Solar drain down plumbing to avoid suction collapse of header????

I will be re-plumbing my pad soon and installing solar. I have a two story roof and live in florida where the solar may shut off during the day with water in the panels allowing the pipes to be exposed to high temps without circulation. I am concerned about vacuum leading to pipe collapse. The header could easily see minus 8 or 10psig. The manufacturer's published install instructions demonstrate no concern about collapse. (They suggest putting the vacuum relief valve at about 5ft above...

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