A little more info would be required. There should not be any cross-coupling between hydronic and potable water. Do you have say a demand type system in your boiler you shut off for the summer and use a tank instead? I'd consider just using a valve(s) to shut one off and open the other.
Check valves can (normally) go horizontal (properly oriented) or vertical. If this is for potable water, you'll need probably a bronze, SS, or plastic approved for potable water use. Being hot may create some other problems.