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(Just found, and LOVE, this site. Been distracted reading other people's questions)
I'm planning to replace 2 zone valves and these other 4 valves... but don't know what they are exactly.
I assume they're used to balance the flow through the two 'halves' of the zone. I thought I just turn the slot vertical for open, horizontal for close, but a friend told me it's more like they screw in to close. So I'm curious what they're called, how they work, etc., but also wondering if I can replace them with ball valves. They are 30-50 years old, and so corroded they're hard to even turn.
Thanks for any help/advice!