Ventilation specialists think of the conventional American home with a forced-air system as a single well-mixed air zone. Whenever the heating or cooling system is on the system’s blower mixes the dwelling’s air.
High-Performance Dwellings are Different
High-performance buildings are different in several ways.
- New high-performance houses need less heating and cooling and therefore have less mixing from a central HVAC system.
- Next, tighter new homes have less mixing from natural infiltration though envelope leakage compared to conventional homes.
- Finally, many advanced new homes don’t have a central forced air system and instead have local heating and cooling mini-split units.
For these reasons, indoor air quality (IAQ) could be a problem in high-performance homes with ventilation systems.
A multizone system offers the capacity to treat each room according to its individual IAQ needs. Lead by the SVACH team, smart ventilation controls may soon regulate multi-zone ventilation systems.
Baseline Situation: Continuous Ventilation with ASHRAE 62.2
The American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) maintains a ventilation standard known as ASHRAE 62.2 for residential buildings. ASHRAE 62.2’s primary ventilation option is continuous ventilation and air flow rate determined by the standard. You determine the ventilation rate by using a table or formula, which considers bedrooms and the square footage of the home. Scientists use the studied and measured IAQ-plus-energy consumption at this level of ventilation as a baseline to compare various advanced single-zone and multizone ventilation strategies.
Pollutant Concentrations Vary from Room to Room
According to research, many parameters influence pollutant concentrations and corresponding ideal room-specific ventilation rates.
- Differences in pollutant emission rates
- Differences in ventilation rates
- Presence of a DCV sensors and controls
- Presence of an air recirculating system (such as forced-air HVAC)
- The position of the indoor doors (open or closed)
To evaluate differences between well-mixed single zone and multi-zonal situations, we use the pollutant-concentration metric called the absolute average fractional differences. We define this metric as the concentration difference between the two rooms divided by the average of the two.
According to other research, VOC contaminant concentrations in manufactured homes varied from 0% to nearly 60% between rooms. Carbon dioxide concentrations varied from 30% to 70% between rooms. Radon fractional differences between rooms in new energy-efficient homes varied in a similar range of percentages.
A sensitivity analysis using the two-zone model demonstrated that the key factors influencing pollutant concentrations are the following.
- Emission source strength and location
- The air change rates
- The inter-zonal airflows
Multizone strategies often depend on zone dampers.
Single-Zone / Multi-Zone: How to Choose?
Designers and installers implement the single-zone strategy using a single zone ventilation system, designed to meet a single IAQ standard for the entire dwelling. Here are a few examples of single-zone dwelling-unit ventilation systems.
- An exhaust-only ventilation system in a conventional home or apartment
- An exhaust only ventilation system in a fairly tight home or apartment that uses trickle ventilators that are part of windows
- A balanced ventilation system that isn’t designed or commissioned to consider the pollutant concentrations on a room-by-room basis but only as a single mixed zone
Designers and installers implement the multi-zone strategy using a ventilation system designed and commissioned to meet IAQ standards in the dwelling’s different rooms. Here are a few examples of multi-zone dwelling-unit ventilation systems.
- An exhaust only ventilation system in a tight home or apartment that uses auto-adjustable trickle ventilators
- A balanced ventilation system, operating intermittently or continuously, that provides airflow to the dwellings rooms and meets particular IAQ standards for the dwellings rooms rather than for the entire dwelling unit
- An HVAC economizer delivering mixed return-outdoor air to dampered registers controlled by occupancy sensors or pollutant sensors
Further Reading
“Impacts of Mixing on Acceptable Indoor Air Quality in Homes.” HVAC & Research Journal (2010).