Borderline of Balanced Home Ventilation: When Do I Need an ERV because Bath Fans Won't Work?

We almost never recommend that our clients go with Energy Recovery Ventilators (ERVs) for efficiency reasons- it always comes down to the air tightness of the home. At a certain point, bath fans all over the home stop being helpful and actually become a serious liability for indoor pollution, humidity control, and predictions of performance overall.
Let's dive into the free online calculator (www.redcalc.com/depressurizat...) that I use in most of my consultations, and how to evaluate the numbers it generates.
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[@jeremyjedynak]

Yet another great video! It would be interesting to see a video showing the differences in ventilation and H/AC mechanical equipment requirements for one specific house around 2k sqft at different blower door scores, perhaps starting at very leaky at 10 ACH50, then going to the various regional code minimums at 5 and 3, and then 1 and 0.1 at the extremely tight end.

[@MrCannonls]

Corbett. Thanks for the analysis! I knew academically about depressurization but we just did our first blower door test after the framing stage and blew 1.04 ACH. So glad we took your advice to do balanced ventilation and no exhaust units.

[@HomePerformance]

Good work Cannon

[@Xyrium]

This is interesting. I start thinking about the radon mitigation system that is pulling air from the French drain in our home. It must be pulling air out of the home in one way or another.

[@racerx1250]

I had an apartment in downtown Chicago where the bathroom was designed with the register coming out of the wall near the mirror. This prevented the mirror from fogging up. No bathroom fan installed.

[@D2O2]

A few nuance questions that will have a big impact on actual performance. Have you seen a PV curve for a bath fan? I haven't. I suspect the 50cfm rating is a zero static and the curve is very steep from there. So, likely not going to deppressurize to the values shown. Also, how much of your envelope leakage is from the bath fans/dryers/ vent hoods/ fireplace, etc. that dissappears when they are running? If you block all those leakage paths during a blowerdoor, then the ACH50 will be much higher during real conditions when they are open. Dampers on bath fans are only good for keeping you up at night when it's really windy out and they are slamming open a d closed.

[@davidarnold344]

Have you seen the aldes brand h/ERVs that have bath fan replacement units that allow for independent boosting of each room. It uses a air foil that keeps constant air volume when boosted at at the ERV and desired vent will bypass that foil so only it is boosted. Very genius. Never seen a review on these.

[@HomePerformance]

Have not seen those yet, I’ll reach out to Aldes again (for the 5th time).

[@2ndChanceAtLife]

I'm in Phoenix and it's impossible to get this "luxury" apartment down to less than 40% humidity indoors. I bathe in 2" of water because the shower steams over the mirror even when I only turned water on to get wet, then turned it off to soap up, and turned it on again to rinse off. Every single apartment I've lived in here has been like this. The home I owned/sold had a fan in the bathroom but it wasn't connected to any exhaust! I hope someday you have an opportunity to start educating the International Building Codes. This "sick" 480 square foot apartment is costing me close to $1,800/month. It's INSANE!!!!

[@petersallay5221]

Fantastic information. I recently had a blower test on my newly constructed home. The ACH 50 was 2.2. I have an ERV but it is integrated into the main ducts for my geothermal system. I tried to get a separately ducted ERV system but my contractor had no experience with this. I have separate bath fans and one low CFM exhaust for my induction stove. This information may give me a starting point to understand how the fans could effect my pressure. Thanks!

[@josephmerritt1411]

IMHO this is one of your best videos. You succinctly described and mathematically demonstrated the types of problems a new home built "airtight" could experience. Your discussion has made me feel better about adding a filtered fresh-air makeup register next to the electric clothes dryer. I will interlock the dryer with a damper and an inline axial fan similar to the Fantech kitchen hood makeup air system.

[@HomePerformance]

Hey, thanks for saying so Joseph! Glad to be getting more useful to you all.

[@adambergmann7716]

Hi there, thanks for sharing .Where is Ideal location for the bath fan ?

[@HomePerformance]

So glad you asked: Best Place to Put a Bath Exhaust Fan kzbin.info/www/bejne/nKLainmaqrR-lbM

[@5starCA]

I love what you do here, because nobody else dives into these details. I’m building a new tight ICF home in a dry high desert climate and have considering exactly this. I am looking 1) conventional ERV system feeding fresh air into two air handlers with ducted bath and laundry stale air returns, and 2) ductless wall penetrating Lunos e2 60 system with bath fans where if there is positive or negative pressure inside the home, the fans will stop blowing and simply let the pressure leak in or out. Obviously if this happens a lot it would reduce energy/heat recovery. Have you ever evaluated Lunos and do you see any pros or cons? Total cost, quiet, air quality are my goals.

[@HomePerformance]

Thanks 5starCA! Video forthcoming, but short answer is avoid the twinned ERV systems like Lunos in general.

[@sp1200M3D]

I like those ICF homes, congratulations. One thing you said sparked my interest, which was having returns in moist areas like bathrooms and laundry rooms. Maybe you meant exhaust fans, excuse me if I’m mistaken.

[@adambergmann7716]

Hi there, thanks for sharing .Where is Ideal location for the bath fan ?

[@mere_cat]

Thanks for sharing this. It finally “clicked” for me what all this ACH 50 stuff was all about. 😂

[@HomePerformance]

🙌🏽

[@brucecampbell6133]

Great comments/discussion.

[@racerx1250]

So is the depressurization in a home similar to depressurization in an aircraft? The airplane gets pressurized above 10,000 ft. The commercial jets have blowout panels in case of a depressurization event. I'm debating whether to install a bathroom fan in a very small bathroom about 50 sq ft. One of my concerns is just another access point for pests to come into the house. My background is i'm an electrical engineer with an A&P license, pest control background, framing background, and paint and body.

[@court2379]

You need to look at the fan curves. The 50cfm is at ambient pressure. Once the fan is fighting the pressure of the other fans their output will fall off. So that depressurization number is grossly over estimated because the fan output is over estimated. Doesn't change the risk of natural draft things not working though.

[@HomePerformance]

Sounds like we need a video test to prove how bath fans really work under pressure

[@stevengruner6435]

​@@HomePerformance the Panasonic whisper select fans ramp up with backpressure to ensure rated flow; they're really loud with the typical "slinky" exhaust ducting - almost silent with rigid pvc ducting.

[@smitm108]

Hi Corbett. We are in agreement that 31574.7 (vs. 31575) is more PRECISE but I question whether it is more ACCURATE ... 🙂

[@HomePerformance]

Haha point taken, but I think you know who I’m talking to with that

[@manuelmacias3]

Hi yay! To science in the house. Question, completely agree that make up air is needed to aliviate depressurization, but if the return for the ERV is the air from the shower (hot + humid) wouldn't that bring hotter and more humid air that what we want in the house?

[@HomePerformance]

Glad you asked: ERV vs. HRV Mythbusting: Which Ventilation is Safer for Moisture and Humidity? kzbin.info/www/bejne/h4DIZZ-iqqyqeck

[@daktusdurggery5505]

I need like a ventilation consultation, I want a Broan ai ERV but I can’t find where to purchase the model I need, seems can only get through. I’ve talked to a few local HVAC dealers who are up charging because they don’t know about ERV use in my area. They say to just use an intake fan. I have a spray foam sealed envelope with 18k BTU dual head mini split system

[@HomePerformance]

You can get it online direct from SupplyHouse or HVACdirect

[@gingerznz5742]

I like what you are saying, good to have the math as well. One question, isn't this a momentary pressure problem? The bathroom fan is going for 10 -15mins or are you expecting them to be constantly extracting at 50 cfm /25 L/s 24/7? I'm all about balanced airflows, for we install we balance the air with a filtered PPV system with a filter to control the source air. But it is never fully balanced because of people!

[@HomePerformance]

Perhaps momentary, but backdrafting chimneys is no joke, even for 10-15 min. And repeated wetting from humid air condensing in building cavities likewise.

[@gingerznz5742]

@@HomePerformance Very true,

[@joeshmoe7899]

Depressurization can be measured. Exhaust cfm is known. Blower test cfm can be computed by this online tool. Why do a blower test?

[@sirfishalot6992]

Do you use Wrightsoft or right-CAD at all? What is your go to man j software??? Thanks.

[@HomePerformance]

I use Sketchup to plot a 3D model, and then Wrightsoft (or Elite in some cases if I need it faster).

[@TagiukGold]

Why would a high efficiency home have a fireplace that uses inside air for combustion? I can see someone wanting that old fashioned fireplace feel, but drafting from the house for a new build doesn't make sense to me. My house is very air tight and so I use HRV for all the bathroom fans.

[@bahopik]

By saying you like heat pump dryers, do you actually live with one or you just like the concept?

[@HomePerformance]

Yes, we own one and we have a 20 min video about it

[@sp1200M3D]

I’ve been thinking about cracking the door of my laundry room to the garage when the dryer runs. I can’t measure it though.

[@HomePerformance]

Ooh, we don’t like garages. Lots of pollution sources out there. I’d recommend any other window in the house instead.

[@sp1200M3D]

@@HomePerformance great, thanks. It’s a gas dryer, does that make a difference on the amount of negative pressure it creates? Thanks for the help as always.

[@HomePerformance]

Not much testing on these, unfortunately. I’m working on something.

[@Iuliancik]

So what is the number that we need to look for in that calculator to know when to go for an ERV, 2 pascals depressurization? 1? 3?

[@HomePerformance]

That’s the whole video, Julian- it’s a system. If I say ‘2 Pa’ it’s only because I’ve answered s bunch of other questions first: -Where is the home, what climate? -How will the home be operated? -Who will live there? -What other equipment or features will interact with the pressures?

[@Iuliancik]

@@HomePerformance well yeah, for sure, and all the details are the builder or architects job of calculating, etc, but I was expecting a conclusion towards the end of the video, or an explanation to put things in perspective, 0.5-1 is normal 1 to 1.5 is pushing it in a humid climate, up to 2 is pushing it in a dry clean air climate. Over 2 you have to have make up air. And these are the consequences of not having it, like dryer not drying. And I mean this is an over simplification, but a lot of times people need layman explanations, and it also makes it easier for me to send this video to someone that is not into building science. Thank you for all your hard work, this is just a suggestion

[@HomePerformance]

Thanks Julian, I hear you, but this approach is fundamentally opposed to generalizations unfortunately.

[@stevengruner6435]

​@@HomePerformance OK then, -North East -raised ranch, finished basement -mini split+sealed combustion furnace -2 Panasonic 50cfm fans (2 baths) -1 exhausting clothes dryer (150cfm?) -1 200 cfm range hood 4 occupants 2.93ach50 (900cfm@-50Pa)

[@HomePerformance]

Happy to consult with you. You can book a time at: buildingperformanceworkshop.com/video-consulting

[@Ariccio123]

One thing I do kinda insist on, even if you have the directional airflow that you are excellent at designing to get low ERV exhaust rates in bathrooms, is to build them with the option to temporarily boost to 2x the normal ventilation rate with a manual switch. Anybody who's ever needed to wear a respirator to clean a bathroom with bleach will appreciate it! From experience, even 150 CFM in a small bathroom isn't enough to keep the odors and fumes of heavy duty bleach usage from drifting to the rest of the house. It's also an excellent option to have during a pandemic... You touched on this in one of your other videos, but it'd be a damn shame if we all didn't internalize that aspect this time!

[@maurozammarano6651]

150 cfm effective flow is definitely enough. A 150 fan might not be enough depending on house tightnes and fan installation.

[@Ariccio123]

@@maurozammarano6651 it sure *seems* like it *should be enough*, but if you're trying to clean a bathroom that's very moldy (hard water causes soap scum), sometimes you really need a ton of bleach, and the fumes will spill out into the house 🤮 In this context, I'm thinking of a bathroom that has a nice fancy Panasonic Whispergreen select that is supposed to actually flow 150 CFM even with higher static pressure than intended.

[@HomePerformance]

You guys, slow down, let’s plan for the bathroom not to have mold in the first place.

[@Ariccio123]

@@HomePerformance I'd like more tips if I can get them! This specific bathroom... It's a challenge because of the hard water combining with natural soaps to form a ton of scum... And also the tile being cooler than it should be because the contractors who built the place were drunk at the time (as evidenced by beer cans from the 80s buried in the insulation) and punched holes everywhere

[@HomePerformance]

Then all you can do is bandage the wound, AR. So many things wrong with that bathroom, the way to truly fix it is to truly fix it. Take down the drywall, airseal, insulate, put fan in proper place with straight duct, water soften, etc.

[@f.villanueva]

👍🏼👍🏼👍🏼

[@jetthardin4927]

Including the .7ft^3 is pointless. Sure your cad program makes it seem exact but the builders won't be. And if even one stud cavity is sealed enough to not have airflow it would change that. Should use sig fig to get an accurate measure of volume without the implication of such precision

[@HomePerformance]

Precision wins arguments was my point, Jet

[@mungs1104]

ERV's are the best way to go. But the payback is more than 10 years. They simply aren't cost effective for that vast majority of homes no matter how tight the home is.

[@HomePerformance]

Not sure you watched the whole vid, ashcam

[@caustinolino3687]

I'm curious, let's take for granted that the payback period for something is 12 years. That is 8.34% return per year. In almost every other aspect of life, this would be considered a great return. You can't get this high of a return on "safe" investments. So why is 12 years spoken of as a bad thing?

[@jetthardin4927]

I don't see the point of an eru for the makeup air. Your could run a return duct to outside air with a zone damper on it controlled by how many exhaust fans were running. Then you'd get cleaned conditioned makeup air to relieve the negative pressure without needing an extra piece of equipment. If your trying to sell them on energy efficiency the increase in cost of the eru will our way any energy recovery gains and it's their environmentaly concerned then the pollution from producing the eru will be way higher than that produced by the minimal energy efficiency gain. This seems like some snake oil to me. Conceptually interesting but the kind of crap you try to upsell high end customers on.