How to Read HVAC Drawings 2: Ventilation System


I learned to read different kinds of HVAC drawings through years of working in the HVAC industry. I thought if junior engineers are able to know how to read HVAC drawings earlier, they are able to come out with a better HVAC system. So, I decided to create a course with a series of posts to explain how to read HVAC drawings.

In this course, I have 5 posts on how to read HVAC drawings. These 5 posts are:

  1. Title block & symbols
  2. Ventilation system
  3. Air conditioning system
  4. Control panel diagram
  5. Section, schematic & elevation drawings

In this post, you’ll learn the common shapes drawn in an HVAC ventilation drawing. Most workers install ducts and grilles exactly as per the shop drawing. Hence, it is very important that you pay attention to exactly what has been drawn. Every line and shape mean something, especially for HVAC ventilation ductworks.

Navigation Guide

I'll be zooming in to different sections of a basement smoke extraction and mechanical ventilation shop drawing for the explanation. So, you can refer to the below full drawing to navigate around.

Navigation Drawing

Equipment Tag and Specifications

A good HVAC drawing includes an equipment tag and a few important specifications of the ventilation fan. The equipment tag is usually set by the M&E consultant and you should follow the format to avoid confusion.

Tag Name

Usually, an equipment tag name includes the system, where does it locate, exhaust or fresh air and the fan number. Some tag names are longer depending on the complexity of the project.

An example of an equipment tag name is shown in the above drawing as: “SSF/B1/EA/3”. It reads as: “Smoke Spill Fan / Basement 1 / Exhaust Air / Fan Number 3”. By reading just the tag name, we know that it is a smoke extraction fan located at basement 1 and there are two more of the same fans.

Specifications

After the equipment tag name, we have a series of specifications that are extracted from the fan's specification sheet. The reason why we “duplicate” and put them in the shop drawing is that it allows workers to only use shop drawings for installation without having to bring a catalogue along.

Most of the time, such detailed specifications only occur in shop drawings and as-built drawings. Having the fan's specification stated in shop drawings reduces the chance of installing the wrong one.

Brand and Model

The fan brand and model are straightforward from the catalogue but the model must be precise because some model names are very similar. For instance, CPF500FE vs CPF500FA. A single word difference can mean one has an extra casing and the other one doesn't.

Capacity

In the above drawing, notice that the capacity is shown as two different airflow rate; 17,500 cfm and 8,750 cfm. It's because smoke spill fans have two speeds, one for the normal mode and the other for the fire mode.

During the normal mode, smoke spill fans are ventilating the basement space. In case of a fire, these smoke spill fans will run in fire mode to extract smoke out of the basement. Thus, they have two different airflow rates.

Static Pressure

Static pressure also has two different ratings. The fan has 4″ water-gauge (WG) of static pressure in fire mode and 1″ water-gauge (WG) of static pressure in normal mode.

Some static pressures are written as 1 in.WG and it's the same as 1″ WG. However, some people may prefer to use other units of measurements and you can easily convert them using online unit converters.

The “EXT written after the static pressure indicates that both the 4″ and the 1” WG are EXTERNAL static pressure rather than internal static pressure. External static pressure is the actual static pressure used to push the air instead of overcoming the internal fan motor and blade's air resistance.

Common acronyms used for different units of measurements for the static pressure are as follow:

AcronymDescription
in.WGinch of water gauge
in.WCinch of water column (same as WG)
in.Aq or in.H2Oinch of water column (same thing)
PaPascal (SI Unit)
kPakilo Pascal
psiPound per Square Inch (Imperial Unit)
atmAtmospheric Pressure
Unit of Measurement Acronyms for Pressure

Fan Type

Smoke spill fans and other mechanical ventilation fans have many different types to choose from. Each type has its own uniqueness. Some of the common fan types for smoke spill fans are square-shaped cabinet fans (shown in the above drawing), round-shaped axial fans and vane axial fans.

The “motor at separated compartment” written in the above drawing is a remark on the construction of the smoke spill cabinet fan to meet the fire-rated standard. All smoke spill fans are required to run continuously during a fire and thus, they obviously need to be able to work in an extremely high-temperature environment.

Remarks

Remarks are additional information that you want to include for the readers. It's not compulsory to have but it's good to have more information. However, we don't want to jam up the drawing and make it difficult to read.

Duct Size and Fittings

Duct size always starts with the duct width before the duct height. For instance, the above drawing indicated that the duct is 350mm in width and 250mm in height. After each grille, the airflow is reduced inside the duct. So, the duct size should also be reduced using a duct reducer to save cost.

Sometimes, duct size can be written in the imperial unit such as 14×10 (in inches) which is the same thing as 350x250mm (SI Unit). Duct sizes written in feet are less common.

Ideally, the duct should be reduced in size after each grille. However, putting a duct reducer increases the overall installation time. Therefore, you need to strike a balance between installation speed and material cost. More often, having a duct reducer is better.

List of Common Duct Fittings:

  • 90 degree bend
  • 45 degree bend
  • Reducer with slopes on left and right side
  • Reducer with slopes on left OR right side only
  • Reducer with slopes on top and bottom
  • Reducer with slopes on top OR bottom only

Look closely and you'll see the duct branches out with a slope. That indicates that there is a guide vane inside the duct to better steer the air through the 90-degree bend. Sometimes, guide vanes are drawn as curve lines if there is enough space in the drawing.

Grilles and Dampers

Grilles can be drawn on the duct to indicate bottom air suction/discharge or on the side of the duct to indicate side air suction/discharge. Arrows indicate the direction of the airflow.

Common acronyms used for HVAC ventilation grilles and dampers are as follow:

Evolution of ERP systems x
Evolution of ERP systems
AcronymDescriptionFunction
EAGExhaust Air GrilleExhausting out air
F.A.G.Fresh Air GrilleSupplying fresh air
EALExternal Air LouvreExhaust/fresh air louvre at the exterior wall
WPEALWeatherproof External Air LouvreAble to prevent rainwater from entering
ISInsect ScreenA steel mesh that prevents insects from entering
OBDOpposed Blade DamperA basic damper that can alter airflow direction
VCDVolume Control DamperAdjust airflow manually
MFDMotorized Fire DamperAutomated airflow control via an electrical signal
FDFire DamperClose under high temperature automatically
NRDNon-Return DamperEnable one airflow direction only
Acronyms for HVAC Ventilation Grilles & Dampers

Acronyms are used heavily in engineering drawings so that more information can be included without making them difficult to read.

Some of the common acronyms you may encounter are:

  • F.A.G. c/w OBD – Fresh air grille complete with opposed blade damper.
  • WPEAL c/w IS – Weatherproof external air louvre complete with insect screen.

On a side note, lines are drawn before and after a fan is known as a canvas. A canvas is some kind of cloth that allows the fan to move and bounce around without pulling/pushing the entire ductwork.

Airflow and Direction

HVAC Ventilation drawings always have arrows to indicate the airflow direction. Some drawings have an airflow rate indicated for installers to meet the requirement through air balancing work.

Some of the common acronyms used for airflow in the HVAC ventilation system are as follow:

  • EA = Exhaust Air
  • FA = Fresh Air
  • OA = Outdoor Air

The common unit of measurements used for HVAC airflow are in the following table:

AcronymDescription
CFMCubic feet per minute (Imperial Unit)
CMH or m3/hrCubic meter per hour (SI Unit)
L/minLitre per minute
L/sLitre per second
Unit of Measurement Acronyms for Airflow Rate

There are many ways to indicate airflow rate but most of the time, CFM and CMH are used in HVAC ventilation drawings to indicate the airflow rate of grilles and fans.

Slab and Wall Openings

There will be many slab and wall penetrations in the HVAC ventilation system. Whenever a duct passes through a wall, a duct sleeve and a fire damper are needed.

If a duct is not drawn inside a concrete shaft as shown in the above drawing, it means that the air travels inside the concrete shaft, not needing a duct.

Slab and wall openings are the two most important things to pay attention to during the initial stage of construction. Missing openings are very time-consuming and costly to redo. Sometimes, it's impossible to redo an opening and I've seen many junior engineers get screwed hard because of openings including me in the early days.


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