Typically only around 90% of the energy is recovered, even in ideal conditions.
That sounds good, but considering that for 'good air', you really want to be replacing the air fully every 10 minutes. That means after ~1.5 hours, you've lost nearly all the heat in your home.
Combine that with the fact the 90% is an ideal figure - in more typical installations it might be more like 50% because the incoming and outgoing airflows are not balanced, the heat exchanger is full of fluff and dust, and the humidity of the air is such that lots of energy is lost to the latent heat of vaporization.
Is it worth having one if you want a well ventilated house? Yes. Will it be worth replacing it in 5-10 years when more efficient models get designed...? Probably also yes.
That obviously does not make any sense. You at least want to know the volumne of the room/house before giving any number...
What "Replacing the air fully" equates to in terms of volume is already a factor of the size of the room/house.
You might want to know what volume of air that actually was to understand energy usage, but gp was talking about air quality, not energy efficiency.
Exactly. If one person lives in a castle, do we need to replace all the air every hour? Certainly not. If we are talking about a person in a 5sqm room (for sleeping) then replacing all air every hour won't be sufficient.
There are more factors besides the number of people and the air volume, but I really didn't want to go into so much detail.
As opposed to the 0% of energy recovered when a house "breathes" (i.e., leaks like a sieve) and lets out all the conditioned air?
> That sounds good, but considering that for 'good air', you really want to be replacing the air fully every 10 minutes.
[citation needed]
ASHRAE 62.2 does not mandate nearly that much air exchange. A 2,000 sq. ft. (200 sq. m) home is about 20,000 cu. ft. of volume, and needs about 100 cfm of ventilation. And some folks (e.g., Lstiburek) think ASHRAE (at least the newer revisions) is too high:
* https://www.energyvanguard.com/blog/lstiburek-has-new-ventil...
* https://www.greenbuildingadvisor.com/article/how-much-fresh-...
It should be tied to the ACH value of the construction, as well as tied to occupancy sensors in each room.
What country are you in that has such standards? In Sweden the official recommendation in the building standards is once every other hour.
As far as I know most of the heat in my home is stored in solid objects like the walls and not the air. Replace all the air and you still have the heat
It's possible to mess up a very good old house made up of breathing materials with adding some plastic for example.
Was it purposefully desgined to do this, or did it just happen because of construction methods of the time?
Modern designs use direct vent systems, where instead of using inside air for combustion, they bring in outside air (and then exhaust externally as usual):
* https://www.efireplacestore.com/five-things-about-direct-ven...
The warming of the house occurs via radiant heat and through any conduction of heat via the actual material of fireplace or stove.
Also they built homes much smaller (the average home was less than 1000 sq feet 100 years ago, today it's 2400) and they tended to have smaller, compartmentalized rooms. This allowed for the inefficient home to use less energy anyways.
Where I live there are quite a few ~120 year old homes that are about 2400 sq feet. These would have been built by fairly wealthy people of that time as evidenced by old directories which indicate a live-in servant at most addresses, rift sawn moldings and floors, and stained glass windows on landings/in dining rooms. And although inefficient, they are smaller so they use similar amounts of energy to much larger modern homes.
https://buildingscience.com/documents/insights/bsi-001-the-p...
You could take an old house and bring it up to a modern standard but you would never recoup those energy savings both in terms of the cost to upgrade and the energy used to create and transport those materials.
Even just replacing the single glazed, wood windows that have an uninsulated weight box with modern windows is probably not worth it if the existing windows are weatherstripped (the most important thing) and have storm windows.
Most old homes in SF originally had (or at least were modified 100+ years ago to have) gravity fed heaters--no forced air, and no return registers in each room; just a single giant return register at a low point on a bottom floor. I've spoken with A/C contractors who say that there shouldn't be any serious problems rigging up a forced air system to the output registers, even without proper return registers. And plenty of homes do this. But I guess maybe the real problems come if you then being insulating the home--can the forced-air heating system circulate air quick enough without return registers to compensate for the fact the building no longer naturally ventilates? I imagine in most cases it works well enough, but you're still moderately more likely to see mold problems.
[1] To varying extents. My house was built in 1926, and it seems they used a relatively thin tar paper to wrap the house, or at least part of the house. (Unless that was somehow added much later, but I doubt it as the wall facing an adjacent house a few inches away is papered, and the siding is original on that wall.)