Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
Ask a Question or Search InspectAPedia
InspectAPedia ® Home
AIR CONDITIONING & HEAT PUMP SYSTEMS
A/C - HEAT PUMP CONTROLS & SWITCHES
AGE of AIR CONDITIONERS & HEAT PUMPS
AGE of HEATERS, BOILERS, FURNACES
AIR FILTER EFFICIENCY
AIR FILTERS, FIBERGLASS PARTICLES
AIR FILTERS for HVAC SYSTEMS
AIR HANDLER / BLOWER UNITS
ANIMAL ALLERGENS / PET DANDER
ANIMAL ENTRY POINTS in buildings
ANIMAL ODORS IN buildings
APPLIANCE EFFICIENCY RATINGS
ASBESTOS IDENTIFICATION IN buildings
BACKDRAFTING HEATING EQUIPMENT
BACKUP HEAT for HEAT PUMPS
BIOGAS PRODUCTION & USE
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING
BLUE vs YELLOW COMBUSTION FLAMES
BOOKSTORE - InspectAPedia
BTU USAGE MONITORS
BUILDING SAFETY HAZARDS GUIDE
CAPACITORS for HARD STARTING MOTORS
CLEANING & Legionella BACTERIA
COOL OFF HEAT, Thermostat Switch
COMBUSTION GASES & PARTICLE HAZARDS
COMBUSTION PRODUCTS & IAQ
COMPRESSOR & CONDENSING COIL, A/C
CONDENSATE HANDLING, A/C
CONTROLS & SWITCHES, A/C - HEAT PUMP
COOL OFF HEAT Thermostat Switch
COOLING CAPACITY, RATED
COOLING COIL or EVAPORATOR COIL
CRITICAL DEFECTS on A/C SYSTEMS
DATA TAGS on AIR CONDITIONERS
DEFINITION of Heating & Cooling Terms
DEW POINT CALCULATION for WALLS
DIAGNOSE & FIX AIR CONDITIONER / HEAT PUMP
DIRECT VENTS / SIDE WALL VENTS
DRAFT HOODS - gas fired
DRAFT REGULATORS, DAMPERS, BOOSTERS
DUCT SYSTEM & DUCT DEFECTS
AIR FILTERS for HVAC SYSTEMS
AIR FLOW IMPROVEMENT, HVAC
AIR FLOW MEASUREMENT CFM
ALLOY SYSTEMS FLEXDUCT
ASBESTOS DUCTS, HVAC
ASBESTOS PAPER on DUCTWORK
ASBESTOS TRANSITE DUCTWORK
BALANCING AIR DUCT FLOW
DUCT & AIR HANDLER ODORS
DUCT in CONCRETE FLOOR
DUCT DAMAGE, MECHANICAL
DUCT INSULATION - Asbestos Paper
DUCT ROUTING & SUPPORT
FIBERGLASS DUCT, RIGID CONSTRUCTION
FIBERGLASS HVAC DUCTS
FIRE DAMPERS in DUCTWORK
FLOOD DAMAGE in DUCT WORK
GOODMAN GRAY FLEXDUCT
INCREASING RETURN AIR
LEAKY DUCT CONNECTIONS
LOCATION OF REGISTERS & DUCTS
MOLD in AIR HANDLERS & DUCT WORK
NOISES in DUCT SYSTEM
ODORS in AIR HANDLERS & DUCT WORK
OWENS CORNING FLEXDUCT
RETURN AIR REGISTERS & DUCTS
RETURN DUCT AIR LEAKS
SOUNDPROOFING for DUCTWORK
SUPPLY DUCTS & REGISTERS
SUPPLY DUCT AIR LEAKS
TRANSITE PIPE AIR DUCTS
UNDERSIZED RETURN DUCTS
UNSAFE DUCT OPENINGS
WATER & ICE IN DUCT WORK
WET CORRODED DUCT WORK
ZONE DAMPER CONTROLS
DUST, HVAC CONTAMINATION STUDY
DUST SAMPLING PROCEDURE
EDUCATION, HVAC SCHOOLS
ELECTRIC HEAT, DIAGNOSIS, REPAIR
ELECTRIC MOTOR DIAGNOSTIC GUIDE
ELECTRIC MOTOR OVERLOAD RESET SWITCH
EVAPORATIVE COOLING SYSTEMS
EVAPORATOR COIL or COOLING COIL
EXPANSION VALVES, REFRIGERANT
FAN, AIR HANDLER BLOWER UNIT
FAN AUTO ON Thermostat Switch
FAN, COMPRESSOR/CONDENSER UNIT
FAN CONVECTOR HEATERS - HYDRONIC COILS
FAN LIMIT SWITCH
FILTERS, AIR for HVAC SYSTEMS
FILTERS, OIL on HEATING EQUIPMENT
FIRE SAFETY CONTROLS
FLOODED HEATING EQUIPMENT REPAIR
FREEZE-PROOF A BUILDING
FUEL OIL TYPES & CHARACTERISTICS
FUEL UNIT, HEATING OIL PUMPS
FURNACE CONTROLS & SWITCHES
FURNACE EFFICIENCY, HIGH vs MID
FURNACE HEAT EXCHANGER LEAKS
FURNACE OPERATION DETAILS
FURNACE OPERATING TEMPERATURES
GAS BURNER Flame & Noise Defects
GAS PIPING, VALVES, CONTROLS
GAUGE, REFRIGERATION PRESSURE TEST
GEOTHERMAL HEATING SYSTEMS
HEAT EXCHANGER LEAKS
HEAT LOSS in BUILDINGS
HEAT PUMPS, DIAGNOSIS, REPAIR
HEAT TAPES & CABLES on Roofs for Ice Dams
HEATING COST SAVINGS METHODS
HEATING LOSS DIAGNOSIS-FURNACES
HEATING OIL PIPING TROUBLES
HEATING OIL TANKS
HEATING SMALL LOADS
HEATING SYSTEM INSPECT DIAGNOSE REPAIR
HEATING SYSTEM NOISES
HEATING SYSTEM SERVICE & MAINTENANCE
HEATING SYSTEM TYPES
HIGH EFFICIENCY BOILERS/FURNACES
HUMIDITY LEVEL TARGET
INDOOR AIR QUALITY IMPROVEMENT GUIDE
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS, A/C SYSTEMS
LEED GREEN BUILDING CERTIFICATION
LOST COOLING CAPACITY
LOW VOLTAGE BUILDING WIRING
LOW VOLTAGE TRANSFORMER TEST
LP & Natural Gas Safety Hazards
MANUALS & PARTS GUIDES - HVAC
MOLD in AIR HANDLERS & DUCT WORK
MOTOR OVERLOAD RESET SWITCH
Natural Gas Combustion
NO HEAT - FURNACE
NOISE AIR CONDITIONER / HEAT PUMP
NOISE, DUCT VIBRATION DAMPENERS
ODORS in AIR HANDLERS & DUCT WORK
ODORS FROM HEATING SYSTEMS
OIL FILTERS on HEATING EQUIPMENT
OIL FUEL TYPES & CHARACTERISTICS
OIL ODORS, LEAKY OIL TANK PIPING
OIL PUMP FUEL UNIT
OIL SPILL CLEANUP / PREVENTION
OPERATING COST, AIR CONDITIONER
OPERATING DEFECTS, AIR CONDITIONING
OPERATING TEMPERATURES, AIR CONDITIONER
PORTABLE ROOM AIR CONDITIONERS
PLASTIC HEATER VENT
PRESSURE READINGS, REFRIGERANT
PULSE COMBUSTION HEATERS
REFRIGERANTS & PIPING
RELIEF VALVE LEAKS
REPAIR GUIDE, AIR CONDITIONERS / HEAT PUMPS
REPAIR & DIAGNOSTIC FAQs for A/C
REFRIGERANTS & PIPING
Reset Switch - Heater Primary Control
RETROFIT SIZING for A/C or HEAT PUMPS
SAFETY, HEATING INSPECTION
SAFETY RECALLS, Chimneys, Vents, Heaters
SEER RATINGS & OTHER DEFINITIONS
SOOT on OIL FIRED HEATING EQUIPMENT
SPILL SWITCHES - Flue Gas Detection
SPLIT SYSTEM AIR CONDITIONERS & HEAT PUMPS
STACK RELAY SWITCHES
STAIN DIAGNOSIS on BUILDING INTERIORS
THERMAL TRACKING & HEAT LOSS
THERMOSTATS, HEATING / COOLING
THERMOSTATIC EXPANSION VALVES
VIDEO GUIDES: Heating System Videos
WATER COOLED AIR CONDITIONERS
WINDOW / WALL AIR CONDITIONERS
WINDOW / WALL A/C SUPPORTS
WINTERIZE A BUILDING
HVAC ductwork in floor slabs: this article describes heating and air conditioning ducts that have been placed in or beneath concrete floor slabs. HVAC air ducts located inside concrete slab floors invite a surprisingly broad range of building problems that fall into two broad categories: functional troubles such as lack of air flow or collapsed ductwork, and environmental problems such as radon, odors, flooding, mold, insects, and where transite - cement asbestos - ductwork was used, asbestos particle contamination. Here we catalog and illustrate the common problems found with in-slab ductwork & how those hazards may be recognized. We describe steps taken to repair or abandon in-slab air ducts.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
How to Recognize that heating or air conditioning ducts have been routed through or beneath a concrete floor slab
HVAC air ducts located inside concrete slab floors invite a surprisingly broad range of building problems that fall into two broad categories: functional troubles such as lack of air flow or collapsed ductwork, and environmental problems such as radon, odors, flooding, mold, insects, and where transite - cement asbestos - ductwork was used, asbestos particle contamination.
It's easy to recognize that in-slab or below floor slab HVAC ducts are or were used in the design of a building's heating or cooling air duct system, and it's not difficult to evaluate the condition of those ducts by a combination of visual inspection and observation of operating problems (lack of air flow) or environmental problems such as odors.
We have had good success inspecting the condition of in-floor and in-slab HVAC ducts using a simple digital camera that can be placed or held into the duct to take a quick look where a person's head cannot possibly fit for inspection. Our photos just below demonstrate exploring the condition of an air duct in a concrete floor slab by inserting our camera (with wide angle lens) into the duct system through a floor register. Naturally you won't see every inch of the duct this way and you could miss collapses or other in-slab duct problems.
An additional and important indicator of a building HVAC duct design that makes use of in-slab air ducts is illustrated at our photo (left) and is explained in more detail just below.
Take a close look at the air handler (blower unit) located on the lowest floor of the building, often in a basement or on occasion in a crawl space.
Inspecting the warm air furnace shown in the left side of our photographs, we noticed that both the bottom of the furnace itself and an air duct appeared to penetrate the floor slab of the building.
Below in an additional photo you'll see what we found - the return air plenum of the furnace was located below slab and below ground, as well as return air ductwork - both had been flooding.
Our photo just above and our investigation photo (shown at left) illustrate how you may spot an air duct routed through the building floor slab and how you may spot trouble too.
In this case the furnace return air plenum was also located below the concrete floor. The air duct system in this building had been subject to periodic flooding, as illustrated in our second photo (below right).
A description of the health and functional problems that may be traced to air ducts that were routed in a concrete floor slab as well as our advice on how to properly abandon and seal in-slab air ducts are found at DUCT in CONCRETE FLOOR.
There we describe concerns with ductwork run in floor slabs in the article above, including risks of air duct collapse that interferes with air flow through the system, water leaks into the in-slab duct system (not a problem unique to transite ducts), and rodent or insect infestations or even mold contamination.
Odor complaints may be traced to the duct system because of these problems (DUCT & AIR HANDLER ODORS).
Hopefully needless to say, flex duct should never be buried underground nor set into concrete slabs. 
Air flow rates of heating or cooling air delivered by in-slab ductwork can become substantially reduced and ultimately blocked completely by
At left and below, reader-contributed photographs of problems in spiral metal ductwork routed in a concrete floor slab illustrate collapsed blocked ductwork (photo at left) and severe rust, and HVAC duct flooding history (below left) .
In both of these spiral metal ductwork photos (left and below left) you can see actual holes in the duct bottom - admitting ground water, vermin, other contaminants.
We discuss and illustrate disconnected heating or air conditioning duct defects at DUCT CONNECTIONS. We also show additional images of the interior of crimped or squashed flexduct at DUCT DAMAGE, MECHANICAL.
Rust flakes from rusty heating or air conditioning ducts themselves are unlikely to be much of a health hazard - these particles are pretty big, not easily airborne, and probably won't be found at high levels in indoor air except in unusual circumstances. But rust in ducts is a problem indicator, showing quite clearly that the duct system has been wet.
Dust & normal air duct debris: The chief components of house dust, which will certainly collect within a duct system include fabric fibers and skin cells, often also including starch fragments and other organic debris.
Watch out: The combination of organic debris within a duct system and water (indicated by rusty ducts or duct registers) indicates a possible risk of mold or bacterial hazards within the air conditioning or heating system. Since blowing air through the system can pick up and distribute these hazards to occupants of the building, wet or previously-wet duct work is a potential health hazard to building occupants.
In addition to blocking air flow, moisture or water in the duct system invites allergenic or pathogenic mold growth therein as well as bacterial or other health hazards that can be transmitted to the occupied space in the building as air moves through the duct system.
Inspect in-slab ductwork first through the floor registers (above left) and better, using a good light or mirrors or a camera system such as the Chim-Scan or plumbing drain camera.
The mud in the air duct shown at left confirmed a history of building flooding.
Air quality hazards of in-slab duct systems include rodent infestations in ductwork, dirt and debris accumulation which then is returned to the building indoor air or that serves as a breeding ground for pathogens, radon gas hazards, and water leaks into the duct system which can in turn generate a mold or bacterial hazard or can cause softening, collapse, and blockage of the in-slab transite pipe duct. All of these are described individually and in greater detail within this article.
While there are companies offering duct cleaning and duct sealing services, we remain cautious
that such a "sealing" project creates a false sense of confidence that no remaining duct issues exist, causing the
occupants to miss the discovery of future leaks and in-duct problems. A description of the health and functional problems that may be traced to air ducts that were routed in a concrete floor slab are found at DUCT in CONCRETE FLOOR.
Transite pipe, which contains significant percentage of asbestos fibers, was often used for heating ducts and on occasion heating and cooling ducts in older buildings. Transite pipe used as HVAC ducts or air ducts for heating and air conditioning was often installed buried in a concrete floor slab - methods that placed the asbestos-cement transite piping below or in a building floor slab. Asbestos-containing transite pipe HVAC ducts were also used in exposed areas such as shown in the crawl space photograph at the top of this page. And transite pipes were also used as flues or chimneys for some heating equipment, usually where gas fired heaters were installed.
Transite Duct Asbestos Hazards: if used for air ducts transite pipe may be a an asbestos hazard, particularly where the ducts become softened by water exposure (such as air ducts located in floor slabs), potentially releasing asbestos fibers into the building air.  See TRANSITE PIPE AIR DUCTS for details.
Also see ASBESTOS DUCTS, HVAC where we describe other sources of asbestos in HVAC duct systems. Readers concerned with ice or water leaks into or out of HVAC ductwork should also see WET CORRODED DUCT WORK and see WATER & ICE IN DUCT WORK as well as FROST BUILD-UP on AIR CONDITIONER COILS where we describe build-up of ice on the cooling coil in air conditioning air handler units.
Photo of transite duct material courtesy of Thomas Hauswirth, a Connecticut home inspector.
Our photos below illustrate very dirty, debris-laden HVAC air ducts. At below left you will also see evidence of rodent infestation in the duct - a collection of seeds probably left by a squirrel or mouse. At below right the fiberglass-duct was invaded by plant roots and appears to be collapsing.
Don't panic about dirt or debris in HVAC ducts. It is normal for some dust and debris to collect within the air handling system, though by installing and maintaining air filters at the air return registers you can minimize debris and dirt in the ducts themselves. (See AIR FILTERS for HVAC SYSTEMS). "Normal" dust and debris we find inside of air ducts is dominated by fabric fibers, skin cells shed by human occupants, perhaps some food or starch granules, and if the ducts are made from fiberglass, fiberglass particles will be present.
Watch out: however if these duct debris materials are wet from any water source they may become a breeding ground for mold or other pathogens, and of course if the ducts are invaded by rodents or flooded by sewage they are unsanitary and are a health hazard to building occupants.
Because an air duct routed through a floor slab is more likely to contain moisture from condensation or water from leaks or floods, that moisture, combined with normal organic dust and debris found in ductwork can increase the chances of mold contamination in slab-ductwork.
Watch out: don't assume that because mold has been detected in an HVAC duct system that the mold is the principal source of IAQ hazards in the building. Modest amounts of Cladosporium sp. are quite common in air handlers where condensate blows off of the cooling coil.
And mold isolated from a duct system may have entered from a far more serious mold reservoir located elsewhere in the building. Expert investigation may be warranted. See MOLD EXPERT, WHEN TO HIRE.
The catalog of odors and gases in buildings that may be traced to an origin in the ductwork is lengthy and includes both odors/gases that originate within the duct system itself and odors or gases that are transported between building areas by the HVAC system.
In addition to the aesthetic issues of mold, rodent, or sewer gas smells, these, along with odorless gases such as carbon monoxide can be serious even potentially fatal
Take a look at the dirt and black sludge on the bottom of the air duct visible through this floor register. We suspected that the duct system had been repeatedly flooded by ground waters, possibly including organic debris or even sewage that could contribute to an indoor odor complaint.
Odor complaints may be traced to the duct system because of these problems (DUCT & AIR HANDLER ODORS).
Radon entry through in-slab duct systems: can occur in areas where radon is present at problem levels in the soils. In particular, because a return air duct is often at negative pressure (when the blower is operating), the movement of radon gas from the soil into the building air through a leaky in-slab duct can be significant, certainly greater than the movement into the building from other openings such as through a basement slab crack.
As reported in Best Practices Guide to Residential Construction:
The EPA and the U.S. Geological Survey have rated every county in the United States as Zone 1 to 3 for radon risk. Links to state maps with county by- county risk levels can be found at www.epa.gov/ radon/zonemap.html.
The EPA recommends that all homes in Zone 1 counties be built with radon-resistant features, which can be easily upgraded to a radon remediation system if needed.
Seal ducts and air-handling units Where Radon Gas is Present. Placing any return-air ductwork under the concrete slab is not recommended, since this will tend to draw radon into the ductwork and distribute it around the house.
If supply ductwork must pass through a subslab space, it should be seamless or sealed airtight with durable aluminum tape or duct mastic.
During a careful site inspection you may find reason to look further into the air duct system interior for contamination rodents. At left our photograph illustrates such an example: the presence of maple leaf seeds and other organic debris entering the HVAC duct chaseway. At below right is one of the (dead) mice along with mouse droppings found in an HVAC air duct and air handler system.
Potential health concerns from mice and rats include not only fleas or odors, but potential pathogens such as hantavirus.
Sewer Gas entry into the HVAC duct system through in-slab transite asbestos ductwork:
A reader (Conrad) provided us with the photographs and case history of the successful track-down of sewer gas odors in a building (SEWER GAS ODORS). The case began with a complaint of sewer gas odors in the building's heating duct work system.
As reader Conrad discovered while tracking down the odor of sewer gas that was appearing in a home's in-slab ductwork, placing in-slab ducts close to sewer piping (or septic system piping ) invites any future sewer gases leaking out of the piping right into the HVAC duct system.
Watch out: sewer gases may include multiple hazards including the risk of exploding methane gas and possible bacterial pathogens as well as VOCs that can be a respiratory irritant. Also see SEWAGE CONTAMINATION in BUILDINGS.
Two principal approaches to correcting hazards associated with environmental or heating or air conditioning system functional problems traced to air ducts in floor slabs are discussed here:
Our starting point for proper abandonment of the in-slab air duct system shown at left was this floor register in the home's garage. Years of accumulated organic and other debris (odors, IAQ complaints) combined with a modest radon gas concern to lead a prior owner to begin abandonment of this duct system. But he had left floor registers open to the building - failing to close them off (we fill with concrete) and inviting ongoing IAQ problems.
There are spray coating duct-sealants that some contractors offer as an in-duct sealant/spray. The contractor extends a spray wand into the HVAC ducts to deliver a coating that, if perfectly successful, can prevent or at least reduce the risk of asbestos fiber release into the building air. And Andrew Oberta has described standards methods for repairing asbestos-cement products including underground transite piping.
A down-side with in-slab ductwork is the difficulty in accessing for application of the spray and difficulty in inspection in the future to see what's going on inside the duct: collapsing walls, sealant falling off of duct interior, flooding, mold, asbestos-releasing scraps, rodents, etc.
Our in-slab air duct photo (left) shows evidence of a history of floods in the duct system as well as rodents (the rodent poison).
A second concern is that even if the coated transite air duct interior surfaces appear to have been treated successfully, especially with in-slab ducts (SLAB DUCTWORK) we are not assured that the in-slab ducts remain clean, dry, and undamaged in the future nor that the transite duct interior coating remains bonded to the duct surfaces.
But given the history of concerns with the product, in particular with in-slab ducts, we would give strong consideration to abandoning in-slab ductwork entirely.
We recommend abandoning in-slab HVAC air ducts, including transite asbestos cement HVAC ducts, reasoning that there are multiple indoor air quality and potential health as well as functional concerns with such installations. We described concerns with ductwork run in floor slabs in the article above, including risks of air duct collapse that interferes with air flow through the system, water leaks into the in-slab duct system (not a problem unique to transite ducts), and rodent or insect infestations or even mold contamination.
The photos below illustrate a continuation of abandonment of the in-slab air duct whose floor register was illustrated at the start of this section.
Our photos above illustrate how we prepared for sealing of ductwork run in a concrete floor slab. Working at the air supply register we used metal scrap to form a support for a section of drywall cut to fill the air supply register. We pushed this assembly down about 2-inches into the register in order to allow for a 2-inch thick concrete plug (shown below).
Even if the in-slab transite air duct system appears to be in "good condition" there is risk of asbestos fiber release (though in our OPINION the fiber release from intact cement asbestos materials is probably quite low).
Even if the transite air ducts look ok now, the risk of future leaks, infestations, and damage remain, and risk exposure of the building occupants to the problems described here. In addition, where the transite ducts have become softened by water entry or have suffered mechanical damage, there is a risk of increased level of airborne asbestos in the building served by the ductwork.
We then poured into the air register opening in the concrete floor slab.
In other sections of this home the concrete floor, originally covered with carpeting, was re-covered using ceramic tile, adding a final layer of sealant as well as a cosmetic improvement.
More examples of duct routing & support troubles are provided at DUCT ROUTING & SUPPORT. We discuss and illustrate disconnected heating or air conditioning duct defects at DUCT CONNECTIONS. We also show the interior of crimped or squashed flexduct at DUCT DAMAGE, MECHANICAL.
When abandoned, in-slab ductwork of any material can usually be left in place. However where radon gas is an issue, we seal the ducts at the air delivery registers in the building floor slab, as well as sealing any slab cracks that may allow radon gas to enter the building at increased levels.
Bought a house 8 years ago built in 1960, thought it was great that the basement had in floor ducts so it would be warmer in the winter.
Photo at left of a wet and rusted and grubby in-slab duct supply register bottom is from InspectApedia.com files.
That winter we purchased two furnaces, one for the main floor and one inverted one for the basement. Spring thaw came and we heard water bubbling in the ducts and it was overflowing on the floor. The previous owners didn't disclose of this problem but anyhow, it's our problem now.
For the past eight years we've had water in the ducts during spring thaw or when we've had torrential rain. We've been the human sump pumps with our shop vac that has an aspirator that takes the water out of the ducts out the window to our yard.
We didn't want to fill it in we like the feature and we just bought a new furnace. But fast forward to our current state and we are finally waving the white flag. We've added two sump pits in the house and we're still getting water in the ducts. We've had quite a winter season with significant snow fall, we're getting older and don't want to keep pumping out water not including the health risk we've been exposed. Good thing we're not in the basement too often and the furnace is set at 57 degrees so it only turns on when it's really cold.
We would like to fill it in with concrete, we've had a structural engineer look at our basement and he recommended a certain mix of concrete but didn't mention about the metal rebar. Do you insert that only on the vents or the whole area? There is no one in our area that is an expert on this so we're hoping you can help or direct us to someone that can.
Also, do you think that when we fill the ducts with concrete, would the water seep through our walls instead?
Thank you for your time and we look forward to your reply, - S. & G. L. 3/13/2013
Let's divide your question & our comments into some subtopics:
You may have been misled by a photo I used in the article above at How to Abandon in-slab and other transite asbestos HVAC air ducts.
But the purpose of this material was not at all structural.
Rather it was to hold in place a backer (I used a section of drywall) to place about 3-inches down into the floor register opening so that I wouldn't need much concrete to fill and seal just the floor opening itself.
As you will read below, the decision about whether to just seal the register openings in the slab or to fill in the entire duct passage depends on several variables. In the example home to which my photo (at left) pertains, no area of the original concrete slab was left with cracks or openings that made us worry about contaminants entering the building through the duct passage. The finish flooring (other than in the garage) was ceramic tile as you can see in our photo.
We filled the HVAC ducts at the supply registers and then sealed that surface by installation of a new ceramic tile floor. There were not worrisome slab cracks, the slab upper surface is above grade, and there was no history of water entry coming up through the slab ducts into the building. Your case and your worry are different.
Indeed in some of our photo examples I have sealed in-slab HVAC ducts just at the supply register - but I only did this in a location (a dry garage floor) where there was no evidence that I might be leaving a dangerous reservoir of sewage, mold, dead rodents, bacteria under a floor up through which (via other cracks or openings) contaminants might enter the home.
Therefore your worry about an ongoing water problem is appropriate. Without more details about your home, its site, roof drainage, surface contours, the actual sources of water that has been entering the home through the in-slab ductwork, I can't be confident I've got a complete understanding of your situation, but here are some things you should consider:
Questions & answers or comments about HVAC air ducts placed in or below concrete floor slabs.
Ask a Question or Enter Search Terms in the InspectApedia search box just below.
Related Topics, found near the top of this page suggest articles closely related to this one.