The only blue collar EA.
I have a background in carpentry and HVAC. Co-own an exotic lumber store & furniture shop with a mill and kiln. I do building diagnostics and the technical aspects of design. My company has performed ~2k energy retrofits.
JesseSmith
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Do you have tested numbers? Going from high to low on 4 filter CR boxes shows a reduction in airflow in the range of 30%-40% and db reduction around ~13%.
Ah, sorry, the CR reference threw me off. That link seems to suggest those are in the 40 db range.
Duct design uses equivalent length as a metric across duct types and fittings with 1 foot of straight metal pipe as a ‘1’. The problem with flex is that each foot has an equivalent length of 1.5, which adds a moderate amount to system length (although to keep this in perspective many near air handler transitions are >100′). For something like this I’d probably do a short run of flex near the motor (both sides) to dampen motor/vibration noise and transition to metal beyond. Avoid cornering with flex! In general, remote fans, eliminating line of sight on the motor and oversizing ducts/terminations will be shockingly quiet to most room occupants. If you want to get super clever, you can start doing offset openings in walls by cutting grilles high on one side and low on the other, but it can be difficult to pull sufficient air while keeping velocity low (400 cfm = 6 stud bays).
Btw this might sound trivial, but for people with a central ducted HVAC system it’s quite common for the central fan to pressurize/depressurize rooms with closed doors, which in some cases will be the largest driver of infiltration. So simply having an AC or furnace cycle on and off during the experiment would be quite bad for the results.
It’s an OK experiment, but there could be other reasons for air exchange in the room that vary quite widely. Building infiltration would exchange air with outdoors and varies quite a bit with outdoor conditions. It’s also not very difficult to measure airflow so I don’t see many disadvantages to doing so
Are you limiting your noise estimates to strictly airflow/static pressure? The research I’m aware of suggests Corsi boxes perform at >40db and up to 60db on high speed (~350 cfm). https://housefresh.com/corsi-rosenthal-box-review . During the pandemic this was widely discussed as a major impediment to their adoption.
Upsizing terminations substantially reduces room-facing air velocity. Assuming 400 cfm, the velocity at the termination I link will be roughly 175 fpm. This is extremely low, and I’d expect to achieve 20-25 db. Most residential return registers are undersized and thus outside ACCA’s duct design standard of 500 fpm. It’s also less than standards widely used for quiet spaces (200-300 fpm). You could continue pursuing improvements here via upsizing further if you’re so inclined, or using clever placement to put the filtration system away from the room activities or behind (offset) cabinetry or furnishings.
My experience is mainly in central HVAC systems and I mostly agree that filters could have a much long service life. However, I’ve seen significantly shorter life via fine particulate via the use of tap water in humidifiers. Ime this will clog a MERV 13 filter in a few weeks. If you can afford a manometer (~$100) capable of reading increments of IWC ~.1, testing pressure is quite simple and many more people would benefit from adding this type of measurement to their skillet! I’m happy to provide further guidance!
Fan energy is a mixed bag depending on motor type. Constant airflow fans have a fairly flat pressure:CFM curve and will increase energy but PSC motors will see much lower airflow and reduced energy as a consequence.
In heat/AC systems airflow reductions outside a fairly narrow range will cause shutdown/failure due to high heat/freezing coils. In filter only systems you’d probably see low airflow and premature motor burnout. In standard motor assemblies going beyond .2 iwc increase would widely be considered quite aggressive (I have a fairly high risk tolerance for crazy stuff in buildings and probably wouldn’t do this)
I think to some extent the skepticism is warranted due to a mix of reduced airflow via restriction and the unknown q of bypassed airflow. Measuring airflow in a few locations with an inexpensive anemometer would be sufficient to allay these concerns, but using particulate as a proxy doesn’t seem quite as strong
Increasing pressure drop across a filter is probably the best proxy for whether a filter needs replacement. In the field, a new, high MERV filter will often have a measured pressure drop between .15 and .3 inches of water column. An increase of ~.5iwc is probably sufficient to indicate replacement. Where high merv filters are used, it’s not uncommon to have very fine particulate be the primary source of clogging which won’t be visible so visible dirt isn’t entirely reliable. It’s not uncommon for sophisticated control systems to automate this. It’s also fairly accessible as a diy tech.
Very similar to systems in the US! My sense is that many people have an ugh field wrt embracing basic trade knowledge, which is a significant impediment to even intermediate level projects. Glad you enjoyed the podcast!
Like you, I’m also somewhat surprised there hasn’t been more innovation here. I’m also disappointed by the current market for HEPA filters, and the widespread perception of HEPA supremacy. To some extent, part of the reason why we see low innovation may be connected to the problem of low HVAC competence, which I’ve written about previously
In my view there’s a point somewhere around a corsi box and possibly before where we’re probably better off embracing the HVAC trade and all of the tools and materials this encompasses. This probably means replacing tape with tin snips, and cardboard with sheet metal. These tools and materials exist for the express purpose of reducing time and expense, while delivering effective, aesthetically pleasing results. This should be done in a way that advances diy projects, but opens up an entirely new world of fans, ducts and filters of myriad stock sizes, each as separate components that can be placed together in a variety of configurations and installed virtually anywhere in a building.
As an example, vevor has a 6″, 400 cfm fan for ~$50. This is compact enough to be installed near the ceiling of a closet, which would greatly reduce motor noise in drawing air from/supplying to the adjacent room. Adding a sidewall filter grill and filter will run another $115, and we’d probably need some ductwork and a supply register. But at this point we probably have a permanent, quiet, discreet system for delivering >350 cfm cadr for <$250 in materials.
Interesting post! Is it possible that a partial explanation could be increases in cases via early or improved detection in some way?
My libertarian-ish/yimby sensibilities don’t love it. It also seems to already be a feature of many US states—eg I hold a NJ master HVAC license—and I don’t see it as have much benefit thus far.
Thanks!
Thanks!
Re 1) This is probably a factor, but I’d guess it would have low tractability and even if completely corrected would have limited impact. This was the basis for CheckMe that I mention in the post, and since then there are many technological innovations that make crosschecking really simple but have limited impact. For instance, if properly implemented, digital instruments integrated into platforms like measure quick would fix a ton of problems, but I don’t see much happening irl with this.
2) Exactly! Not sure on test, but operating within parameters doesn’t seem like a crazy ask.
3) Yes, that’s definitely on of the points I was trying to make. If we’re choosing between systems that have the theoretical capacity to work in a highly optimized way but are failure-prone and opaque vs systems that work sub-optimally but are readily verified and less failure-prone then I think we should choose the latter.
Thanks! Tbh I was pretty close to cutting that part due to superfluity so I’m happy to hear you found it insightful.
I agree that there are potential areas where drawing from a wide range of backgrounds could be better than the current structure. Tbc, I don’t want to overrate this and it’s likely highly domain specific. Long before this I started a piece on the widespread conflation of physical skills vs knowledge that characterizes much of the collapse recovery focus. This also also seems like a potential white collar silo phenomenon.
Seems right! I think some of the biggest opportunities are improvements to technology that anticipate and correct for downstream failure. To some extent sophisticated manufacturers already do this—eg constant airflow fans installed in restrictive duct systems tend to fail prematurely. Many of Mitsubishi’s air handlers now limit external static pressure to .8 inches of water column, I suspect in response to these high failure rates.
ctrl f “external static pressure”
Thanks so much for doing that!
Florida is a challenging environment. Right-sizing (manual J) of cooling equipment is especially important in humid climates. In over-sized systems, short run times tend to satisfy the thermostat but not properly dehumidify, because this takes much longer. In addition, Florida has many duct systems in unconditioned attics, and duct leakage draws humid air and contaminants from outdoors. If you have a system in an unconditioned attic this should be at minimum fastidiously sealed. Many people in building geek circles feel that conditioned attics are warranted, but my experience suggests that these aren’t often cost-effective retrofits: https://buildingscience.com/documents/building-science-insights-newsletters/bsi-119-conditioned-unconditioned
The final 2 considerations are 1) turning the AC fan to the lowest possible speed that can be sustained without freezing the indoor coil. This is generally ~325 cubic feet/minute per nominal ton of cooling. And 2) adding supplemental dehumidification. Target <55% relative humidity at ~75F. This can be a simple standalone dehumidifier piped directly to a sump or condensate pump.
For filtration, recommendations largely in line with pandemic seem fine for small particulate too—i.e. MERV 13 filters in centralized systems. IAQ monitors can be great tools as well!
Reach out directly if you need anything!
Bummer. Operating at 25 db would have been really impressive for a room filter but it looks like it’s not even close. Extrapolating from David Elfstrom’s airflow estimates it looks like ~1/2 cfm would put it >40 db, which isn’t great. So if we want to create a satisfactory occupant experience we probably shouldn’t put down the tin snips just yet.