Conditioned space conduit for main water line in attic?

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Austin, TX
Ok, I don't know if this is classic overthinking but I've got an idea. Here's the background/problem.

I'm re-piping an 80s ranch in Central TX and using Uponor throughout. Attic is now the pathway. The house will have a conditioned space attic, so foam on all the under roof decking and gables. Old insulation all removed. When I first started, the idea was that I would include the garage space in the foam encapsulation as I'm going to have a heat pump water heater in there and eventually a mini-split heat pump wall unit. So it was 'kinda' conditioned. However, due to cost and other details about the garage, it's now out of the foam plan. We'll have a foam wall between it and the conditioned house space.

However, I now have about 35' of 3/4" water line going through what is a very hot space. We just finished our hottest May on record and I don't want to be flushing 35' of water all summer to get to cooler water. Too much waste for me. I had read various posts from people in Arizona dealing with this issue as well. While not Arizona hot, it's still hot. I was just going to insulate the line with 3/4" wall pipe insulation and maybe a radiant barrier wrap.

Then I started wondering if I could use 2" dwv pipe and create a tunnel that began in the conditioned space of the attic and traveled across the garage and to the header where the new pex comes up through the wall from the corner of the slab? I could even foam insulate the outside of the 2" pipe or wrap it. I'd have to end the 'tunnel' probably at the header as it gets very tight at the roofline. Not sure I can manage the sweep. But at least I'd have only 7-8' of very warm water to deal with as the rest of the run would be in the cooling 'tunnel'.

The 3/4" pex would be 'loose' in the pipe tunnel but I'd secure it as it exits. Not sure that's even a concern.

So, am I worried about nothing with the heat or could this work? I have almost all the 2" pipe from another project and just need some fittings and foam.

Thanks for your opinions/advice!


HVAC Contractor
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I lived in a house that had the water come up outside of the garage then ran, insulated, along the garage floor to the far corner where it met the HWH. They had a 2x4 chase built around it. Never noticed an issue with warm water from our hot summers with it like that. Your chase idea might help keep it above freezing if that were an issue but That pvc will heat up quickly.

Can you insulate and run along the floor?
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