Forced Air Zoning

In a perfect world, with perfectly balanced ductwork and exactly equal solar load (sunlight) at each wall of the house, with all areas on the same floor, and no intermittent heat loads from cooking in the kitchen or running a wood or gas fireplace, no one would need zoning. But alas… this is not a perfect world. And zoning can solve a lot of issues.

But before we go further, let’s explain what is forced air zoning. Forced air zoning is a mechanical opening and closing of different sections of ductwork to direct conditioned air to the specific areas calling for heating or cooling. Either a sensor or a thermostat is used to sense and request the unit to send conditioned air to the affected zone.

When done right, a zoning system can make it seem like the house has multiple furnaces or heat pumps. Every area is comfortable. When done wrong… it can frustrate the owner tremendously with lots of duct noise, overheating or cooling some zones, or even shorten the life of the equipment.

Many of the potential problems of forced air zoning come from trying to force 100% of the furnace or heat pump’s capacity into a small section of ductwork. In many systems (especially older systems or not well-designed systems), the furnace or heat pump comes on and makes all of its capacity whenever one of the zones calls. Then, it tries to squeeze all that capacity through just part of the ductwork – the open zone that is calling. To compensate, there will often be a barometric damper across the supply and return, or in one or more of the zones (often called a dump zone).

Let’s run this example… Thermostat calls.  Unit makes 1200 CFM. Basement zone is open at just  300CFM. 900 CMF doesn’t know where to go. It circulates across the bypass, overcooling the coil, or dumps into the main zone, overcooling the main floor.

Forced air zoning in this situation is pretty primitive. If an HVAC contractor sold you one of these systems 20 years ago, you have my sympathies. Forced air zoning is not like hot water zoning. In a hot water system the water goes exactly where you want it, with no complaints. Hot water BTUs are well-behaved lemmings. They would jump off the bridge if you told them too. But with forced air zoning, it is like herding cats into the bathtub. You might get one there eventually, but you will end up paying a price!

So… how to fix this situation. A half step is a 2-stage system with a variable speed air handler. The system can partially balance the output with the load.  Zone 3 (300 CFM) calls. Unit comes on at 60% of capacity or 720 CFM. 300 CFM goes to Zone 3. 420 CFM doesn’t know where to go… same problem as before, though less severe.

But the real solution is a true variable speed heat pump / AC with a modulating furnace or variable speed air handler. These systems tie together the zoning controller with the air handler and the outdoor heat pump. Since they all talk, the system can produce exactly the output needed for any zone or multiple zones that are calling for conditioned air.  For example, the Trane XV18 heat pump can adjust its output from 25% – 100%.  At 25%, the system makes 300 CFM. I bet you can already see how this is going to work out. Zone 3 calls, system knows the size of the zone and comes on at 25%. 300 CFM goes to Zone 3.  Perfection!

Such a system works fabulously well and is really the only way I recommend forced air zoning. There are other ways to solve hot and cold spots in your home, but a well designed zone system is a great tool to use!

This brings up a good point. When it comes time to replace your heating and cooling system, don’t just slap in whatever was there before. Instead, have a knowledgeable HVAC company custom design a system that meets the needs of your building and your lifestyle. Given that HVAC systems can last 20 years or more, most homeowners will only ever buy one or two systems in their lives.  So make yours count!

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