The reason we have 2 tankless heaters is so both of them are close to the point of use, which equals less wait time. I also drew our house plans to make sure my hot water use areas were "clustered" because of this. Due to the piping in our double wide, we have to wait up to 5 minutes for hot water from a tank heater. Ridiculous. I have played around with my parents' stuff and have never waited more than 10 or 12 seconds for hot water because their heaters are also near the point of use. Their and our tankless heaters are Rinnai, btw. The only instant hot water I have ever experienced was in a really nice hotel... ;-) I don't know how big our current tank is, but we often run out of hot water for our family of 4. I'm looking forward to never dealing with that again!! Vonda PS Hello to a fellow Arkansan. Might not be the greatest state in the Union, but it's home. :-)

Gary: I'm building a new house next year where this 6-head shower will reside. It's the one big indulgence I'm allowing myself. The house itself will be super-insulated and airtight to the point that it'll require no more than 12,500 BTUs to keep the house at 70F when it's -20F outside. I'll basically be able to heat the house with a Zippo lighter, although I'll be heating the place with wood. The shower that I'm allowing myself will be using well water, coming from and returning to my land, which will be preheated both by a loop from the woodburning boiler stove that heats the cottage as well as by a substantial GFX drain heat recovery system to get the Delta T somewhere between 20 and 30 degree (depending on the time of year.) I'm sorry you're skeptical about the performance specs of the GFX, but that's your issue, not mine. I'll also be using a pair of incredibly energy efficient condensing tankless water heaters to heat the pre-heated shower water to the final desired temperature. I've done the math now and have calculated that to take two 30-minute showers per day, utilizing all six shower heads going full bore (450 gallons per shower!), each will require an average of 1 gallon of propane costing approximately $2.40 per shower. This is less than the smallest Starbuck's Latte you probably throw back on your way to work. I'm hyper-aware of my carbon footprint. And I'm willing to bet that the footprint caused by my home will be less than the one caused by yours. I just happen to place a high priority on having a really, really great shower. Bottomline: The calculations you provide are erroneous and based both on a lack of information and a number of false assumptions. More importantly though, your perception of what I'm doing is really myopic. You should be concerned about ones NET carbon footprint, not about any one individual indulgence. Don't be so quick to jump to conclusions. You just end up looking foolish. John

The disadvantage of tankless is that you may not have enough hot water to supply multiple simultaneous uses (running 3 showers at a time, for example). So what I'm thinking is a tankless and a tank heater feeding into a mixing valve such as a Watts LFMMV. Set the tankless to deliver 110F and the tank to deliver 120F and set the mixing valve to supply 110F. Then under normal conditions the the mixing valve calls for "cold water" of 110F from the tankless but if the tankless can't keep up and falls below 110F then the mixing valve mixes in hot water from the tank to maintain the 110F. Would this work?

There isn't an exact answer to your question, it will vary by how hard the water is, and how much you use. What they are doing (or should be doing) is removing the lime buildup in the heater with vinegar. The acid will dissolve the hard water deposits that will eventually clog a tankless. It's about a 10-15 minute flush with vinegar. You can google it and see many videos on how to do it. Three years is about right in your situation. I wouldn't just let it go. Tankless heaters use small heating coils to expose the water to heat quickly. You don't want those passages to clog. You can call around for a good price over the phone. They don't need to come out to give you a quote, this is a basic job.