Water in floor vents

How does water get into floor HVAC vents during periods of heavy rain? Floor is on slab. Seller installed sump pump under outside deck, but it’s not draining all the water. Possible causes and suggestions to alleviate problem appreciated.


Very strange as I just had a slab inspection last week with the same exact problem. I went and pulled the cover off a return vent (because I was just curious as to why the return vents were installed on outside walls) in the living room and viola…3 inches of standing water in both return vents in this room.

It appears that water from the outside perimeter of the house has seeped through cracked areas of the return ducts. Every return in this house was a solid concrete tunnel (no liner whatsoever) molded into the slab from the furnace location.

I simply told my client that the HVAC return air system (within the slab) may have to be abandoned, but I would rather see him get an HVAC professional opinion.

Thanks, David.

In a perfect world then, if ducts were removed then the ground water would stay outside?

Not if the water is seeping in through hairline cracks in the slab.

The old ducts probably rusted away from the water seepage, so someone simply removed the rusted material, which left a round tunnel for the return.

No they have standing water and a natural humidifier.

Many homes on slabs had asbestos transite pipe used for return ducts. I’ve seen it many times on demolition jobs.

I had an inspection last month that had water in the ducts after the water was turned on to the home (foreclosure). The ducts leaked water onto the kitchen ceiling.

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I had one of those a few years ago and the ducts were rusted out in the slab, there was no visible water but I still recommended a HVAC. In the end they needed to redo the entire system and it made the deal go south because it was a 2 story home and the heating was going to be too expensive for the buyer to redo. They were going to have to tear walls out to install ducts to the different rooms and levels.

One thing I suggested so that they could get a better look was to use a sewer scope camera.

Kick me if i am wrong, If the duct has been rusted or Damaged and full of water would there not be a chance the floor (slab )be weakened to the point of cracking and fallen in ). I would think it would be advisable to fill the void either with pumped in concrete or a injection system like they use for cracked sewers. If water comes in it will go out causing erosion

I have to deal with this on a daily basis 95% of all homes constructed in Oklahoma since the early 60’s are all slab on grade foundation. Not uncommon to see water in the ducts here I am always on guard for this. It most usually starts here by the builder not understanding the 6X10 rule for foundation pads. The word pad to me means elevated most builders here dig a hole build a house in the hole and expect water to run away from the foundation???

Most homes here do get water under the slabs that have poor landscape topography but not always into the ducts. I use a mirror and flashlight to look for past water marks down the duct from the registers to the first turn and then make my call for a camera scan based on what I see.

BTW the duct work is not in the slab it is beneath the slab covered with sand.

You have different sources of water potential under a slab the most common of course is high ground water. Leaking drains and or leaking domestic water supply lines. And don’t forget about a leaking A-coil drain pan during the A/C season I have observed the supply air plenum full of condensate water and overflowing to the supply duct all of the way to the floor registers.

A good contractor does not have to tear out walls if he knows what he is doing. I have re-modeled many 2 stories and never removed any interior wall covering.:wink:


This may be true in many situations but not in this particular home. The concrete was poured around some sort of cylinder (in order to make a concrete duct) because when I look inside the return area, I see concrete access hole all the way through, to include concrete ducts going right to the opposite side of the room.

It was suggested by an HVAC technician to fill the ducts with concrete. This advice coming without going out to the home to check it out first. Is his recommendation the only method of resolving this sort of issue?

No that is not the only option re-landscaping and or French drains is another option but that does not always work either and it is sad to pay for french drains and then on top of that have to convert to an up flow system any way lots of money envolved here. In this area we normally go for the sure kill upflow furnace and fill the boots over with cement. As a Hi I just list the options and let the buyer and seller decide what to do.

David you are loosing me are we talking upflow or downflow. I have seen upflow furnace with poured return chases built in the slab but are unsual. One can find almost anything out there just depending on what the builder had in mind especially with custom builts. Even I stop and scratch my head sometimes trying to discover what the builder was doing. I have no doubt back east where there is a large amount of really old homes that have been re-modeled 10 times over the years that one can see anything.


It was an Upflow furnace. It sucked the return air from the slab ducts (below the unit) and sent the warm air into duct work (above the unit) that went through a chase and into the rooms. Only the returns were embedded into the slab.

Thought that was what you were talking about but was not for sure. I see very few here with the returns below or embedded within a slab most just the supply in the slab with the return in the attic

We get them both here.

I also didn’t like the installation/orientation of the both the supply and returns. The returns (embedded in the slab) were all located at the outside walls, (under windows) and the supplies were all on the inside walls (in wall chases).

I told my client (and noted) to be prepared for cold areas in the rooms due to the improper installation of duct work.

I have just had my home inspected by a potential buyer. Significant rust was found in all 8 ducts (in the slab) on the first floor. The buyer backed out. I am very upset that I will now be paying a significant amount to either have the ducts coated or the ducts run overhead. Running the ducts overhead will be very expensive since it is a two story home.
One duct coating company looked at the video made by the inspector. The company would coat the ducts but would not warranty the work.
I’m curious if one can pursue legal action even though we have been in the home for 32 years. The soil has a lot of clay. I have read that clay can direct water underneath the slab where it can collect and seep into the slap and thus into the ducts.
Seems like the builder should have known that a high clay content in the soil was going to create problems and thus should have run ducts overhead in the downstairs.
Are you aware of any legal precedents for such possible settlements? I live in Tulsa, OK.

I am no attorney but my thinking would be a time limit on your issue.

I am going to be in Tulsa tomorrow @ St Francis Hospital give me a call 580-761-8477 we can talk. Water in the ducts is old hat for me been there and done that