There’s a saying, earthquakes don’t kill people—buildings kill people when they collapse.

In the open air on a grassland, covid won’t kill a population. The design of our hermetically sealed buildings is what’s killing us.

Back in the 1970s the air outside was full of lead and smog, so the safest thing too was completely seal all office buildings.

Today the air inside probably contains more hydrocarbons (from volatile organic compounds offgassing from carpets and furniture) so the best thing to do is vent air to the outside and keep the windows open (assuming you live in a mild climate)

Of course, that flies in the face of all efforts to make things green…which involve better insulation, sealing windows against leaks, more air filtration systems and etc.

Here in Canada the temperature drops like 5 or 10 degrees when you walk into a forest. I’ve stepped into a traditional mud hut and experienced a similar cooling effect. The common denominator is that both environments have a high surface area compared to say concrete. This acts as an efficient radiator so that when water to evaporates it removes heat. In low humidity climates this is one way to reduce cooling demands. Beyond that if we cool micro zones or interstate cooling into clothing itself through hoses or something that might be the next step in efficiency…until it gives birth to the next problem.

>Of course, that flies in the face of all efforts to make things green…which involve better insulation, sealing windows against leaks, more air filtration systems and etc.

If anyone has a half-hour of time to waste, you can look into the science of "countercurrent heat exchangers".

It's fascinating to see how 2 pipes of air current switch temperatures when they're in contact and moving in opposite directions.

So if you want to be green AND have fresh air constantly circulating through a building, then forcing the warm inside air to pass by the cold outside air as they leave/enter the building can lead to somewhere around 95% energy recovery.

Add in a humidifier to the incoming air, and you'll have a comfortable room with humidity that will attach to airborne droplets, prevent evaporative release of a raw airborne virus, and help pull the droplet down to the ground via gravity.

Wikipedia claims "Heat recovery systems typically recover about 60–95% of the heat in exhaust air".

But even recovering 95% of heat isn't exactly great. If you live in Canada and have a 2600 sq ft house with 8 ft ceilings and 10 changes of air per hour, and the indoors is 70F and outdoors is 0F, thats 2600810 cubic feet of air/ per hour, or 12000 BTU/hr. That means you are more than doubling your heating costs compared to sealing the building and just having conductive losses through the walls.

This is true for most climates - even the best energy recovery systems still double heating/cooling costs compared to having barely any fresh air.

Something must be off with your math. I’ve tested opening a window a crack in my building in montreal. The co2 gets low even with two people present. Heating costs did not double.

Or maybe I had less than ten air changes?

It can’t be that hard to focus on the rooms with people in them, reduce the other rooms to eg one change an hour.

This is already done when calculating ventilation rates. It won't matter much because the majority of space in an occupied building is going to be classified as, well, occupied.

I always wonder how much I save by blocking off the vents in under-occupied rooms. They end up sitting a few degrees cooler in winter.

The problem is very seldom the house. The problem is shared spaces.

10 air changes per hour is a lot; I believe the current recommendations for average climates is 3.

And we're looking at the margins; 1 air change per hour can do wonders for preventing the spread of disease and improving air quality, compared to 0 air changes per hour.

Nevertheless, I redid your math, and found that your hypothetical home has 26,728 more BTUs of energy in its air, compared to the outside air[-1][0][1], and that the recommended furnace for such a climate should be 160,000 BTU/hr [2].

Using those numbers as a base, I find that, with an efficiency of 95%, it would require 120 (that's one hundred and twenty) air changes per hour to double heating costs.

With 60% efficiency, it would require 15 air changes per hour to double heating costs.

And it only takes a few air changes per hour to increase comfort and prevent illness.

[-1] I suddenly realized that the google search links that I used for these calculations are messy and unwieldy, so I'll summarize my calculations: (2600 ft^2) * (8 ft) represent the volume of the house; (1.225 kg / m^3) represents the density of air, (1.005 kJ/(kg * Kelvin)) represents the specific heat of air, (70 * (5/9)) represents the temperature difference you gave me, in Kelvin. Multiplying everything together, and letting Google automatically take care of unit conversions, gives me 28.2 MegaJoules of energy in the house.

[0] https://www.google.com/search?q=%282600+ft%5E2%29*%288+ft%29...

[1] https://www.google.com/search?q=convert+28199142.9+joules+to...

[2] https://www.pickhvac.com/calculator/heating-btu/

My first calculation using entirely imperial units didn't go well... I think I'll stick to metric!

It's unavoidable when discussing residential heating. Just use google for automatic unit conversions, and be grateful that we didn't need to invoke the cumbersome unit of "R-value" to measure insulation!

> "countercurrent heat exchangers"

Very interesting! The concept makes a lot of sense, and I was able to find some companies online that produce such a thing, often under the phrase "counterflow heat exchanger".

Frustratingly there's little details online for actual pricing, but you can find the bare exchange units on ebay for $200-300.

> It's fascinating to see how 2 pipes of air current switch temperatures when they're in contact and moving in opposite directions.

Shouldn't the air ideally move in the same direction?

Nope, because they will both end up at the same temperature, namely, the average temperature halfway between the two previous temperatures.

In a countercurrent heat exchanger, the hot air will get colder and colder until it is at the same temperature as the cold air, and the cold air will get hotter and hotter until it is the same temperature as the hot air.

This is because the hot air is not warming up the same air molecules that it relies on to cool down, and the cold air is not cooling down the same air molecules that it relies on to warm up. Each current of air gives up a part of their heat/coldness, becomes colder/hotter, and then moves towards a section of pipe that's even colder/hotter than the previous section of pipe.

A picture is worth a thousand words, although it takes some staring and studying before the concept finally clicks: https://en.wikipedia.org/wiki/Countercurrent_exchange#/media...

Ah ye thank you makes sense.

There is a diagram illustrating why not in the wikipedia article: https://en.wikipedia.org/wiki/Countercurrent_exchange

With a controlled ventilation system you have better indoor air than with leaky old houses and it's also more energy efficient.

People talk all the time about air changes per hour. Even a recent Nature.com article only discussed this metric. However, if you look at suppliers of heat recovery ventilation units, a different metric is liters/sec/person. About 10 liter/sec/person seems to be pretty good for commercial spaces. For home environments people can adapt to smells a little better, so the requirements can be lower.

Also: heat recovery percentages don't always include the energy required for forced air movement through the exchanger.

The highest infection rates are still short range though. This, combined with being a new virus would still be similar to today.

Short range could be explained by airflow rates in most buildings being near nil. Someone talks in one spot, and pathogens hang around that spot in the air for many minutes.

With say 10 air changes per hour (injecting at the ceiling and extraction at the floor), and a 3m high building, you can expect airflow downwards at about 1 cm per second. That means there is perhaps only a 30 second window for a particular pathogen to hit someone else's nose before being pulled to below face level.

It's not nil, but I think that would lower transmission rates by a sufficient factor for COVID (and many other diseases) to become extinct.

Well the spanish flu happened and they did not have HVACs back then and all the windows leaked.

Spanish Flu is a really bad name for this disease.

From what we know it started in Kansas. It should be called 1918 Flu Pandemic.

It's called Spanish Flu because Spain was the first country not to cover it up to avoid adverse morale effects and revealing potential weakness because of WWI.