Want to change out a standard tank water heater for a Noritz tankless NR83 with minimum 18,000 BTU and max 180,000 BTU rating. I know that the gas line will have to be 3/4 instead of 1/2. The Noritz unit will be located just on the other side of the concrete foundation wall from the gas meter... about a 4 foot run max. Can a "T" just be added after the meter and before it goes to the main line into the house? Should I assume that the natural gas supply coming straight from the meter will be a pressure suitable for the Noritz? Why does the pipe coming out of the meter start with 1 inch, then reduce down to 3/4 inch, THEN go UP to a 1 1/2 inch main supply pipe for the house? The plan is to add the T at the 1 inch pipe before it reduces to the 3/4. I keep hearing that the MAIN problem everyone has with tankless is incorrect gas pipe/pressure issues. THANKS!

No offense to your skillset, But I highly recommend that you have a licensed plumber make any modifications to your natural gas lines. In most areas it is mandated that work be performed by a licensed plumber, either by local ordinance or the gas utility itself. A T immeadiatly after the meter will work just fine, the possibility of inadaquate pressure is unfotunatly one you cannot control. Pressure is set by the gas company and if you make any change to it, it can be classified as tampering with a utilities equipment and can be punishible by fines or possible termination of service form the gas company(usually reserved for repeat offenders). If you are concerned about pressure requirements being met, you should contact your gas utility and inquire what their regulated pressure is, If it isn't enough I would recommend you search for a different model of tankless waterheater that is supported by avalible pressure in your area.

Hope this helps.