Measuring Static Pressure Drop in HVAC Ducts

Heating, ventilation, and air conditioning ducts, or HVAC ducts, channel the flow of climate controlled air throughout buildings with centralized systems. Return ducts collect air in rooms and carry it to the HVAC system where it is conditioned. Forced-air ducts then take the conditioned air back to the rooms. HVAC systems must have restriction-free ducting for efficient airflow, and balanced proportional air flow rates among different sized rooms. Measuring static pressure drop in HVAC ducts is a proven method to check both of these variables. You can easily make your own inexpensive manometer to do it.

U-Tube Manometer

Check static pressure drop in HVAC ducting

Step 1

Purchase a plastic U-tube manometer set for checking HVAC systems or make one. Cut the stud to six feet long and starting in the middle, mark it in inches from the middle towards the top and then the bottom. There should be a 35 and a scant inch left, both top and bottom,.

Step 2

Form a narrow U in the exact middle of the tubing, hold the U at the bottom and attach the tube to the stud with the staples so the graduations are centered between the two sides.

Step 3

Draw some colored tap water into one end of the tube like a straw, until it reaches zero on both sides of the U. File or cut one end of each copper tube on a sharp slant. Insert their other ends into the plastic tubing. The manometer is now ready.

Measuring Static Pressure and Static Pressure Drop

Step 1

Hang the manometer up straight near the ducting being checked. Make sure the HVAC system blower is running. Insert one of the sharp copper tubes into the duct until its tip is in the middle of the duct. For sheet metal ducting, drill a ¼-inch hole. Read both sides of the manometer and add the high and low side readings. The sum is the static pressure.

Step 2

Straight metal ducting drops.

Insert the other sharp tube into the same duct further downstream or around a 90-degree bend. The sum of the high and low readings is the static pressure drop in that section of ducting. It should be somewhat less than the static pressure reading.

Step 3

Remove the first tube you inserted and note the sum of the high and low readings which is static pressure at that point. Subtract this value from the first static pressure and the number should be equal to the static pressure drop you measured in Step 2.

Step 4

Experiment with the measurements. Measure static pressure in a forced-air duct with the room register open, and then with it closed. There should be a significant increase in the static pressure with it closed due to the restriction.

Step 5

Develop a profile for the entire house to isolate restrictions. The most efficient systems operate with the lowest overall drops. You can also measure static pressure drop across the air filter in the return plenum. As filters fill with dust, the static pressure drop across them increases significantly.