thanks for reply
After much research yes your are right using any recirculated water with this tankless will cut warranty 7 years. However, the warranty does state if only potable, non recirculated water is used - warranty ok.
I believe I will put a check valve on hot side of unit then add 2.5 gallon hot water heater to recirculated loop.
Hot water 24hrs a day at every faucet, no extra wear and tear on tankless.