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Originally Posted by Mee-n-Mac
Huh ? I think it costs less to make the hot water than it does to make and maintain it, which is what your typical system does. Am I missing something here ? If you're constantly using the hot water then the difference may be small and not enough to make the extra cost of a tankless system pay for itself. We'd have gone tankless except that the plumber guy said the gas line (from the LP tank) would have to be redone as the on demand systems required more flow to make the BTUs they do. This made some sense and then again .... How many more BTUs, say over a gas range, does a typical 3 GPM tankless system produce (note: not used in winter) ? What kind of "special" pressure regulator and gas line diameter (if any) does an on demand system need vs a typical tank system ?
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You have to remember you are making the hot water in both cases to begin with, its the different of maintaining or constantly making. A modern day stone lined water heater will hold the heat in the water for an incredible amount of time, the $200.00 units at big box stores, not so much. Just about every manufacturer of Tankless water heaters states in there product brochures about the need to pay attention to usage (almost all recommend a dedicated system for dishwashers or large soaking tubs), Rinnai has a section in there site that will actually calculate the size or number of units needed based on zip code. In our area the ambient temp of the incoming water has to be heated to a certain degree for comfort, tankless units (within the spec sections) will give you the output temp at certain GPM's (what your faucet is rated for at max output), add this to the incoming water temp and that is what you can expect at the tap. You then need to size the unit appropriately based on need.
On Rinnai's site when you plug in your information, as an example I plugged in a local zip, 3 baths, 5 shower heads (hand helds with fixed in two units) and two of the units being tub/shower combos. The result was to contact a local dealer as more than one tankless unit would be required. That gets expesive quick and there is nothing stating how many faucets being used at once, but your usage needs to be based on total possible output.
The total cost of energy used goes beyond the gas being burned to heat the water and store, but electricity used to run the well pump, circulators, etc. This is exactly what the LEED construction points are based on, not how "green" one item is but how everything works as a total package and most the "green" products that are marketed as such are not so when added into the big picture.
As for regulator and line sizing that is a question for a qualified installer or manufacturer, its a basic physic calculation but they use software to prevent the headaches. I am just a Project Manager that has a particular interest in researching and educating myself on new and old construction practices. Contact The Granite Group and talk to someone in retail sales (not the counter) they would be able to tell you the line sizing and regulator needed for a particular system.
The btu question is a little easier and a little complicated, all manufacturers post btu ratings for gas appliances, the 3GPM btu rating for a tankless will vary depending on model. 3GPM on one model will be different on another, because a larger unit can heat water hotter at 3GPM than another. You have to remember that the water at no point in time is stopping to be heated, so it has to absorb as much heat as it can as it passes through the heat chamber, the faster it moves the more energy is required to get heat into the water.
These long posts are killing me, sorry for the technical long winded explainations, hopefully I am actually answering your questions.