Why do solar panels drain when the pump is off?

[jeremymiles]

I have a pool with solar panels on the roof of a three storey house and a variable speed pump.

Why is there a need to drain the solar panels through the vaccuum relief valve?

It seems to me that if you primed the panels at the beginning of the season, the pump would be able to run a lot slower (because it wouldn't need to maintain the head), and it would be more efficient.

But there's probably something I'm not thinking of (so I thought I'd ask, before I tried blocking them.

Thanks!

(Edit: Clarified title).

 

[JamesW]

The vacuum would damage the panels and cause them to collapse.

 

[chem geek]

In particular, without water flow, the panels can get VERY hot and hot PVC is far more likely to collapse under low pressure. When you've got water flow through the panels, you still have somewhat low pressure starting from the top on its way down the return side, but the temperature of the PVC is near water temperature.

 

[ps0303]

To answer the question on the topic "Why do solar panels drain when the pump is off?" is caused most likely by a leak in the system. If your pump is losing prime you have a leak you need to find and fix. The only time panels are drained is if there is a leak in the system like at the pump, or if like here in FL there is going to be a hard freeze, then we drain the panels so they don't burst.

 

[mgianzero]

To answer the question on the topic "Why do solar panels drain when the pump is off?" is caused most likely by a leak in the system. If your pump is losing prime you have a leak you need to find and fix. The only time panels are drained is if there is a leak in the system like at the pump, or if like here in FL there is going to be a hard freeze, then we drain the panels so they don't burst.

Sorry, but I don't agree with this answer. They are "designed" to drain when the pump is off. That's why they have check valves at the top of every set of panels.

See the answers above about hot temperatures with no flow not being a good combination.

 

[jblizzle]

I agree ... having a setup without the ability to drain (due to lack of a VRV) is likely to result in damage to the panels/plumbing. They will get very hot and the vacuum created by the weight of the water can crush the pipes.

Jeremy is correct that the most work for the pump is getting the panels primed, once primed the pump speed might be able to be reduced. You could always try running on a higher speed for a few minutes and then backing the speed down. Although realize that the heating efficiency is higher as you increase the flow rate ... for best heating, you want to run as much water through the panels and their design can handle.