With the goal of making structures more well sealed and a desire to know where our "makeup air" is coming from we are doing more amazing air sealing and adding tech, like HRVs and ERVs, to supply tempered & filtered air to these hermetically sealed spaces ... BUT, we're still installing high power bath fans (some that run 24/7) to ventilate not only the bathroom but sometimes the "whole house".

Is this the best we can do?
Is simply connecting a leg of the HRV to the bathroom the answer? Does it move enough air, is that what it was designed to do?

What are you guys and gals doing in the field (most specifically in retrofits) to address ventilation in client homes, evacuate warm moist air in a bathroom, especially in combustion homes where depressurization is an issue?

What do you do for Range hoods?
What about clothes dryers? We're exploring the possibility of condensing dryers in our weatherization program.

What about an installed (in wall) dehumidifier in the bathroom to replace a fan? Anybody try this yet? Would it be enough to keep the mirror clear?

Thanks for reading and let me know your thoughts.

Michael J.


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  • We know what the definition of "not going negative is..." but the definition of "good residential IAQ"  has and continues to be a subjective definition.  Even ASHRAE has danced around it.  EPA (mostly for political reasons) has stayed out of the definitions.   The Europeans do have ranges for what they consider to be good IAQ in homes -- and they are far tighter than anything proposed by even the most "concerned" states.

    Thus as long as we (US) can't seem to get our act together - using the IAQ for justifications of ERV/HRV are problematic and will continue to be for some time.   BTW,  I installed an ERV.   I've been building instrumentation (sensors) to measure AQ. 

    1)  In wall dehumidifier.  Ugh.  Interesting idea,  but long term this could be a home owners nightmare.  I have a NuTone  Audio System that was already installed in the house when I bought it.  Maintenance and repairs for most home owners are impossible.   The technology is obsolete.   Built in dehumidifiers to handle the bathroom humidity is a technology offering that simply kicks the problem down the road.  When they fail,  they won't be replaced (likely) and you'd be getting mold on the walls because there is inadequate bathroom ventilation -- not to mention dealing with the smells.  

    2) The value/benefit of condensing and heat pump dryers values to solve depressurization problems are being oversold!  While they are common in many European countries - they are not as wide spread as many think.  The use of clothes lines is still very common in Europe.  The use of electric drying bars in bathrooms are very common -- and the method to control the humidity - open the bathroom window.  It also solves the depressurization problems.   a) We can solve the depressurization problem with vented dryers in the US by changing their designs to direct vent dryers that bring in fresh filtered air while running.  b) Or we can move the laundry (washer/dryer and chemicals) into a mud room or space between the house and garage where air doesn't come from the climatized portion of the building. c) Or if an ERV/HRV that is smart and capable to adapt airflow to rebalance when dryer is running - that could solve the problem also.  (Currently I don't know of any really "smart" ERV/HRV systems that can dynamically balance air flow based on unvented appliances.)  

    3) Range hood ventilation issues are like that of an vented dryer.... but worse in the IAQ because people simply forget to turn them on.  Most kitchens also have windows in them and people simply forget to open the windows (in climates that can use natural ventilation).  Range hoods are a people problem as much as IAQ.  And it's really hard to sell people on the need to improve their IAQ if we don't have a uniform agreement (across the US) on what good IAQ is...  unfortunately that moves solving kitchen venting problems into almost a niche market -- for those that understand the issues.  FWIW - I have gas stove,  with label on the vent that says turn it on and open window.  My wife and I do so.    But then left discuss and out of the scope of the original posters question -- is what is the range capture range...

    4) I know installers capture air from the bathroom and feed through a hose to an HRV.  But I'd really worry about the run for that return line!   I've seen water build up in vent lines that sag and run through an attic area that in the winter is colder.  Eventually the line sags enough it fails.  The high moisture load on the HRV exchanger core may be more of a problem long term.  I don't know of any detailed study that has looked at the impact.  For much of the western US,  our climate zones are rated as "dry" and we want to retain moisture and try to keep the indoor RH slightly above 40% (less static electricity,  fewer sinus problems, etc).  In the midwest and eastern US the residential homes often already have one or more dehumidifiers installed in the house.  Move the air and the moisture moves also - even without waiting for the normal diffusion.

    5) Bathroom vent fans should either be put on a timer -- or better yet, just look at a "Dew Stop" humidistat that can be triggered when needed (smells) or it runs as needed to address humidity.  These devices could be updated so they could be paired with a "smart" ERV/HRV that changes the ventilation balance when a bathroom vent is running.

  • Great discussion topic - and great title! I have a few thoughts, 

    • First - is being negative so bad? All atmospheric appliances ought to go. As Sean suggests, direct vent or electrify. There are efficiency gains and carbon savings to be had. After all that is taken care of, maybe there is a wood stove at risk but those can be ducted with outside air and that's only an issue when you get very tight, <2 ACH50? Fireplaces = most susceptible to backdraft and ought to go as well.
    • Yes - depressurization will force air leakage - in our zone6 climate we prefer negative to positive to reduce driving forces into the enclosure. Yes - a H/ERV would be more efficient and reduce energy waste as well as provide fresh air to sleeping rooms which has cognitive health benefits.
    • New construction E/HRV's are no-brainers. Retro-fit's are tough value propositions.
    • Yes - H/ERV's can be thoughtfully ducted to remove air from a bathroom. Getting 50 cfm on boost in most installs is very difficult but it can be done. Getting 20 cfm continuous is pretty easy with thoughtful duct design. Is that enough? Maybe. I've found ERV's tend not to remove enough moisture for most tight homes to avoid condensation issues - HRV's do. Many builders have reverted to adding bath fans even when there is an E/HRV to ensure the mirror gets cleared.
      • keeping the bathroom door open a crack does wonders to keep the mirror clear
      • Sean, I'm curious as to why you say to supply and not exhaust from bathrooms?
    • No to the dehumidifier - less reliance on refrigerants which have very high CO2e when there are other options … but they probably work well.
    • Rangehoods (less than 400 cfm) are fine as intermittent pollutant exhaust ... because we eliminated any appliances that can be backdrafted.
    • Problem with some baths is they don't get enough supply air in so that cuts down on the amount that actually does get exhausted over time - well that & crappy ducting installs

      2nd if one goes with an ERV is the air mixing & getting pulled back in (smells) but in higher humidity climates not only are you pulling in humid air but also still keeping some of that humidity in instead of exhausted out

      3rd & yes I know some systems have dehu's & heaters inside of them but you can freeze up either type of core with moisture with most off the shelf units

      4th well if I can do a bath, surely I can do the kitchen... Sorry but for both these spaces, a dedicated exhaust is better

    • Matt,

      Thanks for taking the time to respond.

      The reason for not wanting negative pressure on the envelope for me, aside from the older combustion appliances (many times we replace), is that I find when doing IAQ testing, post air sealing and fan replacement, the indoor air quality suffers. I find that as hard as we try, air sealing wise, when the house is put into 'worst-case' we get nasty air from crawl spaces and/or attics. Particulate and VOC levels are many time off the chart.

      Yes, the HRV retrofit installs have been difficult and pricey. But, I have seen amazing results both conservation and IAQ wise. Many times IAQ is poor (and CO2 is at high levels) so with the addition of fresh, filtered & tempered air people's lives improve greatly. 


      • Hi Michael - yes - I couldn't agree more with the health benefits you describe. Definitely a reason to consider ducted HRV.

        I would also ask - at the cost of a ducted system - do you think the weatherization could go steps further to eliminating the source of the bad air? Leaks to attics can be eliminated (yes - you might have to vaccum what's there out for a clean slate) and crawlspaces can be encapsulated/weatherized as well. Even so, I do hear you and agree, CO2 levels will be high in bedrooms which will also show up as high VOCs and PM.

        • Matt,

          We air seal the ceiling and floor plane but, WX isn't quite there as far allowing us to vacuum the attic out or encapsulate the crawl ... but we're working on it. 

          Again, thanks for the input, brother.


          • Keep working on it! and Good luck to you!

  • Dehu in a bath - no you still need to remove odors

    HRV to bath - only if supply - yes I know some use them for exhaust but that can cause issues in some climates (ERV is definitely a no-no for exhaust)

    Range hoods - if over 400cfm a makeup supply is required and as I recall it has been discussed on here

    Clothes Dryers - condensing can work but really I would consider doing something like the range hoods if 200 cfm is really an issue which for most weatherization programs will never be one

    The best thing you can do is make sure naturally drafted or power vented appliances are not in the house & only use sealed combustion where the air is pulled from the outside or... look into Electrify Everything

    • Sean,

      Thank you for taking the time to give your input. I am grateful.


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