I wanted to post some rough schematics that I've sketched up to show what I've been coming up with, but since this is a paying customer's project, I probably shouldn't. Non-disclosure statements and all that jazz, you know.
"It's all very 'hush-hush' and 'hoosh-hoosh', you know"
(Terry Thomas as Lt. Col. Hawthorne in "It's a Mad Mad Mad Mad World")
Three different ideas;
One: Collected rainwater is pumped from a cistern to a small tank in the house (not the attic, due to possibility of freezing). Then gravity fed to a valve that would mix the city and rainwater roughly 2:1 as it goes to the toilet. An RP backflow preventer would be needed on the city water supply line to prevent the possibility of rainwater contaminating it. And they ain't cheap.
Two: Rainwater pumped into a small interior tank as before, but topped off with city water when rainwater is depleted. An airgap (cheap!) at the top of the small tank would prevent backflow/backsiphon. Another pump in the small tank would provide pressure to operate the toilet. Is a second pump cheaper than an RP device?
Three: Junk the small tank altogether and pump the rainwater straight from the cistern to a solenoid-actuated valve that chooses between city water or rainwater, depending on availability of rainwater. Also requires an RP device on the city water supply line.
Hopefully this is sufficiently detailed to get the conceptual idea across, without disclosing too much to get me in trouble.
