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

100K should be big enough, but there's no way to know for sure without the calculations. Maybe they offer various gallon per minute coils. 5 gpm should be more than enough.

Several people have had questions about the venting of these tankless units -- usually along the lines of "I know the instructions say not to do ___________. But it's Okay if I do what they tell me not to do, right?" Do not share your tankless heater vent with any other appliance. The instructions are very clear about this. Don't get creative with venting on these units. You run the very real risk of CO poisoning. There are several reasons, the most important one is that the exhaust is under pressure (from a fan) you don't want the vented gases flowing back into the room through the other appliance. The vent materials for these vents are stainless steel and are gasketed with silicone gaskets to prevent fumes leaking at the connection points. Likewise, you don't want the flue gases from the other appliance backing up through the tankless heater. Also, these units have vent airflow detection that detect an obstructed vent. Having another appliance could cause problems with that. Pay close attention to the placement of the vent termination in relation to other appliance vents, windows, or air intakes. Follow the instructions on these vents very carefully and don't get creative. Use the proper materials, don't substitute and don't share the vent with other appliances.

Funny to be accused of evangelism by poster that incited an "Amen", a few posts up :) Always appreciate feedback and constantly working on preachiness of writing style. I think including links or reading opines without sourcing helps others decide on misleading comments. It looks like Stiebel updated their Accelera HPWH to include an electronic control screen since we bought one a couple years ago for parents house. After an updated check on reviews I still feel safer with Accelera but it's hard without knowing volume of units sold. Surely Rheem has sold more HPWH than Stiebel at this point, despite Accelera being around longer. Reliability is a compelling reason to go with Rheem Marathon an electric resistance tank. Best warranty and most well-reviewed water heater available. Tank size plays a role here too. Bigger tank means better hot water performance, with minor hit in efficiency. The bigger the tank, the better for grid storage and demand control. Demand control has been around a long time with the most basic strategy using a timer and time-of-use billing options through utility. Studies will continue forever on more advanced grid-smart water heating technology but at this point, it's a time-tested technique with this smart water heater article quoting up to one million in use already with potential savings for homeowners and grid. Another article from same site points to irony of battle between grid storage potential of simple electric resistance tanks and the efficiency of heat pump water heaters. In that first article the Hidden Battery link seemed misdirected but it was put out by the NRDC pointing to potential of electric tanks to reduce the need of new power plants and grid storage for renewables. Large, reliable electric resistance tanks are becoming more of an asset while gas is becoming more of a liability. I think most of this forum's posts could be described as meaningless or feel-good-only but definitely not water heating. It has large environmental and social consequences. It's one of the only home purchases capable of beating the stock market for returns on investment. I think the ROI currently lies with HPWH but the second best choice could easily be a good ole electric tank. If an electric utility offers good incentives, electric tanks like the Marathon, could beat tankless gas or HPWH in total lifetime costs including repairs and replacement. A home equipped with PV panels and Marathon tank could also be in the running as best long-term water heating investment.