DIY solar heating questions about vacuum relief valve

[virtualpaul]

I have a read a few sites describing installation of solar heating panels and I am wondering:

1- For the vacuum relief valve (vacuum breaker), it seems to have two purposes.
a) To prevent pipes from collapsing from a vacuum created when the pump stops and water goes back from the out side (heat) down to the pool.
b) To help flush all the water from the pipes for winterizing.
I see that some design put the vacuum relief valve near the out (heat) side of the solar heat panel which seems to be best for 'a)' since this is where the vacuum will build first. Assuming an anti-return valve is already installed before the in (cold) side of the solar heat panel.
I see other design put the vacuum relief valve near the in (cold) side of the solar heat panel which seems to be best for 'b)' since this would probably fill most of the solar heat panel with air when emptying the pipes from the return side.

What would you recommend?

2- Which type of vacuum relief valve (vacuum breaker) should I get? I see some in PVC for CAD$60. and some other in brass for CAD$16. But most of them don't mention at what PSI they will start to relieve the vacuum.
From what I could see, they just look like an anti-return (spring + ball) connected upside down. Could I just put one of those instead? The ones I saw seem to open at about 5 PSI.

 

[virtualpaul]

For 1, I have found a good reason to put it on the top return side from this site:

Vacuum Relief Valve for Solar Pool Heaters

A solar pool heater Vacuum Relief Valve, or VRV, is not to relieve excess heat and pressure, but to allow air into the system and avoid negative air pressure.

floridasolardesigngroup.com

According to the thread:
No, you want it on the return line – at the highest point. You are supposed to be letting air into the system. Air will follow water down both the feed and return lines. If you put it in the feed line there will be a column of water weighing down on the valve, not allowing it to open and allow air in.

 

[mas985]

Complete nonsense. The column of water does not create pressure on the VRV because there is actually a negative pressure (i.e. suction) at that location. There is a taller heavier column of water on the opposite side of the panels which creates a partial vacuum at the top of the panels so water travels away from the VRV up and over the panels down the return side. It works fine on the supply line. I had mine there for over 15 years.

Having the VRV on the supply side has the added advantage that you can run at lower RPM with a VS pump. Some people have actually installed theirs about 6' above pad level in order to set the RPM to a very low value. Plus it is less likely that the VRV will suck in air when the filter gets dirty.