Over the years I've had 3 tankless water heaters in my sugarhouse. The first one only heated 1 gpm up to 120F, from 35F incoming temp. It was good but my problem was that 1 gpm was not fast enough. I then took that one out and bought one rated at 3.3 gpm for 110F. That worked very well until one time I forgot to drain it. It froze and burst one tube (my sugarhouse is not heated unless the evaporator is boiling). Draining them is very easy and only takes about 30 seconds, I just forgot, at any rate, I tried to fix it using plumbers epoxy, it held for that season, but failed soon after. That was $220 in the trash. Soldering was considered, but I couldn't get the corner that had burst back close enough to even try soldering a patch on. vI then Googled and found several that were even better and at about 1/2 the cost. Made in China, but the others had all been made in China too. The first two were bought from Sportsman's Guide, the 3rd was not. My current one even has a built in digital thermometer (celsius only) and it works perfectly. I plumbed mine to use permeate and I pump it with a 115V diaphragm 3.3 gpm rated pump. When I don't have permeate I haul municipal water and use that. I have mine hooked up so it pushes the water into a cold line, then a tee with a ball valve to send water up to the heater, and another ball valve to get cold water at the sink. As hot water comes out of the heater I have a potable water hose connected, and a hot water valve to the sink. There is a drain plug on the heater but I learned it was not necessary when I tried to get a replacement O ring because the original started to leak. At the full line plumbing outlet I was at, the clerk said he knew exactly what that plug and damaged O ring was from, he had the exact same thing at his camp. He told me to just drain both the cold in and the hot out and the heater would be fully drained. That's how I've done it for about 5 years now, without issue. I don't seem to find a link right now, try a search on this site about tankless water heaters, I've linked to them a few times in the past. Mine cost about $105-110. What I find now are more than 2x that and up to well over $1000. Good luck.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.