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Building envelope / HVAC retrofit for wildfire season

dsmyers | Posted in General Questions on

I’m in the Seattle, WA area (zone 4 / marine) and hoping to prevent my house from filling with smoke this wildfire season.

Are there any suggestions on how to upgrade our HVAC system to help here? We have a pretty conventional forced-air system; I was thinking of a combination of a HEPA bypass filter to eliminate smoke that gets in, plus some sort of a filtered fresh air intake to give us positive pressure and help keep smoke out.

I’d also be grateful for any recommendations on contractors in the Seattle area who can handle this work, as well as contractors who can help find and eliminate building envelope leaks to limit smoke infiltration.



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  1. Expert Member
    Dana Dorsett | | #1

    >Are there any suggestions on how to upgrade our HVAC system to help here?

    First, run some load numbers, and you're likely to find that the existing equipment is 3x oversized for the load. Replacing it with RIGHT SIZED and MODULATING equipment will result in lower cfm, which means lower duct velocity and lower static pressures. If you then install a 4" or 6" pleated filter it can go way up on the MERV scale without running out of static pressure, and if the filter is oversized for the system it will work even better. A MERV 13 or higher filter running at the duct velocities (or lower than) it was tested at it's MERV rating will BEAT it's filtration numbers, and remove nearly all the PM2.5.

    When oversized a MERV 10 filter running at the minimum cfm of a modulating right sized heat pump would have a low enough velocity at the filter to catch the lion's share, but being both oversized and deep- pleated also means an even higher MERV won't be too restrictive. (It also means you won't have to change the media very often.) The system being right-sized and modulating mean it will run nearly constantly even at low load, and at the lowest speed most of the time.

    Several in-line air filters with pleated media (opposed to cardboard framed filters) have media ratings available as high as MERV 16. They don't rate HEPA filters with the MERV system, but they will usually hit the MERV 17-20 range. The as-used performance of a MERV 16 filter will also hit that effectiveness when running at a fraction of the duct velocity used in the MERV test.

    More on filters: (read the discussion in the comments too)

    How to estimate the sizing correctly using wintertime fuel bills:

    Manual-J heating & cooling load calculation would be good too, but needs to be sanity checked against fuel use based numbers. If you're nerdly enough to want to run your own Manual-J, there are some OK freebie tools on the web such as LoadCalc(dot net) or CoolCalc (dot com). Be extra aggressive on the R-values and air tightness (of both duct and house) if you hope to come close. I've recently been playing around with the simplified load tool portion of HVAC.BetterBuiltNW(dot com) which seems to be spitting out realistic numbers, which has reasonably aggressive default U-factors and air leakage numbers (unlike the other freebie tools), but I haven't vetted it against pro tools (yet).

    If I remember correctly that BetterBuiltNW design tool kit was put out by the Northwest Energy Efficiency Alliance (NEEA: ) consortium as reasonable quick-design tool targeted at HVAC contractors for sizing ductless (and ducted) heat pumps correctly, but the load calc portion is dead-easy to use, and oversizes less than some other free tools.

    If you're NOT interested in DIY load tools but willing to share your ZIP code (for more accurate weather data) and some wintertime fuel billing dates and amounts, along with your thermostat settings I could run the fuel use based load calculation here on the forum. You would also need to look up the nameplate input & output BTUs of your existing system.

    If the current system is the typical insanely oversized system the ducts are probably oversized enough to even use ducted modulating mini-splits, which are ultra-quiet and lower cfm than most gas furnaces (by a LOT!)

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