Mixing valve

I'm hoping this falls under the category of "controls". I've posted a few times on the wall and the feedback has been helpful, but although we're getting closer to a solution, I've got one last stumbling block. Short version: my wife and I moved into our house last March and inherited an outdoor wood boiler that supplements a gas boiler in the house and heats a 4 car garage via radiant floor heat. The former homeowner ran the water temp at 130 for both loops - the problem being, at that temp, they were actually robbing heat from the gas boiler in the house. Now I'm trying to get a mixing valve installed for the garage so I can run the wood boiler at 180 for the house, and mix down water temp for the garage so I don't damage the concrete. We had a contractor come out and although he seemed knowledgeable, his plan "A" was to tap into the cold water feed in the garage and mix this with water from the boiler to bring the water down to temp. We were going to go ahead with that plan until I realized a big problem: under this "open" system we'd be introducing new water into the system each time the thermostat kicked on, which would eventually overflow the boiler. On my own, I've done some research and I've read about mixing valves that take the water which has already circulated through the loop, and mix this cooler water with hotter water from the boiler, thus keeping the overall system "closed", which is what I need. It seems to me that this is a more sophisticated type of control, and so probably will cost more - but how much?  The original estimate for plan "A" was well under $1000, how much more can I expect for this type of control? Several thousand? Because at a certain point, it makes more sense to just isolate the garage loop, fill it with glycol, drain the toilet in the garage and shut off the cold water rather than break the bank so I can have a warm garage. The contractor was going to do some research of his own and get back to me, but I feel like I need a better knowledge base to make a decision here. Thanks for any replies, we've spent two months trying to get this squared away before the cold weather hit, and from the looks of the weather forecast, I think winter is finally here.<a href="http://72.3.142.104/forum-reply/1246909/max-water-temp-for-concrete">Reply</a> <a href="http://72.3.142.104/forum-edit-post/1246909/max-water-temp-for-concrete">Edit</a>