How low in the plumbing will a VRV allow the solar system to drain?

[hwy17]

I'm building a DIY solar system with this style of VRV at the top of the system:

https://www.amazon.com/SolarPoolRepair-Vacuum-Relief-Valve-Heaters/dp/B01HZXGFIA/

I'm wondering how low it will drain down, specifically if I have a diverter valve 1-2 feet above the water line, will it drain when the pump is off to provide frost protection for the diverter valve.

The spring that keeps the ball valve in the VRV is quite stiff so I'm thinking it might close at several feet or higher of water still in the plumbing, but hoping not.

 

[ajw22]

@Dirk has studied solar heating designs a lot.

 

[hwy17]

Well I got the system running and didn't quite find the answer, the ball valve was sprung so tightly that drain down was only going at a barely audible hiss, no palpable flow out the return. It doesn't seem right to me, I wonder if these vendors got so many complaints of VRV's not shutting that they started making them with tighter springs.

First thing I did was take apart the valve to see if it was stuck, ball was slightly adhered to the oring but after reassembling same result.

So now, not wanting to commit to cutting the spring yet, I'm actually running with no spring at all and liking it so far. System drains like someone's flushing a toilet up there and this way I know it's draining all the way down to pool water level. I was expecting a sputter out of it on startup this way but so far it's not even doing that.

So far, doesn't seem like it's bubbling out the return, but I'm still peeking under a winter cover to check so we'll see how it looks in broad daylight.