Our heater is a Rinnai that has a coaxial vent duct and forced fan cooling. It makes a little bit of noise, but nothing you would hear unless you're right next to it.

It's way quieter than the old tank heater, which sounded like a large popcorn maker.

When I started working from home, I found the tank heater was firing up about three times in two hours, heating water that no one was using. It was also down to about 10 minutes of hot water, so it was really just heating calcium deposits.

Our first instant heater was one I smuggled in from Taiwan. It's a really simple thing, with one big knob to adjust the fire. I could easily boil water in the pipes. Initally I thought that if I was mixing in cold water at the tap, that I was wasting heat, but I found that the heater gets way more efficient the slower the flow, so we ended up setting the heat to near boiling and mixing in cold to get the desired temp. The flame setting was quite low doing this.

That heater lasted about five years (it was only $200) and I had to replace it with a locally available unit, so I got the Rinnai. This being the USA, with litigation always on the minds of manufacturers, so the Rinnai worked differently. It's got an electronically regulated temperature output capable of only 140f max, so we set it to 130f and leave it there. It uses a much larger flame than did the Taiwan one (Sakura), so it must be far less efficient. It's also way more complicated, but it's also a "set and forget" device. With the Sakura, we had to remember to adjust it for shower, dishes, laundry, etc... I think the Taiwan water devices must all have about the same flow, but that's not the case here.

If I was running off a propane tank, I would definitely want the Sakura type.

I also found that hard water was a big problem for this kind of heater. I ended up having to put a water softener in to keep the heater clear. What I did is install it upstream of the heater only, so my cold water is not softened. I find this is a great arrangement.