If not installed or maintained properly, forced-air heating and cooling systems can perform poorly, creating air quality problems and unnecessary expense. To optimize a forced-air system, you need to design or refine it to prevent leaks and pressure imbalances, protect it properly during construction (or make sure the ductwork is clean and sound), and use the right filter.
The performance spec of the heat (or cooling) source is also important, but it is critical to address the distribution system first before investing in better furnace and air handler performance. This article is the first of four focused on optimizing forced-air systems on the topics of Distribution, Filtration, Heat/Cooling Source, and Ventilation.
Leaks in the distribution system – the ductwork – can quickly undermine the efficiency of a forced air heating system. Ideally, the ductwork should be air tight and well-insulated. A “duct blaster” test, performed by a qualified technician, is the best way to find out how leaky the ducts are. In new construction, a “total leakage” test can be done at rough-in when all the ductwork is still accessible to measure tightness. If needed, theatrical smoke blown into the ducts can help identify leaks to facilitate sealing. If the ductwork is already hidden, a “leakage to exterior” test will identify how much heated air is leaking outside the building envelope. These are the most important leaks to fix since they represent wasted heat. A qualified home performance contractor (see resources below) can perform these tests, explain what the performance targets are for the home and how the system is actually performing. As a rough guide, the Northwest Energy Star target for new construction – a good benchmark for any home - is 6cfm@50 pascals per 100 sqft of conditioned floor area.
In an existing forced-air system, it can be very difficult and time consuming to access all the ductwork to fix leaks. In addition, old ductwork may be full of dust and other contaminants. These factors might lead you to consider replacing the whole duct system – an often overwhelming proposition with ducts buried in the building structure. There is an alternative: First, replace the accessible parts of the system, often located in unconditioned crawl or attic spaces, where in many homes up to 30% of the heat loss occurs. Second, strategically block and air-seal where ducts penetrate the building envelope and disappear out of sight, thereby ensuring they are enclosed in the building envelope. This can be confirmed with blower door and duct blaster to perform the “leakage to exterior” test mentioned above. (NOTE: If unlined building cavities are used as ducts, these must be lined or ducted)
If the thermal performance of the building envelope is significantly improved through weatherization and/or remodeling, this will affect the whole system. Reduced heat loss/gain will result in smaller heating and cooling loads. So a smaller furnace/heat-pump will keep the house comfortable. Smaller volumes of air can be delivered efficiently through smaller ducts. We will discuss this further in the heating/cooling source article coming up (number 3 of this series).
A qualified technician or HVAC contractor should perform a “Room by Room” load calculation (ACCA Manual J or equivalent) and a duct system design calculation (ACCA Manual D or equivalent). This will show if your existing duct system is appropriately sized for your heating and/or cooling loads – and will inform the design/sizing of a new or replacement system.
You should also consider the path of air from the supply registers to the returns. Obstructions to free airflow along this path, such as closed bedroom doors, will cause pressurization of the bedrooms and depressurization of the core of the house where the return registers are located. These pressure imbalances can cause:
- Reduced supply air flow which may make the room uncomfortable;
- Increased leakage from supply ducts, wasting energy; and
- Most importantly, the risk of back drafting of naturally-ventilated gas- or oil-fired combustion appliances, as well as wood-burning stoves and fireplaces located in the core of the house. The negative pressure sucks flue gases into the living space instead of letting them exhaust to the outside—not a good situation. As a rule, combustion appliances installed inside conditioned space should be power-vented or, ideally, closed combustion.
Pressure imbalances can also cause poor-quality air to be drawn into the house from attics and crawl spaces or force warm, moisture-laden indoor air through leaks into wall cavities, where it may condense against the cooler surface exposed to the outside and cause serious moisture problems. You can mitigate small imbalances by installing return ducts in each bedroom, by installing transfer grills (a small duct often installed above a door connecting it to the next room or hallway) or jumper ducts (a duct that runs over the top of a partition wall, bypassing a bedroom door, for example), or by undercutting doors to allow some air to pass from room to room or to return ducts in each bedroom. A rule of thumb for sizing is one square inch of opening for each CFM of air supplied to the area “behind” the door.
During construction, don’t forget to protect the system from particulate pollutants by sealing supply and return registers and not storing ducts in the construction area before installation. Periodically inspect and clean registers and ducts if necessary.
Performance Testing Resources: To find a qualified Home Performance Testing contractor, check out websites for Building Performance Institute (BPI) or RESNET. If you are located in Washington see Home Performance Washington or Northwest Energy Star.
Alistair Jackson, LEED AP, CSBA is Principal in Charge of O’Brien & Company’s Residential Technical Services, and is a LEED for Homes Rater & Faculty, Energy Star Verifier and Performance Tester, ARCSA Accredited Professional, and Built Green Verifier. Alistair was also a major contributor to the Northwest Green Home Primer where much of the information for this article originally appeared. It has been expanded upon for this publication.
Did you enjoy this article? You might also like these Building Capacity Blog articles:
Tips for Optimizing Forced-Air Heating and Cooling Systems Part 2
Tips for Optimizing Forced-Air Heating and Cooling Systems Part 3
Tips for Optimizing Forced-Air Heating and Cooling Systems Part 4
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