I thought this was simply multiple air handlers serving different parts of a building. Recently, I read multi-zones refers to one air handler serving different parts of a building. I don’t follow…why is that different than one air handler feeding a network of ducts?
There are automatically-controlled dampers (basically valves for air) in the ducts. If there’s a call for heat in Zone 2, the damper for Zone 2 is opened and the furnace fires up. The other zones’ dampers stay closed.
If the system is designed as a multi-zone, rather than as a retrofit to an existing system, the air handler can be run at different speeds proportional to the number of zones calling for heat so the one active zone doesn’t have tornado-force winds blowing through. Some systems can also proportionally throttle the burner - if there’s a call to heat only one-third of a building, you can run the burner at one-third power and save some gas.
For air conditioning, the concepts are the same. Some newer AC compressors are able to run at different speeds, but older units generally can’t, so you may run into issues with short-cycling and potentially icing of the evaporator if the retrofit isn’t designed and done properly.
Forgot to say:
What you initially describe - multiple air handlers - would be a multi-system installation.
If you have the physical space for multiple air handlers and AC compressors, they’re a fine solution, but multi-zone systems don’t need as much space. The equipment may be more complicated, but you don’t need multiple blowers, multiple compressors, multiple furnaces, etc. Multi-zone does more in less space, which may be a key consideration in commercial space, where smaller mechanical spaces translate into more income-generating space.
My house has a multi-system, and as the two separate systems have no communication to each other, it’s difficult to keep them balanced, and it’s impossible to have the lower level warmer than the upstairs. Plus, the two separate “half house” systems occupy a lot more space than one system sized to handle the entire house would need.
Is it correct to say that multi-zone systems are, by definition, HVAC systems with VAVs in the ducts? Would it also include VVTs? What about a dual-duct design? Are there other examples you could name that fall into this category?
Multizone air handlers don’t have vavs in the duct.
VAV air handler delivers constant temperature air at variable volume.
Multizones deliver constant volume air at variable temperature.
Multizone air handlers have a ‘hot deck’ and a ‘cold deck’ and each zone consists of a dual damper that may modulate between full heat from the hot deck to full cool from the cool deck or any combination between. As the hot deck side of the damper opens the cool deck side closes. Each zone has it’s own damper and thermostat control.
That at least is the form I’ve always seen. There may be other definitions.
In operation it is similar to a dual duct vav, but instead of running two sets of duct into the zone, the the dampers are at the air handler and only one duct with variable temperature air is ducted out to the zone.
Beware that multizone HVAC may refer to the refrigerant side of DX systems. Some years ago Lennox had a system with one large condensing unit connected to several zone-size fan coils. The condensing unit staged compressors based on the demand from the coils.
A similar method is being used today with some mini-split systems.
Multi zone systems on comercial HVAC (and that is HVAC not H vac) systems varry.
They can be multi unit systems with a BMS system operting them.
They can be VAV systems either single duct or duel duct.
They can be duel deck with mixing boxes either in the space or the fan unit.
They can use either DX cooling or chilled water and a chiller for cooling.
With a multi zone system each seperate zone will have a thermostat trying to control the temperature.
On a system with multi units normally each zone will have a stat turning on or off or modulating the cooling or heating. The fan on the unit may or may not cycle with the cooling or heating.
On a VAV system when the space is cold the amount of air is at a maximun and is hot air. As the space worms up the amount of air decreased to the mimumn until it reached set point. As the temperature climbs cool air in admitted to the space. On a two deck (duct) system there may be a point where no air is being admitted to the space. On a single deck system outside air or a cooling coil is the source of the air to the zone. If the zone needs heating then a reheat coil is used at the outlet of the VAV box or Fan Powered Box in the space.
On a VAV system the amount of air being delievered to the building varies therefore the supply fans have some method of varing the output.
On a duel deck system there is a hot deck and a cold deck. The out put form these decks are mixed according to the stat and delievered to the zone. If the mixing dampers are at the air handler then a single duct goes to each zone. If the mixing boxes are near the zones then a hot and a cold duct will go from the air handler to the mixing boxes and the out put from the box will be ducted to the registers in the zone.
I work in two buildings 15 stories. Each building has only one set of air handlers. Each floor has many zones.
I have also worked in some buildings with many completely different air handlers and each air handler divided into many zones.
Multi zone in a commercial building generally refers to having more than one zone on a floor or air handler
:smack:
Sh! [Psst, I’ll forgive your typos, if you forgive mine!] 
Thanks for the info, y’all! Someone once threw me to wolves in HVAC, and learning by the seat of one’s pants is no way to learn something. The idiot was simply too proud to be bothered with mentoring me. If he had, he could have been a hero.
I was not meaning you had a typo, infact I had one it should have been HVAC not H VAC. HVAC means Heating, Ventilation, and Air Conditioning.
Where H VAC stands for High Vacuum systems. (I depend on a lot of forgiveness for my typos and misspellings here so I would never point out others0:rolleyes:)