This home radon planner treats the entered basement or crawlspace as one well-mixed zone. Once mitigation starts,
the modeled radon concentration moves exponentially toward a steady-state value determined by total air changes per
hour and the estimated rate at which radon enters the space.
For a 12,000 ft³ basement with a 220 CFM fan and 0.3 ACH infiltration, mechanical ACH is
220×60/12,000 = 1.10 and total ACH is 1.40. With a source rate of 1.2 pCi/L per hour, the
predicted steady-state concentration is 1.2/1.4 ≈ 0.86 pCi/L. From an initial reading of 12 pCi/L,
the model reaches a 3 pCi/L target in about 1.18 hours.
For an initial reading above the target, a target below the predicted steady-state concentration cannot be
reached with the entered airflow and source rate. Increasing effective total ACH and/or reducing radon entry
lowers the modeled equilibrium level. If the current reading is already at or below the target, the planner
reports zero time even if the modeled concentration could later rise.
Introduction: Why home radon mitigation timing matters
Home radon mitigation timing matters because a fan does not make an indoor reading fall by a fixed amount each hour.
Radon is an odorless, radioactive gas generated by the decay of uranium in soil and rock. When it accumulates indoors,
especially in basements and crawlspaces, long-term exposure increases the risk of lung cancer. Many homeowners test for
radon with a two-day charcoal canister or a digital monitor, receive a result above the U.S. Environmental Protection
Agency action level of 4 pCi/L, and then install a mitigation system. The process often stops there: once the fan is
running, people assume the problem is solved. Yet radon behaves like any other contaminant subject to ventilation.
The concentration changes exponentially over time and approaches a steady-state level determined by the balance between
the removal rate (air changes per hour) and the entry rate from the soil. Estimating the modeled time to a target and the
residual level helps frame follow-up measurements and system adjustments.
This radon runtime planner treats a basement or slab-on-grade space as a single well-mixed zone. While real buildings have
dead zones and pressure gradients, the one-zone model provides a transparent way to connect the entered volume, initial
measurement, target level, fan airflow, infiltration, and source estimate. The results are an estimate of the time to the
selected target and the concentration the model approaches over time. Use that timeline to compare assumptions, plan when
to collect another measurement, and identify whether the entered fan airflow and source estimate are compatible with the
target; it is not a substitute for post-mitigation testing.
The radon clearance curve makes the role of air changes per hour explicit. With every air change, a fixed fraction of the
difference between the current concentration and the steady-state concentration is removed, so the time constant depends
on the combined air-change rate from the fan plus natural infiltration. If the fan provides 1.1 ACH and natural leakage
adds 0.3 ACH, total removal is 1.4 ACH and the time constant is roughly 0.71 hours. The concentration therefore moves
quickly at first when it is far from equilibrium, then changes more slowly as it nears the level sustained by continuing
radon entry. That behavior explains why a lower source rate matters just as much as more airflow for the final reading.
Equations driving the home radon mitigation forecast
The home radon forecast uses the well-mixed room equation for a contaminant with a constant source. Let
represent radon concentration at time
,
the initial concentration,
the source term expressed as an equivalent concentration increase per hour, and
the total removal rate in air changes per hour. The differential equation is
The radon concentration solution combines exponential movement and the steady-state concentration
:
For this radon planner,
equals the source term divided by the removal rate:
.
The fan airflow rate expressed in cubic feet per minute converts to air changes per hour by multiplying by 60 and dividing by the zone volume.
The entered infiltration rate adds directly to that mechanical rate.
To find the time required to reach a target concentration
,
the calculator rearranges the solution:
When the initial concentration is above the target and the target is below the steady-state value, the logarithm does not produce a future crossing time: the modeled system cannot achieve that target with the entered airflow and source rate.
The planner flags that case so you can reconsider fan capacity, infiltration assumptions, or the estimated radon source.
Worked example: a 12,000 ft³ basement with a 220 CFM fan
Consider a 1,500-square-foot basement with an eight-foot ceiling, for a volume of 12,000 cubic feet. Pre-mitigation testing shows a radon level of 12 pCi/L,
and the homeowner selects a 3 pCi/L target. A 220 CFM fan and 0.3 ACH of background infiltration give a mechanical air-change rate of 1.10 ACH
(220 CFM × 60 / 12,000) and a total removal rate of 1.40 ACH. With an estimated radon entry rate of 1.2 pCi/L per hour, the inputs match the calculator defaults.
The predicted steady-state concentration is 1.2 / 1.4 = 0.86 pCi/L, which is below the selected target. The time constant is 1 / 1.4 = 0.714 hours.
Substituting these values into the target-time equation gives approximately 1.18 hours to reach 3 pCi/L. The curve does not stop changing at that point;
it continues toward 0.86 pCi/L, but at a progressively slower rate because the source continues adding radon.
The calculator's table reports the modeled concentration at the interval selected in the form and includes the first interval at or beyond the target time.
Its CSV download contains the same summary and timeline rows. Treat those values as a scenario calculation rather than a clearance certification: compare
the assumptions with post-mitigation radon measurements, particularly if actual fan flow or soil-gas entry differs from the estimates.
Comparison of home radon fan configurations
This radon fan comparison holds the 12,000 ft³ basement, 0.3 ACH infiltration, and 1.2 pCi/L-per-hour source estimate constant while changing fan airflow.
It shows how airflow changes total ACH, the predicted equilibrium concentration, and the modeled time to a 3 pCi/L target.
| Scenario |
Total ACH |
Steady-state radon (pCi/L) |
Time to reach 3 pCi/L (hours) |
Time constant (hours) |
| A: 220 CFM fan |
1.40 |
0.86 |
1.18 |
0.71 |
| B: 150 CFM fan |
1.05 |
1.14 |
1.41 |
0.95 |
| C: 300 CFM fan |
1.80 |
0.67 |
0.93 |
0.56 |
In this home radon scenario, the 300 CFM fan reaches the 3 pCi/L target sooner and produces the lowest modeled steady-state concentration,
while the 150 CFM fan takes longer and leaves the highest modeled equilibrium level. The table is not a fan-sizing recommendation: actual
sub-slab pressure field extension, duct resistance, and measured airflow determine whether a configuration performs as assumed.
Limitations and assumptions for home radon mitigation runtime estimates
This home radon runtime estimate simplifies building physics into a single-zone model. Stratification, closed doors, sump pits, and HVAC ducting can create pockets with higher radon that take longer to clear.
Continuous radon monitors may show changing readings as HVAC operation alters pressure. The calculator assumes the radon entry rate remains constant, but seasonal soil moisture and stack effect can cause substantial variation.
It also assumes the entered fan flow remains steady; actual suction-fan flow can change with system resistance or moisture in piping. Opening windows changes air exchange and may also change soil-gas entry, neither of which this fixed-input model predicts.
Use the results as a planning baseline, pair them with long-term testing, and consult certified mitigators for site-specific adjustments.
To refine a home radon mitigation scenario, measure actual system airflow where appropriate, document the conditioned volume, and compare repeated radon readings with the modeled curve.
Sealing major foundation openings or improving sub-slab suction can reduce the source term, while verified airflow affects the removal term. If the predicted steady-state remains above the target,
the model indicates that changing the source estimate or effective removal rate is necessary; field diagnosis is needed to determine which system change will accomplish that.