Zone sytems are rarely installed properly but can be efficient if properly designed. The goal is to be able to set back the temperature a few degrees in a zone that isnt being utilized while keeping another zone comfortable that is being utilized. A given zone should never be totally turned off. A single unit with a zone system will have a by-pass duct sytem to dump the air thats been cut off from one zone back into the duct system to be used in the zone that is being used. Zoned systems are more of a detriment to oversized equipment, hope this helps.
Homeowners with forced air duct systems always want to close the registers in rooms they aren't using, and close the doors to those rooms, in the belief that doing so will save energy and money.
I always thought doing this was over-rated and unlikely to save a significant amount of energy or money. I could see doing it for one or two rooms perhaps, but sometimes homeowners close off half of their house or more.
This creates unbalanced air flow in the duct system and likely results in over heating the furnace heat exchanger, and or short cycling the furnace. So the furnace spends a lot of time in start up mode and less time at peak efficiency.
I have not seen any real studies done on this however. I'd like to see some data or research on this.
Adam, most furnaces will move the proper amount of air through the blower and heat exchanger if the furnace cabinet pressure doesnt exceed .5 IWC and the furnace capacity was chosen based on a manual J calculation. This information can be found in the furnace installation manual or IOM (installation,operation,maintenance) that comes with a new furnace. However, this does not mean we are getting adequate air delivery to all of the rooms in the house due to excessive air leakage in the ducts and/or incorrect duct lengths or diameters routed to each room.
ACCA manuals briefly touch on the pro's and con's of zoning. ACCA's Manual D warns against using one system to condition two levels of a house without zoning, but also acknowledges that zoning can lead to its own problems.
In my opinion, zoning is generally an ironic end to any thorough residential mechanical design process that includes a properly calculated Manual J. It inevitably leads to "oversizing", short cycling, an overly complicated duct design (with sizes and velocities that are only correct for one of many potential operational scenarios), and a significant increase in HVAC install costs. I'm not convinced that zoning saves any energy and i'm not aware of any study proving that it does. The only real advantage is comfort - if you ignore humidity!
The ideal scenario for a zoned system, with respect to "right sizing", is a house that is split in half, each zone having an equal load, and perfectly matched by the low stage of a multiple stage heating and cooling system. This scenario doesn't come about very often.
There are provisions in the Manual J for zoned systems, but they don't lead to a downsizing of the system, as folks sometimes suspect / suggest. ACCA specifically says that equipment should NOT be downsized due to zoning unless the specified zones will never be conditioned at the same time. In other words, if you split you house up into 2 sections and planned condition one zone OR the other, but never both at the same time, then yes, the system could be downsized.
My dream research project would be a study on how zoning impacts HVAC system performance..... Hey grant writers...!
Have you ever been driving down the interstate hwy and come up on an exit ramp that has traffic backed up? As the traffic backs up and eventually fills the exit with cars you end up with a slight back up on the entire highway. this is the closest scenario i have been able to come up to help people understand why closing doors and vents is a bad idea. if you have a run off of a main trunk and you close the outlet or register you will cause the air to back up into the trunk causing turbulence in the trunk line resulting in a decrease in air flow in other areas on the system. this can also cause multiple other problems like pulsing and excessive noise from the increase in pressure.
another thing to remember when you shut doors in a home with no return air you cause a difference in pressure in the home. The room you have closed is now significantly positively pressurized and the house is now under a negative pressure. This why some homes have the doors undercut by 3-4 inches trying to allow the air to get back to the return.
Ok as far as the zone system goes... There is so much more thought involved in properly setting up a quality zone system that i see very few done correctly. If you want to do one correctly you first need to make sure you are using a multi-stage HVAC unit so the unit can run on low speed if it is only conditioning one zone. A bypass or crossover duct with a barometric damper is needed if you are conditioning a small zone and the air handler is still supplying too much air, but if you are going the zone route don’t use contractor grade units and piece it together with a simple zone controller. Get a high efficiency v speed unit that is designed for this application.
But what happens when you change the temps in a zone in the house 4-5 degrees?? Energy moves from hot to cold so you end up indirectly heating or cooling the entire space regardless. This brings us to another scenario. The return air in the zone that is off. My experience with working on correcting issues with homes using zone systems has been the returns are never dampered so if the zone is completely shut off you have just installed a permanent blower door. if the zone is off one you are returning hotter or colder air to the system and two you are causing that zone to have a negative pressure.
Just some things to think about !!
Zoning properly IMO requires communicating equipment and really thorough design, as airflow to match equipment size and zone load is key to efficient, quiet comfort. Without communication and good design, equipment usually cycles more, dramatically forshortening its useful life, increasing maintenance and capital cost, and reducing efficiency.
Zoning is a solution for large, slightly occupied homes that offers control and energy savings. Homes 1500 sf per floor and greater.
The primary reason people add zoning is to treat temperature imbalance. The cause is usually leaky shell, insufficient insulation, and oversized equipment (short cycles generally means delivering conditioning to areas close to AH before ducts can deliver uniform temps through out).
The energy efficient and elegant answer is usually to CURE THE DISEASE RATHER THAN TREATING THE SYMPTOM. Often, fixing envelope, then adding multi-stage dramatically downsized long running equipment will fix temperature imbalance comfort and high energy cost issues.
Zoning has benefits beyond temp control. Most often the home does not require the full 2 stage heat or cooling as the system only needs to satisfy a portion of the home. So, then if 1/2 the heated air goes through a properly sized and balance bypass, the return air (from home) will mix with warm supply (from bypass) before going thru the system again. You may not need to go beyond 1stage heat (or cool) except in extremes. This is most beneficial in raising supply temps from heat pumps when in winter mode. Most supply vents feel 'cool' to homeowner. Not so when zoned and not call on all zones.
Flip it to cool, and the bypass send dehumidified cooler air across the coil. Decreasing high side pressures and amp draw of compressor. Then the air gets more heat extracted and further dehumidified. Pulling out additional condensate and running less. Remember systems have to be careful not to oversize as dehumidification is crucial to comfort. Dry and cool temps out supply vent. Not too shabby.
I like EWC equipment. See this very good article...
Bypass ducts are beginning to be recognized as a very energy inefficient design and bad practice to be avoided whenever possible. I see you've just joined Allison's group - I think he might have a blog about this.
2 systems is normally not much more if any more expensive than a properly designed zone system. Upstairs systems can use options such as heat pumps or air handlers with hot water coils. Even electric strip heat could be an option in areas where little upstairs heating is required. Rarely is a full gas furnace needed for upstairs. Downstairs systems can be designed with heat as a primary concern and a small AC to go with it. If one system goes down the homeowner can go to the part of the house that still has heat or cooling until repairs can be made to the other system.
Other issues with zoned systems are the typically LONG duct runs that drive static pressures and ductwork losses very high. If you are going to install a zoned system size the AC for the upstairs cooling load, and the heat for the downstairs load. Only a small amount of air will be needed for the "secondary zone".
In the winter, I'd like to recapture the heat lost from the first floor that rises upstairs. A return at the top and bottom helps average the stratification.
Even using the "Circ" feature on newer thermostats in a zoned single system will bring this 'lost' heat back throughout the home. "Circ" for circulation. The thermostat if it has not run in 30 min with run the fan for 10 min. If the heat or ac had been been 'on' during the last 30 min then it will not run the fan.
Having the 2nd system just means that in the winter the upper unit won't run since the lower unit is sending it all the heat. See what I mean?