Hi I am working for Amor Electric Electrical contractor, Electrician Santa Monica CA, Venice CA, Mar Vista, Culver City Westchester CA (http://www.amorelectric.com)
My boss told me to connect a thermostat to an existing wall heater. There were 2 wire connected to the existing thermostat which act like a one pole 240 switch. The owner of the house brought me a Honeywell RTH230B thermostat. Does anyone know how to connect this thermostat? It said on the package that this thermostat is good for variety of heater and cooler and also well heater. Any help would be greatly appreciated.

Warning: If the old thermostat was mounted onto an electrical box,
It was probably powered by 120/240 volts. In this case, this thermostat cannot be used.

SEE here. Read this file to explain why you cannot do what your boss wants.
http://customer.honeywell.com/techlit/pdf/69-0000s/69-1800.pdf

Sorry but you will have to find another type thermostat. You have the wrong unit for this application.

Thank you for your answer

My boss actually told me to tell the customer to get a one pole 240v 30 amp thermostat but I didn't believed him because it said on the package that this thermostat is OK.

CHECK YOUR VOLTAGE ,this wall heater may be an electric heater and the control voltage may be line voltage and not a milivolt system commonly seen on gas wall heaters. The thermostat will say what it is and they are NOT interchangeable. If you have line voltage at the stat you will need a line voltage stat. (115V or 230V) Mike