oscillation in pool loop with above-roof solar panels

[mas985]

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

I disagree. As I stated before, the VRV is much higher than the filter so the water above the filter will create higher pressure at the filter than at the VRV. This is static head. At the instant the pump shuts off and right before the VRV opens, the pressure at the VRV is ~ -6.5 PSI and the pressure at the filter is ~ +3.5 PSI (pool level is 0 PSI). As air enters the VRV and the water drains, the pressure at the head of the water is related to the height of the water.

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.

The panels are plumbed after the filter so air should not be forced backwards into the filter. When priming, all air in the panels should be forced out of the panel return and then go forwards out the pool returns. Do you have a filter air purge valve? When the pump is running and when you open it, does air come out?

Also, when the equipment pad is well below the pool level, it is a good idea to install the solar valve and check valve on the side of the house above pool level instead of at the pad. Unless that is they way it is done now, that could help. The pad water pressure being higher than atmospheric could be causing an issue with the draining water. But that is the only thing I can think of causing the issue unless the filter accumulates air when the pump is running.

[semenzato]

>>> I disagree. As I stated before, the VRV is much higher than the filter so the water above the filter will create higher pressure at the filter than at the VRV. This is static head. At the instant the pump shuts off and right before the VRV opens, the pressure at the VRV is ~ -6.5 PSI and the pressure at the filter is ~ +3.5 PSI (pool level is 0 PSI). As air enters the VRV and the water drains, the pressure at the head of the water is related to the height of the water.

That's fine, thanks. I was making a feeble attempt at analyzing the dynamic behavior, but I am far from being sure on any conclusion I have made.

>>> The panels are plumbed after the filter so air should not be forced backwards into the filter. When priming, all air in the panels should be forced out of the panel return and then go forwards out the pool returns. Do you have a filter air purge valve? When the pump is running and when you open it, does air come out?

Now that you mention it, you're right, I am not so sure that there is any air in the filter housing at this point. I get air occasionally in there under other circumstances.

I don't have a filter air purge valve. I wish I could install one, but I didn't come with the filter housing, and I didn't find it as a possible accessory.

>>> Also, when the equipment pad is well below the pool level, it is a good idea to install the solar valve and check valve on the side of the house above pool level instead of at the pad. Unless that is they way it is done now, that could help.

No, right now everything is below the pool level, and I don't think it is feasible to change that---I would have to run many pipes a long way.

The only thing I could do is install another check valve higher up. Who knows, maybe it would subtly alter the dynamics and prevent the oscillation.

>>> The pad water pressure being higher than atmospheric could be causing an issue with the draining water. But that is the only thing I can think of causing the issue unless the filter accumulates air when the pump is running.

Thanks!

[mas985]

No, right now everything is below the pool level, and I don't think it is feasible to change that---I would have to run many pipes a long way.

Actually shouldn't have to run anymore than you are now (2). The solar valve would just bridge the two pipes running up the house in a H configuration. The check valve would just be before the H on the supply side. Similar to below. The only difference is the pump and filter is lower than what is shown in the picture. But you only need a supply pipe and a return pipe to and from the house with the diverter valve. Also, they don't show the check valve but you should have that right before the diverter.

[semenzato]

Yes, the picture corresponds to what I have now, except that pump, filter, and diverter are about 8 ft lower. Also, as you said, I have a check valve between filter and diverter.

I also have an additional check valve on the heated water return, near (4) in the picture, probably to prevent backflow into the panels when the diverter is in the non-solar position. But since i am leaving the diverter in the solar position, that shouldn't matter.

Thanks!

[mas985]

So what I was suggesting was to move the solar valve and both check valves so all are above water level. That should not require any additional piping. It is basically moving the H ~9' higher.

[semenzato]

The house is on a hill, with the main floor level with the pool. The equipment is in a room in a floor below. There is no easy way of moving the position of those valves.

This didn't use to be a problem. I am not sure when or how it started. I made small changes here and there. For instance, I added a separate loop, with two diverter valves, to reuse the same pump, filter, heater, and chlorinator, with a hot tub, so some of the pipes changes. Evidently some of those changes were enough to create the oscillation.

[mas985]

OK then pictures might be useful.