Blowing the lines...depends on the depth of the lines. If above the frost line then they have to be dry or they will probably burst when they freeze. Horizontal pipes are prone to bursting because you can get localized freezing and ice plugs, then the water between them freezes and with nowhere to go, the pipe breaks. Just maybe, your return pipes are angled downward so you could get an ice plug at the top but at a lower level the pipe is open and the water will progressively freeze and always have room to expand.
Al
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