I want to tackle some of these zoned HVAC questions/misconceptions. I have owned and lived in homes with zone control. The net out is my energy costs were reduced (significantly) and comfort in the home improved. For example, when the upper floor needed cooling (nearest the roof), why am I also sending cool air to the main floor which is perfectly comfortable? With guest bedrooms, when not being used I could allow temperature swings from 60 to 80 degrees ... when occupied, I could make the room comfortable for the guest (or uncomfortable when I wanted them to leave
). With South facing rooms and North facing rooms, I was not heating or cooling rooms which would be made uncomfortable to satisfy the comfort of the opposite side of the house. The baby's room (that was a long time ago) could be set to temperatures better for the child without needing to heat/cool the entire house.
Zone control boards can handle Heat/Cool "calls" in two ways. One is round robin. The first room calling for service gets serviced and once satisfied the next area is serviced, etc. The other is "priority". You can assign the master bedroom as Priority 1, the family room as Priority 2, etc. The area with the highest priority is serviced first. Most zone controllers are priority based. If you have two areas assigned the same priority, then all of those areas are serviced "round robin" before moving on to the next priority.
Let's deal with this heat and cool at the same time issue. Let's acknowledge you cannot send both heated and cooled air down the same duct at the same time and get the results you want. In any given zone (even it is only one for the entire house), your system has been designed to raise, or lower, the temperature in the house by x number of degrees over y number of minutes. At the same time, your thermostat has been set up such that it will not call for heating/cooling unless the temperature is more than +/- n number of degrees off the set point (this can vary from 3 to 5 degrees and with electronic T-stats, you can control this (within reasonable limits).
So, let's back up into this matter of the theater wanting cooling while your living room wants heat. Here's the reality (with entirely made up numbers). Temperature swings with any given room don't occur rapidly (in the absence of a mass ejection event). You could reasonably expect to see a 3 degree swing over, say 15 minutes (bad day). On the other hand, your HVAC system will make up that swing in, say 5 minutes. So, let's say your living room is demanding heat, your theater cooling. Your living room is cooling at the nasty rate of 1 degree every five minutes. Your theater is heating up at a rate of 1 degree every five minutes. What that means, in the end, is your theater will heat up by one additional degree in the time it takes to make the living room toasty. You really won't notice this. (Understand this example is "made up" ... the rate of temperature change will vary by climate and how well your home is insulated. None-the-less, your HVAC system design is such that it will heat/cool far faster than what is lost/gained.)
While one zone could want heating, and other cooling, your system will not do both at the same time. By the same token, you're not going to notice this (one exception). What you will notice is the reduction in your heating/cooling costs. (The one exception could be where you have a room you've allowed to swing from 60 to 80 degrees and now you want it up to 70 for your guest ... the time it takes to service this could result in more than a 1 or 2 degree difference elsewhere).
On final thought .... you really, really need to know what your system is doing with respect to the number of minutes it will take to raise/lower the temperature. Many people will change the temperature setting in their office or home during the time it is not occupied ... I cannot tell you the number of times this was taken to such an extreme that it actually took more energy to recover than it would take to have just left the T-stat alone.
Oh. And I have now reached the age where I can be a grumpy ol' man. When I see a rabid environmentalist zealously conserving non-renewable resources, I get to thank him/her for their efforts and then call them a hypocrite if their HVAC system is not zone controlled. Edited by Dennis Erskine - 2/13/13 at 6:25am