Heaters on One line Diagram

What did your instructor tell you to do?

Can I just use a box and write Heater in it.

Generally, no (perhaps for a rough or concept draft).

It really depends on what drawing standard you are working to and what type of heater it is.

For example drawing standard EN 60617 (fairly general and common) shows a rectangle with lines for a resistive heater

http://www.roymech.co.uk/Useful_Tables/Drawing/Electical_Control_symbols.html

And a circle with lines for a water heater

http://electrical-symbols.info/browse/indoor-installations/water-heater.html

Thyristor symbol

Heater Symbol 1 Heater symbol 2

Heater symbol 2 can be represented by a rectanguler box and a heat wave on the top.

Heater symbol 1 is the approved cct. symbol and can be used in line diagrams.

A circle with a thyristor symbol in it is the aproved cct. symbol for thyristors and can be used in line diagrams

Dickson.