Step 9: Pressure test
Connect a water hose to one of the 1 1/4 pipe and a small piece of hose on the other side.
You can mount the 1 1/4 adapter to bring it down to the 3/4.
Turn on the water to flush out the air.
When the air is out, bend the exit hose to stop water from coming out.
Check every soldered joints for leeks. If there are any, completely empty the heater, and retouch.
For those of you unfamiliar with soldering pipes, If there is water inside the pipes, you will not be able to solder them properly.
Redo the test until it's perfect.