Residential ventilation is really a tricky topic. But if you’re looking for a practical, cost-effective, holistic solution, go somewhere else. This post offers none.
Hopefully I can dig into practical solutions in future posts, but I think it’s important to be clear about why we ventilate and what an “ideal” ventilation system might look like in a new, efficient home. My ideal system is similar for both single-family or multi-family (though practical issues can be very, very different).
Purpose of ventilation: Remove contaminants that can compromise health, comfort, productivity, durability, etc. I’m sure there are more rigorous definitions out there, but this will work for now. There are other ways to lower contaminant levels:
- Emitting fewer contaminants from materials and activities is obviously good. Do this.
- Actively filtering, adsorbing, or otherwise removing contaminants from indoor air can also be good. There’s talk about doing more of this, but I’m tabling it for this discussion. This may be something to keep an eye on down the road.
For most new residential buildings, mechanical ventilation is still be the primary means to remove contaminants. Or at least it’s the primary method that designers/developers need to plan for now.
If building a new, efficient home in Shangri-La, my ideal ventilation systems would look like this:
Each bathroom would have an exhaust fan that draws 80 cfm or so. For decadent bathrooms with multiple showerheads, big whirlpools, etc., you may want more, but for most of us I think 80 cfm is fine. These bathroom fans would remove moisture, odors, etc. directly to outside. They’d have some control so that they’d run for several minutes after a shower, or perhaps they’d make use of humidity sensors. Occupants could always turn on fans when they want, but in a perfect world fans would come on automatically when they “should” (e.g. lots of moisture in a bathroom).
Kitchens would have hoods over ranges with good capture efficiency that exhaust 200-250 cfm directly to outdoors – in keeping with LBNL findings and advice. Because we’re talking about new, efficient, air-tight homes/apartments, you can’t just suck 200-250 cfm without providing makeup air. This would probably require a separate duct to outdoors with a damper, a fan, and an electric resistance element to temper the air when it’s very cold outside. As we’re still in Shangri-La, this heating element would only operate when outdoor temps really are cold (<40°F or so) and it would modulate to maintain discharge temperature of so that electricity wouldn’t be wasted. The range hood (and makeup air system) would activate automatically when heat and/or cooking contaminants are detected above the range.
That’s the key local ventilation, but you still want some continuous ventilation to dilute contaminants throughout the home. For this you want an ERV or HRV (depends on several factors, but I’d probably lean towards an ERV in most applications when we’re exhausting moisture from bathrooms and kitchens separately). You want an H/ERV with low power consumption and good heat recovery effectiveness (at least 80% sensible, 60% total).
How much air flow should this H/ERV deliver? At a minimum, it would provide what’s required by ASHRAE 62.2 (I’ll leave the debate over which version aside for now). But in our ideal home we also want to be able to boost this rate by at least 2 times – maybe 3 times (if we have a party, or burn something in the kitchen, or whatever). ASHRAE folks are clear that 62.2 numbers are minimum values, and we want occupants to be able to ventilate more if they want/need to. Even better, there’ll be a control system that senses concentrations of key contaminants and can boost the ventilation rate automatically. If we’re in a multi-family building, each apartment will still have its own H/ERV so there are no issues balancing flow rates across dozens or hundreds of apartments.
Fresh air from an H/ERV would get distributed around the home by ducting or some other mechanical mixing system. If local exhaust is taken care of, I don’t think it makes much difference where you exhaust and supply. It really doesn’t take much air flow to equalize contaminant levels throughout a home.
For all ventilation systems, fans would be powered by very efficient, variable-speed motors. All systems could be boosted or manually operated by occupants as needed.
So there it is: a quick summary of my ideal ventilation system …if money was no object and I could ignore a host of practical issues. I don’t see systems like this often; when I do they’re typically in custom, high-end homes.
If you were reading this and becoming more and more incredulous, I’ve imagined some typical responses below (actually, I didn’t really need any imagination).
“You know how much this will cost? You want me to pay for dedicated kitchen and bathroom exhaust PLUS a fully ducted ERV?! Plus a make-up air system?! …in every apartment!?!”
2. Wall penetrations
“You know how many wall penetrations you’re talking about?
- 1 for each bath fan
- 1 for the kitchen hood
- 1 for the makeup air
- 2 for the ERV
That’s 5 holes if there’s only one bathroom! For an apartment there’s not even remotely enough space to run these ducts to the outside wall! And it would look horrible! And it’s not even possible to keep the clearances from inlets & exhaust terminals we need for code compliance!”
3. Wall penetrations (#2)
“You know how much air leakage all those wall penetrations will let in?!? We’ll never meet our infiltration targets for Passive House!”
“You want me to get into every apartment 2-4 times/year and replace ERV filters!?! And how am I supposed to make sure the outdoor air inlets (for ERV, makeup air) don’t get clogged? …in a 20 story building?”
“You expect me to find room to fit all this equipment!? The ERV, all the extra ducting, the makeup air?”
Those are probably the biggest challenges I hear about. And the answer to all questions is: No – I don’t expect my “ideal” ventilation system to become standard anytime soon. It’s not practical/possible for the reasons above and more. Many of the ideal controls I talk about aren’t readily available. But – from IAQ and energy perspectives – it’s what I’d shoot for if designing a new home for myself. And I think it’s useful to keep an “ideal” in mind when designing something realistic.
Hopefully more to come on some practical systems/solutions, but I’m definitely interested in others’ thoughts on “ideal” systems. And mentioning Shangri-La makes me want to read Lost Horizon again. I recall it being a good read, but it’s been a long time…. Or I could just listen to Kashmir. Definitely good.
By Robb Aldrich, Principal Mechanical Engineer