I find that I don’t quite understand how vestibules should work.
Say the outdoor air is 90F, and the desired indoor air is 72F. Should the vestibule be somewhere between 72-90, or should it be below 72?
Similarly, if the outdoor air is 30F, and the desired indoor air is 72F, should the vestibule be somewhere between 30-72, or should it be above 72?
Obviously in each case the actual temp will fluctuate a lot based on use, but I’m interested in the theory.
Thanks.
Should be? I’m not sure what you mean. Vestibules (from a heating/cooling standpoint) are there to provide a buffer zone so that the main structure loses its heat/cool less quickly than if the door opened directly on to the main room. In winter, for instance, you don’t want a gust of arctic air shooting through the front door and into your living room. You’d much rather that the gust of arctic air shot into your tiny, unheated/underheated vestibule where it can do less damage. Then, the guest enters the main house, whereupon the heat loss to the tiny vestibule is not so severe.
I’m thinking of larger commercial/institutional buildings where (I think) the vestibule would be zoned & conditioned separately from the main indoor space.
Large commercial buildings have many different ways to heat and cool. That being said, vestibules generally do not get separate zoning for cooling. Generally.
A very large vestibule might get it’s own zone. (usually through something called a VAV box) For heating purposes [only] a small wall mounted wall heater is common. Most have an integral thermostat built in. Sometimes a baseboard heater is used in a vestibule. (for heating only)
A good way to see if the vestibule is zoned is the presence of a thermostat in the vestibule; although if there is a baseboard heater or wall unit that thermostat may be for heating purposes only.
The setpoint of the vestibule is generally the same as the buidling itself; recognizing that the nature of the vestibule is such that temperature swings are unavoidable. Nonetheless, the thermostat “sees” the hot/cold air and responds to [try to] maintain setpoint. Since no one works in the vestibule, the temperature swings are not an issue to most.
When a vestibule does not have it’s own zone, temperature management is done by calculating the needed air flow (expressed in CFM) and a different zone (usually the lobby zone) simply delivers that CFM to the vestibule. (once again, recognizing that “tight” temperature control is difficult and unnecessary.)
I’m not sure what you’re getting at, but I suspect that you may be really wanting to know about the pressurization of the lobby, not temperature.
Don’t really know what I’m getting at…
I was in a vestibule today which was very hot b/c it had a lot of glass - hotter than the outdoor air - and I was just trying to figure out how it should have been.
In the past I think I’ve experienced a lot of vestibules that were purposely overcooled in summer and overheated in winter, which seems logical. But I might be wrong.
Post #4 was very informative - thanks.
The vestibule today likely had problems; a VAV box that wasn’t working, inadequate air volume (CFM), poor design (including the wrong diffusers (registers)), an A/C system that wasn’t working, poor stat location, someone propping the door open and 50 other potential reasons.
The vestibules you saw previously were probably not purposely overcooled or over heated. (although it seems logical)
More than likely the vestibules were not “air balanced”; which is a procedure in which the correct volume of air (without regard to it’s temp) is delivered into the vestibule. It’s not quite enough to give cool air when it is needed. (or hot air in the winter) If you give too much of it, you will either overheat or overcool the space.
As you noted, this can be corrected by “zoning” the vestibule. But most designers will not specify a separate VAV/ AC zone for a vestibule. The cost just isn’t worth the added benefit of tighter control. So…most of the time you calculate the needed CFM, install “linear” diffusers that “bathe” the glass, and install a wall mounted heater in the vestibule to take off the edge. Most of the time, that will work acceptably, and is a decent balance between cost and control.