Multizone HVAC: Forced Air Systems

Forced Air HVAC systems are an ideal candidate for a multizone installation or even retrofitting the necessary controls to an existing system. With air-based systems, leaks are easy enough to fix and it’s pretty straight forward to add in dampers, sensors, and the central control hardware without majorly disrupting the system. In contrast, water based heating systems require the lines to be drained, soldering, and a lot of effort to seal leaks in tight spaces.


The Basics

Your standard, single zone/whole building forced air system is going to come down to the furnace/air conditioner, a bunch of ducts, some sensors, vents where it feeds into the rooms, and probably a single thermostat. By adding some sensors and controls, this easily turns into a multi-zone system. All we have to do is block the flow of air in strategic locations and have some central hardware manage the strategy for where air should and should not flow.

Blocking the air flow is achieved with a damper and an actuator to control it. These come in a variety of shapes and sizes to accommodate near any system. They fit in over other pieces of duct work and get wired into a central control that sets how opened or closed that damper needs to be, to achieve the desired temperature. In principal, these can simply be added as part of a new installation, or put in place of an existing piece of duct work for an existing installation.

The control situation however, is a little more complicated. Wiring needs to be added for multiple thermostats and there needs to be a centralized controller that manages when to run the furnace or air conditioner and manage the logic for which damper is opened when. This becomes slightly more annoying as, that could potentially be a lot of wiring to run, holes to make in the walls, mounting to do, and careful testing until everything works just right.


The Complicated Hurdle

For retrofitting an existing forced air system, there are some other challenges to tackle that a fresh installation would solve during the design phase. That main challenge is airflow and pressure within the system. It’s established that when air is flowing to every vent, your system works just fine. What if air didn’t flow to every vent? Closing off parts of the duct structure would decrease pressure in some places and increase it in others.

This has the potential to end in tears and lots of big repair bills if the existing duct work cannot accommodate the increased air pressure. If half the rooms are at a comfortable temperature, and therefore, their dampers are shut, perhaps even dampers to whole stretches of the system, then we can expect pressure inside the ducts to double. This increase in pressure is not necessarily a good thing. If we put 100 PSI of pressure through a pipe designed for 50 PSI, something’s going to explode. The same tends to be true of duct work.

Over pressure in the duct system could cause:

  • Air Leaks
  • Premature Hardware Failures in Fans, Dampers, and  the Furnace or Air Conditioner itself
  • Rapid, Unscheduled Disassembly (Boom)

To overcome this pressure issue, it’s important to determine the pressure ratings of all the components involved, the way air flows within the system, and perform extensive testing to confirm that everything is appropriately spec’d for the job at hand.

In short: Consult an Expert and Keep Everything Above Code.


Is It Worth It?

This question always varies by the people it effects. Going forward, we expect multizone systems to become the norm for residential and small business installations in the coming years. There are some additional costs for installation and maintenance over time, but you get a lot of comfort from the zoned system and there’s the potential for energy savings by heating or cooling only as much of the building as is necessary.


The Wrap Up

What did you think? Did we get something wrong? Got something for us to cover next time? Let us know in the comments below.

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