Solar panels with VS pump - HELP!

[chiefwej]

Solar system experts here's your question.

I have Heliocol solar system on my roof to heat the pool. I have recently switched to a VS pump which also handles circulation in the solar system. There is a relief valve at the top of the solar panels located furthest from the riser to the roof. My understanding is that it is there to allow air to enter and bleed the panels back to the pool when the pump is shut down. My question is does it also bleed the air out on start up? I know that I see a lot of air bubble out in the pool when the site first starts, but is air also bled through this valve?

I understand that in a closed system, once the panels and all plumbing is filled with water, there is no loss due to elavation. I also understand that I need to initially ramp up the speed of the pump to fill the panels and push out any air. My problem is that I need to continue to run at a relatively high rpm to keep that valve closed to prevent air from entering the system and stoping circulation. If that valve is there only to allow for draining the panels on shut down, why couldn't it be relocated to a much lower position, like the riser going up the side of the house. That way it would take about 6 psi less pressure in the system to keep it closed. But if I do that will I be trapping air in part of the system on the roof, preventing circulation in a potion of my panels?

 

[jblizzle]

The VRV should not really allow air out of the system very much. But you are right you will have to run at a higher speed to keep that valve closed unless you lower it down. I installed mine the lower header. I think other members have installed it down near the equipment pad. That would allow you to run at a much lower speed without getting air sucked into the system. Of course realize that solar heating is most efficient at higher flow rates.

 

[chiefwej]

I just worry that lowering it will trap air in the highest area of the panels. Would running at high speed for 30 min at the start insure that ALL the air is out of the system.

BTW Heliocol says it will void all warranties if I move the relief valve.

 

[chem geek]

The reason the panel manufacturers don't want you to lower the relief valve is that it is there not only to allow for draining of the panels but also to prevent negative pressure in the panels. If your flow rate isn't high enough where the restrictions of piping and inlets into the pool build up pressure, then there can be less than atmospheric pressure inside the panels and this can cause them to collapse. The panels are designed to handle substantial burst pressure from inside the panels but they are not designed to handle having significantly lower pressure inside the panel compared to the air outside and can collapse (get crushed from the atmosphere) as a result, especially when the panels are hot which they would be when the flow rate is too low.

If you were to have a minimal flow rate, then if the panels were full of water the pressure at the top (without a relief valve) would be minus the weight of the water. If that's 20' high on a roof, then that's 20 feet of head or 20/32 = 0.625 atmospheres (9.2 psi) of negative pressure. In fact, you couldn't go higher than 32 feet because above that you would be creating a vacuum (assuming the panels didn't collapse).

As for air in the panels, that should not be an issue unless you have a dead spot at the top which you should not have. Any air in panels should get pushed out and down out of the returns in the pool. This is why turning on solar results in a lot of bubbling in the pool for a short time. If you have a flow rate high enough to keep the pressure relief valve closed, you likely have a fast enough flow rate to not have air in the panels. And quite frankly, that air would only be in the top header anyway so I'm not sure of what problem you are trying to solve. Basically, if your flow rate is not high enough and the vacuum relief valve opens, you will pull air in and the bubbling in the pool will not stop.

 

[chiefwej]

So, is it safe to move the relief from the high point on the panels, to the side of the house on the riser approx 5 ft above grade?

Will that allow me to run the system with about 6 psi less pressure? It should since I now need 6+ pounds for the elavation plus whatever it takes to keep the valve closed.

If the answers are yes and yes, I'll move it tomorrow.

 

[mas985]

I think if you actually measured the pressure required to collapse a panel, it would be much higher than 14 PSI (theoretical maximum). I have tested mine and they require quite a bit of force to squeeze the tubes closed. I have stepped on them with a small board in-between and exerted well over 25 PSI and the tubes still did not collapse. Not all panels may be this robust but you might be surprised at how much load they can support.

However, the real issue in my mind is the PVC piping. If you leave water stationary in the panels and pipes, the pipes may get soft and collapse. This has happened in many areas with high temperatures where solar gets shut off but the panels do not drain (pump remains running).

 

[chiefwej]

I have no concern about the panel draining. The valve will be on the riser that is attached to the top of the panels and there is a small crossover pipe between the two risers just below where I will be locating it. That will allow for it to drain quickly once the pump is off.

As far as the panel draining when the solar valve closes (temp goal met) and the pump continues to run, this change would have no effect either way on that. I can see the issue. The only thing that may help just a bit is that all my PVC is painted a light tan color. If I had an automation system, I could program it to shut down the pomp for a short period each time the solar valve closes, but without automation that's not an option.

Question remains: is it safe and will it help?

 

[mas985]

Painting the PVC light color doesn't help because it is the super heated water in the panels that travel through the PVC once the pump shuts off that causes the issue.

As to safety, it may never happen to you but I just wanted to let you know that there are documented cases of this happening in climates such as yours. If you are going to do this, then you might want to use CPVC instead of regular PVC for the solar return. That should hold up much better under intense heat.

 

[chiefwej]

Painting the PVC light color doesn't help because it is the super heated water in the panels that travel through the PVC once the pump shuts off that causes the issue.

As to safety, it may never happen to you but I just wanted to let you know that there are documented cases of this happening in climates such as yours. If you are going to do this, then you might want to use CPVC instead of regular PVC for the solar return. That should hold up much better under intense heat.

Fine, but none of this has any relationship to my original questions, since the change I'm considering has no effect on this issue.

 

[chem geek]

I can definitely answer the question "will it help". Yes it will. Having the vacuum relief valve lower will let you run at a lower flow rate without it opening. As for whether it's safe, I'd let Mark comment on that as he has.

As for whether your panels will be able to drain, that's a new issue because if you move the valve down then when the pump shuts off there will be pressure from the water above it to keep the valve closed. Mark, is that an issue with lowering the position of the vacuum relief valve too much? I'm guessing that one shouldn't lower it below the point where the weight (height) of water above it would keep it closed. I suppose the valve is shut at some amount of pressure that is above zero (relative to the atmosphere) but I don't know how much. In theory, if the valve shut at minimal net pressure, then the valve could be kept near the top and we wouldn't be having this discussion so the valve must need some minimum pressure to be shut.

See Vacuum/Breaker Relief Valve Location for a very similar discussion.

 

[mas985]

I would think you want at least 1 PSI at the VRV no matter what height to keep it closed.

As for draining, the panels will drain at any VRV height. The VRV simply allows air into the lines to break the siphon so all the water will drain unless there are any local high spots in the plumbing. Most of the water will drain away from the VRV (yes up and over) although some in the supply will simply drop straight down.

 

[chiefwej]

Ok, based on this discussion I will plan on moving the relief valve to the side of the house. My guess is it should be installed on the return riser since that one is plumbed to the top of the solar panels.

 

[Isaac-1]

In most setups it should be on the feed side , but this depends on how yours was plumbed and if you ave a way for the feed side to drain

 

[mas985]

It is much better to put the VRV on the supply side. On the return side, you would need higher RPM to seal the VRV.

 

[chiefwej]

Ok, supply side 4 ft above grade.

 

[mas985]

I think you would want it higher than the filter.

 

[chem geek]

I would think you want at least 1 PSI at the VRV no matter what height to keep it closed.

As for draining, the panels will drain at any VRV height. The VRV simply allows air into the lines to break the siphon so all the water will drain unless there are any local high spots in the plumbing. Most of the water will drain away from the VRV (yes up and over) although some in the supply will simply drop straight down.

I understood that the water would drain by pulling water up and over, but I didn't realize the VRV trigger pressure was so low which makes my height concern pretty irrelevant (1/15 * 32' = 2' so not very high).

 

[mas985]

Although too low and there could be some splash out.