Patent ID: 8215380

Claim:
A hot water heat transfer pipe structure comprising: a first heat transfer pipe that exchanges heat between an interior and an exterior thereof, the first heat transfer pipe being disposed to carry a first fluid and including a plurality of first projections each having a height of 0.8-2.0 mm disposed at at least one part of an inner surface of the first heat transfer pipe, and a plurality of second projections each having a height of less than 0.8 mm provided between the first projections, a ratio of a pitch to an inner diameter of the first projections being 0.5-10; and a second heat transfer pipe disposed in the exterior to carry a second fluid that exchanges heat with the first fluid, the second heat transfer pipe contacting an outer surface of the first heat transfer pipe, the first projections being formed on the inner surface of the first heat transfer pipe by indenting the outer surface of the first heat transfer pipe, and the first projections being formed at a location outside of a portion of the first heat transfer pipe that contacts the second heat transfer pipe.