Managing Airflow In A Zoning System

July 21, 2005
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A zoning specialist from Bolls Heating and Cooling, Indianapolis, is shown installing a Comfort Systems™ Z-300 series zone control system from Jackson Systems (above and below).
Many homes around the United States have only one centrally located thermostat by which homeowners can control their heating and cooling. Having a single thermostat in a home, unless it's a very small home, almost guarantees that the occupants will not be comfortable in every part of the house.

A good example of this is a two-story house in the summertime. The upstairs is almost invariably warmer than the downstairs. To cool off the upstairs, the thermostat can be turned down, but by the time a comfortable temperature is reached upstairs, the occupants who are downstairs are usually freezing. This scenario can result in higher energy costs and frustrated, uncomfortable homeowners.

The solution to this problem is to install a zoning system, which divides a home into sections so the temperature can be controlled more accurately. A properly installed zoning system will ensure that the correct amount of airflow is delivered to each zone, so that occupants in every part of the house are comfortable at all times. There are several methods by which airflow can be managed, and it is up to the contractor to decide which technique should be used, depending on the application in question.

When installing zoning, it is always necessary to employ some way to relieve the excess air pressure that may build up in the system. There are several ways to do this, according to David Arneson, zoning product specialist, Honeywell, Golden Valley, Minn. "These methods include installing a bypass, oversizing ductwork, allowing damper leakage, or creating a dump or wild zone."

A static pressure bypass routes the extra air from supply to the return through a bypass loop. The volume of bypass air is controlled by a barometric weighted damper or a motorized damper controlled by a pressure switch. Bypass not only ensures that the right amount of air is delivered to each zone, it also ensures good airflow over the heat exchanger or A coil and prevents noise at the register.

"Bypass is recommended by Honeywell because it keeps homeowners comfortable and protects the equipment," stated Arneson. "Bypass works equally well with standard blowers or newer variable-speed blowers," Arneson continued.

Dennis Laughlin, president of Arzel Zoning Technology, said, "It should be noted that there are different types of bypass technology, and contractors should do their homework to ensure compatibility with all of today's sophisticated equipment."

In some cases, the bypass method may overheat or overcool the delivered air. "In this scenario, Honeywell recommends a discharge air temperature sensor, which senses the temperature of the discharge air," said Arneson. "If it gets too hot or cold, it will turn off the equipment but continue to allow the blower to drive air into the calling zones until the discharge air temperature moderates."

Using a discharge air temperature sensor is generally considered a good idea for many applications. However, it is subject to the individual design and must take in to account the size of the zones being served.

A bypass isn't the only method available, though. "If you only have two zones that are pretty evenly split and the contractor can upsize the ductwork, then a bypass damper is not required," noted Tom Jackson, CEO, Jackson Systems LLC, Indianapolis. "But it's important to note that if the ductwork can't be oversized, doing so may introduce velocity and throw problems."

Laughlin said, "Most systems that use leaving air temperature sensors will not have any problem with the amount of air over the coil; then it all comes down to how much noise is acceptable by the homeowner, thus dictating some air management control."

Another way to relieve the excess air pressure is to allow the zone dampers to leak. This method, which is often used when there is no room for a bypass, prevents static pressure from rising when few zones call, as the excess air will bleed into non-calling zones.

"These zones then can become over-conditioned," noted Arneson, "but this method does manage airflow. Unfortunately, homeowners may complain that air is being delivered to zones that are not calling, and they are uncomfortable."

Dump and wild zones are areas in a home where the excess air is delivered to relieve static pressure. As with oversizing ductwork and damper leakage, dump and wild zones do manage airflow, but they often generate complaints and callbacks because occupants are not comfortable.

Utilizing one of these methods is crucial when installing a zoning system, because without proper airflow, occupants may complain that the system is too noisy or drafty. "It is also possible that improper airflow could slug a compressor or cause the furnace to trip out on high limit," stated Jackson. "For these reasons, it is always recommended that high and low limits are installed."

Retrofitting

Louis Agnolutto, president, Louis Heating and Cooling Service, Raleigh, N.C., installs zoning systems in new and existing homes. "In a new home, we do a Manual J and Manual D calculation in order to figure out the proper sizing of the ductwork to handle the airflow. If the ductwork isn't right, zoning isn't going to do the homeowners any good, because they will have a warm spot, or a cold spot, or a humid spot," said Agnolutto.

In an existing home, that's a little trickier because ductwork is often hidden behind walls. "We almost always have to do something to the ductwork in an existing home," stated Agnolutto.

"If we can get to it, most of the time we can do enough to make zoning worthwhile. We're not going to tear the walls up to replace what's embedded in there. We'll make that work out, then we'll go to the attic and pick that up again and redo it up there."

Agnolutto stated that homeowners often want to zone the upstairs from the downstairs, but they don't want two systems. If the system is over 15 or 20 years old, he encourages them to upgrade to new equipment, as well as add more insulation and fix leaky windows, if needed.

If the customer isn't willing to upgrade the insulation and windows but still wants a new system, then Agnolutto will size the ductwork to handle the airflow and base the heat load calculations on the projected losses, which are normally around the 40 percent mark, he noted.

Modifying the return air is something else Agnolutto often ends up doing in existing homes. "I'm big on proper return air," said Agnolutto. "There's no sense blowing air through a straw if you don't take in a deep breath - there's nothing to blow. That's my theory. If I can get that air to come back to the equipment, I can get the temperature difference a little closer to the comfort zone that we need to have. With proper return air, zoning and a seven-day programmable thermostat, homeowners will be very comfortable, and it's very economical."

Publication date: 07/25/2005

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