I just recently relocated just outside of Ottawa, and the "open loop" twin well type of geothermal system is very common here.
Take a look at
this map. The shaded blue areas indicate aquifers that yield a minimum of 0.4 litres per second. (About 6 gallons per minute)
Anyway, back to tankless water heaters. At work, we exclusively sell the Rinnai, as they tend to be one of the most trouble free brands. Having a water softener is a must, as well as doing a "flush" of the unit yearly. Past that, I don't find the other issues a problem. I currently have an R75 that I'm thrilled with.
Wait time for hot water:
In our last house, the conventional tank was on the opposite side of the home, so we were used to waiting the 30 to 40 seconds anyhow. This has been an absolute non-issue for us.
Dishwasher / Laundry:
Again, I was used to letting the water run at the old house, so I do the same here. Turn on the kitchen tap until the water runs hot, then start the dishwasher. Once I am positive it is filling, I turn the tap off. Same for the washing machine.
In my opinion, not running out of hot water is worth the potential drawbacks. And whether it is more efficient or not, I still really like the idea of not keeping a tank of water hot while I am sleeping, or at work.
