The Rinnai has a continuously modulating gas valve, goes from low to
high flow with outlet temperature + or- two degrees. I don't know what
the minimum flow is. They do have a very large size burner, 199,000
BTU/Hr max. You will need a large gas pipe for that. Not simple to
work on either. There are screens that can clog up from crud in the
water. Very efficient however, no standby losses either. Some good &
some bad to them.

Stretch