Is it possible to create circulation of water in bypass pipe without water pump? There is a pipe of 32mm in diameter. Water flows from left to right.
There are two branches in that pipe. To one of them connected input of water heat exchanger, to another - output of water heat exchanger.   Pipe inside of heat exchanger has size of 3/4 inch. That means there is reduction of diameter. Is it possible to create circulation without water pump in that heat exchanger by playing with pipe size, angles of branches, etc.?

Update:

I need water to get into heat exchanger, so that it could boil water inside.
 A: reduction of diameter will probably not make a difference. what will is if the boiler outlet pipe is significantly hotter than the inlet pipe, and if the pipe lengths are vertically disposed. In this case, the hotter outlet water will be less dense than the colder inlet water and it will try to rise up in the pipe. as it does, it will draw cold water into the boiler and establish circulation in the loop.
this is called a thermosiphon and is used here and there in the heat transfer world to avoid the expense of a pump to force circulation or where the amount of heat being handled by the system is small, but it is not nearly as effective as pumped circulation.
Early Subaru cars used this method so as to eliminate the need of a water pump to move coolant through the engine, but they abandoned it when they re-engineered their engines in 1973.
A: This is not an advice, but just a consideration.
For a desired flow rate through a heat exchanger, a certain pressure drop between its input and output is needed. 
The section of the $32$mm pipe running in parallel with the heat exchanger will limit this pressure drop. The smaller the diameter of this section, the greater the flow rate through the heat exchanger will be.
