It's been interesting reading this old thread.
I felt I needed to respond to the last post -- about how much cost is increased by having a vacuum breaker in a heat exchanger boiler setup. The answer is: not much. You can buy one of these valves at retail for less than $10. I bet the manufacturer's cost is probably 1/3 or less than that given the quantity they order.
At first I thought I would discuss the details involved in adding one to a machine here, but after I wrote it I realized it had become kind of a primer on how to install a vacuum breaker in an older machine -- so I decided to split it into it's own thread -- which is here: All about adding vacuum breakers to older hx boiler machines without them
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