I'm here to help
How long of a run does the 3/4 pipe make from tank to heater?
I think that's illegal here in Santa Fe--all interior gas piping has to be low pressure, as I understand it. Going back to your resizing idea, if your tables show that 12-13 feet of 3/4 pipe won't handle the gas flow, can you let me know what size would be adequate? I'll try that first. Thanks for your help so far.
Just found out from my buddy on phone, who had his books in his truck. I gave him your situation and he says that in order for the heater to operate at the max BTU load, you will need a 1" run, if it has three 90 degree elbows. I think that may be a bit of overkill, but the pipes should not cost that much more than 3/4". I think he is figuring in some really cold water in the deepest part of winter. It gets pretty cold in Santa Fe when the wind blows down off the mountains, YES?
One last piece of advice
Have the unit checked by using one of these. http://us.ebid.net/for-sale/gas-lpg-line-pressure-test-kit-manometer-gauge-15-wc-ac-23922150.htm
It may POSSIBLY be the unit and not the gas volume. A manometer is used to measure the pressure at the unit (most tankless have a built in port for this testing). This way cost you a few bucks for the tech to come out, but may be worth it if you don't have to re-pipe the run... Dave
I'm wishing you good luck and a mild winter. Thanks for using Just Answer! Dave