Actually never thought of recirculating the water in the gravity holding tank to the demand heater and keeping the float valve with city water feeding it. Excellant idea! I guess the old Mark VII systems was set up this way with ginormous Jarco Webn boilers attached to them. I should have remembered that! You would want to have an aquastat with the ability to set differential for flexibility to keep the heater from short cycling. I get 6.6 gpm flow with (2) Taco 013's in series to get 40 psi to the heater inlet. You might get another type pump to get you 40+ psi and the rated flow, but this works for me...