You don’t have to poke around very much to discover that most HVAC equipment sizing is still mostly a rule-of-thumb calculation based on outdated information and performance specs using high, medium, and low guesses. Accurate design, sizing, and installation of these systems needs to be improved — both to meet the performance levels of today’s more efficient homes, but also the comfort and efficiency expectations of the people who live in them.
Here are five steps that will help you achieve your goals.
1. Include functional specs for the insulation and sealing package. Specify the correct level of insulation and the installed quality, not just the R-value. Specify high-density batts or better yet, use blow-in insulation to maximize the insulation in the walls; shoot for minimum R-22 in walls and R-50 in attics. Also consider using one of the new air-sealing systems that sprays a thin layer of foam to seal gaps between sheathing and framing in the stud and joist bays prior to installing cavity insulation. Let your HVAC contractor know about any advanced framing techniques that reduce thermal bridging and increase actual insulated surface areas.
Finally, verify the enclosure tightness of the building. It needs to be equivalent to 0.25 cfm per square foot of building surface area at 50 pascal. Then make sure your system designer enters all these specs into his or her Manual J calculations.
2. Use correct window U-value and Solar Heat Gain Coefficient (SHGC) specs in load calculations. Too often a default value or incorrect data will cause an increase in the A/C sizing. Placement details such as orientation, overhangs, and shading can also affect this information.
I recommend U-values of 0.30 and SHGC of 0.30. We can fine-tune the last value to maximize south-side winter gain and also reduce the coefficient on western exposures to reduce summer heat. This is a discussion for another time but specifying the right glazing system will have lasting effects.
3. Design for ducts and distribution. HVAC equipment and ductwork should be located within the conditioned envelope of the home, and can be accommodated by using available framing systems such as open-web floor trusses or I-joists engineered for pre-cut duct openings. The only thing that should go into the attic is more insulation.
Right-sizing typically leads to a reduction in equipment size and, with it, lower total air flows and reduced duct size. The improved thermal enclosure and improved access to equipment for future repairs, replacement, and filter changes makes this one the only choice.
4. Seal ducts and air-handling equipment. We expect our plumbers to install leak-free systems, and we should do the same with our HVAC installers. The problem with leaking ducts is the damage they cause both in energy loss and potential indoor air quality issues. Seal ducts and air handlers to the same level required in plumbing, using mastic (no, not duct tape) at all joints, elbows, connections, and terminations. A duct blaster test at 25 Pa should not exceed 3% of the gross air flow at high speed.
5. Test the system for performance. Finally, test the systems to perform as follows:
Duct tightness: Pressure test at 25 pascal.
Room-to-room pressure: Test the room-to-hallway pressure with a digital manometer; it should not exceed 2.5 Pa with the doors closed and the air handler running.
Delivered air flow: Proper air flow leaving HVAC registers will define the level of comfort you can expect. Verify the Manual D design calculation using air-flow measuring devices such as flow capture hoods, flow grids, and calibrated fans.
A/C system charge: National studies have shown a significant number of installed systems have the wrong refrigerant charge and therefore operate well below manufacturer specifications. Be sure that every project has a final check of the system charge and that it is done according to industry standards.
—Mark LaLiberte is a green building consultant. construction-instruction.com; laliberteonline.com. Adapted, with permission, from the July-August issue of ECOHOME magazine, a sister publication of REMODELING.
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