Problems with Mechanical Ventilation

Essentials of Mechanical Ventilation for Homeowners/Residents

With the increasing tightness of homes, mechanical ventilation is a necessary feature for building design to ensure acceptable indoor air quality and maintain occupant comfort. However, mechanical ventilation is affected by many factors that determine its efficiency and effectiveness, including building tightness, duct systems, and outside air conditions. Understanding these factors is beneficial for homeowners and residential occupants to maintain a healthy and energy efficient space.

Why the QuFresh does not violate code for 2009 ICC

This article is intended to address some common concerns that code officials have when a single family builder or multifamily developer either chooses to introduce outdoor air into residential dwelling units or is required to by the codes as determined by a municipality

Regarding the devices ability to operate intermittently through the Eco-Saver mode, Chris Holland of the International Code Council expressed this Technical Opinion, “If the system meets the ventilation airflow rates indicated in Table 1507.3.3(1) <IRC 2012> then it would appear to meet the requirements in the code. It should be noted that while the code states that manual override controls must be provided there is nothing in the code which states what the manual override is supposed to do.” With the design of the system being done under the assumption of continuous operation, the Eco-Saver mode is only a manual override option for the owner.

This technical opinion is further strengthened by the ASHRAE 62.2-2010 User’s Manual for item 4.4 Control and Operation in which they state, “The occupant must be able to modify the settings or override the system if he or she so chooses. While the standard requires the whole-building ventilation system, the occupant has the option of choosing not to operate the system.”

ASHRAE 62.2 user manual also states that supply ventilation can be provided through dedicated supply fans or through the central HVAC system air handler, and can supply air directly into the dwelling unit or through the HVAC ducting. This indicates that there are quite a variety of ways of installation that would be acceptable.

Section 403 of the IMC 2009 discusses Mechanical Ventilation (the alternative to Natural Ventilation) for all buildings with the exception of detached one- and two-family dwellings and multiple single-family dwellings (townhouses) not more than three stories high. Table 403.3 identifies the amount of needed outdoor air ventilation as 0.35 Air Changes per Hour but not less than 15 cfm/person. With the QuFresh’s max rate of 110 cfm, a single device will meet the requirement for a 2357 sqft residence (assuming 8 ft ceilings) or a residence with 6 bedrooms. These airflow rates are based on ASHRAE 62.1 which is intended for commercial buildings but are applicable for Multifamily Apartments four stories and higher. The more applicable Standard for a detached single family residence is ASHRAE 62.2 and is what the 2012 IRC is based upon. This Standard determines the outdoor air requirements based on both the size of the dwelling unit and the number of bedrooms but requires a lower amount of air which is friendlier to energy efficiency. Under ASHRAE 62.2 a single FAM can serve a dwelling unit up to 6000 sqft and 5 bedrooms.

There have been some concerns raised by code officials regarding the impact of bringing in outdoor air ventilation upon the energy efficiency of the building.

The majority of buildings built follow a performance path calculator which does not factor in these impacts as it is not referenced until the 2012 IECC, but it is not considered to be significant. As an example, the Energy Efficiency Rating Software, REM/Rate, which is used by the Home Energy Rating System industry can model total energy usage with and without an Outdoor Air ventilation system. This software was used to examine a 1653 sqft dwelling unit with 4 bedrooms in Dallas, TX. The estimated total energy with outside air introduced by the air handling unit was 89.9 MMBtu/yr while without it was only 87.2, a 3% increase. It is important to note though that the house achieved a HERS 71 and that the 2009 IECC is roughly a HERS 87 in the same climate zone, so the house is already 18% better than code. A 3% increase from an outdoor air system should be acceptable.

The impact of additional energy generated to operate the fan or Air Handling Unit can be offset by including a timer or controller and allowing the system to operate intermittently. Care must be taken when running the outside air to/near a furnace as the manufactures have minimum return air temperature requirements (typically 55°F) therefore having a controller that limits the air being brought in based on the temperature is a large benefit in these situations.