thanks for all of the criticism! really!
in my climate, i doubt it would freeze, i live in portland, or and it rarely gets below freezing, and if it does, i'm heating my home and the heat loss through the ceiling should easily keep the pipes from freezing. bypass and drain valves are definately a good idea. i don't know what in insolation valve is, but it's probably a good idea too.
i think i read somewhere that the average shower uses around 15 gallons of water. a short one uses 10 and a LONG one can use up to 30 gallons of hot water. i tend to shower pretty quickly, and don't like super hot water, so i'm guessing i'm in the ten gallon range.
i had not thought of the condensation issue. i'm not sure if it would condensate or not. attics do tend to be about the driest place in a home. especially under a heavy blanket of insulation. i don't think it would be an issue, but don't know as i haven't tried it. i'll bet a layer of .6 mil. plastic sheeting stretched un top of the joists would do to keep the moisture from rotting the joists.
my home is old and framed with beautiful doug fir. i've worked on older homes where the doug fir sill plate (the board in contact with the foundation) has lasted almost one hundred years with minimal rot. i think the minimal condensation of the pipe in the attic would take hundreds of years before rotting began. i don't even know if my home will be standing at that point. i know it is not good to get fiberglass wet, but it would work as kinda a moisture sink. it would wick water off the pipe and allow it to work it's way to the top of the insulation.
i have also thought about the copper coils on my wood stove pipe, but dismissed it as the wood stove would have to on for it to work, and i'm not sure if the heat coming off the pipe could keep up with the flow rate of the water. i would thank that after a few gallons went through the pipe could not heat the water due to the volume and velocity.
the best way to do this would probably be to have a hot water heater sitting in your living room as a storage tank to hold water and let it get to room temp before it hits your HWH, but my house is small, and that would be pretty ugly too.
i'm just trying to come up with a very simple system to take the cold edge off water running into the HWH that doesn't require pumps, tanks, roof mounted stuff, electricity, or seasonal adjustments.
thanks again for all the replies!
adam