I am installing a rinnai tankless hw heater, nat gas, 199k btus in my home.

In the installation manual it states that it is imperative that the unit has adequate gas supply. They suggest that it has its own 3/4 line to it.

My gas supply line consists of what looks like a 2" pipe coming in through the foundation which reduces down to maybe 3/4 or 1" line to enter the meter, then comes out of the meter 3/4 (or 1" perhaps) and immediately goes to 2".

The 2" pipe goes for 45 ft to where it ends up at the hot water boiler and hot water heater.

There is a 1/2 tap coming right off the 2" for the hw heater (sort of like a 2x2x1/2 T).

About 2 feet after that the 2" reduces to 3/4, which then reduces to 1/2 to feed the boiler.

Where the 2" reduces to 3/4 there is a short 3/4 nipple that was used for the reduction(maybe 4" long). Would it make sense to repalce that 3/4 nipple with a 3/4x3/4x3/4 T to feed the rinnai?

So it would be 2" reduced to 3/4 which would feed the rinnai, then reduce to 1/2 to feed the boiler.

So I guess the question is, would that be adequate supply for the rinnai and boiler if they both were to run simultaneously, or does the rinnai need a 3/4 port right off the 2"?

NOTE: There are two ratings on the boiler, one is 130k btu the other 110. Also, right off the meter where ther pipe is 2" there is a 3/4 tap that goes completely in the other direction that feeds a gas range and dryer.

3/4 off a 2 inch black iron pipe should do it. It really makes no difference what the BTU imput is since the unit you have needs the bigger line for pressure drop purposes. Most of the tankless units need a good flow along with pressure to operate correctly. I would do it this way. 2 inch to a 2X 3/4X 1/2 tee. Now you might not be able to find that so a 2 inch tee with nipples and bell reducer couplings will work just as well. NOTE: Do not use bushing reducers just bell reducers if you cannot locate that bastard tee. Bastard is a term old pipe fitters like me used at the supply house for that type of tee.

Tees are read on the run first then the center outlet. Like 2 x 3/4 x3/4 or if the same size for the last outlets then it is just a 2x 3/4 tee like below.

Black Fitting - Reducing Tee - 2" x 3/4" $13.15

Black fittings - malleable iron - from FAMOUS PLUMBING SUPPLY (http://www.plumbingsupply.com/blackfittings.html)

BTW droping down to a 3/4 and then also taking a 1/2 inch off that is not a good idea since during the winter there might not be enough flow in just a 3/4 inch pipe to start with. You need to come off the 2 inch.