I'm building a small (160 square feet) granny flat about 20' away from our main house. Our main house has a great blend of pellet stove + solar collection to keep the home warm. Rather than run electricity or install a wood stove in the granny flat I was hoping I could run ducting from the main house.

I'm planning on a simple 12V solar system for the granny flat's electricity, hence nothing powerful enough to run a heater.

I've seen R8 flex ducting, but it seems weird to dig a trench for ducting even if I re-enforced it in a box. Is there any best practice for running ducting through an outdoor stretch? I'd rather have it be underground regarding aesthetics.

Temperature-wise we're around 8,000 feet in the mountains, temperatures dip down to -10°F in the winter, highs in the single digits (~5°F) on occasion. Normally winter days are 35°F highs and 5°F nights.

Is there any best practice for running ducting through an outdoor stretch?

  • You are going to lose a lot of heat from that ducting... 20' is a long stretch outside the building envelope. HVAC isn't supposed to pass through the attic for this very reason, it's protected, but still "outside". – ench Nov 30 '16 at 19:54

I am not going to act like I know the specific codes or whatever, especially for a dwelling 20 feet away from the main house. But I can offer you something similar.

I have flipped many houses in the midwest and on smaller ones we often poured a slab extension. Before pouring concrete local inspectors have asked us to dig three feet down and bury rigid ducts with at least R-40 worth of insulation around it and wrapped.

I thought three feet was a bit extreme but after the insulation, man you are only 1-1.5 feet below the concrete. And several of these rooms have been 20 feet plus away from the house at the furthest point so the ducts were underground for 20 feet albeit maybe insulated a bit by the concrete (same weather variations). Here are the exact guidelines I used when doing mine.

Also I have seen PVC type ducting like this used but not for residential. I would certainly look into something like this if I were doing one today - I would also insulate it too as the R-10 they are promising is a suspect amount of insulation for cold weather climates.

  • That's awesome, thanks for that. Curiously, any thoughts on condensation issues? I'm just thinking cold+moist ground+heat generally involves some condensation. – Eric Dec 2 '16 at 16:59
  • Inside the duct or outside? What is your main concern? – DMoore Dec 2 '16 at 17:24
  • Also read my answer here - diy.stackexchange.com/questions/101662/… - you will have many of the same issues with a long run going through unconditioned space. – DMoore Dec 2 '16 at 17:26

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