My house has a boiler in the attic that provides hot water for taps and for central heating. On the opposite side of the attic is a chimney pipe for the fireplace on the ground floor. I was thinking of running the boiler's intake along that chimney pipe, coiling around it for instance, so that when a fire is burning the heat that leaves the chimney would heat up the cold water that the boiler takes in, thereby reducing the amount of gas the boiler requires to heat the water. This is similar to the idea of a heat exchanger than runs waste water from the shower along the intake water line to re-use the heat that was flushed down the drain.
However, the fireplace isn't always burning, and the chimney is basically an open pipe to the cold outside. I'm wondering if I'm not actually going to cool the water that goes into the boiler most of the time, thereby defeating the purpose. Then again, the chimney is inside, so possibly it's always at more or less room temperature, which is by definition warmer than the water from the water mains.
Is there anyone with experience doing the same? Any calculations I could look at?
Edit: I'm reading up on this some more and realized that cooling the air in the chimney could have an adverse effect on the chimney's draft and even cause condensation within the chimney... not good. Still, I'm annoyed that so much heat is just flying out of the house rather than stored. There must be some way to utilize it.