Retaining "On Board" Water In Winter Conditions

[slimpickens]

I'm a little puzzled? When I close the "shut off" valve to the flat plate heat exhanger located under the sink in the interior cabinet, I continue to lose my on board water if the hot and cold drain valve petcocks are open and the city water petcock is open. I assumed the "shut off" valve was leaking. However, now I'm not so sure. If I open the "shut off" valve, I lose water through the cold side of the flat plate heat exhanger. If I close the "shut off" valve, I lose water instead through the hot drain side of the flat plate heat exhanger. Therefore, the "shut off" valve must be doing something. I hate to replace it and find out later it's not the issue. Does this add up? I would appreciate insightful input.