-=> Quoting Rich Willbanks to Elaine Roberts <=-
ER> Looking for info on a system called "Hot Water on Demand".
  > renovating in November. We are interested in how it works.
RW> As for how they work. You have a coil or series of pipes that the
  > water flows through and a burner under it. The burner is
  > activated when the water in the coil starts flowing. That way it
  > is only heating water when needed or "on demand".
So far, so good...
RW> They only heat water so many degrees above ambient.
Ambient what? Air, water input? You neglected to say. They do heat
water with a fixed BTU input, ie. there will be a temperature rise
according to three factors: Flow rate, input temp, and BTU input.
However, if the flow is low enough the outflow will achieve a given
temp and a gas will be moderated to limit the maximum. However, if the
flow is high (like fillig a tub AND running a dishwasher) the outflow
temp will drop quite a bit, even to the point where the 'hot' water is
merrely warm, or even tepid. I think you pretty much know the basics,
and these heaters are rather basic.
RW> They will save money and fuel but they are expensive. The last
  > time I checked one large enough to handle my needs was over $800.
  > IMO, they only are a good buy if:
Wow! When I bought my Myson it was about $350. I was considering
replacing it with a larger unit. But after seeing the price, I decided
to build a solar batch water heater. It raises the input temp of the
water such that I can get much more and hotter water.
It turned out to be a far better investment. In the winter I get more
hot water for the same amount of gas, and for nine-months out of the
year I have the Myson water heater shut off entirely!
ER> Can it be used with a Propane or Electric Hot Water tank?
RW> I'm assuming that you want to know if you can use propane or
  > electricity to heat the water.
It sounds like she wants to prefeed another water heater. Of course
that would defeat the investment.
I had considered using an electric water heater as a backup should I
run out of gas during the winter. That would be the only reason why I
would do this. After all, I have 140 gals of hot water each day the
sun shines. 
Solar is cheaper, you just have to live where the sun shines. 
 
--- Maximus/2 3.01
---------------