HVAC Unit 37 Air Distribution and Balance Study Guide

Why is it difficult to move air?

3 reasons: Air has weight As it moves down the duct, it rubs against itself and the ductwork Fittings create resistance to the airflow

What is the maximum temperature difference from the inside to the outside in ductwork before insulation is necessary?

A 15 degree temperature difference.

What is used to alleviate excess air when not all of zones are calling for conditioning?

A bypass duct allows the blower to move the required amount of air.

What is the device called that can connect square or rectangular pieces of duct together?

A drive clip.

What type of home would benefit from an extended plenum system?

A long structure such as a ranch house. An extended plenum has a trunk duct running down the middle of the home from the centrally located conditioning system. The room registers have smaller ducts that run off of the trunk duct.

What is the device that measures the actual air velocity?

A velometer. It measures how fast air is moving past a particular point in an air duct.

Define infiltration.

Air that leaks into a structure through cracks, windows, door or other openings due to less pressure inside than outside.

What are the two types of drive mechanisms of a centrifugal blower?

Belt drive and direct drive.

What are the two ways that fresh air may enter a conditioned room?

By infiltration and ventilation

What is formula for calculating CFM

CFM = area x velocity Area is pi x radius squared (circle) or length x width (rectangular) Velocity is the foot per minute flow rate

What are the two devices required for each zone?

Each zone has its own damper and thermostat.

T or F: The motor in direct drive blower is a PSC motor that can start up very fast and get up to speed quickly

False- PSC motors start up very slowly. taking several seconds to get up to speed.

What is another term for blower-assisted airflow?

Forced convection.

What is the most durable material used for ductwork?

Galvanized steel. Unfortunately, it is the most expensive to use.

How is fiberglass duct usually made?

It comes in flat sheets or round. It is has an aluminum foil backing on the fiberglass.

What happens if a duct is undersized?

It will cause excessive velocity and noise levels will increase.

Are fans positive displacement?

No, they are not positive displacement. They cannot build pressure like a compressor can. They can only produce airflow or air movement.

Name the 4 types of duct systems

Plenum Extended plenum Reducing plenum Perimeter loops

What are three pressures in a duct system?

Static pressure - the pressure of the air pushing against the walls of the duct sections Velocity pressure - the velocity and weight of air Total pressure - static pressure + velocity pressure

What is another name for the end cap of a duct where the grille attaches?

The boot.

What is one of the main benefits of using fiberboard duct?

The sections of a duct system can be made or modified on the job site. Another benefit is that the insulation is already on the duct when it is assembled.

What are the two sides of a takeoff fitting called?

The side connecting to the trunk line is the throat. The side connecting to the branch duct is the runout.

Why use balancing dampers?

They aid in directing the correct volume of air to the correct run of duct for better room temperature control. The dampers should be located as close to the trunk line as practical.

Describe a centrifugal fan.

They are more desireable for ductwork. They have a forward curved blade that can build more pressure from the inlet to the outlet and move more air against pressure. They are very efficient and much less noisy.

Why is an oversized duct undesirable?

They cost more and result in the slowing of the air as it flows through the duct.

What do diffusers do?

They spreads air from the duct to the desired air pattern.

What does a supply duct use to distribute air into a room?

They use registers, grilles and diffusers.

For galvanized steel, as the gage number gets larger, the thickness gets _______.

Thinner. Thus 30 gage sheet metal is much thinner than 20 gage metal.

What is the main objective for forced-air systems?

To deliver the correct quantity of conditioned air to the occupied space.

What is used to help with air that gets stuck in corners of a system?

Turning vanes help improve air flow around elbows in a duct.

When would use a reducing plenum system?

When the furnace is located to one side of the building. The main trunk duct reduces in size which keeps the same pressure from one end of the duct to the other, if sized properly.

How do zone dampers differ from balancing dampers?

Zone dampers open and close in response to changes in room temp, they are usually electronically controlled and are either fully open or closed depending on the temp requirements for the zone.

What are the components of a forced-air system?

blower air supply system return air system grilles and registers

If a water manometer has one end that is just connected into a duct and the other end outside the duct, what will it measure?

it will measure only static pressure.

How does zoning increase efficiency?

It ensures that proper amount of air is delivered to each area of a conditioned space and the space is heated and cooled evenly.

What is the name of the fitting that connects branch duct to the main trunk line. What does it look like?

It is called a takeoff fitting. One style has one side larger on the duct side than the trunk side to assist the air to move smoothly into branch duct.

What is friction loss?

It is friction created by the actual rubbing action of air against the side of the duct and the turbulence of the air rubbing against itself while moving in the duct.

Describe a propeller fan.

It is used in exhaust fan and condenser fan applications. It can handle large volumes of air at low pressure differentials. It is set inside a venturi to encourage airflow from one side to another. They can be noisy.

Where should air be directed to maximize efficiency?

It should be directed on the walls. The walls is where the heat exchange occurs. The air can cancel the load and keep the wall warmer or colder.

What is general rule of thumb for the number of BTUs to heat compared to the number of BTUs to cool a room or house?

It takes twice as many BTUs to heat home as it does to cool it.

In a centrifugal fan, if the driven pulley (the pulley attached to the actual fan) is too small, will the amperage draw be higher or lower?

It will be higher since it will need to spin faster to get the revolutions on the fan up to the needed speed.

If a centrifugal fan has less air going through it due to a blockage, will the amperage draw be higher or lower?

It will be lower since the weight of air that it is actually moving will be less.

If a water manometer has one end that is in the stream of air inside a duct and the other end outside the duct, what will it measure?

It will measure both static and velocity pressure or Total pressure.

If a water manometer has one end that is in the stream of air inside a duct and the other end is just connected into the duct but after the other end, what will it measure?

It will measure static and velocity pressure but the other end will cancel the static pressure. Thus it will measure velocity pressure.

When should the plenum style be used?

When all of the room outlets are all close to the unit. The individual duct branches are joined together at the center with the furnace located near the center of the home.