For most buyers, an efficient house is at the top of their wish list. Seeing this demand, more and more builders are incorporating high-performance envelope systems into their homes (SIP panels, ICFs, spray-foam insulation, etc.) Using advanced envelope systems does a great job of addressing air infiltration, heat loss and gain, and the comfort issues that are associated with the lack thereof. But there is another important piece that often gets overlooked completely…HVAC system design.
Not only do homeowners want an efficient house, but they also assume that they will be comfortable in every room of the house. Unfortunately, this simple desire cannot be addressed solely by a great envelope system. To achieve comfort in every room, focused attention must be paid to proper design and installation of the HVAC system. Poorly designed systems produce symptoms such as un-even temperatures from room to room, draftiness, excessive use of electric strip heat, loud noise issues from high airflow velocities, and low system efficiency. Improper design can also lead to low air-temperature in heating mode, insufficient moisture removal in cooling mode, back-drafting of fireplaces & water heaters, shorter lifespan of equipment, and poor air quality, to name a few. So, to create a comfortable home we need an efficient envelope AND a well-designed HVAC system.
What is comfort anyway? How is it defined? For this column, I’ll focus on thermal and acoustic comfort. At the most basic level, the HVAC system should be able to introduce more BTUs (heating season) than are lost through the envelope for each room in the home. Since we typically rely on forced air to deliver BTUs, we need the right amount of air to get to each room. How much air do we need to insure comfort? The required airflow to each room will vary depending on the orientation of the room and the thermal performance of the exterior envelope (air tightness, insulation levels, and window performance). These specific quantities of air are determined by doing some fairly complex calculations based on ACCA’s “Manual-J” method.
While the load calculation is relatively straight-forward, making sure the right amount of air actually gets to the room is the hard part because it involves proper design AND installation of many components. Below, I’ll discuss a few common design issues seen in the field and how they relate to the occupant’s comfort and system efficiency.
One of the most common problems seen in the field is improper duct sizing. When high performance envelope systems are used, the system size and the ductwork size should be reduced due to lower BTU/airflow requirements. This saves money on materials, uses less energy, and generally creates less noise. However, if the ductwork is too small, the overall capacity of the system could be reduced by as much as 30%. It could also cause comfort issues due to lack of delivered BTUs, produce higher velocities (noisy airflow), and could reduce the lifespan of the equipment itself. Duct sizing using ACCA’s “Manual-D” method is the proper methodology for duct sizing.
Another design variable we typically see problems with is the size of the air returns. Most returns are designed to accommodate a flimsy fiberglass filter. When a homeowner installs a pleated media filter, the resistance increases drastically and efficiency tanks. If returns are not sized to accommodate for this added resistance, the lower airflow can result in cracked heat exchangers, damaged compressors, lower system efficiency, and frozen coils. Smart design will ensure these scenarios never happen as a result of the occupant wanting to improve their indoor air quality through better filtration.
The final design component that is frequently ignored is the lack of return air pathways. Since ductwork costs money, and the general tendency is to hire the contractor with the “best deal,” returns are often centralized. This strategy relies on open doorways to act as the return air pathway, instead of having a return-register installed in each room. The problem with this concept is that occupants close doors. That’s why they bought them to begin with, right? If the air that is getting pushed into a room can’t get back out, then there is a clog in the system. When a clog like this happens (closing the door to a room), airflow to the room is reduced. This also has a serious impact on the airflow to the other rooms of the house, with the net-effect being a reduction in total airflow. This situation not only creates comfort issues, but can also be a safety concern, as it causes depressurization in the area containing the central return. If you plan on having rooms with doors, then a return air pathway must be included in the design to ensure proper delivery of air to each room at all times. Jumper ducts are commonly used to provide a pathway for air to bypass a closed door, ensuring its ability to make it back to the central return.
As you can see, there are quite a few design variables that must be balanced and working well with each other to achieve a safe, efficient home that is comfortable in every room. ACCA’s sizing calculation is “Manual-J” and the duct design process is “Manual-D.” These two steps are critical to ensuring the system will work to the best of its ability. By designing/specifying these details before requesting bids from mechanical contractors, you can ensure you are receiving proposals for a system that will actually produce the result you’re looking for – a system that matches the needs of the house, safely and efficiently.