Thread: Crawl space plenum

Crawl space plenum
Hello, my crawl space has no vents to the outside. I has a cement floor (unfinished slurry). This is my return air plenum. No duct work, return air grills go right into the crawl space. I dont think this is very commom has anyone heard of these?

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Without getting into the building codes, I would generally not consider it a good idea. Sounds pretty inefficient. Part of the job of a return air vent is to help circulate air properly. Is it 1-story home? How many return vents are there in the main floor of the house, and where are they?

But more specifically, where is the heating system (furnace), and what type of heating system is it? Gas, electric, forced air furnace, heat pump, etc. Is there a filter somewhere on the furnace?

If it is a gas fueled heating system, where do the gas lines run (crawlspace)? What if there was a gas leak?

You probably have plumbing in the crawlspace. What if there was a plumbing leak, especially a leak in the waste (sewer) pipes? You would have to worry about any chronic leaks that would allow mildew and mold to form somewhere in the crawlspace.

What else is in the crawlspace? Where is the water heater (is it gas or electric)?. Do you have a fireplace in the house that might have a cleanout in the crawlspace?

I have seen a house where the crawlspace acted as the 'supply duct"
see http://www.inspectionnews.net/home_i...l-furnace.html

Originally Posted by Matt Nelson

Crawl space plenum
Hello, my crawl space has no vents to the outside. I has a cement floor (unfinished slurry). This is my return air plenum. No duct work, return air grills go right into the crawl space. I dont think this is very commom has anyone heard of these?

As Terry alluded to there is not much info to go on. Taking return air from a crawl or basement is not good. If you discount sources of flue vent gases their are moisture issues and of course radon. There is most likly a supply vent also.

This could well be a "conditioned crawl space" if the walls are insulated, etc.

Need more detail.

Matt, taking return air from a sealed and insulated crawl space is fine. It actually makes the house more comfortable during the heating season.

It is a problem if you have gas fired appliances (a furnace) in the space or the vent lines from the furnace running thru the space.

As far a gas piping running thru the plenum I see no issue. If you have a gas leak anywhere in your house you have a problem! Being in the crawl space is no different than being in your basement or mechanical room.

The furnace is in the garage but the gas line and plumbing are in the crawl space. It is not insulated but I consider it a conditioned space. Single floor rambler with 3 return grills. We do not have radon issues in Washington state. There is a footing drain around the foundation but we do gett water in there after a very heavy rain but it quickly evaporates. The house was built in 1960, I was just wondering if you have heard of or seen this type of system?

Originally Posted by Rod Butler

Matt, taking return air from a sealed and insulated crawl space is fine. It actually makes the house more comfortable during the heating season.

It is a problem if you have gas fired appliances (a furnace) in the space or the vent lines from the furnace running thru the space.

As far a gas piping running thru the plenum I see no issue. If you have a gas leak anywhere in your house you have a problem! Being in the crawl space is no different than being in your basement or mechanical room.

M1601.4 Under Floor Plenums. An under-floor space used as a supply plenum shall conform to the requirements of this section. Fuel gas lines and plumbing waste cleanouts shall not be located within the space.

Matt, you have a closed crawlspace, not to be confused with a closed conditioned crawlspace. I would suggest installing duct to keep your conditioned air isolated from your crawlspace air. There are other options but they are more expensive. It also sounds like there are a few things that need to be done to your crawlspace to make it function correctly and comply with codes. A little Googleing and I'm sure you will find enough to keep you busy for a couple of weekends.

After installing return air ductwork, I advise you to look at improving the drainage around the exterior of the house to reduce the moisture in the crawlspace.

When you say the water quickly evaporates, remember that directly above the wet crawlspace is your living space.

Have a drain contractor check the perimeter drains. This might cost $200 and will tell you if there is hope for better drainage. It is common in this area for all the downspouts on an older house to empty into the foundation drains. That is a lot of water when it's raining. The old drain tiles can't carry the water away fast enough, so it ends up in the crawlspace.
I had a similar situation with the older end of my house. All my downspouts now empty into a new drain system separate from the foundation drain tiles, which I flushed and repaired. My crawlspace is dry. It didn't cost very much to fix it.

This is the exact opposite of the systems that I have built. Typically the under floor space is no longer a crawl space but a sealed plenum with the outlets located the same as slab on grade with ducts in the slab. When I first used this system there was problems with the A coil freezing up. After visits by the manufactor and everyone standing around scratchin their behinds as to why the problem occured, my installer finally figured it out - there was to much return air. After resricting the return air the system worked like a champ. In your case I can not see why the reverse could not work. Be sure the plenum is sealed, the return air area is sized for the unit and most importantly that you have proper drainage around the house. Without proper drainage you WILL have problems with the plenum. As a note the plenum system is approved in the codes. Yes there is the possability that you could have water line or waste line problems. However, with the use of PEX tubing and schedule 40 waste lines, the homes that I built have not had any problems in over 30 years. The only problems observed was when drainage was not maintained.

Originally Posted by Bill Scott

- there was to much return air. .

??? hmmmm.

Originally Posted by Rod Butler

??? hmmmm.

Humm x 2. The fan chart goes down to .1 wc It might have just been a charge issue.
I've seen where guys come out and add a pound of "freon" and leave, after a few years the HO thinks they need a pound every year.

Sealing the crawl space, that's going to cost more than running 3 new return ducts.
Got's be hell on the heating bill too. Most people don't heat under their house too.
Sealed or not that concrete is cold in the winter.

Glynn Penn
I thank you will find this is called a plenair system and was ued in a limited number of homes and required a water proof membrane on the ground. Must not have worked out to well overall because in 38 years of inspecting in Texas I have never seen one but may be more common in other parts of the country.