Limitations of Centralized Heat or Energy Recovery Ventilators

Due to the air tight design of modern buildings, heat recovery ventilators are utilized to deliver fresh air to indoor spaces and remove stale air and pollutants such as odors, CO₂, particulates, etc. Heat recovery ventilators provide excellent indoor air quality with the additional benefit of heat exchange between fresh (supply) and stale (exhaust) air streams. Furthermore, a special type of heat recovery ventilator which is called an energy recovery ventilator can be used to transfer heat and moisture between fresh and stale air streams. Despite the positive merits of centralized heat or energy recovery ventilators (H/ERVs), indoor occupants still suffer from poor indoor air quality due to some factors which are briefly discussed here:

Inadequate Sizing

While design standards provide the framework to determine the minimum ventilation flow rate that is needed for balanced ventilation, the required air flow rate may be under estimated at the design stage due to invalid occupancy assumptions, unaccounted infiltration/ex-filtration in old buildings, etc. Additionally, the inaccurate application of standards (e.g., ASHRAE Standards 62.1 and 62.2, International Residential Code, Building Science Corporation) or applicable local codes can result in an under-estimation of the adequate air flow rates for indoor occupants.

Flow Mal-distribution

The air flow distribution in buildings depends on the level of air mixing and replacement that can be achieved based on the number, position, and characteristics of supply and exhaust channels amidst other factors. H/ERVs are typically installed by connecting supply and exhaust ducts to all spaces (the bathroom may be exempted), partially connecting ducting to large spaces (e.g., supply air to the living room and exhaust air from the kitchen), or connecting supply and / or exhaust ducts to the appropriate ports of an air handling unit. Depending on the method of installation, the lack of efficient air flow and replacement pathways between the indoor spaces can lead to internal air recirculation or stagnant air zones which may compromise the indoor air quality. Even in houses where the centralized ventilation system is not connected to the air handling unit and is fully ducted to most or all spaces, the temporal use of dryers, kitchen exhaust hoods, and bathroom exhaust fans can result in periods of depressurization where sufficient fresh air is not supplied to living spaces.

Flow Imbalance

H/ERVs are designed to provide balanced ventilation to spaces to avoid the problems of under pressurization and over pressurization which are associated with exhaust-based and supply-based systems, respectively. However, due to the wide discrepancies between manufacturer-based laboratory versus field installation conditions, the supply and exhaust sides of an H/ERV often become imbalanced from the differences in air flow resistance that arise from the ducting, bends, accessories, fouling of filters, etc.

Frost Accumulation

In cold climates, frosting can occur in an H/ERV when the water vapor in the relatively hotter and more humid air stream is cooled below its freezing point. The accumulation of frost in an H/ERV can degrade the heat or moisture transfer performance of the equipment, and is often mitigated by using control strategies that reduce the air flow rate and / or quality (e.g. throttling the supply air or recirculating the exhaust air) or consume additional energy (e.g., preheating the relatively colder air stream). During prolonged sub-zero ambient conditions, the application of defrosting strategies in H/ERVs may compromise the air quality in buildings thereby necessitating the need for supplementary balanced ventilation.

Bibliography

Alonso, M. J., Liu, P., Mathisen, H. M., Ge, G. and Simonson, C. (2015) “Review of Heat/Energy Recovery Exchangers for Use in ZEBs in Cold Climate Countries.” Building and Environment. 84:228-237.
ANSI/ASHRAE (2019) “Standard 62.1. Ventilation for Acceptable Indoor Air Quality.” ASHRAE, Inc., Atlanta, GA.
ANSI/ASHRAE (2019) “Standard 62.2. Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings.” ASHRAE, Inc., Atlanta, GA.
Merzkirch, A., Maas, S., Scholzen, F. and Waldmann, D. (2016) “Field Tests of Centralized and Decentralized Ventilation Units in Residential Buildings – Specific Fan Power, Heat Recovery Efficiency, Shortcuts and Volume Flow Unbalances.” Building and Environment. 116:376-383.
Rafati Nasr, M., Fauchoux, M., Besant, R. and Simonson, C. (2014) “A Review of Frosting in Air-to-Air Energy Exchangers.” Building and Environment. 30:538-554.