Planned mechanical systems 31231 SSDPressurization Systems

One of the most frequently used radon reduction techniques in existing homes is an SSD system. Typical installation costs for a system in existing homes currently range from USD1500 to USD2500.912 If the same system is installed or at least planned for, and roughed in during construction, the cost is much lower; so a prudent builder who is erecting a radon-resistant home should include features that will allow for the easy installation of such a system.

Radon mitigation by SSD has been proven to be very effective, often decreasing indoor radon concentrations by 90% or more following mitigation.

The theory of operation for the SSD system is that by penetrating the concrete floor slab with an exhaust pipe one gains access to the area beneath the slab. The area, often a gravel bed, serves as a

Site evaluation

FIGURE 31.6 Major radon-resistant new construction topics. (Adapted from U.S. EPA, Radon-Resistant Construction Techniques for New Residential Construction—Technical Guidance, EPA/625/2-91/032, U.S. Environmental Protection Agency, Washington, DC, February 1991.)

Planned mechanical systems

Mechanical barriers

Sub-slab depressurization

Site evaluation

FIGURE 31.6 Major radon-resistant new construction topics. (Adapted from U.S. EPA, Radon-Resistant Construction Techniques for New Residential Construction—Technical Guidance, EPA/625/2-91/032, U.S. Environmental Protection Agency, Washington, DC, February 1991.)

collection site for the soil gas-containing radon. The exhaust pipe is then routed to the outside of the building, typically through the roof. The negative pressure provided by the exhaust pipe reduces the convective flow of soil gas into the building and causes the soil gas to be removed from the subslab area. If communication exists between the subslab volume and the walls of the building, soil gas will simultaneously be exhausted from the walls. The exhaust mechanism can be passive, which implies that suction pressures beneath the slab will vary seasonally, with the greatest suction occurring during the coldest weather due to increased buoyancy of the air in the vertical exhaust stack (if it is routed through the inside of the building). Active systems, where exhaust fans are used, were shown to maintain near constant suction pressures under the slabs during the entire year.

The key point to remember, in the merits of year-round radon removal, is that there is no guarantee that radon problems will not be present even in the summer months. The radon levels found in individual houses are a complex result of radon source strength, soil transport, the number, size, and location of entry points, weather, and the way the house is operated.2 To be certain of maintaining low radon levels in the house normally requires that an SSD mitigation system works properly 24 h per day, 365 days per year. It is for this reason that durability and system performance are very important considerations. The performance level goal for the system is 100% on-time operation for the life of the building. This requires excellent durability of system components and a reliable means for determining whether the system is fully operational at all times.

The question of durability of the mitigation system arises not only from the need for lifetime operation in the house, but also from concerns about the environment to which the SSD system is subjected.13-15 Soil gas is often very humid, causing condensation problems in the piping and the fan of the mitigation system. Also, particles can be drawn from the gravel bed or soil; they in turn may line the pipes and deposit on the fan or possibly interfere with the fan bearings.

The moisture removal from the subslab can be very substantial, and could amount to many gallons of water per day.13-15 Unless the piping design allows for that water to drain back into the soil, the water could block flow of air in the piping or interfere with the fan operation. Evidence of moisture and other debris has also been found in the staining of roofs near the exhaust pipes of the SSD systems.

The amount of sand and other particles sucked from the soil must be viewed as a possible cause for bearing failure or for the generation of bearing noise (such effects can also be caused by the moisture). Noise can directly influence the occupant to shut down the SSD system. Sandblasting of the fan blades or plateout on the fan blades by particles sucked into the mitigation system could lead to degradation of fan performance over the long term.

Another environmental effect that should not be overlooked is the amount of airflow through the fan. To remain at an appropriate operating temperature the fan requires sufficient airflow to remove fan motor heat. Fan motor capacitor failure will cause the motor to operate at a lower speed and efficiency, especially after the motor has been shut off by the occupant or electrical power interruption. Operating the fan in either of these modes will lead to higher radon levels in the living space and invites early fan failure.

For a simple view of the SSD system and its operating principle, refer to Figures 31.7 through 31.9.

A subslab pressurization system creates a high-pressure zone beneath the slab. Although this does not reverse the direction of the airflow (the air from the system will still flow into the home through cracks and holes), it does dilute the radon concentrations beneath the slab and may keep the radon that is being produced in the site from reaching the foundation. In a number of existing houses, it has been found that this technique performed better than SSD. In buildings where pressurization works best, there are a few common factors. One is the presence of soil or bedrock that allows air to move very easily through it—so easily, in fact, that it is difficult to establish a low-pressure field by exhausting 100 cfm or so of air from beneath the slab. It is this feature that limits the performance of soil depressurization systems. The other factors that seem important are a relatively low concentration of radon in the soil gas and a remote location for the source radium, with radon transported some distance from the house through the very permeable soil. It is thought that a positive pressure created by blowing low-radon-concentration air under the slab dilutes the

FIGURE 31.7 SSD theory. (Adapted from U.S. EPA, Radon-Resistant Construction Techniques for New Residential Construction—Technical Guidance, EPA/625/2-91/032, U.S. Environmental Protection Agency, Washington, DC, February 1991.)
FIGURE 31.8 Typical interior suction point. (Adapted from U.S. EPA, Sub-Slab Depressurization for Low-Permeability Fill Material—Design and Installation of a Home Radon Reduction System, EPA/625/6-91/029, U.S. Environmental Protection Agency, Washington, DC, July 1991.)

PVC vent pipes to various collector pipes (slight slope away from fan)

FIGURE 31.9 Schematic of the fan placement and roof penetration of a typical installation. (Adapted from U.S. EPA, Sub-Slab Depressurization for Low-Permeability Fill Material—Design and Installation of a Home Radon Reduction System, EPA/625/6-91/029, U.S. Environmental Protection Agency, Washington, DC, July 1991.)

PVC vent pipes to various collector pipes (slight slope away from fan)

FIGURE 31.9 Schematic of the fan placement and roof penetration of a typical installation. (Adapted from U.S. EPA, Sub-Slab Depressurization for Low-Permeability Fill Material—Design and Installation of a Home Radon Reduction System, EPA/625/6-91/029, U.S. Environmental Protection Agency, Washington, DC, July 1991.)

soil gas near the foundation, and diverts soil gas originating farther away. Pressurization has been successfully used in buildings built in coarse gravel, shattered shales, and limestones. This technique has been used in existing homes to reduce radon concentrations; however, there has been no major research effort to verify the actual effectiveness of pressurization. Other factors to consider when installing a pressurization system are the effect the introduction of, in some climates, below-freezing or high-humidity air will have on the concrete floor slab and the effort that must be made to ensure that the air intake does not become blocked by foreign matter.

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