork check. The furnace manufacturer will show the parameters on a blower/cfm chart. The numbers they are listing are only accurate if the ductwork is within the parameters.
If the furnace has air conditioning the first test should be done on the furnace side of the coil and the return air compartment or between the filter and furnace if there is room. You do not want any resistance items such as filters or coils between the probes.
Copied from Kerr Furnace Installation ManualAt a 0.5 static duct pressure the cfm would be 990. This would be good for 2 ton a/c. Take note the reference to fan speed to achieve the rated cfm. If the reading is too high it may be a duct design problem. It may be as small as adding some return air or as major as redoing some of the duct system. When the reading is high let's first eliminate the a/c coil and air filter as the problem. The coil may require cleaning or the air filter is too restrictive for this application. To check the pressure drop across an a/c coil use test holes drilled as shown below. A dirty coil will affect the pressure reading you will get on your a/c gages. This will tell you if the coil is dirty and require cleaning. It will require to have the pressure drop charts from the manufacturer's manuals which most supply in their I&O manuals. If you cannot find the manufcaturer spec's a rule of thumb number that is fairly normal number would be 0.15 to 0.25 depending if the coil is wet or dry. Higher efficiency equipment may be different.
If the pressure drop exceeds the manufacturer's rating you must clean the coil. If the rating is correct it may be an air filter which is too restricted. Do a check on both sides of the filter and see what it is equal to. Does changing the air filter to a less restrictive filter or a good cleaning resolve the problem. There are many air filters on the market that carry a high static pressure due to being very tight. These filters may not be good for your system if the static pressure is too high.