HVACR416 - Design Pressure Measurement Part 2

Pressure Measurement Several pressure measurements exist. They all measure the same things, the force of air with regards to something else.

Static Pressure Static pressure is the outwards force of air in an object. o House o Balloon o Duct

Velocity Pressure The pressure of the air moving in the duct.

Velocity Pressure to Airflow Velocity pressure can also be calculated and used to calculate CFM. 1.Calculate velocity. 2.Calculate area of duct. 3.Calculate CFM.

Total Pressure The combination of duct static pressure and velocity pressure.

Pitot Tube A pitot tube is a device that can measure all of these pressures by using one tube in the duct.

External Static Pressure The external static pressure of any device is the pressure in the supply and the return ducts added together. This is normally compared to the number on the data plate If over, or seriously under, a problem exists.

Pressures in HVAC Systems Every device in an HVAC system causes a drop in static pressure. Pressure drop across every device can be measured.

Coils To measure the pressure drop across a coil take the pressure reading on both sides and subtract.

Filters Air filters also have pressure drops. You can get a chart from the manufacturer or measure it.

Filter Locations

Driving Forces Stack: o The house acts as a chimney. Wind: o The wind can pressurize one side of the house over another. o It can also make draft change become unstable.

Driving Forces Exhaust fans and combustion venting: o Exhaust fans and combustion venting can depressurize the house. o They pull air out of the house and this air must be replaced. o A wood fireplace can remove upwards of 800 cfm. o A dryer using a standard 4” vent can remove 106 CFM’s.

Driving Forces Duct Leakage: o Duct leakage can pressure and depressurize houses as well as single zones. o A return duct leak in an attic can pressurize a house. o A supply duct leak in the attic can depressurize the house.

Air Movement Combustion air, distribution air, and ventilation air movement are required in very specific amounts for safe, efficient operation of an HVAC system.

Building Stack Effect Warm air rises The air leaks out of the holes near the top of the structure and leaks in the bottom of the structure.

Exhaust Fans Exhaust fans all take air from the house. If 100 CFM is taken out of the house 100 CFM must come into the house. Fireplaces act as large exhaust fans.

Wind The wind blows against the house. The windward side of the house experiences positive pressures. The leeward side of the house is negative.

Supply Leakage Supply leakage will force air out of the ductwork. Depending on where it is leaking it can pressurize and depressurize a house.

Return Leakage Return leakage can draw air from areas such as crawl spaces, attics, and garages which may have contaminants.

Closed Door Effect If a building does not have a return in every room the air will not flow back to the furnace. This closed door effect can allow low pressure conditions.

Outside influences The design of distribution systems is based on calculations which may not take into account such factors as: o Unusual installation practices o Winds outside the structure o Humidity

Ventilation Air All houses and buildings should have ventilation air. Ventilation air is required for occupant health comfort and fire safety. Ventilation air may be fan induced into the structure.

Combustion Appliance Zone The CAZ (combustion appliance zone) is a zone where a vented combustion appliance is located. This zone is the most critical to maintain in proper pressures. A negative pressure in a CAZ can spread carbon monoxide.

Measurement If you do not measure, you do NOT know. DOCUMENT EVERYTHING!!! EVERY TIME!!!