I heard/read somewhere (years ago) about a construction/architectural method where a crawlspace under a house is sealed with a concrete floor and is used as a plenum for air circulation/heating/cooling of a house.

Anyone hear of this and know of some informative sites on the subject?

Dave, I have seen it both ways. As the common return and common supply. This was in real old houses. Not recommended for either. Ducted supply and returns are preferred.

My house, built in the early 50s had, pass tense, a common cold air return. This was change to a ducted return when the addition went on.

Either way, the craw space should still be sealed to prevent moisture and radion gas from entering the house through the ground. The minimum being heavy duty plastic sheeting covered with gravel.

I will be checking for any sites but this is so old, I don't think I'll find any.

There are a lot of reasons why you can't use the crawlspace as a plenum without taking somewhat significant measures.

You can't run sewer, gas or electrical lines (in unprotected conduit) through a plenum. The real obvious one here is the sewer lines because you wouldn't want to draw any of the sewer gases into the plenum return or supply...very dangerous.

There is also a huge risk of rapidly spread fire in this type of system. This is why building departments stopped allowing things like laundry chutes in homes - the fire just shoots right up there and in the case of the crawl space with many openings for vents it would be similar. In general a concealed fire is the most dangerous because it is nearly impossible to fight and causes structural damage quickly. Crawl spaces aren't really concealed but penetrations are generally meant to be sealed with fire caulk at the very least to prevent a fire from getting in there.

Okay so what else... ductwork actually provides fire protection. I was told that most building departments consider the sheet metal to provide 1 hour of protection... basically they allow fire fighters and residents a chance.

So what you can do is create a large duct running through the crawl space and just feed your registers off of the trunk line, and I think that in some areas you can line the joist space with sheet metal (rather than installing an independent duct). If you do line the joist space, then I know that no other building mechanicals can occupy the same space - i.e. electrical or plumbing.

The rest of your question - yes, I've heard of environmentally friendly homes that use a plenum system for heating and cooling. It is pretty cool actually, but all the framing and walls are made of heavy timber. This provides greater fire protection (counterintuitively) than traditional framing because the flame spread is lower on heavy timber. I can't remember the name of these manufactured homes that used this system but I will post it if I do locate it (Try Google: Green Homes California Manufactured Timber). I believe they have done several homes in Northern CA and the homes are quite beautiful. The plenum system is used around the entire envelope to try and capture geothermal cooling and heating and to utilize natural heat flows... it's very interesting stuff and apparently results in energy costs of about $100 per year. This system of building isn't just using the crawlspace but allowing air to flow throughout. I still don't have a great understanding of why building departments are allowing this kind of shift but the company explains that they will present in front of any building department on the customer's part.

I think I've made a mess of explaining this, but hopefully this is helpful in some way.

What Ami is describing is a variation on the Thermal Envelope House concept. Those, and the geothermal heating, became popular in the late 70s and early 80s. Both are good ideas, with some modifications (fire protection being foremost), but are best utilized in new housing rather than retro-fitting, and in new housing, superinsulation provides similar savings in heating/cooling as compared to the double envelope. However, they can be retro-fitted in most existing house, but the pay-back period is much longer.

Thermal Envelope House: That's what I was thinking of.

Well I think it is.

What I recall is that is was a "solar" house. It had a slab floor in the crawlspace, floor vents through out the house. Also had a solarium (attached green house). I know that air passed through a cinderblock and rock wall – the cinderblock was built with flues inside that opened up in the crawlspace. I know that in the winter the air flow was sucked from the top of the wall (which ended about a foot from the ceiling) and flowed under the house. In summer the flow was the other way, blowing air at the ceiling.

The reason why I even thought of it last night was because there was a commercial on for a robot vacuum cleaner http://irobot.com/. Now when I had heard of the crawlspace being used as a air duct, my thought, having been under many houses, was that it would be pretty dirty with a collection of dust through the years. So when I saw the commercial my mind went back in time and I thought “Gee, now one can keep that space clean.”