Return air from crawl space

The air handler for the heat pump was in the crawl space. The return air was not ducted in from the house, but from the crawl space. The man who built the house stated he was a mechanical engineer and was quick to protect his design. The crawl space had no vents to the outside. Instead, air was supplied to the crawl space from the house itself (three vents in all). His reasoning of not putting duct work for the return air was to keep the crawl space the same humidity and temp as the rest of the house.
Will not having the return air come from the house interior cause the system to overwork? What othe problems will come from this design?

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By the way, I am a NACHI member. I have paid my dues, I must have just forgotten to get something completed.


A number of other things need to be taken into account.

  1. Was the c.s. floor sealed or exposed earth?
  2. Is there any known a.q. (air quality) issues within the concealed c.s.?
  3. How much moisture is present? - ranges over 50-60%?
  4. How well sealed is the foundation and rim joist area of the building envelope?

These are just a few quick issues that may impact the the response.

To clawrenson:

  1. plastic was placed from the top of the walls of the crawl space down to the ground, and then on to cover the ground.
  2. I know of no air quality issues.
  3. on the opposite side of the c.s. there stands a small 2’x3’ puddle of water.
  4. The veiw of the foundation is blocked by plastic.

do these answers help with your response.
Thank you,

Sticking to the HVAC side of the problem. It all depends on how tight the crawlspace is. Ideally, the house, return ducting, air handler, and supply ducts are all air tight and you have basically a closed loop system.

If any of the ducts or house leak (and they all do), then you have an imbalance in the system. The question is how much.

For example with an leaky return ducting and tight supply ducts, the air handler is drawing air from a infinite source and pressurizing the house with a mixed amount of conditioned and unconditioned air.

If you have the opposite (tight return ducts, leaky supply ducts), you are drawing air from a closed volume and pumping it out into the atomosphere. This creates a vacuum within the house and you start pulling in air from every crack in the house.

So, unless the crawlspace is entirely sealed off, you probably have more leak sources than you’d probably see in a regularly ducted house. I think the guy was trying to take advantage that most crawlspace floors are at a constant temperature.

If there is no problem with trapped moisture in the crawlspace and your clients really wants to know how well the crawlspace/house/ducts are sealed, you can recommend that a HERS Rater do a duct and whole house leak test. With a little luck, the rater may also be able to determine the leak rate in the crawlspace.

In California, we have HERS rater testing the ducts in houses when someone does a changeout. The average duct leak rate in a older home is about 30%. In new construction, it’s probably in 5 to 6% range.

Another issue to consider is that all wiring in the crawl would need to be in a raceway. No junction boxes would be allowed either.


Recommend further investigation by an independent HVAC contractor. You are not a specialist in design and/or installation of HVAC systems, and you’re not going to win the battle with a claimed “engineer” who has a personal part in the argument (his design).
You and I know we are supposed to be independent, unemotional, objective in or inspecting and reporting for our Client. Do your Client a favor, don’t challenge the “engineer”, let the HVAC expert do the dirty work on this one.
I’ve designed and installed “free return systems or ductless return systems” in commercial and seen it in a few residential. Conditions should/must be right for the systems to work and maintain a level of safe/environmentally healthy air. Water in the crawl space is not a good sign, and if the wiring existing in the crawl space is rated (ie: plenum rated wiring) I would’nt worry. The question is, how do you tell if the materials installed are correct. I always ask for help.
Good Luck

By definition, return air is taken from an approved conditioned space. The crawl space is NOT a conditioned space unless it is heated, which means it needs an insulation envelope which means crawl spaces rarely ever fit this criteria. Rememeber that exposed insulation is not allowed in conditioned areas. Everyone has seen what can accumulate in crawl spaces (i.e., rat droppings, stagnant water puddles, etc., etc.). Would your buyers want to risk breathing that air - filtered or not?

Also, the IMC 1602.2 #3 states that the minimum volume of the space from which return air is taken needs to be at least 25% of the entire volume being served. How many crawls are that large?

Short of standard aluminum ducts, about the only other method I’ve seen on recent construction is making a gypsum plenum between floor joists. I don’t understand why your builder would be so bent on keeping that crawl space heated. Has he ever paid a heating bill? I would definitely write this up. Very likely will result in higher energy costs and possible air quality issues down the road.

The only problem is that they won’t know what to do either!
Sounds like we have more experts here answering this thread! :wink:

As for the added heating and cooling load of the crawlspace, (if sealed and insulated as already addressed above) the added space will have little effect on the HVAC equipment as the below grade space is already close the setpoint temp your working to obtain (year round). The added load would be in the latent heat (moisture) range, but should go away when the space is conditioned with equipment run time.

What’s with the puddle of water in the space?
I guess the engineer didn’t construct properly.

We are starting to see construction standards changing in these directions with the “Energy Star” program which we will find to be in direct conflict with former standards. The attic and crawlspace areas are being sealed and are becoming “conditioned spaces” in some cases. I see all kinds of problems associated with this, but it is “acceptable”! Energy saving code supersedes current code violations! The problems will likely show up in 7-10 years and we will have a whole new batch of remediation business to deal with.

I remember when they first started using vapor barrier in the walls. They started closing things up so tight it caused all sorts of issues and the walls had to be vented later so they would hold paint!

The principals in theory are OK. The building practices incorporated by these theories is where the problem will be. Like EEFIS siding. There is nothing wrong with the product, it’s how it has been used.

Was looking for something else and found this old thread. Don’t know what part of the country the OP is from, put in my part, the first thing that popped in my head was radon.

What better way to make sure any and all of it circulates throughout the living space!

As an Oklahoma licensed Mechanical Contractor, a NATE certified Senior Technician, and a BPI BAP/EP, I am here to tell you that this MUST be referred to a licenced professional contractor that can explain to the homeowner his folly. Allowing an unsuspecting new homeowner to walk blithely into that cluster would be criminal, literally!

A little late for that Robert this thread was from 07. Some of the licensed professional contractors in this State need to go back to school as some of the junk installed present day is shameful