Earth tube for make-up air
I am planing on building a very tight house. Before I ever heard of “earth tubes” I was planing on providing make-up air. About 90 feet from the home I am going to build an out building. On this outbuilding on an exterior wall i was going to build a box that would hold a furnace filter from which I was planning on using 6″ s&d pipe to run down 8′ into the ground & then over to the house from where it would come up into a chimney chase & ultimately exit at the top of the conditioned attic space. My thoughts: in winter the cold-make-up air would be tempered by going underground & dumping into the bottom of the chimney chase where it would get heated before exiting into the attic. In summer the hot-outside air could be cooled by passing underground & then dumping into the hottest part of the house at the highest point. An added benefit was being able to have filtered make-up air that would be easy to service @ the shed & would not be unsightly on the side of the house. All good right?
…so now I have become aware that condensation can happen in that pipe while underground…something i had not thought of. I live in the central mountains of Idaho at 5,000 feet. Lots of snow in the winter, wet springs & fall but quite dry in the summer. Is there any way of knowing before hand if condensation will be an issue? I can’t have “drain holes” in the pipe because radon is an issue in the area so the pipe needs to be sealed.
Should I just scrap the whole idea & install an ugly filter & air-make-up intake on the side of the house? Martin, I would really appreciate your thoughts on this one…wish i could see the future of any condensation issues in the pipe, because I think it sounds like a great idea. -eric
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Replies
Eric,
1. There are different kinds of "makeup air." If you have a balanced ventilation system (HRV) and sealed combustion appliances, your home won't need any makeup air.
2. If you are planning to install an exhaust-only ventilation system, random cracks in your building's envelope are usually adequate sources of makeup air.
3. If you are planning to install a powerful range hood exhaust fan, you may need a dedicated makeup air system. But these systems are tricky to design, and I think it's better to avoid powerful range hood fans.
4. In no cases would an "earth tube" be advisable in your climate, because of condensation and mold risks.
5. To read about a recent disaster involving earth tubes, see Belgian Passivhaus is Rendered Uninhabitable by Bad Indoor Air.
Thanks. It was after reading the Belgian experience that i was informed about possible condensation in earth tubes.
Martin, could you expand on this point. It isn't something i've ever heard before.
"If you are planning to install an exhaust-only ventilation system, random cracks in your building's envelope are usually adequate sources of makeup air."
Malcolm Taylor,
For more information on the issue, see Designing a Good Ventilation System.
Here is a summary of the relevant information:
1. Properly designed ventilation systems have low air flow rates. For example, according to ASHRAE 62.2, three people who live in a 2,000-square-foot house need a ventilation system that operates at only 43 cfm. That's not much.
2. It's extremely hard to build a house that is so tight that a 43-cfm fan can't find enough makeup air. Windows, doors, mudsills, seams, and penetrations are all available when makeup air is needed. Ordinary stack effect and wind pressures result in infiltration and exfiltration rates that often exceed 43 cfm.
Thanks Martin, that is very reassuring. i thought that with the level of air sealing being done now that some sort of outside air source was necessary. As you say 43 cfm isn't much at all.