The pipe should be sized based on the maximum potential demand. Doesn't matter if multi-stage or single stage burner.
1/2" copper is at the limit of typical system pressure drop (0.3" pressure drop per 100') for low pressure gas service.
I would recommend 3/4" if starting from scratch, or look for a smaller heater.
As always, offered only as opinion
Edit:
If you have a 2 psi gas service, you will have more than enough supply. If you are dealing with a typical residential gas system of 5-7" pressure, the 1/2" pipe is too small.
Rant=ON
As someone who works for a gas company, let me just tell you how much I "Love" customers who are so cheap or shortsighted that they didn't run big enough pipe and want me to come in and redo their meterset for a 2PSI system. It may not seem like a big deal, but just the correcting meter index alone costs about $50 and I have to replace the regulator too, with something like a sensus HP 143...which likely costs another $50. Meanwhile...the customer is suddenly taking eight times as much gauge pressure into their home where they are going to need regulators to handle that pressure, because the appliances need to see 7 inches w/c or so. That's more expense on their side that likely eats any savings they achieved by staying with the smaller pipe, and if they get a LEAK on their piping it will leak more than it would have at a lower pressure.
Meanwhile, our meter set is no longer standard, meaning if there is a problem someday, there is almost a 100% probablility that the serviceperson responding will NOT have the correct parts on his truck to fix the problem.
If someone can save 50-cents they want to save it, even if it costs us $100 to save them that 50-cents.
For God's sake people. Just run adequate sized piping.
Rant = off.
Phil
