Sealed crawl space…the next step
I am about to submit a bid on my first seal up the crawlspace job (assuming I get it!). So I spent some time this weekend digging out all of the articles from FH and JLC that I have and reviewing them.
All of this time thinking about sealed crawls brought an idea to mind…
Why not use the sealed crawl as the cold air return? No ducts. Just a vent in each room to the crawl and the furnace pulls the cold air from the crawl.
The only commercial job I’ve been involved in had the HVAC designed to use the space between the suspended ceiling and the roof in just this way. But the roof really helped heat that air in the summer (at least the ac guy complained that it did) and (I assume) chill it in the winter.
The crawl would, at least, help cool the air in the summer.
Is this a nutty idea?? Or is it the FUTURE!!??
Rich Beckman
Another day, another tool.
Replies
Not an expert, but my guess would be that all of the return air would come from ducts/registers/vents closest to the furnace. This would lead to more air "exchange" close to the furnace and less from the far rooms.
Just a guess on my part tho.
I don't know what the details of the specs, but there is an admentment to the IRC that address this.I looked at the older 2000 IRC and it said that you did not need outside vents if there conditioned air circuilated in there among several other exceptions.Not exactly what you where asking about, but using the air as a pleumn would be one wat to circulte conditioned air.
novel idea about the return plenum. One thing though. My guess is that all wiring down there would need to be plenum grade, which basically means that it has a different kind of sheath.
Edited 12/13/2004 7:21 am ET by DIRISHINME
Interesting idea, but I don't think I'd risk it.
Let's say the crawlspace wasn't sdealed PERFECTLY. Then the HVAC system would be puming moisture up into the house.
Also - The crawlspace would have to be heavily insulated. Since the crawlspace would be the same temperature as the house, you'd have to insulate the perimeter to the same R-value as the house walls, or you'd be wasting energy.
I don't think there would be any real advantage, other than eliminating the return air duct.
Crawl spaces can have musty odors. Even a sealed space. I suppose if you have a 2 inch slab in there it might be odor free.
Jim,"...my guess would be that all of the return air would come from ducts/registers/vents closest to the furnace. This would lead to more air "exchange" close to the furnace and less from the far rooms."I would think this is easily compensated for by making the vents in the rooms closest to the furnace smaller. The further from the furnace, the larger the vent (since there would be no "friction" from the duct, could the vents be smaller than otherwise?Matt,My guess is that all wiring down there would need to be plenum grade, which basically means that it has a different kind of sheath."Interesting point. I was unaware of a different requirements for wiring in a plenum.Boss,"Let's say the crawlspace wasn't sdealed PERFECTLY. Then the HVAC system would be puming moisture up into the house."From the articles I've read in FH and JLC (and threads here at BT), everything that is in the crawl ends up in the house anyways. No one seals the crawl ceiling."The crawlspace would have to be heavily insulated. Since the crawlspace would be the same temperature as the house, you'd have to insulate the perimeter to the same R-value as the house walls, or you'd be wasting energy."Yes, the perimeter is insulated. One of my thoughts on this is to save energy. I'm not sure about winter time savings...but it seems to me that air conditioning costs could be cut quite a bit. Crawl spaces are always cooler than the outdoor air. It could be that using the crawl as the plenum only pays off where the heating season is short and the cooing season long.Of course, in the heating season the floors would be a lot warmer!Bill,"I looked at the older 2000 IRC and it said that you did not need outside vents if there conditioned air circuilated in there among several other exceptions.Not exactly what you where asking about, but using the air as a pleumn would be one wat to circulte conditioned air."That is one of the considerations that led me to think of this.Jackofall,"Crawl spaces can have musty odors. Even a sealed space. I suppose if you have a 2 inch slab in there it might be odor free."Crawls are musty because they are damp. A well sealed crawl with lots of ventilation would not smell musty.Rich BeckmanAnother day, another tool.
Edited 12/13/2004 9:21 pm ET by Rich Beckman
Is there an HVAC guy around that could chine in on plenum "static pressure"?
There may be something to this . . .
In commercial buildings, the return plenum ceiling is a common cost saving method of construction. The static pressure drop for the return side is minimal and takes place primarily in the registers/grilles, which BTW is how individual air flow rates would be adjusted, by registers.
In commercial applications, due to fire codes all materials within the plenum must be rated to limit smoke production and fire spread.
The greatest concern with plenum returns is that the plenum is seldom if ever cleaned and becomes a great dust collector, and because they are always at a negative relative pressure, they are chronic souces for infiltration and/or unwanted and uncontrolled humidity (at times). Add to that, the fact that most of these ceiling plenums are enclosed via acoustic ceiling tiles, i.e. cellulose, and you have created the perfect environment to grow fungi (mold, mildew, etc.). You could say the same for ductwork, and for exposed ducts in unconditioned speces, this is true. It is reasonably possibe to clean ducts and very difficult to effectively clean a plenum
In a house, the construction joints will allways fall in the areas that might make a good plenum, however, if sealed reasonbly, there is no reason this can't be done.
If you think about, most houses ARE plenums already. How many times have you seen house that had return for every supply? Never. How many times have you seen a house that had a single return for large areas (say multple BRs)? Probably as many times as you cared to notice. So what would be the difference between using the hallway as a giant return duct or the crawlspace as one? None, except those already pointed out.
"... and because they are always at a negative relative pressure, they are chronic souces for infiltration and/or unwanted and uncontrolled humidity (at times)"I imagine that that is the obstacle that would prevent the crawl as plenum."If you think about, most houses ARE plenums already. How many times have you seen house that had return for every supply?"I think I remember posts here complaining about that very point and claiming that insufficient returns can screw up the house.Rich BeckmanAnother day, another tool.
The low static pressures involved with residential systems would pose little real problems if someone inteneded to use a plenum return and make a reasonable effort to seal the space.
Insufficient returns is a chronic problem in residential forced air systems. Using the hallway as a plenum is an acceptable way to address returns in a house, though, in most jurisdictions, would be a code violation in commercial or institutional applications.
I wouldn't want to constantly breathe air pulled from a crawlspace.
Yeah, crawl space air slowly migrates up through the house, but that has a completely different dynamic than actively pulling it from there.
In my experience, conditioned crawlspaces rarely have moisture problems, but that doesn't equal never.
I recommend leave one of the vents only partially "sealed" so thet the conditioned air pressurizing the crawl goes outside rather than up.
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