A Technician Tagging a Zone in a New Furnace Install.Forced air gas furnaces and air conditioning systems – and their accompanying maze of ductwork – have become nearly ubiquitous in U.S. housing stock. Many larger homes feature multi-zone heating systems, where a single furnace and duct system utilize a system of dampers and thermostats to allow the system to heat different parts of the home to different temperatures and on different schedules. A typical multi-zone duct system might have one zone for the living area, another for the master bed and bath, and another for the bedrooms and bathrooms. Multi-zone duct systems are touted as a way to save energy in larger homes by only heating or cooling the parts of the house that are being occupied, while keeping unoccupied parts of the home at a temperature setback – for example, 60° F instead of 68° F in winter – to save energy on heating and cooling. However, most traditional multi-zone duct systems were and are designed and installed in such a way that they don’t save as much energy as they should – if they save any energy at all! In addition, most traditional zoned systems’ design leads to operating conditions that are very hard on equipment and lead to decreased reliability, more frequent service and repair, and premature failure. If your home currently heats or cools with a multi-zone duct system, or if you are considering changing from another type of heating system, be sure to avoid the pitfalls common with most multi-zone systems when it comes time to upgrade the HVAC system.
Is a Multi-Zone duct system the right solution for your particular home?
There are a number of factors that determine the viability of different types of heating and cooling systems. If your home’s need for multiple heating systems is based on the fact that certain parts of the home doesn’t stay comfortable when a single zone serves the entire home, your home probably needs improvements to its insulation (“The True Impact of Sub-par Insulation” and “Surface Temperatures and Comfort”), air leakage problems (“Seal Tight and Ventilate Right”), or heating system (“Should You Care About an Oversized HVAC System” and “What’s the Deal with Duct Leakage”), not a fancier heating system. A two-story, 2,000 square foot home often does better with two HVAC systems – one serving the downstairs and one serving the upstairs – due to the difficulty of routing ductwork through wall, floor, and ceiling cavities. This is especially true when the system is being installed in an existing home. A house with a rarely-used, medium-sized space, such as a large sunroom or a basement in-law suite, would probably do better with an auxiliary heating and cooling system such as a ductless minisplit heat pump(“Alternatives to Forced Air Heating”). But for a 2,000 square foot or larger single-story or split level home, a properly executed multi-zone ducted heating and cooling system can greatly reduce the energy wasted in heating and cooling seldom-used or differently-scheduled parts of the home.
What’s Wrong with Most Traditional Multi-Zone Systems, Anyway?
Most traditional zoned systems are powered by a single-stage furnace and/or ac unit that is already oversized for the whole home. The ducts for each section of the house are grouped together by zone, and electronic dampers are installed where the main duct trunk for each zone splits off from the main duct plenum. When any individual zone, or combination of zones, calls for heat, the furnace switches on and the dampers feeding those zones open, while the other dampers remain closed.
Remember, that furnace is already too big for the whole house, and it can only run on one speed – full blast. So when a furnace is oversized by 50%, and only 1/3 of the house is calling for heat, the furnace is producing almost five times as much heat as is needed to heat the space that needs to be heated, and pushing all that hot air through only a portion of the already-undersized duct system. This method of zoning produces all of the symptoms of an oversized heating system, on a grander and more detrimental scale (“Should You Care about an Oversized HVAC System”). In summary, the system works a lot harder, uses more energy relative to the amount of usable heat that is delivered to the home, makes more noise, and wears out a lot faster.
Since the huge furnace is running full blast and pushing all that air through a duct system that is way too small, all the pressures in the duct system are a lot higher. As a result, any leaks in the duct system leak even more air than they typically would. It works just like deflating an air mattress – apply more pressure, and the air moves out of the deflation hole faster than it would if left sitting.
To deal with this huge volume of conditioned air at high pressure in a small space, most traditional zoned systems utilize a jump duct with a barometric bypass damper. This is a fancy way of saying that when the pressure on the supply side of the duct system gets insanely high (which is often, when an oversized, single-stage furnace or AC is serving only part of the home), a damper opens to allow the conditioned air to move straight from the supply side of the forced air furnace back to the return side. In effect, the furnace is on “recirculate” mode, so when in heating mode, the incoming air is ever-hotter, and when in cooling mode, the incoming air is ever-colder. This causes short –cycling – rapid, inefficient on-off operation of the furnace or AC. In heating mode, bypass dampers also frequently cause furnaces to trip their upper limit switches and shut off the system because the heat exchanger is in danger of overheating. In cooling mode, bypass dampers can cause the incoming return air to be so cold that the evaporator coil freezes and the unit sustains damage.
The return ducts on most multi-zone duct systems can also be a source of problems. In homes with an open floor plan, a single well placed and properly sized return duct can do a good job of managing airflow on a two-zone system. However, once you get into very compartmentalized homes, multiple return ducts become necessary to manage airflow and pressure. These compartmentalized, multi-return systems are almost never dampered on the return side, so even if only one part of the house is calling for heat, return air is being pulled from all parts of the house. The part of the house receiving heated air becomes over-pressurized, while the parts of the house that are not calling for heat become depressurized. These pressure imbalances cause hot air to be circulated through the entire home, not just the part where heat is called for. More detrimentally, these pressure imbalances also cause cold air to be pulled from outside into the unheated zones, and cause hot air to leak outside from the heated zones. The consequences of the pressure imbalances reduce or even negate the energy saved by having the zones in the first place.
The bottom line is that almost all older multi-zone systems, and even most installed today, cause the heating equipment to be drastically oversized almost all the time. The system is designed to compensate for the symptoms of its flaws, rather than designed to run properly, to the detriment of energy savings, reliability, equipment longevity, and comfort.
What are the Components of a Properly Executed Multi-Zone Ducted Heating & Cooling System?
Accurate Load Calculation & Equipment Selection
In order to avoid short-cycling, high static pressure, temperature imbalances, and other “oversizing” symptoms that are exacerbated by traditional multi-zone systems, it is critical to know the total heating and cooling load of each individual zone, as well as the load of the entire home. The most effective method for accurately calculating the heating and cooling loads of the entire home is the ACCA Manual J method, which takes a variety of factors into calculating winter heat loss and summer heat gain to determine the proper size of equipment to install in the home. Factors under consideration when calculating heating and cooling load include the climate where the home is located, the surface area, orientation, and insulation value of walls, floors, ceilings, windows and doors, the air leakage rate of the home, and the number of occupants the home is designed for. ACCA Manual J is most often calculated using software such as EnergyPro or WrightSoft. The overall Manual J load is an aggregate of the entire home, and is commonly broken down by room. When designing a multi-zone duct system, each individual room’s load should be grouped together into the planned zones to determine the heating and cooling load of each duct zone.
Proper System Layout & Duct Design
Once the whole home load and the zone loads are calculated, ACCA Manual D should be used to design the duct system. Manual D uses the room-by-room load data to calculate the amount of airflow each room needs to achieve adequate heat and cooling, and the size and length of duct required to provide the correct amount of airflow. The entire system should be designed so that the heating and cooling equipment are centrally located. This helps prevent certain zones from having excessively long ducts, which incur more distribution losses due to having more surface area where heat can be lost, more joints which can leak, and low air pressure, which leads to less-than-adequate airflow into the faraway rooms. Return ducts should be laid out and sized to provide adequate airflow through the furnace and AC unit while minimizing pressure imbalances inside the house. Finally, as with any duct system, the installation should be very close to airtight, with a leakage rate of 6% or less confirmed by a duct blaster test.
Manual J load calculations for the total home as well as each zone are necessary to select the correct equipment for the home. Manual J calculations, whether for a whole home or a zone, represent peak loads – in winter, the coldest night of the year; in summer, the hottest day of the year. Most of the time, the furnace and AC don’t need their full capacity to heat and cool the individual zones of the home. Consider, for example, a home with an overall heating load of 100,000 Btu (British Thermal Unit), comprised of three zones with equal loads of 33,333 Btu. Only one of these zones may need heat at any given time, and usually the zone will only need a portion of its peak load. So, though the home may need 100,000 Btu occasionally – and the furnace must be large enough to meet this need – usually, the furnace need only run at 20% to 60% of its full capacity. Clearly, a single-stage furnace – which is either running full tilt or not at all – is not a good fit for a multi-zone ducted heating and cooling system.
Two stage furnaces and air conditioners are capable of switching between a high and a low output. Modulating furnaces and air conditioners are capable of running at a variable output ranging from very low to full blast – kind of like burners on your gas stove. Two stage equipment is an OK solution for a two-zone duct system, but modulating equipment becomes almost essential to achieve good performance in a home with three or more zones. It is practically impossible to design and install a reliable, efficient, quiet heating system with multiple duct zones that does not also utilize a modulating furnace. Two stage furnaces usually cost between $500-$1,000 extra to install compared with single stage furnaces. Modulating furnaces often require proprietary thermostats and other controls, and are generally installed as components of larger, more complicated heating systems, so a heating system featuring a modulating furnace can be thousands of dollars more than a basic single stage, single zone system once all additional installation costs are factored in.
Installing right-sized, modulating heating and cooling equipment will allow the system to adapt to the wide variety of heating and cooling needs it will be tasked to provide. A well thought out duct design and zone layout, coupled with appropriately sized modulating furnace and AC units, will minimize short-cycling, noise, and temperature swings and eliminate the need for energy-wasting and equipment-damaging jump ducts and barometric bypass dampers. If your HVAC contractor mentions installing a jump duct or bypass damper as part of a proposal for a multi-zone duct system, run the other way!
Return Paths & Dampering
While sorting out equipment and duct sizing and modulation are definitely the most important factors in implementing a long-lasting and efficient multi-zone heating and cooling system, it is also important to consider how return air makes its way back to the furnace. In many homes, particularly those with open floor plans and only two proposed zones, a single return duct in a strategic location – for example, in the hall between the living area and the bedrooms of a 2,000 s.f. ranch home. To get the best performance out of a return path like this, it’s important to make sure that the return is open to whichever zone is being heated or cooled at any particular time, and separated form whichever zone is not being heated or cooled. However, if the home is truly compartmentalized, multiple returns will be needed, and in some cases one or more of those return ducts will need to be dampered. The different layouts of zones, doors, and equipment is as varied as the housing stock, and each situation is unique. There is not one “correct” way to develop a return path for every system – rather, it is up to the contractor to make the decision about how to design the return path(s) and whether to damper any, or all, of them.
Multi-Zoned Systems are Complex
The bottom line is that a multi-zone ducted heating and cooling system is a complex project that requires taking a step back from the furnace and looking at the whole house. If you are in the market for such a system:
Do Work With a Company That Will:
Calculate heating and cooling loads for each zone as well as the entire home
Put thought into designing the supply and return duct system
Pay close attention to detail when installing the duct system
Commission the system after installation is complete
Don’t Work With a Company That Will:
Utilize jump ducts or bypass dampers
Install single-stage equipment
Use “rule of thumb” equipment sizing guidelines
Not give consideration to length of duct runs and/or return paths
A company that is well-versed in building science and the myriad ways that different components of a home interact, such as a home performance contractor, is the best type of professional to work with when installing a multi-zone heating or cooling system.
Copyright / Courtesy of Advanced Home Energy. September 2014. If you would like to use this article on your own website, please give Advanced Home Energy credit and a link.