If you have a cold water pressure washer it is possible to turn it into a hot water washer with a modular heater.  Modular heaters come in various size and fuel choices.  

If you regularly wash with cold water but occasionally need hot water then a modular heater may be just what you need.  But, there are some things you need to consider before going out to buy one.

The cost of a cold water washer and separate modular heater may be more expensive the buying an all-in-one hot water washer. 

Most modular heaters require an outside power source to run the blower and igniters.  All-in-one hot water washers have their own power source from a charging coil on the engine. Separate oil fired  heaters will need either 12 volt or 110 volt power to operate.

The exception is a propane or natural gas fired burner.  These burners are naturally aspirated,  meaning there is no blower required, due to pressure forcing the fuel into the burner, much like a backyard BBQ.  Ignition is created from a standing pilot, much like the way the furnace operates in your home.  

For portability propane modules are ideal.  Natural gas modules are best suited for fixed in-plant locations.