In-Slab Ducting Problems?

I have performed hundreds of in floor duct video inspections through the years in Albuquerque and surrounding areas. This is some general information about possible in floor duct problems that could be occurring in certain neighborhoods in the Albuquerque area. This might be the most important inspection to have done before any other inspections, only because there is no easy fix or answer to this very expensive problem. This only applies to ducts in the ground located beneath the slab. Here are some videos of  problem ducts in Albuquerque.

General Theories of Water Infiltration: Due to the geographical characteristics and terrain of the Albuquerque and surrounding areas, rain water/run off flows westward spanning the width of the Sandia and Manzano mountains working its way to the Rio Grande RiverValley. Depending on the grading of the lot that the home sits on and other factors, water could run under the homes foundation regularly or intermittenly slowly soaking through the concrete that surrounds the ducts and then the ducts themselves.  This on going process deteriorates the ducts, crumbles the concrete around them causing permanent damage to this major part of the heating/cooling system.

The In Slab Duct Installation Process: Trenches are dug in the soil according to the HVAC blueprints. The ducts are then pieced together and placed into the trenches on top of wooden blocks. Often times the ducts are in direct contact with the soil. Concrete is them poured around the ducts as they lay in the soil. Then the slab is poured on top of the ducts/trenches to form the foundation etc.

Two Types of Duct Work Used: Sheet metal and Sonotube. Sheet metal ducts. (spiral or locking tab style). Metal ducts deteriorate and flake after continuous exposure to moisture/water. All joints, fittings and registers are metal. Often times metal and sonotube ducts were used in combination. Sonotube Ducting, A thick gauged cardboard with a foil lining. Sonotube becomes mangled, deformed and breaks down more readily when exposed to water on a regular bases. Sonotube was code and used as in floor ducting from the early 1970’s to the mid 1980’s and it was originally used and designed for concrete pour forms such as pillars and posts etc. Sonotube was also used in various homes dating back to the 1950’s. Both types of ducts when exposed to moisture/water deteriorate! Metal ducts take a little longer to break down.

Ductwork Damaged By The Following Or A Combination Thereof: A naturally occurring high water table. Watering or over watering of shrubs and bushes around the perimeter. Negative slope or inadequate yard drainage. Step down lots where water flows from neighboring property. Irrigation leak.

Other Causes: Poorly designed roof drainage –lack of gutters and splash blocks etc. Water entry at ground mounted swamp coolers. Indoor atrium watering. Ongoing or “one time” plumbing leak. Water follows loose fill or gravel and permeable soils underneath the slab creating trenches and pockets that channel and collect water.

Signs or Symptoms of Water Damaged Ducts: A mold or mildew odor in the home. Rusty boots and registers. Foil peelings from sonotube present at registers. Limited to no air flow in areas. Tree or plant root growth in ducts. Gurgling sound may be present after a heavy downpour of rain.

How To Correct The Problem? Determine if the water problem is widespread or isolated. If the problems are widespread, most heating contractors opt to abandon the in floor system and go overhead. This would be the most practiced remedy now a days. If the problems are isolated AND the water source is stopped, a repair to the effected area is typically the remedy.

Abandoning The In-Floor Ducts: This could be the easiest fix if the house has both sets of ducts (overhead and in floor). A new up flow furnace is usually installed thus sharing the overhead ducts. Concrete is then used to fill in the floor registers, thus abandoning the system.

Redesigning A System: If overhead ducts do not exist, a new overhead system can usually be designed to work using BTU and square footage formulas. In most cases, new systems must be engineered by a certified HVAC specialist. This process is costly and neighborhood covenants can effect design such as visible ducts from the street, firewall height etc. Jack hammering or wet sawing the floors is the only known method of accessing the damaged areas. Very messy and inconvenient for occupants. Not a viable option in Tanoan due to post tension slabs. Often only a temporary fix if water source has not been determined. Jack hammering floors can also compromise the structure.

Other Potential Problems That Can Occur: Deteriorated ducts effect the quality of air. Mold issues from damp/moist ducts could exist. Termites are attracted to wood/paper products such as sonotube thus making a lifetime of meals out of the ducts. Permeable soils underneath rotted/deteriorated ducts allow mice and other rodents and insects to enter. Respiratory related health risks. Radon gas entry via the heating and cooling ducts. Effects negotiation of and terms of the sale of the home. Bad ducts located beneath the slab could and often effect a real estate transaction.




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