Underslab Duct 2

Buildings and homes where the return duct of a forced air furnace or air conditioning system can be very difficult to mitigate.  With return duct being on the suction side of the blower very high negative pressures can exist within the duct.  When this duct is buried in the soil, very small holes in the ductwork can cause radon laden soil gas to be drawn into the ductwork and distributed into the building.  An active soil depressurization system that attempts to draw radon laden soil gas from beneath a foundation often does not have enough vacuum capacity or pressure field extension to be able to overcome the severe vacuums created within the vicinity of the buried return ducts.  This was the situation for the case below and also in Case 1  for a commercial structure.

Case 2: Home with Under-slab Return Ducts

Radon mitigation contractors were asked to provide a bid on a home that was for sale where a NEHA certified tester and home inspector had measured radon levels in excess of 4.0 pCi/L.  The contractors estimated as many as four separate active soil depressurization system would be needed to overcome the effect of a buried return duct under the slab.  Other than the fact the house would have looked like a pin cushion with all the vent stacks, they were also unwilling to guarantee the reduction.  This condition brought the real estate transaction to a screeching halt.   Technicians utilizing E-RPISU monitors were called into investigate the concern.  The results of the investigation are shown in the thumbnail gallery below (click on each thumbnail to enlarge the picture).

Lower-Level Layout Finished basement with bedrooms and living area above. Return duct for basement was below slab.	Forced Air Unit Buried return duct was a concrete channel with several gaps to sub-grade. FAU set on top of return duct channel.	High Efficiency Filter A high efficiency filter was already on return side of furnace. Deep pleated filter needed to be changed.	Test Devices Lower Level E-RPISUs were deployed in two locations of lower level. Continuous radon and radon decay product monitors were also deployed.	Test Devices Upstairs E-RPISUs were deployed in two locations of upper level. Continuous radon and radon decay product monitors were also deployed.	High Efficiency Filter Replaced As part of the investigation, the filter media was replaced.
Results 1 The results of the continuous monitors indicated: 1. Elevated radon but much lower RDPs than expected. 2. Increased air movement with filter out decreased RDPs 3. Replacement of filter reduced RDPs to less than 0.02WL	Room to Room EF Variations With filter out, a test was done to determine if significant variations were seen in Equilibrium Factor. Conclusion: No significnat variations with furnace operating. Note: EF values are significantly below 50% EPA assumption.	Filter Impact on RDP Reduction The graph shows a reduction in EF (also RDPs) with an increase in air circulation and further reduction when filter was replaced.	Impact of filter on RDPs The replacement of filter, with fan operating constantly, reduced levels both upper and lower level of home to less than 0.02WL.	Unattached Fraction A concern is sometimes raised that the use of an air filter may increase number of RDPs that are in small particle range (unattached fraction). The E-RPISU has the ability to qualitatively measure unattached fraction. The results of before and after the addition of the filter did not show a statistical increase in unattached fraction.

Discussion: By measuring  both radon decay products and radon, researchers were able to determine that although radon levels were very high due to the presence of the buried return, significant plate-out was occurring due to the operation of the forced air unit at the same time it was importing radon.  The presence of the filter system improved radon decay product levels without an increased in unattached fraction.  (Note an increase has been observed  in unattached fraction in cases where the filters have been extremely tight, but not in filters with a rating of MERV 10 or less).  This investigation also revealed that equilibrium factors did not vary from room to room, or from level to level when the furnace fan was operated continuously.  This investigation, as has others, also points out that the majority of the  radon decay product reduction is a result of simple air circulation, with the use of filters to further reduce levels as well as provide other indoor air quality benefits.

Outcome: Home buyers were thrilled that exposure levels were not as high as indicated by radon gas measurements alone.  Their children also suffered from allergies and asthma.  One of the attributes of the house that they perceived as a positive was the presence of the high efficiency filter on the furnace to reduce respiratory stress on their children.  It was their plan to operate the fan unit continuously prior to knowing it would reduce exposure to radon decay products.  They purchased the home based upon these findings.  A year later they conducted a major renovation that included the removal of the under slab return duct and re-ducting of the furnace system to further improve indoor air quality.

Caution: When doing follow-up measurements using radon decay product levels as a criteria it is important to also measure radon.  This insures that the lower radon decay product levels are a function of lower equilibrium factors and plate-out, rather than an a lower radon gas level during the time of the follow-up measurement.  That is why PGL provides devices through its manufacturing partners that measure BOTH radon and radon decay products!

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Last modified: 06/05/08