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How do you optimize a forced-air system to maintain a green home? Alistair Jackson, LEED AP for Homes Responds: Part I: Distribution

AlistairJackson-web 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.

Christesting 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.

Iaq 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
Mechanical Systems and Fuel Choices for the Warming World
Creating Effective Energy Efficiency & Conservation Strategies Part 1
Type of Construction for Passive House System
LEED EB: O+M's Energy Threshold

4 Responses to “How do you optimize a forced-air system to maintain a green home? Alistair Jackson, LEED AP for Homes Responds: Part I: Distribution”

  1. Al D'Ambola

    Would it not be easier to test the duct system the way it functions? Without hooking up a secondary device (duct blaster). Why not verify fan speed, test actual static pressure and use the manufacturers fan chart to determine the fan flow. Once this is done use a flow hood (commercial type) for your readings at the register and grilles. The difference is the real duct leakage in CFM, not sq inches of leakage.
    Example, the fan curve tells you the fan can move 1000cfm, your flow hood tells you that all the supply register air is 850cfm and the return air is 500cfm. I now know that the supply ducting is leaking 150 cfm and the return is leaking 500cfm. Once I have a cfm total and if you knew temperature you actually could calculate the btu loss/gain through the ducting.
    What does an HVAC contractor do with sq inched of leakage? A duct blaster is a crude test at best. I have seen where we have moved the pressure sensor a few feet and got totally different leakage results from the duct blaster. Hard to get repeatable readings with it.
    Temperature diagnostics determining the percentage of BTU loss/gain through a duct system is a phenomenal test that only takes about 10 minutes.

  2. Alistair Jackson

    Al – thanks for your comment. There is definitely great merit to your approach – and I agree the duct blaster test has its limitations. Understanding the real word performance of the system is obviously important when looking at upgrades. I wish there were more qualified, experienced balancing contractors around interested in working in the residential sector.
    One attraction of the duct blaster and blower door approach is the ability to isolate the leakage to exterior. This focuses sealing efforts on getting ducts inside the thermal envelope as much as feasible and then sealing up those left outside.
    I’d be very interested to know if there is any way to make this differentiation with your approach?

  3. Danny Gough, Energy Solutions, Inc.

    Al,
    Why do you have such confidence in the accuracy of a flow capture hood? I have 3 Alnors and 1 TSI and can tell you they are much more finicky than a calibrated fan. We have simply rotated the hood 90 degrees and get 10% to 15% variation in the readout. LBL also published a study showing their accuracy to be highly suspect. I agree that flow capture hoods are a great tool for air balance. But measuring airflow accurately has never been their strong suit.

  4. Al D'Ambola

    Danny, the LBL study 7 or 8 years ago has a lot of flaws. TSI had one of there engineers watching how LBL performed there test, he was not impressed with how they used the hoods. They did say if my memory is correct using a garbage bag with a stop watch to calculate the flow is more accurate. No way would that ever happen on a balancing project. When it comes to research who is paying for it? And who benefits from the report?
    As professional commercial balancer’s we are often asked by engineers to prove our readings in the field, using hoods and pitot tube traverses or hot wire anemometers. We are always within a few percentage points using different methods (repeatability)is what we strive for, and achieve.
    My calibrated hood matches my traverse readings, so I do not no why your hoods are giving you different readings. If you are using the Low Balaometer from Alnor it is of 30-40% off it is not a product that a professional balancing firm would use. Saying that hoods are not accurate I guess all the systems that have been tested by commercial balancers for hospitals, industrial facility’s and office buildings and commercial kitchens, we are all wrong…is it time for garbage bags and stop watches?
    Alistar, I believe the fan curve method of determining flow is much more dependable and less labor involved in achieving repeatable results. Professional airflow training is most critical.
    You wrote a very good article, glad you’re helping people!!!

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