HVACR416 - Design Pressure Measurement Part 1

Why Measure? Need to Make Sure: All combustion appliances vent properly All HVAC systems are installed according to manufactures specifications Buildings and their HVAC systems produce comfort Buildings and HVAC systems are efficient.

Why Measure? Buildings and HVAC systems are durable Buildings and HVAC systems do not effect IAQ

Reasons Why: Over the recent years more problems exist: o Tighter homes o More exhaust fan o More combustion equipment (wood and gas) o More zones in a home o More multi-story homes

Reasons Why Societal trends: o Increased competition among HVAC contractors for businesses. o Increased third parties looking at efficiency. o Higher homeowner expectations of comfort.

Reasons Why: More homes with forced air heating systems More attached garages Every one of these reasons can effect pressure in a building. Anything that can effect pressure can effect IAQ.

Tightness Tightness of houses is being increased for energy efficiency. Tightness increases pressure differences within a building.

Backdrafting Backdrafting is why we care about the tightness and pressures in a home. The key to understanding backdrafting is understanding the driving forces that create negative pressures.

What is pressure? Textbooks define pressure as the force pressing against a surface as stated in weight per unit area The force acts as right angles to the surface uniformly in all directions.

What is Pressure? Air pressure is the measurement of molecular density. The greater the density the greater the pressure. When molecules get pushed together the pressure increases. When the get pulled apart the pressure decreases.

What is pressure? Normal air pressure at sea level is 14.7 psi (pounds per square inch). In Denver, Colorado the pressure is 12.1 pounds per square inch. At 35,000 feet above sea level the pressure is about 3.4 lp/sq. in.

What is pressure When air is warmed in the combustion process the molecules become spaced further apart. The density decreases and the air rises. This is stack effect.

Pressure Pressure will move from a positive pressure to a low pressure - ALWAYS The only exception is when we are pushing it with a fan.

How to Measure Pressure is measured most often in the HVAC industry in WC”. Several tools are available to measure these pressures.

U-Tube Manometer The u-tube manometer is filled with water and measures how far the pressure pushes the water one inch. This is where “one inch of water column” comes from.

Inclined Manometer The inclined manometer is a more refined version of the u-tube. It measures the smaller pressures more common to air handlers and homes. This has a max of 249 pascals or 1 WC”

Gauge Manometer Gauge manometers eliminate the use of liquids and use gauges to do the same work. Uses high and low pressure taps.

Digital Manometer Digital Manometers are available. They eliminate all mechanical parts and all is done with circuit boards and sensors.

Pressure Measurement The basis of all pressure measurement is to compare the pressure in one zone to another. For example the inside of a building to the outside. The inside of a duct to the inside of the building.

Fans and Blowers In the HVAC industry fans and blowers are usually used to move air. This movement and force provides a positive and negative pressure around the blower.

Blower Assemblies Are constructed to pull air through the return air ducts and filter, then push air around the heat exchanger and through supply ducts. They require careful adjustment to produce enough volume and velocity to maintain comfort at design conditions.

Blower Assemblies There are two types of blower assemblies available: o Direct Drive o Belt Drive

Direct Drive Approximately 1050 RPM on high speed Supported by motor shaft Variable speed requires a multi-speed motor. Used in residential applications mainly

Belt Drive Motor is usually at 1725 RPM Blower wheel is supported by the shaft and bearings Variable speed is obtained by varying the setting of a split pulley More often used on light commercial applications.

Blower Assemblies

Belt Drive Blowers It is important that the drive on the motor and the pulley on the blower wheel be perfectly aligned. Any belt wear, more than normal, is most often attributed to the improper alignment of the pulley and motor.

Fan Pressures When we put a blower in a furnace. The supply side of the fan is a positive pressure. The return side of the fan is a negative pressure.

Fan Pressures Fan and system pressures can be found on the furnace name plate, data plate or rating plate.