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Current on-demand heaters on the market can deliver over 6GPM (http://www.cpotanklesswaterheaters.com/gas_products/whole_house_tankless/natural_gas/2400e-ng.html) of hot water. 3/4" copper pipe can only deliver 7GPM of water anyway, so i doubt there would be a difference. If you could run those applications at once with a tank heater you could certainly do it with a tankless.

To me, GPM or heater cost is not the issue, its putting one in an older house where the electrical or gas needs to be be upgraded or overhauled. These upgrades could easily double or triple the cost of using one of these heaters.

These heaters should be all that are allowed in new construction, they are the future. Sad that European countries have been using them for so long and they are just getting recognized here.

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Oh and i forgot to mention; there is a $300 tax credit for installing these heaters.

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These have limitations because they can only raise the temp of the water so much. The colder the input temp, the faster the flow, the cooler the output. They are making these better at heating the water though. Like the other commenter said...they should mandate that all new residential construction use these. The only problem is that the government will mandate low flow toilets to conserve water but they have no interest in slowing us down in energy consumption. Why the difference? They make their money on energy not water.

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There are several other considerations when contemplating an upgrade, as I am:

1) The trickle problem. These devices sense demand for water by measuring flow, and they don't turn on until the flow is 0.5gpm typical, which means you can't "crack" the hot water on just a little. Some people think this is annoying.

2) Distance to the demand point. If one of these is far away from your bathroom, you have the same problem of wasting all the water in the pipes to get hot water running as with a tank heater. So place it centrally and insulate the hot pipes if you can.

3) Installation costs can be quite high for replacement in existing homes.

4) The rated GPMs (like Doug's 6gpm figure) are *optimal* and depend greatly on the temperature difference between incoming water and set point. So in the winter, you get less hot water because the water coming in from the street is being heated from maybe 50F instead of 70 or 80. Manuals for these heaters show curved graphs of how the flow rate drops, and the rated flow is of course always near the maximum possible under ideal circumstances.

I'm not saying they're bad, I'm just saying, they're not a panacea either.

BTW Doug -- I believe a 3/4" pipe's maximum flow rate would depend greatly on the water pressure which is influence by the source, length of pipe, and other factors such as any height differences (1st floor, second floor, etc).

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