Before I even get started, I want to point out that I am no expert on ventilation. I have learned a lot from (and rely on) many experts, including Paul Raymer, Gord Cooke, John Krigger, Joe Lstiburek, Armin Rudd, and Terry Brennan, among others. I depend on them to fuss about the details of how much ventilation a house needs.
I do, however, have to deal with ventilation requirements when I work with my clients to certify their buildings. Most residential certification programs require that a home or apartment meet the ASHRAE 62.2-2010 whole house ventilation standard. This can be accomplished through exhaust-only, supply-only, or balanced systems.
In the humid Southeast, we tend to discourage exhaust-only systems, but some developers and contractors meet the requirement by using a continuous bath vent fan – not the most efficient method, but it is simple.
Supply-only systems normally have an outside air intake into the HVAC return plenum with a controller that turns on the HVAC blower motor and opens a damper when air is needed.
Balanced systems in my climate use energy-recovery ventilators (ERV).
Supply-only system problems
When I test out and inspect a home for certification, checking for an operating ventilation system is a piece of the action. For an Energy Star home, the ventilation system’s flow rate must be tested. Other programs only require that the system be installed and sized properly, without any other field verification.
Most supply-only systems I run into include a motorized damper connected to a control with a ventilation cycle function. I have yet to see an HVAC installer who figures out the CFM coming in through the supply duct, calculates the required ventilation rate, and sets the fan control to meet ASHRAE 62.2. They just put in the damper and controller and assume everything will work out fine.
In several cases, I have seen the dampers installed so that they default to the open position – sometimes causing duct systems to fail leakage tests, particularly if they don’t have a good outside damper. In these cases, no one has any idea if the ventilation system actually does what it is supposed to do.
ERVs: Are they as good as we think they are?
In doing some research on ERVs, I learned that if they are connected to whole-house duct systems, the air handler blower must run to move the outside air throughout the house – the ERV motor can’t move enough air through a duct system.
After several discussions with ERV manufacturers’ support staff, I have come to the conclusion that they should be installed with dedicated duct systems to avoid the complexity of control systems and the energy use involved with running the blower motor.
This leads to the question: where do you put the supply and exhaust ducts in the house? Some people suggest drawing exhaust air from bathrooms and supplying fresh air to bedrooms or living areas. This has some appeal in that you can eliminate bathroom fans. However I have read other opinions that don’t like the idea of exhausting bathrooms into ERVs. What’s a poor, ventilation-ignorant boy to do?
Another minor beef I have with ERVs is how they are treated in the RemRate energy modeling program. The new Energy Star target HERS Index, required for some green certifications, can be a challenge to meet. On several recent projects for which I have done energy models, I discovered that upgrading from supply-only ventilation to an ERV lowers the HERS index by 7 or 8 points, allowing a project to meet the required target index and achieve certification.
I appreciate the fact that ERVs are more efficient due to the motor size and the fact that they do actually recover some of the energy in the heated or cooled indoor air. But do they really increase the energy efficiency of a house by 10% or 15%? The conspiracy theorist in me thinks that there might be some collusion between the ERV manufacturers and the energy modeling software companies. (Only kidding, guys).
So how much ventilation do we really need?
I’ve heard Terry Brennan suggest that ASHRAE 62.2 is too low a ventilation rate. I’ve heard Armin Rudd say it may be too much. Joe Lstiburek recommends installing systems that provide 1.5 times the required rate, but setting them to run initially at ½ the rate, letting the occupants adjust it themselves.
I kind of like this last idea, because the designed rate seems to me to be somewhat arbitrary. It has to be either too high or too low for most people. If you have a bunch of dogs, smoke tobacco, fry turkeys indoors, and don’t vacuum your carpet, then you probably need a high rate of ventilation – maybe even more than the ASHRAE 62.2 rate. If you don’t wear shoes in the house, don’t smoke, don’t have pets, and keep your turkey frying outdoors, then you need less.
Then there’s the thought of occasionally opening windows and doors when the weather is nice. I know that we can’t rely on passive ventilation to guarantee a specific number of air changes, but can’t we please just take a little responsibility for our homes?
I understand that a lot of very smart people have been working on the ASHRAE ventilation standard for a long time, but I am concerned that we just aren’t getting the anticipated results from all the effort that goes into this issue. Like I said, I’m no expert, but something about this whole thing is starting to bug me.