[CAUT] Humidity (HVAC codes)

[Fred Sturm]

On Jan 12, 2011, at 11:21 AM, David Skolnik wrote:

> Room humidifiers and/or DamppChasers might be more effective, at  
> least in maintaining pitch.


Room humidifiers are ineffective with an active HVAC system. The  
system changes the entire volume of air in the room much faster than  
any unit could replace it (a couple string faculty members here tried  
it, without any success whatsoever).
	There is no reason the same ductwork can't be used, depending how the  
system is designed. Typically there are enormous blowers moving two  
currents of air through two sets of ductwork, one for warmer, one for  
cooler air (and a separate exhaust system). Where there is temperature  
control (sometimes individual rooms, sometimes sets of rooms), the  
thermostat controls the mix of air coming from the two systems, by  
means of baffles entering a mixing box. If the air is humidified as  
needed at the source - down in the basement or up in the attic where  
the blowers are - it becomes relatively simple, as long as you aren't  
concerned with a very precise level (the problem being that the two  
temperatures of air will each have a different dew point/RH because of  
the difference in temp). You would control it for a dew point for the  
average target temperature (the two currents of air would both be  
adjusted to have the same dew point, or, to maintain a lower level of  
RH, to have the same minimum dew point).
	Refined humidity control is quite difficult, as it requires refined  
control of pressure throughout the building. It is the pressure  
differentials that cause flow of air into and out of rooms, from and  
to the ductwork. When doors are opened and closed, this changes  
everything. Windows make it virtually impossible, hence the move to  
have windows that can't be opened. This pressure system affects not  
only temperature control, but the effectiveness of the circulation and  
renewal of air. Circulation and renewal are the driving forces behind  
design, for human health reasons, so as not to get buildup of germs  
and toxins. No reason maintaining a minimum of humidity couldn't be  
added as another element of human health. These codes make it very  
expensive to have and run a public building, but they also protect the  
public health.
Regards,
Fred Sturm
University of New Mexico
fssturm at unm.edu