I live on a hillside in Utah, and when I moved here 4 years ago during my divorce I was told by my home inspector that radon "wasn't much of an issue here in Utah" after I asked about it (since I had lived in Illinois where radon testing was required). Last Fall (2023) I read a book about Chernobyl and it piqued my curiosity about radon in my home, so I bought an AirThings radon tester. I was blown away how high the levels were- 30-60 pCi/l in my theater room in my fully-finished basement, and 20 in my walkout basement generally. Upstairs was 13-16ish. I panicked and called a few companies to mitigate, and I had one install a system in the middle of the home from the basement mechanical room up through the roof. They said I'd probably need a second system in my theater room, but I wanted to check the levels first. The basement levels came down to about 10 or so, and upstairs was about 8 or so. I saw how they did the one system, so I installed another system myself in the theater room (it was a lot of work but I now own more tools than before lol). The levels came down a bit more, but since I wasn't sure what was going on, I had the original radon company come out and measure pressures under my basement slab. It turns out my system in the theater room was getting the best (most negative) pressures under the slab- much better than under the professional install- but they attributed that to the likelihood of unsealed cold joints around the foundation in the main part of the home. So I've spent the last month sealing concrete cracks in the basement- and since the basement is finished I've had to cut out drywall and baseboards and pull up almost all the carpet, and I even pulled out a kitchenette from the basement and sealed behind that. My house looks like a tornado hit the basement. I don't know if I've sealed ALL the cold joints, but despite sealing as many as I can so far, it's had a negligible effect on the radon levels. Subsequently I had a plumber run some 3" sch40 PVC to the hallway on the other side of the basement from the main line that was professionally installed, and I drilled another hole in the slab to tie into the original professionally-installed system since they put in a pretty big fan (AGM Festa Legend Extreme). Since I installed this other pit my levels dropped to around 2-3 in the basement and 1-2 upstairs- but ONLY while I was out of town and my HVAC was turned off (or actually on ECO on my Nest). I was gone for about 4 days and my newer Airthings shows a graph of a nice green line while I was away. As soon as I got home from my trip the levels downstairs went up to 3-4 and upstairs 2-3 (angry yellow and red lines). I know this is much better than originally- but I’m super confused why the levels definitely rise when I’m running my York high-efficiency HVAC system.

TLDR: Why are my levels still marginally high despite having THREE pits with active fans, and why do the levels go up when my HVAC is running?

Pertinent facts (at least I think they are):

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1. The home is about 4800 square feet total- so I’m guessing the footprint is about 2400 sq/ft.
2. Building plans show LOTS of footings in the basement- both bedrooms, theater room, main area, and cold storage are completely surrounded by footings
3. There is a lot of wind that comes up the hill to my house. I’m told this is bad for radon levels.
4. I have three HVAC systems (IMO this is ridiculous- I bought the home but didn’t understand why it needs three)- all are high-efficiency York systems. The levels definitely go up when the HVAC is running. Would a blower door test be best to diagnose this issue?
5. Under the slab there is gravel
6. The three pits are served by two fans- the fan serving two pits is the AGM Festa Legend Extreme, with a manometer reading of 1.5” before adding the second pit, and 0.5” after adding the second pit. The theater room pit is an AGM Maverick, and it is reading at 1.5” water.