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Old 11-05-15, 09:35 PM   #4
jeff5may
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This was the ancient way of dealing with excess solar heat. The old systems either had a heat dump (a radiator outside in the shade or nighttime collector cooling) or a quenching setup (dump hot water, replace with cold) to keep the collectors cool. This method is not so effective or efficient.

Newer closed loop systems employ a "steamback" design. The manufacturers have learned how to build collectors that will not melt apart when they stagnate, AND that will hold pressure under stagnation conditions. The loop pump stops when the heat store has maxed out. If the coolant in the collector boils, the water boils first (fractional distillation) and forces the rest of the fluid into an expansion tank.

When this stagnation event occurs, the loop pump is locked out until the collector temperature falls below condensing temperatures. Normally the event only occurs on vacation days, as you mentioned.

Here is a discussion about your question regarding steamback design:
Steamback System Design - Heating Help: The Wall
You have to register to see the pictures, but IMHO It's worth the trouble in that particular discussion.
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