Hi, this may be an odd question, but I've been thinking about how to best organize a potential retrofit from boiler to air-to-water heat pump (already esoteric, I know!). I understand the water temp limitations of the A2W and making sure my emitters are correctly sized; my question is about inserting a buffer tank that only gets 'engaged' in the flow under narrow conditions. The A2W heat pump will operate most efficiently if I let its internal pump modulate up and down to match the load from my zones (kept all open, regulating mainly on supply water temperatrue vs outdoor temperature curve). Under my conditions, this will give me between 4-7GPM flow. But at first startup, the heat pump cranks up to full-capactity for ~10min, and delivers about 10GPM. I want that 'extra' 4GPM to start dumping into a buffer tank, as my zones won't be able to dissipate that much heat. Also, during a 'defrost' of the outdoor unit, it cranks it way up to 100% pump capacity, so it will pump even more. Likewise, I'll want it to flush any warm water right out of the buffer tank when it is pumping that hard for a defrost.
So can I set up a buffer tank just downstream of a differential pressure bypass valve, so that it only opens up and flows to the buffer tank when the total flow is above ~7GPM? I guess I'd have to figure out how to map pressure to flow to dial it in properly? Would that work? When the valve opens, the total head through that buffer tank path would be low, so would I also need to add some flow restriction to keep the valve from chattering open/closed?
I suspect the 'sane' way to handle this is to add a secondary loop with pump that just pulls a fixed ~6GPM off my heat pump primary loop (which would pass through the buffer tank). But I'd be giving up some efficiency, and wondered if the differential bypass/other valve route would be feasible, or if it would open up a ton of headaches.
Sorry for the chicken scratch diagram, but I'm trying to figure out if b) would be viable compared to a) on this sketch.
Thanks for any thoughts!