I did a search, looks like a fun and simple project I will make the manometer this week and check the pressure, I don't see an obvious way to connect it inside the heater so will need to tap into the supply line right before the heater I guess... There is a union there won't be too hard to install a T.Google how to make a manometer. Simple tubing on a board. Water height differential in inches is the pressure. Plus you'll need a 1/8" threaded to barbed fitting to go into the test hole.
You can buy a manometer if you want.
Natural gas can be run two stage with a 2 psi first stage from the meter to a regulator near the heater that steps down to the inlet pressure of the heater. This allows for smaller line for most of the run.
Note: Consult with a qualified gas contractor for anything you're not sure about. Don't do anything that you're not sure that you can do safely.
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