Technical Series 96-215
With new homes becoming increasingly airtight, building codes have been revised to require the supply and distribution of fresh air and the removal of stale air to prevent indoor air quality problems. This has lead to a steady increase over the past 10 years in the use of packaged heat recovery ventilators (HRVs). HRVs are relatively complicated devices that require frequent maintenance and adjustment. Concerns existed that most homeowners do not understand the problems that can occur if their HRV is not maintained and adjusted, such as over-or under-ventilation, backdrafting of combustion appliances or building envelope deterioration due to excess air exfiltration or poorly controlled humidity. Given the increased reliance on HRVs for ventilation, this research project was undertaken to determine the extent of homeowner knowledge and the extent of problems related to lack of maintenance and adjustment. At the same time, the opportunity was used to explore and assess the relationships between mechanical ventilation system design and installation practices as they relate to the effectiveness of ventilation of the dwelling.
The research project involved four phases:
A total of 60 homes were inspected: 15 on the west coast, 15 on the east coast and 30 in central Canada. All installations were at least one year old and ranged up to fourteen years of age. Using an HRV System Inspection Form, the investigator documented the way in which the HRV was originally installed, inspected the system and measured the air flow capabilities of the system at the time of inspection. Major deficiencies and occupant-related adjustments and revisions were also noted.
The Occupant Survey was completed at the same time as the inspection of the 60 HRV systems. The occupant survey was devised to determine the occupants' understanding of the operation of their HRV, their understanding of how their HRV interacted with other systems in their home and to ascertain the way in which the occupant used and maintained their HRV. In addition, the survey was used to relate occupants interpretations to the actual condition observed during the inspections. A supplemental Telephone Survey was conducted with an additional 15 owners of tract-built housing in Ottawa to correlate the results of the HRV inspections.
HRV installation types commonly used in Canada fall into four categories (Figures 1 to 4, respectively): Fully-Ducted, Extended, Simplified Standard, and Simplified Cross-Furnace. Twenty homes in Eastern Ontario, with HRV installations representative of these systems were tested using a tracer gas decay method. Tracer gas (sulphur hexafluoride) was distributed evenly throughout each house prior to testing. By measuring the decay in the concentration of the gas over time in a representative selection of rooms, the air change rate was derived. Measurements were taken:
Figure 1. Fully Ducted HRV Installations are completely independent of any other air circulating devices. Air is exhausted from areas of high contamination and humidity and supplied directly to other rooms throughout the house.
Figure 2. Extended HRV Installations exhaust air from the kitchen and bathrooms and supply air into the furnace return. The furnace fan circulates the ventilation air throughout the house.
Figure 3. Simplified Standard HRV Installations exhaust air from the furnace return air plenum and supply air to the furnace return air plenum (downstream of where air is exhausted). Ventilation air is circulated by the furnace fan.
Figure 4. Simplified Cross-Furnace HRV Installations are a modification of the Simplified Standard installation in which air is exhausted from and supplied to the furnace ductwork and circulated via the furnace fan.
Four experimental systems exemplifying low cost and energy efficiency were also tested. The experimental systems were based on conceptual systems proposed in a report published by Natural Resources Canada - Energy Impact of Ventilation Air Distribution. For this test, a house was selected where the HRV warm-side ductwork was configured in such a way that with minimal effort, the ventilation system could be rearranged from one alternative ventilation strategy to another. The testing methodology was the same as that used for the 20 conventional HRVs. The following alternative distribution systems were assessed:
All of the inspected HRVs have cross-flow heat recovery cores. The Simplified Systems (Standard and Cross-Furnace) are only prevalent in the Central Region. Fully-ducted systems are far more prevalent on the coasts as more homes in these areas are built without forced-air heating systems. In Ontario, until the early 1990s, the majority of HRV installations were the ex tended type. As HRV installations have become standard practice for many tract home builders, simplified systems have become more prevalent.
The majority of the HRV ventilation systems investigated were found to be operating as intended. However, problems were noted in many installations, including the following:
Of major concern is a lack of balancing of the HRV systems, particularly with the Simplified Cross-Furnace systems, and the lack of provisions to allow balancing. Additionally, of the systems inspected, 7 (12%) were not operational due to failure of a component of the HRV, in two of these cases the failed component being the supply motor.
The majority of occupants perceived their HRV to be beneficial. Most occupants understood the general purpose of their HRV; however, comprehension of the technical aspects required to use and maintain the HRV system properly was low, despite more than half having had the operation explained to them and most having been provided an operation manual (only 32% reported that they had read their manual). Further, few occupants understood the potential negative effects of a poorly operated or maintained HRV. Of the 77% of occupants who reported they understood their HRV systems, 55% had unbalanced systems, 60% had substandard ventilation and 55% of occupants with partially ducted or simplified systems were not aware of the need to operate the fan of the forced air heating system.
Most occupants (81%) reported that they regularly cleaned the HRV system components, but many systems were found to require maintenance. Forty-two percent had systems with dirty filters, cores or cabinets, 17% had blocked air intakes and 46% had unbalanced supply and exhaust air flows. Of the occupants that reported indoor air quality problems (26%), 60% had substandard ventilation, 62% had unbalanced supply and exhaust air flows and 56% had dirty filters, heat recovery cores or HRV cabinets.
For most houses, natural ventilation alone was found to be insufficient for the provision of adequate ventilation, confirming the position that houses require mechanical ventilation. The layout of the house and the design of the system were found to be important factors in the equilibrium of the distribution. The distribution, and hence the air change rates in individual rooms, varied widely from house to house depending on the layout of the distribution system. Small bungalows with relatively open plans had the best distribution, whereas air flow rates to bedrooms on upper floors in a two-storey houses tended to be weak
Contrary to anticipated results, closing doors increased the apparent mechanical air change rate in bedrooms where the air flow rates were high in relation to room volume. As might be expected, closed doors also had an effect in houses with fully-ducted systems, but had little effect in open areas, such a living rooms. For extended and simplified systems, the operation of an HRV without a circulation fan to distribute the fresh air resulted in the over-ventilation of the basement and inadequate fresh air distribution to all other areas.
There was very little difference in ventilation effectiveness obtained using a low-speed circulation system with an ECM motor or the higher, low-speed air flow rates associated with the "typical' furnace fan found in Canadian housing stock.
When the circulation system was not operating, the installation of a backdraft damper (Figure 5) to prevent fresh air delivered by the HRV to the supply plenum from entering the return air system improved the distribution of fresh air throughout the house. However, this was still insufficient to meet code ventilation rates on a room by room basis. Central point fresh air supply systems with additional fans to distribute air did not provide adequate ventilation throughout the house tested.
Figure 5. Simplified System with Backdraft Damper located in the vertical section of the furnace supply air plenum between the furnace and the connection of the HRV fresh-air supply.
In some installations, HRVs may not be realizing their full potential due in part to installation faults and in part to a lack of homeowner understanding. Existing information transfer mechanisms need to be improved. As many of the problems noted in the existing systems could have been prevented by proper installation, installers should be required to pass the installation and designers training programs offered by the HRAI. Installers should also be encouraged to offer HRV maintenance agreements to home owners and/or impress to them the importance of proper operation and maintenance.
The issue of most concern is that of properly balancing HRVs. In one house, the supply fan was not functioning. The homeowners were not aware of the problem because they still heard the sound of the exhaust fan. The result was backdrafting of the fireplace and the potential for backdrafting of other combustion appliances. HRV systems should be installed with balancing dampers and permanently installed flow measuring stations. Also of great concern is that filters were found to be dirty and air intake clogged. Clearly lack of owner appreciation of the potential hazards of a poorly maintained and operated HRV is also an issue of concern. The industry should also be encouraged to develop trouble indicating devices (e.g., trouble lights) or fail safe controls to indicate component failure or overdue maintenance.
Other recommendations for improving HRVs include limiting the use of flexible ductwork or encouraging proper design and installation. Cross-Furnace installations, which are most affected by varying the furnace speed, should also be discouraged. Circulation systems must be in operation to distribute fresh air throughout the house when an extended or simplified system HRV is in operation; controls should be provided with all such systems to allow continuous operation of the furnace fan at a low speed. (Such interlocking is now a requirement of the 1995 National Building Code of Canada.)
In order to improve HRV system performance, the regulatory agencies should continue with the current trend towards greater regulation regarding contractor training and certification standards, and HRV system installation practice and system requirements.
Project Manager: Duncan Hill
Research Report: Field Survey of Heat Recovery Ventilation Systems
Research Consultant: Buchan Lawton Parent Ltd.
full report on this research project is available
from the Canadian Housing Information Centre.
The information in this publication represents the latest knowledge available to CMHC at the time of publication, and has been thoroughly reviewed by experts in the housing field. CMHC, however, assumes no liability for any damage, injury, expense or loss that may result from use of this information.
©1999 CMHC-SCHL. All rights reserved.