Towards a better tinyhouse

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Laminar (displacement) flow ventilation system

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What is a laminar flow ventilation system? What is the point?
Well, to my surprise, I couldn’t find and good documents describing the use in buildings with those terms, it turns out laminar flow is an extreme version of what is is called “displacement flow”.

Anyway, I heard about this in a lecture by Amory Lovins, head honcho of the Rocky Mountain Institute. You can watch it here. It was a while ago, and I’m not sure which one it was, but I assume it would be the building one.

I found a good document that explains things much better than I could, including some data, here: http://www.price-hvac.com/Repnet-www/PDFs/BR409_DisplacementDesignGuide.pdf

In the “Ventilation Effectiveness” section, you can see it implies that you get about the same air quality being inhaled with about half the ventilation, if you are sitting down (I assume the total amount per second of contaminants in the exhaust stream does not change, because there is still just as much contamination being produced indoors). Less than that if you are standing or moving around, but still, that’s pretty sweet.

Remember the “thermal issues in a tinyhouse” post; ventilation requires a fair bit of power in the winter. If you can get the same air quality with less ventilation, that’s good.

I don’t think it looks too complicated to do, either. You just need the diffusers. Maybe get some of that sock diffuser fabric stuff, or some stainless steel mesh would be fine. Also, it leads to less noise indoors from the ventilation system. Could be nice to have it basically silent. Also, no drafts.

About heating, that document says you can heat using radiant floor heating, but as the amount of ventilation goes down, the efficiency of the ventilation heat exchanger (“energy recovery ventilator”) goes up. The numbers seem like a close call, but I think a good way to go would be to use a high efficiency exchanger, then what you are doing all year long, is actually cooling the house.

Think about it : heat is being produced inside the house. 400 watts or something, 120 from a human body, and all the heat form the hot water system, ventilation system fan (maybe only a few tens of watts), appliances that draw some power even when turned off, computer, lights, and solar gain.

As I said in the thermal issues pose, the conduction of heat through the walls would be ~90 watts or so at 25 deg in and -5 out. Double that if it is a cold snap. In other words, without ventilation, heat would actually build up indoors normally. This scenario is actually the same sort of situation you have when you are trying to air-condition the place in summer; you need to remove heat, not add it. That means if the output diffuser is at the floor, the air coming in will be colder than the ambient air, which is when displacement flow systems work best, apparently. So a dilution flow system and a reasonably good heat exchanger could be a good combo.

Of course if you simply turn on a fan inside the house, or take the diffusers off the air duct outlets, it becomes a mixed flow system, if you want. Or it says radiant floor heating works okay, but it seems like that would remove some of the benefits, as heat convects up from the floor. Also, I would wonder if sunlight coming in a window, hitting the floor, would produce a thermal plume that would siphon away a lot of your fresh air from the floor region, leaving less for you. Hm. But maybe this is already included in the ventilation efficiency figures, because presumably (hopefully) they are real-world figures, not theoretical.

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Written by gregor

April 21, 2011 at 00:42

Posted in Uncategorized

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