Why would someone use a Honeywell 4219 and not just zone-double?

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You will want to use a 4219 if you want more hardwired zones. First thing that is troublesome about zone doubling is that you don't get a full 8 zones by doing it. Zones 1 and 9 on a Vista system cannot be zone-doubled.

Secondly, zone-doubling in itself can be problematic. We get more troubleshooting calls from people who try to zone-double than from those that spend the extra money and buy a 4219 zone expander.