A year ago, we had a plumber install a Nevian instant water heater at our home. Everything worked fine for about four months.
Now, sometimes when we turn on the bathtub water, it will be cold, and it doesn’t get hot unless we turn the faucet off and then back on.
We had the plumber out who installed the heater, and he said we must be getting a back feed from the washer or dishwasher. He said to check that and left.
We did watch to see, when it did this, whether anything else was running. There is no consistency as to when it happens. Nothing can be running at the same time, or everything can be. The bathtub is the farthest point the water runs to.
Because our plumber pretty much washed his hands of the issue, we are at a loss as to where to turn next.
I think when the plumber left it was for the best. The last thing a homeowner needs is someone who can’t be bothered to solve a big problem with something he installed.
The heater’s name is actually Navien. The problem would best be solved by dealing with the company directly. Make sure you have all the pertinent information about the product you purchased (serial and model numbers) and give detailed information on your problem.
From what you have described, my guess — emphasis on guess — is that it might be the heater itself. Perhaps it’s a relay that worked fine at the start but had an inherent fault that now forces you to try twice for hot water, because the message doesn’t get from the faucet to the component providing hot water on the first try.
Many manufacturers, Navien included, have installers who are not their employees but have been trained to troubleshoot such problems. That may be what it suggests to you.
The heater has a warranty; download the information for your model at the website if you don’t have a copy.