I have a tank less in my house, it was there when I bought the place 13 years ago so Ill estimate the age of the unit now 18-20 years old. At the time it was and is probably one of the cheapest brands on the market. It has a standing pilot light (gas) as most now have a electronic igniter. My showers are at the other end of my house , maybe 60 + feet away from tank. I do get the cold water sandwich effect but only in this situation. Say someone gets up turns on shower ,runs the water for a lil bit to run all the cold water in 60 ft of pipe out then they get hotwater take a shower. A bit later next shower, hot water fast as pipes are full already then a couple minutes in u get maybe a 15 second cool down then hot again. My tank fires up the burner on flow so that few second cool down is the first burst of cold water when the tank fires back up to heat again. I would never go back as they are endless hot water when u want it. They one thing you touched on that I cant give advice on as Im no plumber was the statement "Plumber said i would have more success eliminating my hot and cold knobs and joining them" My showers have a single knob mixing valve all though all my sinks are two handles so it may or may not make a difference but it dosnt in my house.
I always replace the rubber ones with the stainless braided type . Costs just a few bucks more. Amen bro. Some things just go without saying; mea culpa. Yes, I always try to upgrade whenever feasible. ws
If your water temp is adjustable lower the temp to around 120. You'll be moving more water through the heater to maintain your desired temp and then the heater will not shut off during the shower. Mine has to see a min of a 1/2 gal a min to operate the burner. Had it for 15 years so far. only problem was a bad board couple of years ago. Fixed it myself. When installed I used hard copper on all the connections and then into the PVC house system.
I like the option to adj the temp instantly. When I want to fill a mop bucket or get water for the upholstery/carpet cleaner I'll crank that joker up to 180. Just dont forget to turn it back down before you jump into the shower that night. Done that!
I had the same problem with mine on natural gas . I am on well water that is very hard . I even have a filter on my incoming cold doesn’t matter . The super high heat on the coil just turns the hardness into little sand / salt like calcium particles ... so Scaling was a problem with mine as the water pressure would get low and causing the water sensor making the fire go on and off. I since flushed the system with a 1/2 horse pump and 5 gallons of cleaning vinegar recycling it into the same bucket for about an hour. At the end I just divert the vinegar back into the piping leaving the showers and hot faucets open ....helps clean the shower heads and diverters. I now do this procures every six months works fine . These things really shouldn’t be used on hard well water if you can help it. I put it in to save space in my utility room getting rid of the 50 gallon water heater.
Here’s my set up for flushing . The short green garden hoses are used to do the flushing . It’s very simple as I just by pass the main supply and also divert away from the hot water exit pipe with the installed bypass valves that I installed ( they are not cheap ) Attach one garden hose coming from the heater to the outlet of the submergible pump and place the pump into a five gallon clean spackle bucket filled with about 4.5 gallons of cleaning vinegar . Take the other Green garden hose coming from the heater and run back into the bucket. Turn the pump on and let it do it’s thing for 45 minutes to an hour. If you choose you can open your faucets and run it through your piping into the sinks or tubs . I notice you don’t have the bypass valves . You can still do it, just shut your incoming water , disconnect the braided lines from the heater and somehow adapt your open braided line end into the 3/4 hose fitting . Just use the braided lines as I use the garden hose
One more thing as I don’t have a water softener nor want one . I installed this filter specifically made for tankless heater inlets . It has a cartridge life of two years , it seems to help as it has a salt like cartridge . It’s made by aqua sauna I ordered it at Home Depot about 60 bucks and easy install no wiring or anything . It’s about 2ft. Long and maybe 2-3 inches in diameter. The submegable pumps will set you back about a hundo. .....and it’s 1/3 hp. Not 1/2 ....I guess I we always lie about h.p.
