How do you filter air when the electricity is low or non-existent?

In trying  to figure out how to power my CPAP machine in an extended electrical outage, I started thinking about what other essential, electrical gadgets I might need to run. Mostly I’ve been assuming that I’ll just make do with portable battery-driven devices for things like flashlights and solar powered power banks for my cell phone and tablet. But then I thought, it would be really good to be able to run my fan in hot weather. And… I thought, I might really need to run my air filter.

Here, in Northern California, we’ve been having power outages instigated by our power company. Most of these happen in fire season to, supposedly prevent wildfires from happening, but they do anyway. Really bad fires lately. So, it’s entirely possible to have a scenario where the electricity is out, but we’re supposed to keep our windows closed and are supposed to be filtering the air which is full of unhealthy particulates.

How do you filter the air if there’s no electricity?

I recently measured how much power my air filter uses and it’s kind of a power hog. It uses nearly as much as my CPAP, but, unlike a CPAP I would have to run the air filter either all day or at least for 12 or so hours a day. I can’t really afford a battery that keeps both my CPAP and my air filter running every day.

I’m wondering whether any of you out there have any suggestion for how to approach this problem.


  • Comments (3)

    • 2

      I could double-check the generator thread, but if the air filter is in the same vicinity as CPAP for total kwh consumption, you would probably have sufficient capacity for both with the larger yeti and 400 watts of solar input.

      Indoor air quality (IAQ) is an already super-complicated topic that gets even more complicated when you add spikes in outdoor contaminants and power loss. Individual circumstances are pretty different, so I’m going to keep comments broad, and you can add specifics related to your situation.

      What are the contaminants you’re trying to manage that require the air filter? Could you potentially lean less heavily on the filter by addressing other things? A good assessment of IAQ would include:

      1. Source control. Basically avoid bringing bad stuff into the house. If you’re not already familiar with it, check out the lumber liquidators formaldehyde controversy from a few years ago. In your case I would extend this to mean that you need a tight building enclosure to ensure that you minimize the quantity of outdoor contaminants entering the house. And for the love of god don’t let people wear shoes in your house!
      2. Local ventilation. Exhaust fans ducted to outdoors at kitchens and bathrooms. But you have to be careful with this when there are outdoor contaminants!!! When you turn on an exhaust fan the air that’s sucked out is replaced by air from outside. With tight building enclosures some people think the enclosure can have a filtering effect roughly equivalent to a MERV 10 filter, which might be OK in your case. But you want to make certain that the exhaust fans aren’t pulling in air that’s more contaminated than the air the outgoing air.
      3. A continuous supply of fresh air for the building occupants. Again this is very tricky when outdoor air quality is crappy. You can potentially filter the incoming air stream provided the particulate is big enough to be filtered by the HVAC filter. Some heat/enthalpy recovery ventilators (HRV/ERV) have filters, and central fan integrated supplies (CFIS) can be ducted to the filter on your existing heating or cooling system. IMO, the overall best system for most people is a CFIS connected to a variable speed furnace/ac fan with a MERV 14 filter. In tight enclosures this will slightly positively pressurize the house, which buffers against existing the infiltration of contaminants from the outside (including radon). But this can be expensive, so might be more of a long-term thing.

      Working from my memory of the evidence, air filters improve air quality, but not as much as high quality HVAC filters. So if you have ducted HVAC, adding a MERV 14 or better filter will likely have a higher impact than running the air filter. And running a variable speed fan at very low speeds will probably consume less energy than running an air filter (would guess 10-30 watts for VS fan on low). Otoh, connecting these to small-scale back-up power isn’t highly viable for a layperson. And the relative impact of either is low when contrasted with the impact of the other three strategies above.

      Depending on duration of poor outdoor air quality and advance notification you might try to ride out much of the outage with minimal filtration by turning over the air as much as possible in the house prior to the loss of outdoor air quality. Then allow for minimal air infiltration once the power is out and the outdoor air quality is poor. One of the major drivers for air infiltration is pressure problems associated with heating/cooling duct systems, so leave interior doors open to minimize infiltration driven by air equipment. Also, IAQ problems are often significantly driven by ducts located in unconditioned space (esp attics). If you have ducts in unconditioned spaces, at minimum you should meticulously seal them to ensure they can’t drive bad stuff into the home.

      Might be a bit kooky, but IIRC Cresson Kearny developed a manual air pump to turn over the air in the expedient fall-out shelters he developed. He claimed some really high cubic feet/minute numbers. I’ve never built one, but they look super basic, and could be potentially connected to a filter. I believe there are contemporary manual air pumps that you can also buy pretty inexpensively. Would need to connect to a duct and filter on incoming side. Might be an interesting diy weekend project. https://en.wikipedia.org/wiki/Kearny_air_pump

      • 2

        Let me try to make this a little simpler (for me anyway). My overriding reason for having an air filter is that I live in Northern California, about an hour or two away from where some of the worst wildfires have happened in the past few years. In two consecutive years, the air quality in my whole region during these fires was TERRIBLE. At one point the worst on the planet. I am actually not in the areas usually hit worst, but it’s bad enough! In 2017 there were many days when we weren’t even supposed to go outside, and you hardly wanted to because of all the smoke in the air.

        Additional complications: I live in a 100 year old house with lots of air leaks and I am a person who can’t bear to live for long stretches of time with windows closed. I particularly can’t sleep with a closed window. I have to open it at least a little.

        I got the air filter after the last round of fires thinking I really need to be prepared to hunker down in a room and keep the air as fresh as possible there. I haven’t used it for real yet (only to measure electrical load) so I don’t know how long I would need to run it or at what speed. I did a test of the electrical usage for the filter at its highest speed. It was 53 watts over the course of an hour (compared to 41 watts for my CPAP). I haven’t tried testing it at its lower settings.

        CPAP use and air filtration if the air is bad enough are neck in neck for me as first priorities in an extended power outage. If the air is bad enough I would take the air filtration over CPAP, although so far I’ve managed without it, so I guess it has to get pretty bad.

        The solar generator I’m eyeing at the moment is the Ecoflow Delta which has a total Wh rating of around 1300, so about 1000 of that is usable. It does charge very fast if you have it rigged up to enough solar panels — which I probably won’t have all of at the beginning because I can’t afford it. I could run both the CPAP and the air filter for some time on that power, and hope I can recharge it quickly in a day or so before I run out of watt hours. The problem with air filters is that you’re really supposed to run them for a big chunk of the time. Some people advise 12 hours, some 24/7!

        So, okay. If I get out of the technicalities, the basic question for me is, what do I do if there are wildfires, the air has gotten really bad, and the power goes out? This is actually a very realistic scenario since our power company has been shutting off the power at exactly such times. They tell people to stay inside and use their air filters and then they shut off the electricity! My specific house has never been affected by that, but it could be. In the past I’ve managed without air filtration partly by leaving the area (harder to do during COVID-19) and partly by just wearing a mask in the house part of the time and hoping for the best. That’s low tech, but doesn’t feel quite secure.

      • 1

        This is a tough one. To clarify, the air filter uses 53 watts of power (on high) continuously when it’s running, right? So if you ran it 24 hours/day, we would expect it to use about 1.3 kwh/day, right? If you could throttle this back to 12 hours/day you’d likely be fine with solar generator and 400 watts of solar also running cpap. Remember from the generator thread that on an average summer day 400 watts of solar will produce ~2 kwh. My cursory search suggests that being in not very close proximity to forest fires will diminish solar output by ~12%, which isn’t a huge hit. https://s.campbellsci.com/documents/us/miscellaneous/forest-fire-impact-on-solar-pv.pdf

        Other questions:

        1. How do you heat and cool your home?
        2. Any ducts in either an attic or foundation crawlspace?
        3. Do you leave windows open during the forest fire?
        4. House was built in 1920s? Stick-framed? What’s exterior finish? Ever had air leakage tested with a blower door?
        5. How long do the fires generally last?

        This is probably nothing ground-breaking, but worth a read: https://newscenter.lbl.gov/2019/10/25/improving-indoor-air-quality-during-wildfires/ Does’t solve the problem of power outage though.