HVACR416 - Design

Pressure MeasurementPart 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.