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Our basement has low-moderate radon levels, fluctuating typically between 1.5 - 4. We do have a subslab mitigation pipe, but it doesn't work great. For other air quality reasons, we decided to install an ERV in our basement. After the ERV is installed, the HVAC guys will "balance" the pressure in our system, to ensure that the ERV is not putting an excessive pressure on the system.

For example, if the ERV put a negative pressure differential into our basement, this would cause the basement air to essentially "suck" in new air from under the house, effectively increasing radon level. Therefore, it's standard to make sure the air pressure is balanced.

My question is - is it possible to actually do the opposite? Adjust the air pressure in my hvac system so that the basement floor is kept at a slight positive pressure? My logic here is that the positive pressure might act as a barrier to radon that otherwise would seep into the house through the basement subfloor and cracks and so forth. Is this possible/sensible, or do I just not understand what I'm talking about?

PS - I do understand there is mixed information about efficacy of ERV for radon mitigation.

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  • This comment would be a good addition to your question instead of a comment. – FreeMan Feb 10 at 15:49
  • When you pull air out of an enclosure (negative pressure) then the makeup air can come from any source, e.g. sub slab, sill gaps, HVAC, ..., you just want it removed . If you pressurize an enclosure (positive pressure) then you have chosen the opposite: force the air out through any means possible. In the case of radon, you want it out of the house (negative pressure), not out of the basement (positive pressure). – HABO Feb 10 at 16:24
  • Thanks @HABO - so you are suggesting to just have no pressure difference? – Adam Hughes Feb 10 at 16:41
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    No. If you want to remove it from the house then negative pressure with the exhaust being directed outside is the way to go. An ERV is a more efficient way to do that. Directing the incoming air from the ERV into the living space, so as to create a positive pressure there, will help to encourage flow from the basement to the outside world. (Or at least minimize flow from the basement into the living space.) – HABO Feb 10 at 16:44
  • Thanks - this is what I had in mind, and probably wasn't articulating it well. – Adam Hughes Feb 10 at 17:25
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Yes you can create a positive pressure and that may be a good thing as more of the air intakes will be on the upper floors. The difference between positive and negative is the amount of intake verses outlet vents. More vents pushing air than intake creates a positive pressure but you don’t want a large difference or the system will not heat well.

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  • Thanks Ed. So once a system is balanced, do you suggest not opening/closing vents during different seasons then? – Adam Hughes Feb 10 at 16:03
  • I do change the balance of flow 2x a year but you can still keep the basement positive in the summer as it needs very little cooling so very little intake it may require an additional intake damper to maintain positive pressure downstairs. That or a portion of the intake grill covered. – Ed Beal Feb 10 at 16:11
  • Ed - first let me say thanks for a always answering all my questions here. You are a hero. Any recommendations on how an average joe like me can know the balance of my system without professional tools? Like after these guys get it set, if I wanted to keep on it myself later... – Adam Hughes Feb 10 at 16:21
  • I use a hand held anemometer the pro ones are spendy true but a while back I picked one up as a backup it is quite accurate and only has the fan turbine not a hot wire but for checking flow rates it is great. I don’t remember the brand but a friend picked one up and said it’s battery’s lasted longer I have had it for 5-6 years and it has worked well. I just checked on line and I can’t find the one I have but there are some for 14$ With shipping. adding up the square inches and speeds you can calculate flow. Once I balance I don’t mess with the rooms but damper ducts not everyone puts them in. – Ed Beal Feb 10 at 16:38
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Ventilation/ pressurization based solutions can be advised, and often are in commercial design. In residential a reduction of 50% average Radon readings is usually considered an upper limit with HRV/ERV mitigation. Modifications to the recommended HRV install with some exhaust drawn from near the basement or crawlspace floor can improve this. If you have measured your intake/exhaust volumes and adjusted for slightly more fresh air in , say 15%, than indoor air out (and you don't want to T into to your HRV pipes to modify) then you've done all you can with your HRV. Volume is not pressure. Measuring airflow alone gives you zero idea of across slab or exterior wall pressure differential. This is the force behind soil gas, Radon, entry. Maintaining a small level of vacuum, just a few Pascal usually, under the slab through your Radon pipe should not take more than the installation of a small quiet fan and should yield 75 - 95% Radon reduction.

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  • Thanks for your answer. We have both, an newly installed ERV and a radon pipe. The fan on the radon pipe, when added a while back, didn't make much difference and we think maybe the pipe isn't properly installed (was part of initial home build). The ERV is installed and balanced and did improve our radon home-wide, so I probably won't mess with it for now. – Adam Hughes Feb 19 at 15:14

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