8″ passive air supply for 400 CFM 6″ range exhaust fan?
mikeysp
| Posted in General Questions on
Zone 4a.
Am I good to go with a 8″ passive air supply?
My exhaust fan is 400 CFM, 6″ duct, and it is a pretty tight house.
I has read that passive air supply is normally done by skipping one size which would mean a 10″ exhaust which I am hoping to avoid this but will do if it is needful.
My thinking: I am at the 400CFM that does not require makeup air according to code, but my build is pretty tight, so it seemed that a single step up to 8″ duct with a motorized damper and a merv 11 filter box, would probably be a good fit. I am dumping the makeup air on the leading edge of the range hood.
I would appreciate a sanity check.
Thanks. -Mike
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Replies
Do you have combustion appliances in the house? If not you don't need makeup air.
I'm generally skeptical of the need for makeup air. If you look at what are considered tight houses, in absolute terms they're not that tight. It's not like you have to close doors slowly to avoid popping your eardrums.
Let's say you have a house that's 1.0 ACH50, 3000 SF, 24,000 cubic feet volume. So at 1 ACH it's venting 400 CFM. That's at a pressure differential of 50 Pascal. That's about quarter inch of water. It's imperceptible without instruments.
Several years ago I designed a house about 2200 sf plus a full basement. I had spec'd 1.0 ACH50 but the builder got it down to 0.1 ACH50. The 600 cfm range hood or even the dryer operating at about 125 cfm would depressurize the house enough to make it hard to open the outswing exterior doors; it felt like someone was pulling on the door handle. And that was WITH the 4" passive air inlet I had spec'd.
Lessons learned? A tight house needs a way to balance the pressure between indoors and outdoors, and a small passive air inlet is not the way. Maybe an 8" version would have worked. Maybe 1.0 ACH50 would have been dramatically different than 0.1 ACH50. I actually can't recall now what the builder ended up doing but I think he added a powered makeup air fan, as I now spec on most projects. Even if you don't have combustion appliances and are not required to have makeup air by code, there are often concerns about radon infiltration, rain infiltration (I have a depressurization nightmare story on that topic!) or other unintended consequences.
Then again, I've designed or built plenty of projects with no discernable problems with depressurization. But when it's a problem, it can be a big problem.
I have my own horror story of a project that was a large (6000 sq.ft.) house with an 800 CFM exhaust hood that ended backdrafting the plumbing stacks. It was a perfect storm of a house on a septic system and kids on antibiotics, and it was MESS.
8" will probably work. The MERV 11 may be interesting-- go with a 3" filter to manage that friction. In the end, you could probably take out the filter if you have issues. If you don't really run the fan on high all the time, then it'll probably work well. The backup solution of just supplementing with some window operation (which is what I do in my galley kitchen) for those really intensive cooking sessions will be good.
Big picture-- perhaps find a hood (if you haven't already) with really good capture efficiency. the more it can look like a laboratory fume hood with a couple of pascals pressure difference at the aperature, the less CFM you need. Also, good placement of the makeup air will help too-- see Brett Singer's research at LBNL
I didn't put any makeup air in my house thinking that since the house is all electric and range hood is rated under 400cfm I won't need it. My house end up testing at 0.6ACH which is 95cfm for the volume in it. Dryer runs at about the same cfm rate. Entrance door in my house is 3'x7' which results in about 22lb of force on that door. that would be 10lb of force at the door handle. That is quite noticeable and I can hear air whooshing through the not so tight weather stripping. I wish i had put make up air provisions in but for now cracking a window open while running one of these two appliances does the trick.
It is all electric.
Sound like, I can get by with a whole lot less, like a 6" inlet. The motorized damper will operate when the range hood exhaust fan turns on.
Probably. The limiting factor would be the filter box anyways.
The reason for the MERV 11 is the massive amount of pollen that turns all things green here in middle TN. Perhaps it is not needed since it is coming in at the leading edge of the range hood and will be sucked out immediately? Another thought is to eliminate the makeup air and see what happens. Since this is a living quarters at the end of a shop building with a 13ft+ ceiling, I am doing a somewhat industrial look, so adding a makeup air system after the build would be quite easy. I just did not want the system messing with my Broan AI ERV and doing some sort of back draft or a negative pressure. Maybe I install without and then check pressure with my manometer to see how things look.
This is a timely post, as I was going to post a related question.
My house tested out at 1.1 ACH, and when I run the 300 cfm hood at max, not much is pulled into it. If I open a door or window when the hood is on, the hood is IMMEDIATELY more effective. This leads me to believe makeup air would improve the hood operation.
The builder installed provisions for makeup air, but never finished the system. There is a standalone 6" intake duct from the exterior that goes to a 6x10 ceiling return register located halfway across the house from the kitchen hood. In between, in the attic, is a motorized damper. There is also control wiring from the damper to the hood. In the paperwork I acquired, I found information on an AirCycler RangeRelief device. Apparently that was the plan to control the damper. I recently bought and am installing the RangeRelief device.
QUESTION: What is the best method to filter the incoming air? Like the OP, I cannot simply allow unfiltered pollen laden outside air to enter when the damper opens. The intake vent at the exterior only has an insect screen. The most direct way seems to be to replace the fixed ceiling return air grill with a filter grill. 2" is the deepest that would fit. What MERV rating filter can I use that would would not obstruct this system from supplying make up air?
Here is a house with passive 8" duct makeup air and a high MERV filter. It is where I go the idea for my range hood makeup air approach.
https://youtu.be/Gyph72_y8wQ
With an 18 x20 inline filter, it seems that duct system is (alot) larger than 8"...
Yes. My initial concern was about mounting the intake and the hole in the wall. I have plenty of space for a large filter box. Also, to account for long term cost, I planned to check pricing for bulk (6 pack) filters at filterbuy.com because certain sizes are cheaper per square inch. I assume the common sizes sell a lot more + a larger filter does not need changing as frequently.
I also have a Broan AI ERV that can handle a MERV 13, but proprietary stuff is $$, so I am going prefilter the Broan to use a common size that is larger than the filter in the ERV. It is much smaller, so probably something in the 12x12 14x14 for the ERV.
Mind you, I have not done my diligence on filterbuy.com filter quality yet. So, buyer beware. However, I remember the good prices I found on filters before my building science journey.
Although the main topic of this thread is range exhaust makeup air, I'm chiming in to reinforce that you are on the right track to also consider your ERV filter. Give more consideration to your ERV filter! After all, your ERV runs 24/7, your range makeup air is only used when the range is on. In many homes that is about 1-3 hours/week vs. 168 hrs/week for the ERV.
Also, I've found there is a huge difference between Filterbuy.com MERV 13 filters and others. I have three 20x20x1 MERV 13 filters in front of me, on of which is I think is a Filterbuy filter and there is a great difference in quantity of fabric used among the 3. The Filterbuy one does not have the most. Maybe purchase a few filters and compare them...
No the first time this type of MUA comes up, it doesn't work, don't do it:
https://www.greenbuildingadvisor.com/question/kitchen-exhaust-and-make-up-air-all-in-one-hood
Thank you Akos! That info alone saved me much grief. I am going to comment on the Matt Risinger video right now to look at that info here. While I owe Matt's video channel for setting me on my DIY Building Science journey, I am so grateful for folks like you who correct any bad information floating around.
I will look at the info on releasing the make-up air behind the stove when I get home.
mikeysp,
Mat Risinger seems like a good guy, and puts out a lot of valuable material, but it is sprinkled with a fair amount of poor advice. I think he is maybe just a bit too credulous about things people (and suppliers) tell him.
Mike, you might want to watch this episode of BS+Beer: https://youtu.be/OYyS-be3TA4. Ross, Ben and Kyle all know a lot about this topic.
If you are going with a passive 8" and filtration, then I'd highly recommend you install a fan as well. 10" is the recommended size for 400 CFM, but as soon as you put filtration (even a larger filter) in there you increase static resistance to the point where the makeup may only be flowing 150-200 CFM or less without a fan.
I'm using a 14x14x4 prefilter for my HRV setup and you are correct in that an external filter like that will have much less resistance than the Broan supplied filter. I'm seeing a drop at 90 CFM of 25-30 Pa (0.1 in.w.g) across a 14x14x4 MERV13 supplied by HVACquick with the filter box.
Dennis, maybe I use use a 6in intake with fan then and balance the pressure with speed controller once I have the system installed.
I had figured if I am on the edge of CFM to air leakage, any additional hole would suffice. I hate guessing. :)
Honestly, I need to read up on range hoods more. Perhaps I do not even need 400 CFM, if I put a better hood on. I will look for some articles this evening when I get home, before I lean forward. I don't have much time before I hang my drywall, but if I don't figure it out, I can just install the wall intake after drywall.
If I ever build again, I will definitely not touch a piece of building material until I have built the structure in CAD beforehand with every detail.
I've been working a lot over the last few months to measure effectiveness and differential pressure with a range hood that is only flowing 160 CFM (measured) at max setting in a heavily retrofitted, 100 year old home.
A six inch fan with an inline EC fan is pretty easy to control with a 0-10 volt dimmer, or via automation (which is how I do it). If you do that, you'll be able to dial in makeup air quite nicely. I'm providing about 60 CFM makeup air but doing it via an HRV with two ECM fans and speed controls on both...so running an asymetric balance profile which is triggered by the kitchen exhaust fan.
It's hard to guess without measuring but your 400 CFM exhaust may flow quite a bit less with ducting/vent, as it's likely rated at 0 static pressure. I've found that my exhaust fan (over an induction cook top) only needs to flow 110 CFM to capture effectively, but again, it's a modified hood with the stock fan removed and replaced with a remote inline ECM fan.
https://www.garagejournal.com/forum/threads/automate-kitchen-exhaust-hood-fan-using-inline-ecm-fan-turbocharge-yer-exhaust.491330/
Btw, I think makeup air in the hood can work fine, but I'd sort the slot width and use a make up supply fan so that air velocity (edit: see comments below..you likely want low velocity and careful consideration to design) would be higher using the same concept as an air curtain in an entry door.
I really appreciate your detailed notes, both here and in the other forum. I'd love to hear about your automation setup. Are you using a CT clamp to measure voltage, and that then kicks on the supply air? Or does the exhaust fan transmit a signal to the automation app / controller and that then kicks on a fan for the make-up air? Any thoughts about hooking the fan up to cut on automatically when the cooktop is engaged?
On the Terrabloom fan … if you were tackling this again would you go for the inline fan with the silencer? Or would you stick with the one you got?
Finally, do you have any sort of filtering screen on it to prevent grease from getting to the fan? Any concerns / evidence of buildup on the fan?
I would not use the Terrabloom inline silencer as it uses acoustic foam and would be a code fail for sure. You could 100% though use a fantech inline silencer as it is fire rated. If your fan is remote and say 10-15 ft away in the attic, you likely would be ok with noise levels with no silencer in play. Fantech's EC fans can be controlled with the 0-10 volt automation bits as well, just like the Terrabloom, or AC infinity one's I've been playing with.
Charlie, no CT clamp. I'm using an Aeotec ZW078 zWave 240 volt switch with power monitoring on the induction cook top. I've left the hood's grease screen (maybe 12 x 12) in place. I did have to replace a fan after 1 year (0-10 volt control stopped working so fan ran at 100% speed) and after a year in use, it showed about zero signs of grease on the blades. Terrabloom sent the replacement with zero hassle and was in my hands in 3-4 days.
I am using an inexpensive Hubitat C7 automation hub to manage all the vent stuff (entire home automation on one box actually). The setup goes like this:
1. ZW078 switch detects that the induction cooktop has turned on and starts sending power use updates to the Hubitat hub.
2. Hubitat Hub turns on the inline ECM Kitchen exhaust fan and sets speed to 100% for a minute to "pop" the ball in the Broan Ecovent (will stick a bit in sub zero temps). Then, the hub sets the kitchen inline exhaust fan speed from about 40% to 90% speed depending on power use by the induction cook top. 50% on the kitchen exhaust fan corresponds to a measured 110 CFM flow.
3. My HRV setup uses two ECM inline fans, again controlled by Hubitat. When the kitchen fan goes on, it ignores the house CO2 levels (which normally control HRV ventilation rate), and fires up a "make up air" profile where supply is about 110 CFM, and exhaust (stale air out) is about 50 CFM. Since doing all this, you'd be hard pressed to figure out the kitchen was in use at all, anywhere else in the house...and PM2.5 levels bump to just 6 ug/m3 (from a 2-3 ub/m3 average) during cooking on the main floor sensor.
So far the system is working extremely well. More details here on the HRV setup including early bench testing etc.
https://www.garagejournal.com/forum/threads/super-efficient-hrv-retrofit-with-smoke-filter-for-300.505161/
Amazing. Thank you so much for that detailed write-up! I love that you were able to create the unbalanced HRV profile to provide the make-up air. Also — even independent of the "ball pop" benefit — the 100% kickoff seems like a great way to "gather" air molecules and initialize the capturing flow so the air particles from the cooktop start heading in the right direction.
Will read through that extensive thread and get more familiar with Hubitat.
Thank you again!
No worries Charlie :-) I understand that my approach is not for everyone, however the Hubitat setup is pretty much key to minimising consumption in our home with respect to lighting, so it make sense that it would extend over to ventilation too. All of this has become a lot easier (and more importantly, reliable) in just the last few years. There are other automation hubs out there of course, but I've chosen Hubitat for now as it has good support, and runs everything locally (so no internet required). They also have a backup mechanism and free migration tool to get onto a new hub if required.
I look at this HRV control panel quite often (iOS app, or web browser) as in a glance I can check on sensible efficiency, wattage on HRV post heaters, air quality and CO2, etc.
I'm adding an Airthings device in shortly which will add VOC, Radon etc. into the picture.
That video Michael linked ( https://www.youtube.com/watch?v=OYyS-be3TA4 ) actually showcases the hood within a hood setup ( at 19:22 ) which was designed by one of the guest speakers...I think it's the NS builders version. Again, I see no issue with that as you can see that capture is good.
Dennis,
What do you think of the criticism of that design in the discussion Akos linked to in post #13?
Malcolm, see Michael's comment below ... I really like that hood in a hood design as I think with the correct setup, you could likely avoid pre-heating altogether. I see no reason why with a smart design that this would not work well. As Ross mentions, it's pretty standard in commercial hoods to have make up air delivered via a perforated slot as the front of the hood.
I also 100% completely agree with Ross Trethewey on his comments with regard to hood depth and size vs capture efficiency. I've taken some time to browse hoods at various outlets and have honestly been slightly aghast at some design considerations...like the flatter "designer" hoods. If you just fired up a few with a smoke pen, in some cases you would see the in-hood fan actually throwing air back into the room (at the periphery of the collector) simple due to edge turbulence on a cheap blade fan. Using an inline fan means all you see in the hood is suction. Deepen the hood and recess the grease filters as much as possible (or angle them as they discuss in the video) and you have a low velocity capture area where there is some time for collection. This in turn means you can extract with higher efficiency at lower CFM values.
Dennis,
I should have been more specific. The link Akos posted seems to show these designs don't work: https://s3.amazonaws.com/greenbuildingadvisor.s3.tauntoncloud.com/app/uploads/2020/01/16133334/Optimizing-Makeup-Air.pdf
Malcolm, that's an excellent article!
It highlights that some care must be taken on the "hood in hood" setup. I'd be tempted to start with the "Perforated Perimeter Supply " setup on page 8, taking the plenum around the sides as well. I think that is what Ross was referring to in the podcast. Contrary to what I said earlier they stress low velocity in pretty much every method and they provide good data to back that up.
It's also interesting that this paper suggests what Ben Bogie (again, from that podcast) prefers, which is makeup air supplied from behind or under the stove. My makeup air is supplied essentially from our "dining area" via the HRV system which was just easier as I already have filtration, heat exchange and post-heat in place on that system. At -25 C, you can't just dump cold air in.
That article highlights too that if you start with an efficient hood (deep hood, full coverage) your actual make up air needs can drop a lot.
Ross speaks of using exhaust when using the oven (which my system does not currently automate) which is interesting as I have seen elevated PM2.5 after oven use, and have been looking at various ways to integrate that without spending another $200 on an aeotec switch. I likely will do it just based on temperature as the wall oven shell heats up and fires up a thermostatic cooling fan. Our oven is separate from the induction cook top, which perhaps makes another good case for MUA delivered under the appliances which would help direct oven "effluent" during cooking to the kitchen hood.
Dennis,
I did a lot of work for a nearby resort which had a restaurant. The make-up air was located on the ceiling about six feet in front of the hood, and the air movement between the two was sufficient to cool all the food the chefs had just prepared and placed on their line. The owner's response was to close off the make-up air supply!
After that it was impossible to keep the swing door between the kitchen and dining area shut, and after a few months black streaks appeared on the walls around every opening and crack where the outside air infiltrated through the old, leaky walls.
I think a lot of the considerations that influence the location of make-up air in commercial settings probably just don't apply on much smaller residential set ups.
Look at the PDF I linked to in the previous discussion. The setup is exactly Fig 6. The description even says that this is not a recommended setup. A bit of steam and smoke does not show the real picture like a Schlieren image does.
All it does is create extra turbulence under the hood and reduce the effective flow rate of the vent hood as most of the air flow is now from the MUA. You can get same capture efficiency by turning off the MUA and running the exhaust fan at much lower speed.
Akos, it does highlight short circuit challenges, but does not rule out hood in hood (ish) designs altogether as in Figure 9. Lowering velocity of MUA at the hood, and considering adding in behind range MUA was my takeaway. They also specifically mention air density as quite important which means that in a cold climate like ours, make up air via the hood in hood (when it's -10 C) outside might actually work quite well.
It looks like these guys read that paper :-) https://www.captiveaire.com/CATALOGCONTENT/HOODS/SUPPLYPLENUMS/index.asp
You're 100% correct though in that if you do MUA outside the hood, that paper certainly suggests there is less room for error. As a general recommendation, I can see why folks would go there. That just makes me want to design a better hood with MUA integrated...ha.
I don't know if that is possible. If you look at the 150FPM velocity guideline a 36"x1.5" slot diffuser should only supply 56CFM. That isn't going to get you far with MUA even if you have a low flow hood.
That probably was an NS Builders project. Our guest was Ross Trethewey, a mechanical engineer and member of the This Old House team. You can see in the video that it's working, but Ross did mention several caveats.
Malcolm, in response to your post 33 (sorry the reply thing here is wonky), I saw the same black streaks on a room in my commercial building, but it was in a room that was designed to maintain negative pressure for painting and VOC management. It had a makeup air louvre system (motorized) that was intentionally undersized, and a large in wall exhaust fan in the 2000 CFM range. That was a brand new build so the streaks from the door frames and receptacles etc. was a good reminder of what happens (at the extreme) when a space is under negative pressure.
What I've learned from all the responses/research posted here is that the ideal residential scenario looks like multiple MUA supply points, at the lowest velocity possible. That's a hard task with in a cold climate while keeping costs low. I've noticed that since firing up the HRV make up air profile automation that the air quality (PM2.5) on our main floor has improved as MUA flow is from outside the kitchen, into the kitchen. I'm going to be resizing my HRV supply/exhaust ports to 6" on the exterior this summer which should allow me to get to 100 CFM MUA via the HRV system, with filtration and post heat already in place.
The information on the impact of oven use has me motivated to sort that via automation now as well :-)
Akos, or anyone who understands:
What are: ["low-flow, high-velocity air jets” along the perimeter of the hood.]
From the Design Guide for Optimizing Makeup Air you provided link to.
https://s3.amazonaws.com/greenbuildingadvisor.s3.tauntoncloud.com/app/uploads/2020/01/16133334/Optimizing-Makeup-Air.pdf
Reference paragraph from the document:
"Hood Geometry
Interior angles close to, or at, the capture edge of the hood improve C&C
performance, allowing reduced exhaust by directing effluent back towards the filters.
Hoods designed with these better geometric features require as much as 20% less
exhaust rate compared to hoods identical in size and shape without these features.
Capture and containment performance may also be enhanced with active “low-flow,
high-velocity air jets” along the perimeter of the hood."
It’s not clear by this statement if the “jets” are MUA at the periphery or ducted exhaust ports. High velocity but low volume MUA along the entire hood periphery is not something that paper tested.
Common sense says that you want the lowest pressure possible in the capture area of the hood, and short circuiting MUA evidently defeats this. If the “air curtain” though is low volume, but higher velocity, then that would be worth experimenting with in my book. You’d still need MUA somewhere else though to make up the volume target.
I think I found the part they are referring to. It looks like a fan with a jet nozzle installed inside the canopy:
https://www.halton.com/products/cjp-en_gb
Doesn't look like it has anything to with MUA.
Google did show some residential hoods with something similar(hood with a 2nd fan to feed an air curtain around the edge of the canopy) but they are older and can't find any data sheets or info on them. I'm guessing they were not a commercial success.
I have been thinking how to get the air to the back of my stove. It will be easy to get it there in my situation. However, since filtering the air seems important in my location, I am struggling with the passive (no fan) verses active (fan) solution.
If I use a fan, I will be blowing the air in, which seems like it would create an airstream behind my stove. if my fan is blowing through a 6" replacement duct that equals a duct area of 28.3 square inches. If I put a 30" wide x 1" (30 sq in) duct channel in the back of the stove, it is about the same square inches, so it won't slow the air down.
How do I determine the size of MERV 11 air filter I need to do a passive air supply? Surely, if I go large enough, CFM throughput is no longer an issue. I can use a very large filter if needed. 20x30x4?
How do I determine what I can reasonably expect through a passive system with a partially clogged (working conditions) air filter of a particular size?
At this point, I am glad to put any size hole in the wall.
I have read through all the links and comments.
I have not listened to the Beer & BS, but I will in a few minutes when I head back outside to get ready for drywall delivery tomorrow.
-Mike
Ok, let me simplify my larger question:
What FPM or MPH is acceptable for MUA behind a stove? If I have this number, I can work out a solution.
With a 400CFM hood, your solution would work well. Even in a well sealed house not all the airflow is supplied by the MUA so the velocity through the 36x1 duct should be pretty low. Also since the place is very large, I can't see a 400CFM hood depressurizing the house enough to nee MUA.
As someone with allergies, this is mostly me thinking out loud.
To get proper filtering in house, you need a HEPA filter somewhere. You can get a ducted HEPA unit (ie Fantech HS300 or Lifebreath TFP3000 HEPA) with a built in fan.
If you look at the install manual of the TFP3000 on P10 it shows how to hook it up to an ERV:
https://www.lifebreath.com/wp-content/uploads/2023/02/69-TFP3000-RTO_EN.pdf
The one change to their setup is to add a motorized damper to the breather T so if the blower of the filter box is off, the air from the ERV would still be pushed through the HEPA filter by the ERV blower. You only really want the filter fan to run during allergy season or if there is outdoor smoke.
You could also add another T and run the outdoor MUA to it. When the range is on, open the MUA damper and trigger the HEPA fan on. Assuming the ERV is running around 70CFM, this would supply about 200CFM of makeup air to the house which should be enough in most cases for a 400CFM hood. If you are in cold climate, a post heater might still be needed. Simpler is dumping it into an air handler return and use the HVAC to heat the MUA supply.
Mikey, I don't think you'll get a definitive answer on the passive/filtration/CFM questions as there are so many variations with respect to filters, even of the same size. You can always leave room for a fan in your setup, and try it static first to see what kind of flow you're getting, but I would go with a 10" duct to start, and go with the largest 2 or 4" filter that you can accommodate. That should give you the best chance for success in the fanless MUA department.
From the research reviewed so far, I don't see a problem with 400 CFM delivered behind the stove, as long as the exit point is 12-18" below the cook surface. I don't imagine pre-heat will be a requirement in TN :-) Keep in mind that the exhaust hood will not flow anywhere close to 400 CFM actual once ducting etc. is attached, so your MUA flow will be less than you think.
OK, when installing the behind the range air flow, I only saw one illustration, but common sense seems to dictate I have the opening for air entry facing down, so nothing obstructs it and grease or spoons don't ever fall into duct?
I have finalized my initial install. I will use 10" passive MUA input. Once installed, I will run some tests with my manometer and if the numbers dictate, I will install a fan and balance the system.
I guesss that will be the science, test, observe, modify if results dictate, repeat. :)
Thank you all for the great input.
I'd have the MUA exit pointing down if running behind, or exit under the appliance.
Test and post back your findings :-)
It will be a few months, but I will post my setup, results, changes.
How do I determine filter size?
I am definitely using the passive 10" MUA supply. I will add fan later if needed. I have purchased all hardware except filter box.
Is there a calculation or will I need to experiment with a manometer and a taped-on filter on temp plywood filter holding jig?