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AGE of AIR CONDITIONERS & HEAT PUMPS
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BACKUP HEAT for HEAT PUMPS
BLOWER FAN CONTINUOUS OPERATION
BLOWER FAN OPERATION & TESTING
BOOKSTORE - Air Conditioning "How To" Books
CAPACITORS for HARD STARTING MOTORS
CIRCUIT BREAKER SIZE for A/C or HEAT PUMP
CLEANING & Legionella BACTERIA
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
DATA TAGS on AIR CONDITIONERS
DEFINITION of HEATING & COOLING TERMS
DIAGNOSTIC GUIDES A/C / HEAT PUMP
DUCT SYSTEM & DUCT DEFECTS
DUST, HVAC CONTAMINATION STUDY
EDUCATION, HVAC SCHOOLS
ELECTRIC MOTOR DIAGNOSTIC GUIDE
EVAPORATOR COIL or COOLING COIL
EVAPORATIVE COOLING SYSTEMS
EXPANSION VALVES, REFRIGERANT
FAN, AIR HANDLER BLOWER UNIT
FAN AUTO ON Thermostat Switch
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FAN CONVECTOR HEATERS - HYDRONIC COILS
GAS DETECTION INSTRUMENTS
GAUGE, REFRIGERATION PRESSURE TEST
HEAT LOSS (or GAIN) in buildings
HEAT LOSS (or GAIN) INDICATORS
HEAT LOSS R U & K VALUE CALCULATION
HUMIDITY LEVEL TARGET
INSPECTION CHECKLIST - OUTDOOR UNIT
INSPECTION LIMITATIONS, A/C SYSTEMS
LOST COOLING CAPACITY
LOW VOLTAGE TRANSFORMER TEST
MANUALS & PARTS GUIDES - HVAC
MOTOR OVERLOAD RESET SWITCH
MOLD in AIR HANDLERS & DUCT WORK
NOISE AIR CONDITIONER / HEAT PUMP
ODORS in AIR HANDLERS & DUCT WORK
OPERATING COST, AIR CONDITIONER
OPERATING DEFECTS, AIR CONDITIONING
OPERATING TEMPERATURES, AIR CONDITIONER
PORTABLE ROOM AIR CONDITIONERS
PRESSURE READINGS, REFRIGERANT
REPAIR GUIDES A/C / HEAT PUMP
REPAIR & DIAGNOSTIC FAQs for A/C
REFRIGERANTS & PIPING
RETROFIT SIZING for A/C or HEAT PUMPS
SEER RATINGS & OTHER DEFINITIONS
SPLIT SYSTEM AIR CONDITIONERS & HEAT PUMPS
THERMOSTATS, HEATING / COOLING
THERMOSTATIC EXPANSION VALVES
WATER COOLED AIR CONDITIONERS
WINDOW / WALL AIR CONDITIONERS
WINDOW / WALL A/C SUPPORTS
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.
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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.
[Click to enlarge any image]
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.
Steps in detecting the presence of in-slab HVAC ductwork
Using a hand-held camera for HVAC duct inspection
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.
Check the air handler and other mechanical system components
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 above confirmed a history of building flooding that sent flood waters inside the HVAC duct system.
Catalog of Environmental Problems Found in In-Slab Air Ducts Used for Building A/C or Heating Systems
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:
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