>> #3 - That should happen immediately. Are you saying it doesn't?

The check valve closes quickly, yes, but not "immediately", unless that means "a short time". The pump slows down and stops, and the water has to change direction. It could be a fraction of a second.

>> #4 - If true, any suction would happen at the top of the panels near the VRV, not at the filter.

Any suction near the VRV will produce a lower pressure at the filter.

>> It is the low pressure that opens the VRV but pressure at the filter should be higher because of the weight of the water? Is the equipment much higher than the pool?

You're right, statically the pressure at the filter on the panels side cannot be lower than on the pool side. Dynamically, I don't know.

The equipment is actually lower than the pool, say 8 feet or so below the pool surface. The panels are something like 15 feet above the surface.

>> Even if the VRV was very restrictive and only allowing a small amount of air into the system, it just means the water would slowly drain from the panels. That shouldn't allow an oscillation to be set up.

Yes, I would expect that too. But the oscillations are there and I am trying to explain them.

>> My VRV is quite restrictive as well but I never experienced what you are. So I wouldn't expect that to be the issue.

>> Do you have a solar valve that shuts off when the pump shuts off?

Yes, but at this moment I removed the valve actuator, so the solar valve doesn't change position when the pump shuts off. The loop remains the same.

>> Do you have air in the pump basket when running?

No. Any air in there gets purged quickly.

>> When the pump is running, is there air in the filter? This could be causing the oscillation.

Probably. When the pump starts, there is air in the panels, and it is forced into the filter. Eventually it all flows out into the pool and bubbles away, but there could still be air in the filter housing in many cases, and you are right that this could cause the oscillation.

>> Can you describe the solar setup in detail?

Pump (at -8 feet) --> filter --> check valve --> solar valve --> solar panels (at +15 feet) --> SWG --> pool --> back to pump.

>> Height of the panels above equipment

23 ft

>> Height of equipment above pool

negative 8 ft

>> Height and location of VRV

At the top of the highest and furthest away panel.

>> Pictures of equipment might help too.

I can send those when I get home, but I am not sure they'll help much. It's just the usual stuff.

Thanks a lot!