June 2020 Focus Section
Without a Qualified HVAC Design Team and Testing, A Perfect Storm is Brewing
by Beverly Smirnis
As energy codes require homes to be more air-tight, there’s a need to make sure every room in a home gets the proper amount of heating, cooling and ventilation. Though you may have taken proper steps to provide the right amount of conditioned air to each room, comfort issues may still exist because of oversized equipment. Other problems that may foil the best of intentions could include poor velocity at the register causing poor mixing or drafts from improper register placement. Getting the system’s static pressure just right is also key—if it’s too high, the mechanical equipment has to run longer and start and stop more often, reducing airflow and the life cycle of the system itself, and possibly drawing moisture-laden air into wall cavities. An unsealed supply duct in an unconditioned space can waste as much as 500 cubic feet of air per minute or more than a ton of heating or cooling capacity. And finally, poor venting can lead to negative pressure in a house that draws in hot outside air, soil gases, garage fumes, and pollens.
The SLZ-KF Four-Way Ceiling Cassette, a ductless split system made by Mitsubishi Electric Trane HVAC U.S., earned a 2019 National Association of Home Builders (NAHB) Global Innovation Award. Its 3D i-see Sensor calculates the room occupancy rate, adjusts conditioning accordingly with direct or indirect airflow settings and dozens of different combinations. The result is improved energy savings with a SEER rating over 19.8 and even temperature distribution.
Healthy and high-performing heating, cooling, and ventilating systems start with a good design team that may include an engineer, a physicist, your HVAC contractor, and a home energy rater. Following protocols from all four of the manuals developed by the Air Conditioning Contractors of America (ACCA) will help the team choose wisely.
The analysis from Manual J estimates how much heat the house loses in winter and gains in summer and thus how much conditioned air each room needs, expressed in cubic feet per minute (cfm), to help designers choose heating and cooling systems that are sized properly.
The Manual J calculation factors in all the surfaces of the building envelope and the insulation levels for each type of assembly, the location and tightness of the duct system, the internal loads (appliances and people) and where the house is located to provide room-by-room cfm requirements.
While many contractors start right with Manual J they neglect to use Manual S, D and T. Manual S is the reference tool for selecting the right equipment for the home and the budget. While the general rule of thumb says that 350 to 400 cfm per ton of cooling is required for proper air conditioning system operation, there are other considerations to make. In particular, this diligence can help avoid the common mistake of oversizing the AC. A savvy HVAC design team recognizes that each piece of equipment has different capabilities for handling sensible heat gain (related to temperature) and latent heat gain (related to humidity).
Central air conditioners are typically installed with sealed combustion gas furnaces and use the same ducts and blower. Variable speed motors allow the units to run the blower longer at lower speeds for increased efficiency and comfort. Heat pumps provide both cooling and heating and are more efficient than standard electric air conditioning and electric resistance heating but may require electric resistance or gas furnace backup in temperature extremes.
Ductless heat pumps, sometimes referred to as min-split heat pumps, consist of a single outside compressor/condenser unit connected to wall- or ceiling-mounted indoor air handler units. They use inverter technology with a variable-speed compressor that varies the refrigerant flow to continuously match the heating/cooling load. Without ducts and mounted inside conditioned spaces, a ductless heat pump can provide zoned heating with increased energy savings over standard heat pumps. Advanced units offer SEER ratings up to 26 and can perform at a much wider temperature range than standard heat pumps, eliminating the need for backup heat sources in many locations. Geothermal or ground-source heat pumps use the constant temperature of the earth as the exchange medium instead of the outside air temperature making them very efficient for heating and cooling, but initial installation costs can be prohibitive.
Manuals T and D are your guides for designing the air distribution system. Manual T provides best practice tips about where the supply registers, diffusers, and grills should be located, and how big they should be. Locating air handlers and return ducts in the garage is not recommended because of the potential for drawing carbon monoxide and hazardous fumes into the home. IRC says that ducts located in the garage may not have any openings in the garage and furnaces and air handlers that supply air to living spaces shall not supply air to or return air from a garage. All ducts, air handlers, and filter boxes should be air-sealed and joints and seams should comply with the 2009 UL 181-approved sealing methods. The Building America Best Practices Series recommends water-based mastic and caulk, foam sealant or mastic for duct-drywall connections.
Manual D details the duct system design, helping designers to evaluate duct lengths, fittings, and turns and select the best ducting material with the goal being to deliver the correct cfm to each room against the ideal friction created by the ducts and fittings with the static pressure available from the blower. Numerous studies have shown significant energy savings by placing ducts and air handlers inside the conditioned space. Ducts may be run through open-web floor trusses in a two-story home, through a dropped hallway ceiling in a one-story home, or through a conditioned attic. Duct runs should be as short as possible, and duct sizes and layouts should be shown on plans, per Building America’s recommendations. Alternatively, ducts can be located on the ceiling deck of an unconditioned attic and “buried” with blown-in insulation, however, condensation may occur at the duct exterior surface in humid climates; buried ducts may be permissible if they are directly insulated to R-8, apart from the piled-on insulation. But since the mixed-humid and hot-humid climates often exceed Building America’s maximum permissible dew points for this option, it specifically does not recommend buried ducts.
Building America states that all new homes should be equipped with whole-house mechanical ventilation and meet The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 62.2. A supply-only ventilation system that brings filtered outside air to the intake side of a home’s central air handler offers the advantage of positively pressurizing the home and reducing the intrusion of moisture and radon. Building America also recommends the addition of electronic controls, including a motorized damper and timer to open the damper in sync with operation of the HVAC fan for automatic ventilation of the home at regular intervals throughout the day. This approach would run the central fan on a timer basis, even when the compressor is not running. Exhaust only ventilation may draw too much air out of a home, creating negative pressure that pulls outside air in through leaky walls in older structures and potentially backdrafting combustion from fuel-burning appliances or fireplaces in a tightly sealed newer home.
Awarded a 2019 IoT Breakthrough Award, Rheem’s EcoNet Smart Thermostat is Alexa-enabled so users can control air conditioning, heating, and water heating appliances through voice command. EcoNet is capable of managing 65 percent of the energy used in a typical home.
A finalist in the International Buiders Show’s “Best of IBS” Awards, Enginuity’s multi-fuel residential Micro-Combined Heat and Power system (Micro-CHP) is a water heater that also provides most of the electricity and heating needs for your home. The Enginuity platform is built on the technological foundation provided by the world’s first production-ready 4-stroke inwardly opposed piston engine.
The best means for providing both supply and exhaust ventilation is with an energy recovery ventilator (ERV) with a heat exchanger that uses the heat or cooling from the outgoing exhaust air to warm or cool the fresh incoming air. ERVs are preferable over heat recovery ventilators (HRVs) in the humid climates since the heat exchanger transfers both water vapor and heat energy while HRVs only transfer heat.
Residential hot water energy use accounts for approximately 19% of the residential energy consumed in the United States, according to the Energy Information Administration. Building America advises builders to install high-efficiency, sealed combustion, power-vented, or direct-vented gas water heaters or to consider alternatives like tankless gas heaters, solar thermal water heaters, or air-source heat-pump water heaters. Other recommendations include insulating supply lines to R-4 and installing tanks that have at least R-12, consolidating bathrooms and other hot water-consuming activities into the same area of the house and placing the water heater In a central location, using a central manifold distribution system, locating plumbing pipes in the attic and covering them with insulation, avoiding oversizing the piping, avoiding continuous recirculation pumps, and sealing around plumbing penetrations in all exterior surfaces.
Every new system should be tested and properly commissioned. Duct airtightness should be verified with duct blaster pressure testing. at rough-in when leaks can be easily accessed and sealed. Building America notes that the time-delay relay on many newer air conditioning units is set to keep the fan running for about two minutes after the compressor shuts off. The relay can help boost efficiency in dry climates but allows some of the moisture on the evaporator coil to evaporate back into the air stream, contributing to increased indoor humidity in humid climates. It recommends resetting the time-delay relay to 30 seconds or less in humid climates. Also, the fan on central air conditioning systems should always be set to “Auto” rather than “On” for most efficient humidity control. Some air conditioners allow the compressor to start about 30 seconds before the blower starts and this is also recommended in humid climates to reduce “hot” blow and provide better dehumidification. Building America further advises making sure the drain pans are correctly installed and drain all the condensate water rather than creating a puddle. The EPA reports that 75% of installed air conditioners had the wrong amount of refrigerant when tested. Incorrect refrigerant levels can lower efficiency by 5% to 20% and can cause premature component failure, resulting in costly repairs.
The bottom line: Even with the best HVAC design, if it’s not installed as designed, the system will perform poorly. A savvy design team and tests to confirm proper installation and commission of the system cannot be considered optional!
Beverly Smirnis is the co-founder of Building Savvy magazine and publishes its flagship Dallas/Fort Worth edition. She and her business partner and husband, Steve Smirnis, have served as judges for the International Builders’ Show Best of IBS Awards and judged numerous homes for builders association parade events across the country.