Not being snarky but why don't you just open the window for that?

I have a CO2 detector that I believe is a reasonable proxy for stale air. When it goes above 1000 I simply open the windows. By the time I remember to close the windows the reading is almost always below 500.

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.

They're obviously no replacement for exhaust fans in bathrooms and kitchens, but there's a number of ductless energy recovery ventilators that can give you fresh air in your bedroom(s).

https://www.buildwithrise.com/stories/ductless-heat-recovery

Thank you - this could solve a problem in my house. We had CO2 build up in our bedrooms and the only way I have found to ameliorate the problem is to have our HVAC fan running continuously at night to circulate the air in our house.

Agree, ducts are wonderful! If you don't have ducts, go ductless for sure.

I paid real money to have my ducts removed. They cause issues installing comprehensive insulation and caused hot/cold spots on the ceiling.

Honest question: Why would one care about cold or hot spots on the ceiling? I don't spend much time up there.

It’s still a thermal path between inside and outside, and thus wastes energy if you’re trying to maintain an internal temperature that is different from the external temperature. Also there are radiative effects - hot spots on ceiling will radiate IR into your space. Just try standing under a corrugated steel roof in the summer.

Approximately 70% of the heat loss from your body is as radiated heat. Ceiling insulation is critical for thermal comfort. Have a look at the Efficiency Matrix YouTube channel videos on insulation consistency.

The average temperature sure. But would uneven spots have that much effect on the thermal radiation I get sitting over a meter away?

Less that 2% of insulation being out of place can waste something like 40% or some ridiculous amount of energy. It's quite nuts how even just a little break in insulation can have a significantly negative effect. A few years after I moved into my first home - a townhouse - I added some R-16 bats to the already existing R-12 insulation in the attic and it more than cut my heating bill in half. It was a three story narrow, townhouse - not that much square footage of ceiling compared to the walls/rest of the house but it made ALL the difference. If I didn't live it, I wouldn't have believed it. And I kind of did it on a whim since I didn't need that many and Lowes was having a sale on insulation. I would have done it the first week I moved in and had a couple far more comfortable winters!

Okay but that's not what I was asking. That much heat loss will affect the overall temperature.

He did answer what you asked, but I think you may be losing sight of the forest for the trees here. Maybe re-phrasing will help?

They explained how something could cause uneven spots and low average, as far as I can tell. Because those spots leak so much.

But I want to know if there is a way uneven spots could cause a problem without some effect tanking the average. What do uneven spots themselves do?

The uneven spots may indicate the presence of leaks. The leaks can have an effect on the average temperature.

Hot air rises and thus heat escapes through the ceiling if there's spots that aren't well insulated.

Because in the winter the thermal energy within your room will “flow” out of those “cold spots” at surprisingly high rates; in summer those spots act as space heaters. In both scenarios they will adversely affect the ambient temp of the room. In short, whatever method you use to heat/cool the room will have to work harder and will take longer. Here’s [0] a good overview of heat flow in this context, with some hints towards the math behind if you wish to go deeper.

[edit: add link]

[0] https://www.energyvanguard.com/blog/Flat-or-Lumpy-How-Would-...

I might be totally misled, but I think of it as a parallel resister circuit. One area of low resistance is extremely significant.

Am I crazy, or are those two sentences completely contradictory?

I think he means having a duct work for the whole house for a central heat pump is good. But having a heat pump per zone is also good if you don't have ducts for the whole house. Some newer home designs separate heating and cooling from the air exchange systems so there are multiple zones of heating and cooling.

I thought it was a joke. If you don't have ducts, you are by definition ductless. So that's how you must go.

> So that's how you must go.

No, you can go to ducts by installing them. I believe the comment is advising against doing so.

I read it as: Ducts are better but not so much that it's worth tearing everything apart to install them. If you don't already have ducts, you should definitely use a ductless system. If you do have them, you get some other benefits.

Ducts are great with a heat pump and an air handler with variable fan speed.

A recirculating central heating system doesn't do that. As mentioned below you either need exhaust only ventilation, balanced ventilation or ideally ERV/HRV. All of the above are available in both ducted and ductless forms.

>Ducts are still needed to circulate air

>A recirculating central heating system doesn't do that.

They definitely circulate air, and they definitely filter the air. Whether there is fresh air depends, mostly no.

Good callout. Some people have the strangest reasons to hate high-efficiency ductless systems.

Except it's incorrect. The best system is a "ducted ductless" system that circulates and filters air throughout the house.

I've seen that terminology used on US websites, it took me a while to get my head around. In Australia it would just be called ducted reverse cycle. In the vast majority of cases it would be "ducted ductless" I guess: outside unit connects via refrigerant lines to a heat exchanger and fan in the roof cavity, when then distributes the heated/cooled air to rooms via conventional ducting.

Nobody calls it that, it would just be called a ducted heat pump. The refrigerant lines just run to a central air handler where the ducts start in the house, instead of the air handlers that mount on the walls.

How do you go ductless in a 6000 sq ft home?

Two multi-head units should work just fine.

If it's an existing build with ducts, stick with that. If it's a new build, then like the other poster said, you add more units. See the top of any commercial building for example: heat pumps all over the place.

Another option is to supplement an existing ducted system with a ductless one adding additional zones where you might want conditioning over night. E.g. bedrooms where you want to sleep in cold A/C in the summer where the rest of the house can warm up a bit, or a person in the household who likes their room to be warmer than everyone elses in the winter.

A recirculation system will prevent high levels of CO2 buildup in occupied rooms, particularly with closed doors. I've measured the difference in our bedroom overnight between having the fan on and off, and it was dramatic (if I recall correctly, something like 700ppm in the morning vs 2000). Even if it's only keeping the level consistent throughout the house, that would be a benefit. But every house will have some amount of air leakage as well; a recirculation fan with a fresh air intake (which all modern ones have) will pull in fresh, filtered and conditioned air to replace air that leaks out. If that's not sufficient for indoor air quality, then you can crack a window or set an exhaust fan to run periodically. I haven't generally found that necessary though.

ERV/HRV has its place it less moderate climates where there is significant efficiency loss from pulling in outdoor air, especially if indoor conditions require a significant amount of fresh air turnover. But a central HVAC system with a recirculation fan most definitely can be beneficial in a lot of cases.

It’s worth noting that ASHRAE ventilation standards are based on “bioeffluence” levels which may or may not correlate well to other IAQ values like CO2, VOCs, etc.

In have ducted mini split units in my home, made by Mitsubishi I designed the system to work with merv 13-16 filters. I can run the fans and filter my air as well as use the units for heat and cooling. I installed fresh air intakes on the return units that can pull external air when they are opened.

It's really a very good proposition. Has all of the advantages of central heating and the efficiencies the heat pumps