Doug Garrett 0000-00-00 00:00:00
HVAC CONSIDERATIONS FOR SPRAY FOAM HOMES Design engineers for residential HVAC systems always keep a close eye on home construction mores.But these days spray foam and strict adherence to insulation codes is changing the formula for how powerful an HVAC system needs to be. This is an excerpt from a webinar by Doug Garrett, CEM, ACCA Certified Instructor, Building Performance and Comfort Inc. Visit www.huntsman.com/sprayfoam to request a copy of the white paper or to view the webinar. According to the Department of Energy (DOE), uncontrolled air leakage in a typical home can account for as much as 30 to 50 percent of its heating and cooling costs. As a result, air tightness standards are becoming increasingly important in a variety of residential building codes. A tight building, free of air infiltration can help improve energy efficiency and comfort; improve indoor air quality, and manage the moisture content within the building. Spray polyurethane foam (SPF) can help provide a building with the air barrier requirements needed to meet new air tightness standards. However, careful consideration must be given to the building’s heating, ventilation and air conditioning (HVAC) systems to account for the increased tightness. A system-wide approach, including combustion safety, ventilation, HVAC right-sizing, use of Manual J, humidity and moisture management and duct and register considerations, should be used when looking at overall HVAC strategies for homes insulated with SPF. The following article will focus on two of the key components: right-sizing HVAC equipment and use of Manual J calculations. Rules of Thumbs No Longer Apply In the beginning, the residential HVAC industry relied on “rules of thumb.” The most Commonly applied rule of thumb states that one ton of air conditioning equipment was needed for each 400 to 500 square feet of conditioned space. Homes like those in the 1950s and ’60s with little or no insulation, leaky single pane windows, no air sealing package, ducts that lost 1/4 of the conditioned air and other common attributes of older homes needed a ton of capacity for each 400 to 500 square feet of conditioned space in coolingdominated climates. Similarly, each region had rules of thumb for heating requirements. The home building industry has made significant strides in improving energy efficiency with higher R-values, improved windows and improved air tightness in both the envelope and the ducts. As a result, the sensible cooling and heating loads on a home are significantly less than when the rules of thumb were developed. SPF takes energy efficiency to the next level by positioning ducts and equipment in an conditioned space, and by greatly reducing air infiltration. Given that air infiltration and duct leakage often contribute 40 percent or more of the heating and cooling load, tightly-insulated SPF homes have greatly reduced sensible heating and cooling loads. Today, most HVAC units are installed without the contractor performing a Manual J8 calculation. Most contractors just apply one of the rules of thumb. ACCA states that these typical industry practices result in the average System being between 150 to 200 percent oversized!With higher efficiency SPF homes, the old rules result in even greater oversizing. Oversizing equipment will result in short cycling of the equipment and higher upfront costs for the builder and the homeowner. It reduces the efficiency of the units, leading to higher utility costs for the homeowner. In general, heating units and air conditioners begin each cycle at a much lower efficiency than their stated peak efficiency rating. That is the efficiency they reach after running long enough to reach what is called “steady state efficiency.” It takes at least 10 minutes for HVAC units to reach this efficiency level.When they short cycle, they are always operating at the lower efficiency rate, so utility costs are higher than necessary. Air conditioners that short cycle do not run long enough to perform dehumidification, which can lead to high indoor relative humidity, and poor comfort. Performing strict Manual J8 loads gives builders much-improved envelopes. Ideal equipment operation is then accomplished by following the ACCA Manual S for equipment selection (now required by IECC 2009); never oversizing by more than 15 percent over the calculated actual BTU load and reducing the tonnage of the equipment to closely match the now reduced sensible load. Use of Manual J and SPF Spray foam insulation, when properly installed, can be counted on to reliably deliver the R-value that the Manual J8 calculations assume. SPF expands to provide a total wall fill, and it doesn’t compress or settle over time. SPF is its own air barrier, too. It stops convection in the insulation and stops outside wind intrusion, so it produces a tight envelope air barrier and positive control of air infiltration. Right-sizing is thus essential with SPF insulated homes. It is recommended for builders to always require that their HVAC contractor provide the room-by-room Manual J8 before the HVAC equipment is installed. This is especially critical in SPF-insulated homes. For spray foam homes, sizing must be done with the most current version of ACCA’s Manual J, Eighth Edition. This is the only version of Manual J that can accurately model and determine the true heat gain/heat loss of spray foam insulated homes. It is the only version of Manual J to feature SPF insulation as a selection on the drop down menus, in order to accurately model the impact of having the equipment and ducts fully in a sealed, unventilated attic, (as so many spray foam homes do), and to allow input of the actual, tested duct leakage and air infiltration rate. ACCA Manual J load calculations are critical for spray foam houses because the loads themselves are considerably lower than the rules of thumb often used in the industry. As building envelopes continue to improve, this will be necessary for all homes. In fact, under 2009 codes, performing an ACCA Manual J8 heating/cooling sizing software calculation and selecting equipment according to ACCA Manual S are mandatory. Actual field-tested air change rates should be input into Worksheet E > Infiltration > Option 3, instead of choosing from three standard air change rates that are all too leaky To reflect the tightness of a spray foam home. Actual duct leakage to the outside should be input into this version. If the ducts and equipment are all in conditioned space, (as they often are in spray foam homes), this duct leakage should be set to zero. Otherwise, the actual tested value should be entered, as the IECC 2009 requires mandatory duct leakage testing and one could easily enter that value into the program. Mechanical ventilation is an important load in spray foam homes. In fact, the 2012 IECC requires the use of mechanical ventilation meeting ASHRAE Standard 62.2 in all new construction. Inputs can be found on the standard input page of MJ8 software in the box label “Ventilation.” Enter the number Of cubic feet per minute of outside air that will be introduced to ensure excellent indoor air quality. The sensible and latent loads that this air will introduce will automatically be calculated by the software and added to the load using design day conditions. The software links to AHRI/GAMA databases so equipment can be selected with confidence. It links to REM/Rate and FSEC EnergyGauge – the software tools used by the RESNET and Home Energy Raters (HERS) and IECC RES check energy code compliance software. These are all capabilities contractors will need and value, because they are now required by many cities and utilities.Clients will also demand these for super-efficient, green homes. The ACCA Manual J, Eighth Edition is a must in an HVAC contractor’s toolbox, particularly for spray foam insulated homes. Taking these steps will ensure that the sizing of the equipment is best suited to the home, resulting in lower first cost for equipment, lower operating costs for the owner and better comfort and humidity control in the home. Conclusion Homes insulated with spray foam have tight envelopes, and are inherently very thermally efficient. This means sensible (temperature) loads will be greatly reduced, but latent (humidity) loads may remain where they have been. Adequate fresh air ventilation and good humidity removal are essential. In all tightly sealed homes, only sealed combustion or power vented combustion appliances should be installed to ensure safe operation. With these changes, HVAC equipment can be safely installed in spray foam homes with confidence. These steps allow for optimized indoor air quality, moisture control, combustion safety, air mixing and equipment sizing, and help owners achieve an energy-efficient, healthy home.
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