HVAC Symbols: Drawing Legend and Abbreviations (Chart + DWG)

MEPbase Staff — Fri, 16 Oct 2026 12:09:32 +0000

HVAC drawings use a shorthand of symbols and abbreviations for diffusers, ducts, dampers, sensors and pipes. This chart shows the HVAC symbols you will meet most often on plans and shop drawings, with their abbreviations and what they mean, plus free DWG blocks to use in your own drawings.

Quick answer

A square with an X is a supply diffuser, a square with one diagonal is a return or exhaust grille, a line across a duct with a pivot dot is a volume damper, and a circle with T is a thermostat. Every project has its own legend, so always check it first.

HVAC Symbols and Their Meaning

Symbol	What it shows
Square with X	Supply air diffuser (ceiling)
Square with one diagonal	Return or exhaust air grille
Long narrow rectangle	Linear slot diffuser
Rectangle with X (in section)	Supply duct cut in section or turning up
Rectangle with one diagonal	Return or exhaust duct in section
Two parallel lines with size	Rectangular duct, width × height (e.g. 600×300)
Zig-zag line	Flexible duct
Line across duct with dot	Volume control damper (VCD)
Crossed lines + FD	Fire damper
Damper + M	Motorised damper
Curved blades in elbow	Turning vanes
Hatched block in duct	Sound attenuator
Circle with T / H / TS	Thermostat, humidity sensor, temperature sensor
Box with FCU	Fan coil unit
Circle with blades	Fan (exhaust, supply, jet)
Solid / dashed lines with CHWS, CHWR	Chilled water supply and return pipes

HVAC Abbreviations

Abbreviation	Meaning
AHU	Air handling unit
FCU	Fan coil unit
VAV / CAV	Variable / constant air volume box
SA / RA / EA / FA	Supply, return, exhaust, fresh (outdoor) air
CHWS / CHWR	Chilled water supply / return
HWS / HWR	Heating hot water supply / return
CD	Condensate drain
RL / RS	Refrigerant liquid / suction line
VCD	Volume control damper
FD / SD / FSD	Fire damper / smoke damper / fire and smoke damper
EF / KEF	Exhaust fan / kitchen exhaust fan
CU / DX	Condensing unit / direct expansion
BMS	Building management system

How to Read HVAC Drawings

Start with the legend
Each consultant has its own symbol set; the legend sheet overrides any general chart.

Follow the air
Trace supply from the AHU or FCU through ducts to diffusers, then return and exhaust back.

Read sizes and levels
Duct sizes (W×H or Ø), airflow at each diffuser, and bottom-of-duct levels for coordination.

Check the accessories
Dampers, access doors, sensors and attenuators are easy to miss but must be installed and accessible.

Size the ducts too

Convert airflow to duct size with velocity and friction checks.

Open the Duct Calculator →

Download HVAC Symbols (DWG)

All 25 symbols from the chart as AutoCAD blocks (MB_HVAC_…), drawn at paper size in mm: scale them by your drawing scale, for example ×100 at 1:100.

Checking…

Frequently Asked Questions

What does a square with an X mean on an HVAC drawing?

Usually a supply air diffuser in plan, or a supply duct cut in section. The project legend confirms which.

What is the symbol for a fire damper?

Commonly a duct with crossed lines and the letters FD; smoke dampers use SD and combination units FSD.

What do CHWS and CHWR mean?

Chilled water supply and chilled water return, the pipes between the chiller and the cooling coils.

Are HVAC symbols standardised?

Partly. ASHRAE and ISO publish symbol sets, but most consultants use their own legend, so always read the legend sheet.

Related Downloads and Tools

Written by Faisal, MEP engineer with 16+ years in building services design and shop drawings.

diffuser symbol – MEPBase