On the west coasts of Canada (BC) we've had insane forest fires the past few years and you 100% want to filter the internal air when it's extremely unhealthy outside. We're talking air quality index in the 300+ range or off the charts at times. Plus, we've had these heat domes were it's in the high 30+ and just gross inside, where you really want to open the windows, but you're letting in tons of pollution. Basically, you need to cool off and filter the air at the same time.

[1] https://www.airnow.gov/aqi/aqi-basics/

I have a routine for this scenario: open the windows in the morning for 1-2 hours, then close everything and clean the air internally (I have a 24/7 sensor running and know when the air is good or bad).

That seemed like the easiest thing to do without making much more complicated system. Basically risk 1hour of bad air for 23h of "clean" air.

This. We live <50’ from an expressway, so after air sealing and replacing all of our ducts, I added a MERV 16 filter to our air handler. We had the windows open at night a few years back (no ac as well), and I woke up to smell of smoke from a forest fire after the winds changed direction. AQI2.5 was 200+ inside. Closed windows, ran the fan on high and after a couple minutes the AQI throughout the house was within healthy range (tested multiple locations). Unfortunately our furnace model leaves few realistic options for controlling it programmatically, so I still have to manually turn on the fan when AQI drops.

Where I live, it can get very cold. Not always very efficient to open windows for 5 months out of the year.

A great option for keeping CO2 levels down in a house is with an HRV (or ERV) [1] that will heat the fresh air coming in to cycle it throughout the house.

[1] https://en.wikipedia.org/wiki/Heat_recovery_ventilation

There is an open source 3D printable unit design here which I installed in my window and works well for relatively efficiently exhausting the CO2 from one person in cold weather: https://www.openerv.org (license: CC BY-NC-SA 3.0)

I did not 3D print mine, but ordered it, though as an early adopter, adjustment was required.

Do you know, is there a document explaining the design somewhere? I can see from the Google images some of the parts, but not clearly all the parts nor the flowpaths and such.

From the ERV/HRV (Energy Recovery Ventilation / Heat Recovery Ventilation) wikipedia page: https://en.wikipedia.org/wiki/Energy_recovery_ventilation#Ty... :

> During the warmer seasons, an ERV system pre-cools and dehumidifies; During cooler seasons the system humidifies and pre-heats.[1] An ERV system helps HVAC design meet ventilation and energy standards (e.g., ASHRAE), improves indoor air quality and reduces total HVAC equipment capacity, thereby reducing energy consumption.

> ERV systems enable an HVAC system to maintain a 40-50% indoor relative humidity, essentially in all conditions. ERV's must use power for a blower to overcome the pressure drop in the system, hence incurring a slight energy demand.

In Jan 2023, the ERV wikipedia article has a 'Table of Energy recovery devices by Types of transfer supported': Total and Sensible :

> [ Total & Sensible transfer: Rotary enthalpy wheel, Fixed Plate ]

> [ Sensible transfer only: Heat pipe, Run around coil, Thermosiphon, Twin Towers ]

Latent heat: https://en.wikipedia.org/wiki/Latent_heat :

> In contrast to latent heat, sensible heat is energy transferred as heat, with a resultant temperature change in a body.

Sensible heat: https://en.wikipedia.org/wiki/Sensible_heat

There's a broader Category:Energy_recovery page which includes heat pumps and thus mini-splits: https://en.wikipedia.org/wiki/Category:Energy_recovery

IME you don't use an ERV alone. You'd use it _with_ a Heat Pump. The ERV is all about transferring heat from one airstream to another. It's _not_ a device that manages indoor temperatures though, just recovers some of the latent energy in the air. In the process it's also managing humidity mainly as a by-product. The humidification properties of the ERV allow you to run the Heat Pump in a more efficient manner. I have an HVAC geek friend who explained the whole process, but essentially (in non physics/fluid dynamics[?] terminology), if the Heat Pump doesn't need to dehumidify air it can operate more efficiently.

Not to be snarky, but if the weather outdoors is uncomfortable enough to be using climate control, why would I want to open the window?

Because presumably in this case you would want to open a window for fresh air, and simultaneously run your heating system to heat up that fresh air?

I was respond to a comment that presupposes the need for fresh air. If you don't need that, feel free to close the windows then.

Airborne allergens can be an issue, and dust more generally.

Seems like it would be more efficient to wear a climate controlled bubble suit than heating up entire rooms. And you could even go outside when it is cold or hot or dusty.

You do realize that “open the window and hear that air” is massively energy inefficient, right?

I don't know; is it more energy inefficient than having a fresh air intake via a duct? Maybe they are comparable in the absence of a heat recovery ventilation system (mentioned by a sibling comment upthread)?

You don't necessarily need to take in fresh air to freshen air in certain ways. It might suffice to pass it though a good filter.

HEPA and carbon filters do wonders for cooking smells, particulates etc, but unfortunately do nothing for the main reason you want air exchange with outside, which is CO2 buildup.

Our house has a heat exchanger that exchanges air. So the outgoing air changes the temperature of the incoming air.

You want to open windows in opposite sides of the house to quickly circulate air from outside to inside. 5 minutes should be enough. It will get colder, but most of the heat is in walls and furnitures and as long as you only open the windows for a few minutes it should be quick to heat up the air again.

For the fresh air that this thread is about, regardless of whether it comes through a glazed window or a duct connected directly to AC system?

Because it smells bad inside? Because fresh air feels good? Not to be snarky, but how did you miss these use cases?

If it's pretty cold outside, then you're throwing all of the ostensible energy savings of a heat pump out the window the minute you open it to air out the house.

Following the thread of conversation, it just didn't compute to me:

>>>> Heat pumps are great for climate control >>> Yeah, ducts are dead >> What if you want to recirculate air in your house through your central air filter to eliminate smells? > Just open the window

It's like we've hit a contraction: the premise is that we care about energy but the contractions is then that we don't and we open the window while climate controlling the house. So to me it does seem to prove that in some climates, duct work with a central blower and filter mat not be dead.

For an existing dwelling, the best solution is probably mini splits (wall mounted) combined with decentralised heat recovery ventilation as required.

Ducts running in an insulated space are sources of huge energy loss.

*uninsulated

Yes correct! Thank you!

I think there's a middle ground.

We've just today had our first snow of the season here in a lower elevation of the Sierra Foothills. It's been chilly for 3 months or so, and our heating is an 'old school' ducted propane furnace. In time, we'll replace it with a heat pump, but not this year. Anyway, we're sensitive to accumulated odors that go with a well insulated, closed-up home in winter.

Every evening, we open three doors in the house to the outside. This is after the furnace has entered its timed 'off for the night' state. We exchange pretty much all of our air for fresh ambient, which is great when we wake up in the morning.

The impact from doing this on our propane bill is undetectable. This is because air, even humid air, has a trivial heat capacity compared to the warm house structure and contents. Those are by far the greatest energy reservoir in our home. Very little energy is lost in a daily air exchange with the ambient.

*edited for typo

I'm not sure why "Heat pumps are great for climate control" went to "ducts are dead".

I live in Florida, we've used Heat Pumps for as long as I can remember. We also have central air handlers with blowers and ducts to distribute the conditioned air. Mini-splits can _also_ be ducted mini-splits. According to my HVAC geek friend, mini-splits are pretty terrible about humidity control (an important thing in Florida). For proper humidity control you'd ideally have a dedicated set of dehumidification ducts (powered by a central dehumidifier) as well. Mix in an ERV and you have the ability to build a fairly air-tight house with _controlled_ ventilation and very efficient conditioning of the air in the house.

The Melbourne climate I was referring to needs heating 9 months of the year. The vast majority of houses have very simple, inefficient gas ducted heating. Rightly or wrongly, humidity control is generally not well considered.

I was going to install an ERV system but the payback was not within the life of the equipment.

Someday I’ll build a Passivhaus with a system as you describe.

No doubt: majority of the southern US went to heatpumps with central air handlers 2-3 decades ago. Ducts are definitely not dead. Or, if they're dead, then they must be as dead as BSD. :-P

A ventilation system allows you to filter (not a luxury when you live in a big city, where you will be breathing pollutants otherwise) or perform some thermal magic if the outside air is very cold (or very warm), by running the outflow and inflow pipes really close to each other.

Which CO2 detector do you have?

The one recommended by SwiftOnSecurity years ago. https://twitter.com/swiftonsecurity/status/12230876092216115...

Part of it is code. Make-up air systems need a duct for example. Also a lot of people like their air filtered so opening a window isn’t great for that. Also winter.

I used to have a bathroom without an extractor and in the winter with the window open the cold air would just cool the walls and cause more condensation there than I’d get with it closed.

On cold days you should open not the windows longer than 5-10 minutes at a time for exactly that reason.

I have a bathroom without an extractor. If I don't open the window the mirror fogs up in seconds of turning the shower on, and over time I get mould. If I do open the window it stays clear the entire time. By the time I'm out and dry the moist air has left and I close the window.

This morning it was -5C, so not cold by continental standards, but certainly cold enough.

In the UK recirculating air is very rare. In cases that bathrooms don't have a window (or indeed in new houses where they do) there's an extractor fan, but that just vents the air directly outside.

Allergies, humidity, heat, cold, rain, noise, privacy, and the hassle of doing it manually.

digression: Any recommendations for a good CO2 detector?

TFA Dostmann AirCO2ntrol Mini is a relatively cheap one that works well.

AutoPilot APCEM2

The only requirement is "2 channel low drift NDIR gas sensor". Cheap CO2 detectors either have zero IR sensors (instead they guesstimate eCO2 using a VOC sensor) or a single IR sensor that requires weekly calibration outdoors or it will just assume that the lowest sensor reading means 400ppm.