Georg Fischer CPVC ductwork for industrial applications where there are exposures to hot corrosive fumes - at see for more informationPVC Air Ducts for HVAC Systems
Is PVC Ductwork Recommended in Slabs or Above Ground?

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HVAC ductwork in floor slabs: diagnosis & repair or abandonment FAQs.

Questions & answers about how to diagnose, repair, or abandon under-slab or in-slab heating or air conditioning ductwork.

This article series describes heating and air conditioning ducts that have been placed in or beneath concrete floor slabs.

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Use of PVC HVAC Ducts in Residential Applications & In Slabs

PVC ductwork from U.S. Plastic Corporation described at InspectApedia.comThese questions & answers about using PVC or CPVC residential in-slab air ducts were posted originally at SLAB DUCTWORK - topic home.

The illustrations of CPVC duct piping at page top and of PVC duct piping shown just above are adapted from Georg Fischer Harvel, a U.S. producer of PVC and CPVC duct piping in Easton PA. Contact information for the company is given in this article.

[Click to enlarge any image] - permission for image use requested 2017/11/28

Question: will using PVC pipe for in-slab heating avoid issues with water in slab duct?

2017/11/27 Marcia Thompson said:

Would installing PVC pipe for in slab heating system pose the same issues mentioned in your article?

We also have radon and have a mitigation system in place but do not want to increase the radon problem.


No, Marcia, at least not where condensation can remain an issue in duct systems.

IN-slab heating using hot water from a boiler, also referred to as radiant floor heat, is a closed system. You would not expect ground water to leak into the piping distribution system.

The piping or routing of slab heat itself does not affect radon leakage into a building through the floor slab as long as the slab is sound - i.e. not cracked.

Any building heating will create a stack effect that causes warm air to rise in the structure; that pressure differences increases the movement of soil gases up into the building if there are openings (such as slab cracks or porous concrete block) through which it can pass.

Reader follow-up:

Just to clarify, I wasn't referring to a radiant heating ststem in the floor. We recently visited a home renovation project in which the HVAC contractor installed some type of PVC pipe instead of metal ductwork beneath the concrete slab.

The owners wanted to eliminate the metal ductwork in the ceiling and therefore had it installed under the slab. We are considering the same system but we have radon and a radon mitigation system. Sounds like the issue with radon leakage is caused by cracks in the slab not the material used for the ductwork.

Correct? Do you know of any other problems with using PVC pipe instead of metal ductwork under the concreate slab? What would you recommend is the safest material to use for ductwork under a concrete slab? And, if it were advising us, would you even consider putting the ductwork under the concreate slab.

Reply: Advantages and disadvantages of using Sch 40 PVC or Sch 80 as in-slab HVAC air ducts:

NFPA 4.3.1 code on air duct materials at InspectApedia.comI take it you mean that the contractor is using large-diameter PVC pipe as HVAC air ducts? If that's the case I would still be concerned about ultimate leaks into and contamination in the in-slab ductwork, though the contractor and I might agree that PVC is probably less-likely to leak from corrosion than the older traditional metal ductwork.

Where are PVC & CPVC Duct Systems Used?

OPINION: There are certainly applications such as industrial environments where plastic, PVC, or CPVC duct materials are not just appropriate, but critical, such as in corrosive environments.

To that end, we list some suppliers of PVC and CPVC duct materials at the end of this article.

What About PVC & CPVC Duct Pipe Use in Residential Applications

OPINION: For residential applications there are code, fire, and in some situations water entry or condensate accumulation hazards that should be considered.

Water and rodents in air duct (C) D FriedmanI'd want to review also the specific PVC piping used and to check its specifications for tolerance of the temperatures produced by the heating system (probably ok) and for its rating and listing as approved for in-slab ductwork.

Shown here is a metal in-slab duct that makes clear that the duct system had held water at least intermittently for many years. While plastic ducts would resist the corrosion we show here, they would not prevent water from entering the duct system.

An advantage of PVC ducting in slab systems is that it's less temperature-conductive than metal, possibly reducing interior condensation. But still I would want to know how the in-slab ducts are being insulated against un-wanted heat transfer or heat loss into the slab.

If the slab temperature is ever going to drop below say 55°F the risk of condensation in the in-slab ducts may be significant.

Any in-slab duct system can and probably will accumulate condensate inside the ducts even if ground water is not leaking into the system.

In-slab HVAC ducts in areas of high ground water and ducts in flood zones are at extra risk of water entry.

That in turn invites development of mold or growth of bacterial or other pathogenic hazards. Going from metal to PVC does not eliminate that risk. But better insulation and less heat loss might reduce it.

PVC is generally not designed to withstand temperatures over 140°F. CPVC duct piping can handle higher temperatures, up to 200°F.

Because PVC ductwork also will have a maximum allowable internal negative pressure rating (to avoid risks of duct sag or collapse), PVC ducts carrying hot air or hot gases might also need design considerations on the return side or where the ducts are under negative pressure. That's more likely to be a concern in some industrial applications than in a residential application.

For example a 10" PVC return duct at 73°F can tolerate a negative interal pressure of -146 inches of water or about -5.3 psi. You won't ever see that much pressure drop in a residential HVAC system where a typical pressure drop across an air filter might be about 0.20 in. wc. (Richardson 2014) cited at INCREASING RETURN AIR where we discuss negative pressure problems in return air ducts.

PVC and CPVC ducting for air movement in a residential HVAC system may require special adapters to connect it at transitions to the metal components of the HVAC system plenums and air handler - a place to watch for jury-rigged snafus by an inexpert installer.

2015 International Mechanical Code or IMC 603.8 Underground Ducts

Here is an excerpt from one of the model codes, the 2015 International Mechanical Code or IMC 603.8 Underground Ducts, section 603.8.3 Plastic Ducts and Fittings

Plastic ducts shall be constructed of PVC having a minimum pipe stiffness of 8 psi (55 kPa) at 5 percent deflection when tested in accordance with ASTM D 2412.

Plastic duct fittings shall be constructed of either PVC or high density polyethylene.

Plastic ducts and fittings shall be utilized in underground installations only. The maximum design temperature for systems utilizing plastic duct and fittings shall be 150°F (66°C)

IMC 603.12 Condensation

Interestingly section 603.12 of the IMC includes a provision for addressing condensation but only on the exterior of the ductwork.

Provisions shall be made to prevent the formation of condensation on the exterior of any duct.

IMC [BS] 603.13 Flood hazard areas

For structures in flood hazard areas, ducts shall be located above the elevation required by Section 1612 of the International Building Code for utilities and attendant equipment or shall be designed and constructed to prevent wter from entering or accumulating within the ducts during floods up to such elevation.

If ducts are located below the elevation required by Section 1612 of the International Building Code for utilities and attendant equipment, the ducts shall be capableof resisting hydrostatic andhydrodynamic loads and stresses, including the effects of buoyancy, during the occurrence of flooding up to such elevation.

You can read the flood protection provisions of the ICC IRC IBC Section 1612 here: 2009 ICC IRC INTERNATIONAL BUILDING CODE (2009) [PDF] complete

IMC 603.14 Location

Ducts shall not be installed in or within 4 inches (102mm) of the earth, escept where such ducts comply with section 603.8.

Really? In my opinion the model codes should considere the problems that accrue when water leaks into or floods into the duct interior or when condensation forms on the duct interior where the combination of accumulation of water and the passage of house dust that contains organic particles invites bacterial, mold or similar contamination.

NFPA 90A Section 4.3.1, Air Distribution - duct materials

Also see this page excerpted from NFPA 90A Section 4.3.1 AIR DISTRIBUTION [PDF] that does NOT list PVC among permitted HVAC duct materials and review this fire safety question with your local code enforcement officials. Air Ducts shall be constructed of iron, steel, aluminum, copper concrete masonry or clay tile except as permitted in or - see my attached snapshot of this section of the NFPA code below

NFPA 90A covers construction, installation, operation, and maintenance of air conditioning and ventilating systems, including filters, ducts, and related equipment, to protect life and property from fire, smoke, and gases resulting from fire or conditions having manifestations similar to fire. - available at

Bottom line: in my OPINION I would avoid putting HVAC ducts into a slab

Reader follow-up:

Marcia Thompson said:

Thank you so much for taking the time and providing such good references. We will probably avoid taking the risk. Much appreciation and this is a great site. Keep up the good work.

Question: PVC pipes for air ducts in the crawl space?

(July 30, 2015) ed wilson said:

i have a low small space beneath my house . I need to add more heat air ducting , Is it a good thing to install pvc piping for a/c ductv


OPINION: There are risks of

On 2016-07-01 by (mod) why PVC ductwork doesn't solve in-slab troubles

Water leaks, condensation, organic dust and debris combine to grow mold and support pathogens in low-routed in slab ducts.

On 2016-07-01 by wayne kerr

why wouldn't pvc duct work solve most in ground duct problems

Suppliers of PVC and CPVC Ductwork

Georg Fischer PVC duct at at

PVC ductwork from U.S. Plastic Corporation described at


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