What I'm Seeing Now


When HVAC Ducts Run Under The House

When HVAC ducts run under the house.

What?  Ducts that run under a slab?  Ducts that run underground? 

That was very common in the 60s and 70s in Virginia.

Can that be a problem?

Yes.  Such duct work can break down over time and deteriorate.

This house was built with a ground-level slab.  The HVAC duct work for the lower level ran entirely under the slab - entirely underground. 

As such, and if deterioration occurs, the duct work can fill up with water and mud, and even various sorts of animalia that can get inside.

I have seen ducts fill with tree roots!

When full of water, if/when the heat or AC is running the water in the duct work itself will contribute huge amounts of moisture to the house.  And that moisture can be laden with whatever microbial contribution that is amplifying inside the ducts!

Water staining can be seen in this particular duct and water has obviously sat in there in the past.  And where did this mud and detritus come from?

Obviously such this duct work technique is not done anymore, at least in my area.

Can the deteriorating condition of underground duct work be remedied?

Again, yes.  There are spray techniques that can coat and recondition underground ducts that have deteriorated or have filled with mud and moisture, and even molds.

My recommendation:  sometimes when you buy an older house you will find techniques and products that are no longer done or used.   Usually there are reasons for that!   This is one technique that is rife with the potential for future problems, which can develop quickly or over time.  And once such problems begin they don't simply stop or correct themselves.  When problems develop a remedy might be considered - and should be considered sooner rather than later.



Jay Markanich Real Estate Inspections, LLC  

Based in Bristow, serving all of Northern Virginia.

Office (703) 330-6388   Cell (703) 585-7560


Comment balloon 13 commentsJay Markanich • March 18 2017 11:10AM


Yicks...that knd of plugging looks like an expensive/extensive mess to fix !

Posted by Sally K. & David L. Hanson, WI Real Estate Agents - Luxury - Divorce (EXP Realty 414-525-0563) over 3 years ago

I don't know what recoating the ducts would cost, but I expect it would be good to do, S&D.  Such deterioration does not get better.

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 3 years ago

Deterioration should just learn to fix itself!

Oh wait, that's a bit unreasonable.

Underground anything can be a bit problematic with time.

Posted by Kat Palmiotti, The House Kat (406-270-3667, kat@thehousekat.com, Broker, Blackstone Realty Group - brokered by eXp Realty) over 3 years ago

Good Morning Jay - as home building changed you can see things like that.  In our area the practice of putting oil tanks underground is creating problems too.

Posted by Grant Schneider, Your Coach Helping You Create Successful Outcomes (Performance Development Strategies) over 3 years ago

Good morning Jay. Sage advice for those considering an older home! Enjoy your day!

Posted by Wayne Martin, Real Estate Broker - Retired (Wayne M Martin) over 3 years ago

Good morning Jay. I think the key is knowing. Once you know better fix it immediately.

Posted by Sheila Anderson, The Real Estate Whisperer Who Listens 732-715-1133 (Referral Group Incorporated) over 3 years ago

Often ducts are damage in construction as they fill and pack with sand and overtime the water and sand get into the ducts. Many houses in Oklahoma City of that vintage have had them sealed up and new ssytems run overhead.

Posted by Joe Pryor, REALTOR® - Oklahoma Investment Properties (The Virtual Real Estate Team) over 3 years ago

I was not aware that this technique existed.  It's a novel approach but creates a real problem if it fails at some point in the future.   So what tool did you use to get us a picture down there?  Looks like Mighty Mo has a friend in your tool bag!

Posted by Stephen Weakley (Nationwide Mortgage Services) over 3 years ago

I always recommend that my clients that have in floor ducts to have them scoped!

Posted by Fred Hernden, CMI, Albuquerque area Master Inspector (Superior Home Inspections - Greater Albuquerque Area) over 3 years ago

Kat - when I get a cut on my arm it heals itself, sometimes with a doctor's help.  Maybe we should design things that act like the body!

Grant - those are still around here too.  I see them a couple of times a month.  One problem we have had was the earthquake a few years ago which damaged tanks and often unknown to the owner.

These ducts can often become problems Wayne.  I don't see why they wouldn't!

Sheila - there are remedies out there, but unless one has a home inspection they probably wouldn't consider the ducts an issue.



Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 3 years ago

That would have to be the ultimate solution here too Joe.  But if they are not too bad they can be sealed.

Stephen - I was able to get my hand down there with a finger on the camera button.  It took many tries to get a decent photo.

Fred - I scoped a bunch!  This one was representative.  In the past I have seen them dramatically worse, and evidence of animals, trees and some nearly clogged with mud.

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 3 years ago

Just the thought of having buried duct work raise so many health and safety questions.  Nothing good can come of that

Posted by Ed Silva, Central CT Real Estate Broker Serving all equally (RE/MAX Professionals, CT 203-206-0754 ) over 3 years ago

Yet it was real common around here Ed.  No longer done, obviously, but I often still see conduits run under slabs for electrical and data.

Posted by Jay Markanich, Home Inspector - servicing all Northern Virginia (Jay Markanich Real Estate Inspections, LLC) over 3 years ago

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