Tankless water heaters for domestic space heating

Tim R

I am trying to gather enough information about tankless water heaters to determine if they have an application for heating demands of 30,000- 5o,ooo btu loads. So far most info comes from sales people who will promote their unit as being better and more efficient than a conventional boiler. I can accept some of what they say but can't determine if these units will provde economical heat. Combustion efficiencies are high but does this mean that they will provide heat efficiently? I have been looking at a unit called a Luna. This unit has piping outlets for potable hot water as well as a closed system space heating. Unit cost is comprable to a boiler. Fuel is NAT. gas or LP. They are built with 1/2" copper thru the heat exchanger. My understanding of flow rates is that a 1/2" copper pipe will allow 1-1/2gpm. If 1gpm of flow will yield 10,000 btuh at a 20 degree temp drop can more water be pumped thru the 1/2" copper to increase flow and heat output? Is it possible to get 30,000-40,000 btuh out of this unit? The answer seems obvious to me until I listen to the sales pitch and then I am scratching my head and asking "what am I missing here". This unit is being sold as being capable of providing hi temp zones as well as low temp.
My interest in these units is because they can fit into small spaces and combustion efficiencies. I know that there is an application suited for these units I just don't know where to draw the line. All advice or technical info would be greatly appreciated. These units are moving into my market area in a big way. Thanks for the help.