If you have a system that has some fluid that does not freeze easliy this will be better. I have had solar collectors that freeze

Open Mike,
Without reading back I'm not sure which of the postings you're responding to, and you might already be familiar with what I'm about to say, but here goes.

In Oz, there are generally two types of solar powered hot water systems that use thermosiphoning to heat the water. An "open system" supplies water to both the storage tank and the thermosiphoning part of the system; the water is usually potable and thus can have no antifreeze added to it. Frosty nights can routinely cause the water in the heat collecting tubes to freeze and burst, leaking precious water out of the system; they gave solar hot water systems a bad name, as the only way to prevent the freezing was to drain the collectors every night. Pain in the rear end!

A "closed system" has the water in the heat collecting tubes isolated from the (potable) water in the storage tank. Heated water in the collecting tubes goes through a heat exchanger in the storage tank, heating up the water in the tank, before returning to the bottom of the collecting tubes. The isolation means the water in the collection tubes and heat exchanger can contain glycol (quite toxic when ingested) or any other antifreeze. Depending on the lowest minimum overnight temperature expected and the limit (ie, freezing point) of the antifreeze, you may never have to drain the collecting tubes.

And yes, Grundfos do make water pumps to cope with high temperatures.

Cheers, Rowan