If not installed or maintained properly, forced-air heating and cooling systems can perform poorly, creating air quality problems and unnecessary expense. To optimize a forced-air system, you need to design or refine it to prevent leaks and pressure imbalances, protect it properly during construction (or make sure the ductwork is clean and sound), and use the right filter.
The performance spec of the heat (or cooling) source is also important, but it is critical to address the distribution system first before investing in better furnace and air handler performance. This article is the FINAL of four focused on optimizing forced-air systems on the topics of Distribution, Filtration, Heat Source (and Controls), and Ventilation.
Whether in a new house or an existing home, mechanical ventilation is an important component of indoor comfort and building durability. Here in Washington, codes have required it for several years, but that doesn’t necessarily mean the ventilation system in a home is effective.
This is another system that is worth having tested by a qualified contractor to see if it is over- or under-performing. A system that is under-performing (provides less airflow than recommended) may not provide adequate fresh air or remove enough moisture from the home. A system that is over-performing (providing too much airflow) will increase energy consumption, as heating system has to heat or cool the additional volume of air.
The ventilation performance requirements of the IRC 2009 (and the current Washington Residential Code) are consistent with ASHRAE 62.2-2007, a recognized national reference standard, and a great performance benchmark even for a project that isn’t covered by the code. The intent of this standard is to ensure adequate air changes in the home, based on size and occupancy load. It requires mechanical ventilation with dedicated outside air inlets in new construction to increase the likelihood of effective ventilation.
For existing homes, it should be noted that mechanical ventilation becomes increasingly important as the building envelope becomes more air-tight. If a blower door test gives a result of ≥7 ACH@50, the focus should be on reducing envelope leakage rather than adding mechanical ventilation. This test result is roughly equivalent to 0.35 nACH or about one air change every three hours. With this much leakage in the envelope, the house will experience more than enough ventilation through the natural convection caused by “stack effect” as well as those induced by the influence of wind on the structure. But if your envelope weatherization efforts have brought envelope leakage substantially below this level, adding effective mechanical ventilation is a top priority.
Let’s assume we’re talking about a project that has successfully upgraded the building envelope to something less than 4 ACH50. A common practice in forced air homes is to integrate a mechanical ventilation system into the forced air system; a logical approach since you already have a system of ducts to deliver air to all the living spaces. This is called a “Central Fan Integrated System (CFIS)” – Imagine that. It is a straightforward solution that can be properly implemented for around $500 as long as your distribution ductwork is clean and sound and there’s easy access for a duct run from your air handler to an exterior wall, away from other exhaust outlets.
In its simplest form, this approach consists of a duct into the return side of the air handler, drawing fresh air into the system whenever the air handler fan runs. The problem with this approach is that the ventilation rate is totally dependent on how much the air handler runs. So on the coldest days (or the warmest days in a cooling-dominated climate) you get the greatest ventilation. On these days, the outside air also needs more conditioning to meet the indoor set-point temperature – so that means more heating (or cooling) energy; a recipe for higher energy bills.
This can be mitigated by adding a “fan cycler” – a control box connected to a mechanical damper in the fresh air duct. See the mechanics of an AprilAire fan cycler in the photo at left. The fan cycler opens and closes the fresh air duct and monitors how much time the fan runs with the duct open. On cold days, it will close the fresh air duct once the ventilation air volume for a given period has been reached. On mild days, when heat (or cooling) demand is low, it will also switch on the air handler fan to circulate fresh air to meet the ventilation requirement. The disadvantage of this action is that it uses the large air handler fan (and associated electrical consumption) to circulate a relatively small amount of fresh air. If your air handler includes an efficient, variable speed fan this impact is significantly reduced. Aircycler provides an interactive schematic showing the airflow.
Another approach that reduces fan energy is to tie the Fan Cycler to one or more exhaust fans and use these fans to pull outside air through the ducts when the air handler is not running. Energy Star labeled fans from Panasonic and others use a fraction of the energy of a typical air handler fan. I’m not personally convinced this solution provides good outside air distribution but it may be worth further exploration.
Energy (or heat) recovery, where energy from the stale exhaust air is used to pre-condition the incoming fresh air, can also be integrated into this approach. Typically, the air handler fan of an HRV or ERV is not large enough to effectively circulate fresh air through the large, forced air duct system, resulting in inadequate distribution and mixing of outside air. It is better to integrate the ERV/HRV with a fan cycler and use the furnace air handler fan, (Venmar AVS used to sell a passive HRV unit called the Enerflo, that offered all this in one convenient unit, but I’m not sure this product is still available – comment if you know differently!).
You can also install small diameter, dedicated ductwork for the ventilation air distribution. Either approach will involve increased installation cost and control complexity (see the photo). In simple payback terms, this probably only makes sense in very cold or hot climates.
IMPORTANT NOTE: Any CFIS system requires proper commissioning by a qualified technician to ensure it is performing properly and delivering the required rate of ventilation in a range of conditions. Your HVAC contractor may be able to provide this service, or you may choose to use an independent contractor such as those listed with Home Performance Washington, RESNET, Building Performance Institute (BPI), National Comfort Institute Certified Contractors.
Alistair Jackson, LEED AP, CSBA is Principal in Charge of O’Brien & Company’s Residential Technical Services, and is a LEED for Homes, Energy Star and Built Green Verifier, LEED for Homes Faculty and ARCSA certified Rainwater Harvesting designer. Alistair is a frequent contributor to the Building Capacity Blog and was also a major contributor to the Northwest Green Home Primer.
Did you enjoy this article? You might also like these Building Capacity Blog articles:
Tips for Optimizing Forced-Air Heating and Cooling Systems Part 1
Tips for Optimizing Forced-Air Heating and Cooling Systems Part 2
Tips for Optimizing Forced-Air Heating and Cooling Systems Part 3
Interview with LINC’s Maria Elena Marquez-Brookes
Creating Effective Energy Efficiency & Conservation Strategies Part 1
Cracking the Energy Code: What Will It Cost?
Mechanical Systems and Fuel Choices for the Warming World