Most people spend about 90 percent of their time indoors. For home owners, indoor environmental quality (IEQ) is the most meaningful differentiator between ‘high-performance’ and ‘code-built’ homes.
Indoor environmental quality
IEQ describes how well the indoor environment promotes occupant comfort and health. The components of IEQ include thermal comfort, indoor air quality (IAQ), sound and lighting. Requirements for optimal IEQ vary per occupant and household, due to individual health needs and levels of sensitivity to sound, light, color and temperature.
Let’s discuss how to control the indoor environment for IEQ with particular attention to how heat pumps can help improve thermal comfort and IAQ.
Start with the Building Envelope
A healthy and comfortable home with optimal IEQ can be considered a single system
A healthy and comfortable home with optimal IEQ can be considered a single system, which consists of interdependent parts and sub systems. Mechanical system designers give careful consideration to how components perform, in relation to each other and other variables.
Code requirements for tighter building envelopes, improved windows, increased insulation values and more efficient appliances have reduced energy use intensity (EUI), since the 1970s, but also provide the foundation for better performing homes. Optimal IEQ requires control over how air, thermal energy (heat) and moisture enter, exit and flow through the building.
Limit Thermal Bridging
A thermal bridge is an area that has higher thermal conductivity than the surrounding materials, creating a path of least resistance for heat transfer. Thermal bridges reduce energy efficiency and create health and comfort challenges.
When the components of a building assembly are made colder than the air in those spaces, there is the potential for condensation. This can reduce the durability of the building and create potential health hazards, such as mold. When designing high performance homes for maximum efficiency, health and comfort, choose techniques and products, like continuous exterior insulation and windows with low U-values, so as to eliminate limit thermal bridging.
Heat Pumps and Thermal Comfort
Comfort is a subjective experience, affected by variables, including the occupant’s age, level of physical activity and where they were raised. ASHRAE Standard 55 and the Predictive Mean Vote (PMV) concept use five factors to help builders design comfortable environments that are specific to occupants:
- Operative temperature
- Air speed
- Relative humidity
- Metabolic rate
- Occupant clothing
High performance heat pump with variable speed compressor
A high performance heat pump with a variable speed compressor can meet the challenge of subjective comfort, while improving the home’s energy efficiency. Heat pumps use the natural movement of thermal energy from hotter objects to colder objects, in order to heat or cool the home’s zones.
In heating mode, the outdoor unit expands refrigerant gas to make it colder than the ambient air, enabling the unit to extract thermal energy from the outdoor air and transfer it via refrigerant lines to the indoor unit, conditioning the zone. Using this method, a heat pump can provide more energy for heating than it consumes in electricity. Even at low ambient temperatures, modern, all-electric heat pumps can be up to three times more efficient than conventional electrical-resistance systems.
Heat pump systems with individually-controlled indoor units
Heat pump systems with individually-controlled indoor units for each zone create opportunities to customize comfort, for specific occupants and activities (e.g., cooking, exercise, sleep). Using multiple independent units and compact duct runs offers more flexibility to design, according to occupant preferences. If the cost of changing ductwork in an existing house is prohibitive, ductless indoor units may be the solution.
| Performance can be improved by applying a heat pump system with a compact duct design |
Compact Duct Design for Heat Pumps
Performance can be further improved by applying a heat pump system with a compact duct design, instead of a large, conventional duct design. During heating season, thermal energy is lost as conventional systems push conditioned air through long duct runs in unconditioned spaces. This heat loss through ducts can result in comfort issues and poor energy performance.
In compact designs, duct runs are shorter and more centralized, with ducts running to interior walls and blowing toward exterior walls. With shorter duct runs and less opportunity for energy loss, contractors can install smaller indoor units with smaller and quieter fans that use less energy. Ductless indoor units for heat pump systems, such as recessed ceiling cassettes or wall mounts, don’t require ducts. Ducted air handlers and horizontal-ducted units are compact, making it easier to fit all the HVAC equipment and ductwork within the envelope.
Indoor Air Quality
Indoor Air Quality (IAQ) is particular to occupants, but less subjective than comfort
Indoor Air Quality (IAQ) is particular to occupants, but less subjective than comfort. The primary objective of IAQ design is keeping pollutants from endangering occupant health. Contaminants of concern include particulate matter 2.5 (PM 2.5), acrolein, formaldehyde and volatile organic compounds (VOCs). PM 2.5, for example, can contribute to asthma, sinus congestion, coughing, skin rashes, brain plaque and cognitive issues, including headaches and sleep disturbances.
Source control is the foundation of IAQ. In designing healthy homes, builders should avoid materials that off-gas formaldehyde and VOCs to limit the infiltration of pollutants. All-electric heat pumps align with the trend of limiting or eliminating the hazards of on-site fossil fuel combustion in healthy homes. After addressing source control, builders can apply methods and products for filtration, elimination and dilution.
Whole-home Filtration
The Minimum Efficiency Reporting Value (MERV) uses a scale of 1-20 to describe how effectively a filter can capture particles of a given size. A High Efficiency Particulate Air Filter (HEPA) with a MERV range of 17 to 20 and can remove 99.97% of airborne particles as small as 0.3 micrometers.
When determining the level of filtration required, the impact of the static pressure drops associated with increases in the efficacy and depth of pleated filters considered. The duct design and Manual D calculations must account for pressure drops which cause air to move more slowly. Homes with multiple zones may have a mix of indoor unit styles, including ductless and ducted units. If a home owner requires higher filtration levels, HVAC contractors can install a complementary system purpose-built for whole-home filtration.
High performance ventilation systems, ERVs and HRVs
Healthy and comfortable homes use high performance ventilation systems to introduce fresh outdoor air. An energy recovery ventilator (ERV) or heat recovery ventilator (HRV) provides conditioned ventilation air to dilute pollutants and remove stale air, without significantly increasing heating or cooling loads. Humidity-balanced, conditioned fresh air may be directed to the air handler or ducted directly to zones served by ductless units.
With modern construction methods, mechanical systems, knowledge of human physiology and help from immutable physical laws, HVAC contractors can help produce homes purpose-built for occupant comfort and health.