[CAUT] HVAC information

[Fred Sturm]

On Jan 5, 2009, at 8:49 PM, Greg Graham wrote:

> How do the thermostats with two copper air tubes connected to them  
> work? If it is ripped off the wall, and the room is 87 degrees in  
> January, would plugging the tubes with bubble gum help or hurt?


I don't know the answer to the bubblegum part, but I can shed some  
light on the copper tubes. They act a lot like the tubes in a player  
piano. They have a "partial vacuum" due to connection to the air ducts  
through which hot and cold air are being forced by fans. The principle  
is similar to carburetor jets. I don't know the actual mechanism, but  
closing them off or opening them (which is what the thermostat does,  
using a bimetal strips) has the effect of changing the mix of hot and  
cold air. There are separate hot and cold air ducts, and there is a  
mixing box for each area (room or set of rooms) controlled by the  
thermostat. Something on the order of flaps or baffles move to change  
the proportion of hot to cold entering the box and then blown into the  
room.
	The operation of an HVAC system is a very complex thing, involving  
air pressure differentials (which are knocked out of kilter when  
people open windows), outside temperatures (which vary over the course  
of a day, and a proportion of outside air is added to the mix of air  
being circulated through the system), and lots of mechanical  
components. It's a whole lot easier to control an environment if you  
aren't changing the entire volume of air more than once an hour to  
keep things healthy for large numbers of people. It doesn't help if  
you have, like I do, a system that is 45 years old, where, among other  
things, flaps get stuck, meaning that some rooms are consistently cold  
and others are hot, at least until someone complains enough. Then a  
tech guy goes crawling around up in the space between floors, and  
manages to get it adjusted so it is tolerable until the next seasonal  
change.
	About the gum, I'd substitute tape, and experiment with blocking one  
tube or the other. Assuming you can't get someone to replace the  
thermostat <G>.

Regards,
Fred Sturm
University of New Mexico
fssturm at unm.edu