There are three primary ways to provide mechanical ventilation to a home. The first is not good. The second is better but not great. The third is the best choice. Do you know your options?
Mechanical ventilation provides fresh air to the occupants of your home through active systems. It uses a fan to move air and controls to determine when the fan should operate so the appropriate amount of air is moved.
The three primary (popular) means of mechanical ventilation are: 1) exhaust-only ventilation, 2) HVAC bypass or supply-only ventilation, and 3) balanced ventilation through an energy- recovery ventilator1. If it seems like this blog post is already too technical, don’t worry. All three methods are relatively simple.
New building codes and standards require tight construction. Homes are pressurized to see if there are air leaks through gaps, cracks and joints. We can’t control the indoor environment (temperature, humidity and air quality) or the efficiency of the home when it has many air leaks. You may have heard the mantra, “Build tight and ventilate right.”
When we build tight houses, we need to provide fresh air for the occupants. The fresh air dilutes indoor pollutants and replenishes oxygen. This may help reduce allergy and asthma triggers.
Many existing homes don’t have ventilation to provide fresh air. The “fresh air” comes from air leaks. New building codes and standards mandate fresh air be provided through mechanical ventilation systems though your jurisdiction may not be using a new building code or standard. Mechanical ventilation systems have controls that operate on certain intervals to provide fresh air throughout the day so you don’t have to take action.
Bad: You Get That For Which You Pay
Some building codes and standards allow an “exhaust only” solution to ventilation. By running an exhaust fan (like a bathroom exhaust fan) intermittently, air is drawn out of the house and replaced with air that leaks in through the joints, cracks and gaps of the home’s exterior.
This is not a good method for ventilation because there’s not much control over where the air comes through the house. If the floor between the crawl space and the living area has more leaks than the exterior walls, it stands to reason more the of the “fresh” air will come from the crawl space. Air brought into the home through this method isn’t filtered and there’s no control over humidity.
This option is installed because it’s the least expensive yet it may technically meet the ventilation requirements. Avoid this option.
Better: Less Expensive Than Best
If you’ve seen an energy-recovery ventilator1 (ERV) at a home show, you may already know they can cost thousands of dollars installed. That cost can be easily justified if they solve or significantly reduce health problems like allergies and asthma though not everyone can afford that level of initial investment.
A less-expensive solution is to introduce fresh air into your existing central heating and cooling system. This “supply-only method” doesn’t give you the same balanced-ventilation solution an ERV provides but it can be relatively effective when the system is properly installed and checked to be sure it’s flowing the expected amount of air when it’s operating.
To bring fresh air into your heating and cooling system, your HVAC contractor connects an 8″ duct from fresh outside air to your return air duct system, just before the filter. A motorized damper is installed in the new fresh-air-supply duct and the control is connected to your heating and cooling system. When the blower (fan) in your heating and cooling system comes on, the damper control opens the damper in the fresh-air-supply duct to allow fresh air into the system.
The blower creates a pressure difference so fresh air is drawn into the system. The fresh air goes through the filter where pollutants and allergens should be removed. The air then flows through your air conditioner (across the air conditioning coil or heat exchanger) so it’s conditioned before it comes into your home.
When you use this solution in the summer, the humid fresh air from outside hits your cold air conditioner coil and the humidity condenses. Much of it drips off the coil into a drain pan rather than coming into your home.
This solution may cost less than half the expense of an ERV though it doesn’t work for all system configurations. The location of the air filter in your system the location of your air handler (the indoor unit with the blower) may make this method impractical. In those cases, you’re probably best served by an ERV.
The supply-only option is typically installed because it’s better than exhaust-only ventilation yet it’s not the most expensive option. In comparison, it appears moderately priced so you might assume it’s fine without taking time to understanding your different ventilation options. After all, did you ever think you’d be reading 1,120 words about home ventilation?
Best: Balanced Ventilation
An energy-recovery ventilator1 (ERV) offers balanced ventilation: it brings in as much air as it exhausts. It takes a lot of energy to heat and cool the air in your home. That’s typically the largest single use of energy in a home. When an ERV exhausts air from your home, it recovers most of the energy from that exhaust air by putting it into the incoming fresh air. It does this without mixing the exhaust air with the fresh intake air.
ERVs don’t need to be connected to your air conditioning system. They can be installed and function independently. That’s important in shoulder seasons when you may not need much heating or cooling but you still need fresh air.
Whether building a new house or renovating an existing home, sealing air leaks is critical to gaining control of the indoor environment. Once the leaks are sealed, you need fresh air for the occupants. A mechanical ventilation system should be designed to provide the amount of ventilation for the volume of the home, number of occupants and uses expected – there’s some calculation involved. One size doesn’t fit all.
Air sealing and proper ventilation can help reduce allergies and asthma. You can even buy ERVs with HEPA filters that remove “99.97% of allergens and other microscopic particles in the air”.
When presented with good, better and best options… many people would choose “better”. These options aren’t good, better and best. They’re bad, better (than bad) and best. When you choose, please choose the best option: the energy-recovery ventilator. Your health, your comfort and the efficiency of your home are easily worth the additional expense of an ERV. It may be one of your best investments.
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1 Energy-recovery ventilators (ERVs) are not the same as heat-recovery ventilators (HRVs). Though they both typically provide balanced ventilation, ERVs are typically capable of transferring some moisture (humidity) in the incoming air to the outgoing air – they may keep some humidity from entering the home. Performance can vary widely by manufacturer so ask your designer to help you choose the appropriate unit type, size and installation configuration.
Image by Atomic Taco used under creative commons license.