I think what happens is that even though the minimum flow rate is met (0.75 gal/min), our heater seems more “comfortable” when working hard (raising the temperature of the water a lot.) Its performance is much more sporadic at lower flows or temp increases. Our issue seems to happen at the peak of summer, when the incoming water will be at its warmest, meaning the delta between in/out temps is at its lowest. At all other times of year, it seems to do OK. As Stephanie mentioned, our first unit would cycle constantly during showers.

The hot/cold/hot/cold temps are really just a symptom that your heater is turning off during the shower. The cold slug is a result of the time it takes the heater to kick back on. Even if it only takes a few seconds, that’s a lot of cold water flowing through. Not very fun when you’re in the middle of a shower!

I’ve put up a web page to discuss what we learned, as well as some additional tankless installation tips I’ve picked up at my Home Inspection business here in Chicago at:

http://paragoninspects.com/tankless-heater-home-inspection-evanston-chicago-il-faq.html

The Hot Water Lobster uses no electricity and is pump free, so it creates no noise. It is made in the U.S.A., has a 10-year warranty, and can be easily installed in under 10 minutes.

Check it out at:
http://www.hotwaterlobster.com

I can’t really answer your question, because I don’t know much about geothermally-heated water heaters. If the geothermal heat was enough to meet your needs, then I’d connect the input to the cold line. Assuming that it’s not able to keep up with high-flow uses such as showers, I’d connect it to the hot line after the tankless. There is no reason you can’t use a 50 gallon tank in this way, but it’s a different approach with a different result. Ours are set up to both buffer the “cold slug” and to give instant hot water. I wouldn’t classify a 50 gallon tank as a buffer. The big tank would be the primary heater, but you would get the advantage of having pre-heated water flowing into it from the tankless. It will ultimately be less efficient, because you’re keeping a much higher volume on constant standby. But again, I know nothing about geothermal heaters, so that may not be an accurate statement. One things for sure…you will never run out of hot water or be surprised by cold slug!