Ha, that was a subject of great debate in that linked thread! Picture the flow of water without that check valve. If the solar valve is in the "solar off" position, and if there is sufficient head (resistance) between that same valve and the pool (like my heater and SWG and plumbing would provide), then some amount of water will travel up, backwards, through the pipe coming down off the roof. How far up it will travel will depend on how much resistance there is in the rest of the system. I claim that check needs to be there to completely eliminate that "wrong way" water. Not only for the flow inefficiency I suspect would exist without it, but also because you definitely don't want water getting up to the roof during the winter freeze season (which you and I can experience where we live). I suppose the isolation valves could serve that purpose in the winter, but not in swim season, so you'd have to remember to use those valves seasonally. More importantly to me, I don't want any potential deadheading in my system. Especially one that could be caused inadvertently by an actuator valve (either by user error, or system error). I digress. The three-way and the check together solve for everything, without me having to remember to do anything.
Further, if you install a FlowVis flow meter there (as I have), then you can optimize your panel's heating efficiency. The FlowVis doubles as a check valve, so you get a two-fer. I determined my solar panel array's optimum flow rate (all solar panels have one). I got mine from the panel's manufacturer (5GPM per panel, times 8 panels = 40PGPM). With the FlowVis inline, if was a very easy task to adjust my Intelliflo's RPMs to achieve 40GPM through the panels. Done. I now know my panels are heating the pool as best they can, without using any more energy (electricity to the pump) than absolutely necessary. To low a flow, less heat. To high a flow and you risk damage to the panels and the roof-top PVC, which can translate to decreased lifespan of the panels.
There are other ways to simulate the optimum flow rate: complicated calculations, and/or something about using the filter's pressure gauge. Other "pros" will stick there hand in front of the return, while the solar is on, and determine optimum flow based on how warm the heated water is. I didn't like the sound of those methods. The FlowVis takes the guess work and math out of the mix.
I justified it's not-inexpensive cost in several ways:
- I would have spent more in time learning one of the other ways to calculate the flow.
- I needed a check valve there anyway, so that deferred some of the cost.
- I estimate if will only take about 1057 years for the FlowVis to pay for itself in terms of energy savings! (I have no idea how to calculate that!)
- I use the FlowVis often, not just for that one time purpose. I adjust my pump's solar RPM setting a few times a year to compensate for my system's declining flow rate due to the accumulation of dirt in the filter. So not only do I start out with the perfect flow rate for my panels, I maintain it all season long.
- I use the Vis for other things, about once a week (monitoring flow for my suction-side vac and for my filter).
- And most importantly, I want the maximum amount of warmth in my pool that my none-too-cheap panels can provide.
So for me, it was money well spent.