What is more, the very nature of a successful installation of a mitigation system is to make it unobtrusive, so that without specifically thinking about it, the building occupant is not likely to know whether the system continues to perform adequately. That is true of mechanical systems—even though pressure gauges and alarms are sometimes used to signal system failure—and it is especially true of passive and radon resistant systems.

One advantage of mechanical systems is that there are objective tests of whether, for example, a fan continues to operate or, when repaired, resumes correct operation. Many ASD systems are installed with pressure gauges of various kinds that provide a necessary (but not always sufficient) measure of continued system performance. Similarly, one can determine that the fan in an HRV system or a filter system designed to remove radon decay products from indoor air continues to operate.

To the extent that radon-control systems also rely on passive radon-resistance techniques to ensure control of radon entry, failures of these features will be much harder to detect without, for example, directly measuring the indoor radon concentration. Even then, the establishment of baseline radon concentrations in a local region is necessary if one is to be able to estimate the overall effectiveness of radon-resistance construction techniques.

System Reliability and Durability

There have been only limited studies of the continued performance of active (that is, mechanically driven) radon-mitigation systems (Naismith 1997; Brodhead 1995; Dehmel and others 1993; Gadsby and Harrje 1991; Prill and others 1990). For the most part, such studies have identified two major sources of system failure: the fan ceases to operate, or is turned off by the building occupant and not restarted. One limitation to these studies has been that relatively few systems have been operating for periods approximating the mean-time-to-failure data for various fans (typically about 10 y). Buildings themselves are expected to last for many decades, if not a century, so multiple failures of a mechanical system should be expected during the lifetime of a building.

Alterations to the building, such as adding ground-contact rooms, can also alter the performance of a radon-mitigation system. In some cases, no provision for additional pipe penetrations is made so the pressure field established by the existing mitigation system does not extend into the region of the addition.

Periodic Radon Testing

One way of ensuring that systems continue to perform adequately is to conduct periodic radon or radon decay-product measurements. Typically, short-duration radon measurements are done following installation of a radon mitigation system in an existing home as a check that the system, as installed, reduces indoor radon



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