Colorado is a Zone 1 radon state — the EPA's highest-risk designation — and the Colorado Department of Public Health and Environment (CDPHE) estimates that roughly half of Colorado homes carry radon levels above the EPA's 4 pCi/L action threshold. Douglas County, where Castle Rock sits, is squarely inside that pattern. If you've bought a home in Castle Rock in the last decade, your inspection almost certainly included a radon test; if you haven't, your home almost certainly hasn't been tested since it was built.
This isn't an HVAC company's usual specialty — radon mitigation is its own licensed trade — but every HVAC design choice we make in Colorado either helps or hurts a radon situation. Here's what a Front Range homeowner should know before the next HVAC upgrade, and what to ask a mitigation contractor when you need one.
What is radon, briefly?
Radon is a colorless, odorless, radioactive gas produced by the natural decay of uranium in soil and rock. It enters homes through cracks in slab foundations, pipe penetrations, sump pits, and any other pathway between soil gas and interior air. Colorado's granite-heavy geology produces meaningful amounts of it — hence the Zone 1 classification statewide.
Long-term exposure to elevated radon is the second-leading cause of lung cancer in the U.S. after smoking, per the EPA and Surgeon General. The 4 pCi/L (picocuries per liter) action threshold isn't arbitrary — it's the level at which lifetime lung-cancer risk exceeds a defined threshold in EPA modeling. Below 2 pCi/L is considered "as low as practical." Between 2 and 4 pCi/L is a consider-mitigating range. Above 4 pCi/L, the EPA strongly recommends mitigation.
Testing before mitigation
Every mitigation decision starts with a test. CDPHE and the EPA both recommend long-term (90-day+) alpha-track tests as the most accurate approach for a homeowner decision. Short-term (2 – 7 day) charcoal tests are useful for a real-estate closing but can miss seasonal variation. Radon levels are typically higher in winter than summer because homes are more tightly closed and the stack effect draws more soil gas up through the foundation.
Test kits are available inexpensively from Douglas County Health & Human Services, the CDPHE, or hardware stores. A confirmed reading above 4 pCi/L is your trigger to bring in a mitigation contractor for a plan.
How does HVAC design affect radon?
Several ways, some of which make things better and some of which make things worse.
1. Return-air location and duct leakage
A leaky return-air duct running through a crawlspace, basement, or slab cavity can actively pull radon-laden soil gas into the airstream and distribute it through the house. This is the worst HVAC-induced radon scenario — an HVAC system that turns your radon problem into a whole-house radon problem.
Fix: seal return ducts with mastic (not tape — tape fails at temperature) at every joint, and route returns through conditioned space wherever possible. Every Colorado Bear duct-inspection walk includes a check for return-duct leakage into potential radon-source cavities.
2. Combustion-air makeup on gas furnaces
An 80 % AFUE gas furnace pulls combustion air from indoor space, which creates a slight negative pressure that can increase radon infiltration. A 96 % AFUE condensing furnace with dedicated outdoor combustion-air intake doesn't do this — it takes its combustion air directly from outside.
Upgrading from an 80 % non-sealed to a 96 % sealed-combustion furnace is a modest radon-friendly move as a side effect of the efficiency upgrade.
3. Whole-house ventilation strategy
A tight, modern Front Range home hits ASHRAE 62.2 mechanical-ventilation triggers well before older stock does. A balanced ERV or HRV exchanges stale indoor air for fresh outdoor air without creating a pressure imbalance that draws more soil gas in.
Contrast with a bathroom exhaust fan or kitchen range hood: those depressurize the interior and can actively increase radon infiltration during operation. Adding a balanced ERV or HRV neutralizes that pressure imbalance and improves the whole IAQ picture.
4. Air handler location
An air handler located in an unconditioned basement or crawlspace, with unsealed return-air pathways, has the highest potential to interact badly with radon. An air handler in conditioned space (a utility room upstairs, a closet on the main floor) has minimal radon interaction.
This isn't usually a fix-it item — moving an air handler is expensive. But it's worth understanding as a factor when planning a full-replacement retrofit.
HVAC doesn't fix a radon problem — a licensed radon mitigator does. But bad HVAC can amplify a radon problem badly. The two decisions belong on the same table.
What sub-slab depressurization does
Standard radon mitigation is called sub-slab depressurization (SSD). A licensed radon mitigator drills through your slab, installs a suction pipe below it, and vents the pipe above your roofline with a small continuous-run fan. The fan creates a slight negative pressure under your foundation, reversing the pressure gradient that pulls radon indoors.
Installed cost: typically $1,500 – $3,000 for a standard Castle Rock single-family home. Ongoing electric cost: about $8 – $15 per month for the fan.
Post-install, a follow-up radon test confirms the mitigation worked. Well-designed SSD systems routinely take homes from 6 – 12 pCi/L pre-install to under 1 pCi/L post-install.
This is not HVAC work — it's radon-mitigation work. Colorado licenses radon mitigators separately (through CDPHE). We're happy to refer you to reputable Douglas County mitigators; we don't do this work ourselves because it deserves a specialist.
How does mitigation interact with future HVAC changes?
A few points worth planning for:
- Post-mitigation, avoid creating new slab or foundation penetrations without coordinating with your mitigator. A new plumbing or electrical penetration can compromise the SSD system's pressure balance.
- When installing an ERV or HRV, make sure the balanced ventilation isn't fighting the SSD fan. A good HVAC design accounts for both simultaneously.
- When replacing an 80 % furnace with a 96 % sealed-combustion unit, you may need to re-test radon post-install because the combustion-air pressure dynamics changed. Usually an improvement, but verify.
- Any duct-modification project (new returns, extended supply runs, sealing existing leaks) should be planned as radon-friendly by default — seal aggressively at every penetration through the mitigation slab or wall.
What Front Range homeowners should actually do
- Test first. Long-term alpha-track test if you haven't tested recently. Buy the kit through CDPHE or Douglas County Health for $15 – $30.
- If above 4 pCi/L, mitigate. Hire a licensed Colorado radon mitigator. Get the follow-up test to confirm the system worked.
- Between 2 – 4 pCi/L, consider. Mitigation isn't required but the reduction to under 2 pCi/L is inexpensive and worthwhile if you're already doing basement finishing or major HVAC work.
- Coordinate the HVAC and mitigation work. If you're planning both, tell each contractor about the other. The best outcomes come from projects planned together.
- Re-test every 5 – 10 years or after major foundation or basement work. Radon levels can shift.
Colorado's radon situation is not going away — geology is patient. But the tools to manage it are cheap, well-understood, and increasingly integrated with HVAC design. Every Colorado Bear replacement quote takes a quick look at your return-duct integrity, air-handler location, and combustion-air arrangement as part of the standard walk. If we see a radon-adjacent problem worth mentioning, we mention it.
Your HVAC and your radon aren't the same system — but they share the same house. Treat them like the connected problem they are.