We run into problems with tankless heaters not having the rite size gas piping or not the correct pressure. We have found that a 3/4" counter strike track pipe run from the gas meter a a dedicated line with 2 lbs. of pressure is more then enough to operate the Rinnai tankless.
You need to install the regulator and this is very inportant at least 4' away from the unit. this will prevent the diaphran of the regulator from opening up full to the max setting of the regulator.
Always check with you local gas supplier to see what pressure they are suppling to you.
If you have any questions please add them to the comments below.
I hope you like the video. Mike