Flow Switch for Domestic cold line w/single pass heat exchanger from Air Source to Water Heat Pump

This is a portion of a SpacePak Hydronic Heat Pump application using a single-pass flat plate heat exchanger on the cold domestic supply to a 50 gallon electric water heater. The 26 gal SpacePak buffer tank at 135 degrees will be the heat source for the other side of the heat exchanger. We’re only looking for a partial preheat at 1.5 gpm for shower (so ~100-110 F target). We just need a 3/4" flow switch to initiate the pump thru a relay or control of some type. A flow switch with low voltage contacts wired directly into a 24vac Taco zone control Thermostat input circuit would be ideal. Simple!!

If we can’t utilize the 'flow switch direct' to zone control method, we may be able to use it as an input to a different device that utilizes flow switches, like the Leridian 'Smart Control’ typically for recirculation system applications. The Leridian activates at just over .5 gpm, whereas others don't. We really like the Leridian on DHW systems with Heat Pump Water Heaters (due to 'the issue' of lowering tank temps) but it seems like overkill since we wouldn't be utilizing the somewhat expensive (~$250) control electronics. Yet, if that's all we can come up with, that's what we'll do. It's flow switch puts out pulses to be interpreted by the control. All we need is an 'ON/Off' (closed/open) feedback. If we can't find what we're looking for we'll use the control too. At first I thought it wouldn’t work since the temperature sensor would be disabled but then realized we could just strap it to the cold supply pipe and it would create an ‘unsatisfied’ load status and keep the pump running as long as the flow switch was calling for pumping. So, we believe we have a solution but we'd rather not put another box on the wall that isn't being fully utilized and try to go for the 'Flow Switch direct' method to the zone control. Any big ears out there connected to puzzle working brains and more experience than we have with flow switches (rated for domestic use!). Thanks! N