HVACR416 - Design

Pressure MeasurementPart 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 Houseo Balloono 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!!!