Here's a question for the Radiant Gods:

I was wondering if it is possible to integrate a gravity fed domestic hot water recirculating loop with a tankless combination mod/con unit - without using a storage tank?  I am planning a new house here so the slate is clean.  I can see how using something like a Polaris for DHW (with a gravity return to the drain) might work when tied to a closed heat exchanger for the radiant loops.  I think a mod/con with a seperate indirect heater tank for DHW would also work.   I'm having trouble determining if the same can be done with a single tankless mod/con unit equipped with seperate DHW and heating coils but no tank.  The minimum flow rates of cobo mod/cons don't seem like they would work for gravity return configurations and the lack of convection within a tankless water tank might stall the gravity loop anyway. 

This all assumes a basement location for the heat source, well insulated hot water supply pipes, less insulated returns and the last 10-20' uninsulated to boost unpowered recirculation.

Any there alternative methods other than using a pump-powered recirculating system to get near-instant hot water.  Any thoughts??

T