If I were you op, since this is supposed to a good weather week, I’d shut ac and see what that does to your reading. Keep record and measure every 4 hours since you have a continuous monitor. It should go down to old level if your ac is causing the problem

If your basement is unfinished, also consider putting DryLok down on the concrete slab everywhere accessible. Doing so often reduces the amount of radon that seeps up from below ground.

Modern houses that are airtight tend to cause a host of more health problems, via radon radiation, mold, negative air pressures affecting both and your health, etc.

Opening windows often is a good thing. Especially if you have granite counters, certain tiles, and basements.

https://www.radonmitigationofmd.com/about-timhughes.html

This guy installed my system a few years ago in moco but not sure if he serves DC. Wouldn’t hurt to reach out. Super nice guy.

Op here, thank you again for all these responses. We do have a super tight row house, no current radon remediation system (we never needed one before) and our HVAC is a Carrier Infinity Greenspeed that the HVAC contractor manually reset to run much higher than it was bc we wanted more airflow (10000-12000? I am not sure I remember the numbers correctly.) But I believe the unit is strong - they sized a larger basement return register when they installed it. The filter is an Aprilaire 20x25” whole home filter. As for a crack in the basement, I haven’t seen any. We are in NW DC and I would love any recommendations. It seems to me that going from a 1-4 to a 28-32 we have to revisit the airflow of the new HVAC system, as well as perform radon remediation. Thanks in advance

Where are you in DC area if you want recommendations??

negative pressure theory sounds possible, but I am having hard time that is the cause unless your house is super airtight, your basement is badly cracked, and your hvac fan is super strong.

did you have a current mitigation system (such as sub-slab depressurization)? If so, is it still functioning? Maybe it got turned off or the fan stopped working coincidentally around the same time as the work.

I vote for the something about your HVAC system is creating a negative pressure, drawing radon into your basement. Did the new system change your filter type? I believe if the filter is overly restrictive as far as air movement through it, it can cause pressure problems as the HVAC struggles to get enough airflow.

Radon level of ~30 is really high.

Op here. Thank you for these v helpful responses. There was a small pipe coming out of the old unit to the backyard and I don’t know if anything is coming out of it now (it doesn’t seem to be). The new basement return is larger than the old one and has a boot attached to it with tape around the joint that is falling off, could that be the leak I wonder? We adjusted the airflow settings on the heat pump to more closely mimic the old unit. We also did a home audit where they depressurized the whole house and saw some leakage from the ducts, and they are likely going to recommend replacing the flex duct in the attic. The home audit made the radon go from about 28 to 32. I would be grateful for any other HVAC suggestions, fine tuning or otherwise as to how we can reduce the radon in the house (in addition to the radon remediation. Thank you!

Anonymous wrote:The return air ducts leak or are likely not properly sealed. This negative pressure draws air from the soil beneath the foundation, which may contain high concentrations of radon gas in your area, into the home.

The continuous cycling of the heat pump compared to a regular furnace, can also alter the home's "stack effect" and infiltration patterns, which are known to influence radon entry.

NP. Above at least sounds plausible.

The return air ducts leak or are likely not properly sealed. This negative pressure draws air from the soil beneath the foundation, which may contain high concentrations of radon gas in your area, into the home.

The continuous cycling of the heat pump compared to a regular furnace, can also alter the home's "stack effect" and infiltration patterns, which are known to influence radon entry.

No. It's gas bubbling out of your concrete foundation and decaying rock below it. Occurs naturally.

Now, if ventilation in the basement was somehow changed from the old to new system, that's a possibility. Maybe they tightened up piping that was serving as a source of ventilation in the house, which they should have if it was found.

Ventilation with outside air and a basement that is not airtight is the solution.

We just upgraded our old HVAC to a top of the line variable heat pump that goes to gas heat after 35 degrees. We don’t like it, as it has to work much harder than our old system to feel moderately cool. But the latest is that our radon monitor (which we have continuously monitored with several different devices for 15 years) went from 1-4 to 29-32! Now we need radon remediation, but my question is: does anyone have any advice on who to work with to figure out what is wrong with the HVAC system? in addition, I would take any radon remediation recommendations for the DC area. I no longer trust the HVAC contractor, who was also super expensive. Thanks in advance.