My plumber installed a gas Navien NR-240A in my crawlspace (November 2010). I ditched my 21 year old electric 65 gallon tank located in a closet in my kitchen.
I read mixed reviews on the Navien product, but my plumber insisted they work well if installed correctly. He changed out my gas line (1”) and I found out through reading reviews I needed to update my gas meter since I also have a gas furnace.
You mention you have “no hot water low volume”. I discussed this issue with my plumber before he installed the NR-240A, since I was aware through reading you need a certain flow rate to get the Navien working (forget flow rate #). One of the criticisms I read about the Navien was it needed a greater flow rate than some its competition. That may be true although maybe the experts can comment on that.
The biggest challenge I have is shaving in the morning. With my old electric tank heater I could get the hot water going upstairs (after about 30 seconds) with the water stream barely moving. I cannot do that with the Navien. I have to run the hot water in the sink (again for about 30 seconds) at a very moderate water flow to get the Navien started. After I get the hot water running in the sink I can slow the rate down somewhat, but if I get it water stream too slow the Navien stops making hot water and the water turns cold again. That’s been my experience with the Navien. The same operation holds true for the kitchen sink. I’m also not sure how our front loader clothes washer and dishwasher work with the Navien, since they call for hot water at various times during their wash cycles. I only hope the cycles stay on long enough so making hot water is not an issue. I haven’t yet seen a problem with the clothes or dishes though.
My overall user experience with our Navien NR-240A has been positive with some of the flow issues noted above.