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Comment by j_walter | Hacker News Reader

My company took steps to address this months ago. The re-circulation rate was set to zero so all air is fresh make up air from outside. This has significantly increased our cooling bill since it's summer, but it was something that we felt important even though there hasn't been much science to back it up. We also have a UV system hooked up to our system, but this has been installed for the last 2-3 years now.

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How does that work? Is it just a setting on industrial HVAC systems or did it require someone coming out and reconfiguring things? I’ve heard some schools are considering leaving the windows open year-round while running the heat/AC to get air turnover. Maybe because those HVAC systems are dated?

Most big HVAC systems have this ability. On a rooftop unit usually it's as simple as a hatch that you slide; a split system would have to have a separate fresh air duct you can control with a damper. Newer systems typically have explicit fresh air makeup systems with heat exchangers.

Most schools with forced air ventilation should already have such a capability, if they bothered to look. If not, if would certainly be cheaper to have one installed than to leave the windows open. But of course, it may be easier to do one than the other bureaucratically.

> Most big HVAC systems have this ability.

This is patently untrue.

It's an industrial HVAC system that has control systems to allow this (simply just changing damper settings)...but ours is also ~25 years old. Schools opening windows would just be a low tech way of looking at how we change the damper settings.

I wonder if air-to-air heat exchangers could address that cost issue.

Surely all industrial / professional HVAC get installed with HRV and the loss here is going from the "100% efficient" recirc to HRV, no?

Not all if it isn't designed that way (ours isn't because we never plan to not use recirc). The loss isn't actually that much and since we are using low temp cooling water to cool the air and we have a significant amount of that on site already we barely notice (still a cost though).

How old is your building? New construction is so weather sealed and lacks fresh air leaks to meet air quality standards. Pretty sure whatever standard to get a green rating on a building requires outside air to be mixed in and due to efficiency requirements there has to be heat exchanger.

25 years...but we also have the ability to open outside dampers on our return air chases to output all air if needed.

Energy Recovery Ventilation (ERV)

https://en.wikipedia.org/wiki/Energy_recovery_ventilation

Yes. I was wondering if the wheel-based ones had some means of killing pathogens on the wheel, rather than transfering them back to the incoming air stream.

I've been thinking about getting two CPU cooling fans that have the heat sink at a distance from the fan and use a heat pipe to move the heat over to the fan and connecting them back-to-back so one fan blows outward and one blows inward. Something like two of these:

http://i.ebayimg.com/images/i/151452729196-0-1/s-l1000.jpg https://prod.scorptec.com.au/10/299/65429/121859_large.jpg https://brain-images-ssl.cdn.dixons.com/7/2/10178327/l_10178...

That way outgoing air would warm (or cool) the incoming air. If a Hepa filter could be added to the incoming air fan it would make an ideal unit to put in the window at fairly low cost. Since they are CPU coolers they could probably be made speed adjustable also.

That would transfer sensible heat, but not latent heat.

Countercurrent exchange is the way to go with a scheme like this, so you'd want a series of pipes at varying temperature.

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