I have never uses a tankless water heater but as I understand how they work, they only turn on when there is enough water flow. The water flows because of pressure, about 60-90 psi at our home. By simply drawing water from the secondary reservoir I don't see how that will create the water presser. I see the need for a primary reservoir, but even if the primary reservoir is significantly higher will that be enough pressure and or water flow to turn the unit on?
I can see having one reservoir higher than the heater with an overflow that runs through the heater and then into the secondary heater might work, if the heater turns on with water flow and presser is not required (I have no experience with tankless water heaters).
If so, I would imagine that when the
pump draws water from the secondary reservoir and fills the system it would run back into the primary reservoir. Then when the primary reservoir reaches the right height it will flow back through the heater from an overflow in that reservoir, then into the secondary reservoir again where it is pumped through the system again.
It seems that there would be a need for 2 pumps to me. The primary tank that the
hydroponic system overflows into needs to be lower than the system or it wont overflow. The heater needs to be below that one to allow gravity to feed it. Unless the pump is in the primary reservoir and runs the water through the heater into the secondary reservoir that would be positioned higher than the hydro system and a overflow in the secondary to feed the hydro system. Then the overflow in the system to drain back into the primary reservoir where it can be pumped back through the heater.
I think I got it unless the heater requires presser to turn on, I am thinking an flood and drain type system also but don't really see a difference in that.