Building a tight house is the goal for most of us. What that means is that the unintended openings, gaps and holes in the home’s exterior shell and in the duct system should be sufficiently sealed to keep outside air from leaking in, and to keep inside air from leaking out. A tight house will be more comfortable and have increased energy efficiency and lower utility bills. That’s because conditioned inside air produced by your HVAC system is less likely to escape and unconditioned outside air is less likely to sneak in through unwanted gaps. A tight house is also quieter, cleaner, and has better indoor air quality because outside noise, pests and pollutants have fewer opportunities to enter the home.
If you’ve listened to past episodes, you know that if we build tight, we should also ventilate right. You’ll hear old school contractors say that building a tight house is not a good idea because, they argue, a house should breathe. Modern building science has proven that old school contractors are wrong to avoid a tight building envelope. But, that old adage that a house should breathe is actually right. However, instead of house breathing through the uncontrolled air infiltration of a leaky house, we want a house with a tight building envelope that breathes through controlled ventilation.
This week. we’ll discuss ventilation systems for the home. Like last week’s moisture control mini lesson, the majority of the information this week comes from the US government’s website, energy.gov.
Let’s get started.
A tight house requires controlled mechanical ventilation to maintain good indoor air quality. A ventilation system takes stale air out and brings fresh air in.
Without controlled ventilation, the air-sealing techniques of a tight building envelope can trap indoor air pollutants inside the house—pollutants like formaldehyde, volatile organic compounds (VOCs) and radon.
Another role that ventilation systems play is in removal excess indoor moisture, which, if left unchecked, could lead to mold, rot and structural damage. We learned last week about how to control damaging moisture in your home, for more detail, take a listen to episode 134 called Control One of Your Home’s Biggest Enemies: Moisture.
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has come up with a calculation that will tell you the appropriate amount ventilation your house needs. That calculation takes into account the number of people living in the home, the number of bedrooms, how many stories a house has and a percentage of the total conditioned square footage of the home. Ask your HVAC guys, home energy rater or builder to help you with the calculation to determine the amount of ventilation you need for your home.
Now let’s briefly go over the 3 MAIN VENTILATION STRATEGIES we can use:
1. Natural ventilation
2. Spot ventilation
3. Whole-house ventilation
Most homes should use all 3 ventilation strategies.
In a poorly-sealed, leaky house, which we hopefully won’t have, natural ventilation occurs with the uncontrolled air movement in and out of the cracks and small holes in the home’s building envelope. In today’s tight homes, though, intentional natural ventilation occurs when opening windows and doors.
Spot ventilation can improve the effectiveness of natural ventilation and whole-house ventilation by removing indoor air pollution and/or moisture where they develop. Bathroom exhaust fans and kitchen range hoods are examples of spot ventilation.
Whole-house ventilation systems provide controlled, uniform ventilation throughout a house. These systems use one or more fans and duct systems to exhaust stale air and/or supply fresh air to the house.
There are 4 basic types of Whole house ventilation systems: exhaust, supply, balanced, and energy/heat recovery systems.
-Exhaust ventilation systems work by removing air from the house.
-Supply ventilation systems work by adding air to the house.
-Balanced ventilation systems add and remove equal amounts of fresh outside air and stale inside air.
-Energy recovery ventilation systems (ERVs) and Heat recovery ventilation systems (HRVs) provide controlled ventilation while minimizing heat loss.
Now let’s talk about each type of whole house ventilation in a little more detail.
EXHAUST VENTILATION SYSTEMS
Exhaust ventilation systems remove, or exhaust air from the house. This depressurizes the house.
Exhaust ventilation systems are best for cold climates. That’s because in hot humid climates, the exhaust ventilation systems can trap moist air in wall cavities, where it can condense and cause moisture damage.
Typically, an exhaust ventilation system consists of a single fan connected to a centrally located, single exhaust point within the house. This simple system is relatively easy and cheap to install. You can also connect the fan to several rooms through a duct system and add window and wall vents to introduce fresh air.
The problem with exhaust ventilation systems is that they suck. No, seriously, they literally suck. And if the system is sucking air in from an attached garage, a damp crawl space, or dirty attic, you could be bringing pollutants like dust, radon, mold, and garage and fireplace fumes into the house.
SUPPLY VENTILATION SYSTEMS
Supply ventilation systems use a fan to bring in, or supply, the house with fresh, outside air. Supply ventilation systems pressurize your home, meaning they add air pressure to your home.
Like exhaust ventilation systems, supply ventilation systems are relatively simple and inexpensive to install. A typical supply ventilation system has a fan and duct system that introduces fresh air into one or several rooms. This system can include window or wall vents in other rooms.
As compared to an exhaust system, supply ventilation systems are better at controlling the quality of the air that enters the house. Air brought in by supply ventilation systems is filtered to remove pollen, dust and humidity.
Supply ventilation systems work best in hot or mixed climates. In cold climates, supply ventilation can cause moisture to condense in attics and the outer parts of exterior walls, causing mold, mildew, and decay.
BALANCED VENTILATION SYSTEMS
Balanced ventilation systems, if properly designed and installed, will neither pressurize nor depressurize your home. Instead, they introduce and exhaust equal amounts of fresh outside air and stale inside air.
A balanced ventilation system usually has two fans and two duct systems. Balanced systems allow the use of filters to remove dust and pollen from outside air before introducing it into the house.
Balanced ventilation systems are appropriate for all climates. But because they require two duct and fan systems, balanced ventilation systems are usually more expensive to install and operate than supply or exhaust systems.
None of the 3 ventilation systems that we just talked about remove heat from fresh air before it enters the house. Therefore, supply, exhaust and balanced systems can cause higher heating and cooling costs, unlike energy recovery ventilation systems, which is the last type of system on our list today.
ENERGY/HEAT RECOVERY VENTILATION SYSTEMS
Energy/Heat recovery ventilation systems provide a controlled way of ventilating a home, while minimizing energy loss. In the winter, these systems transfer heat from the warm stale indoor exhaust air to the fresh, cold, outside supply air. In the summer, the inside air cools the warmer supply air. This exchange of heat helps to reduce heating and cooling costs.
There are two types of recovery ventilation systems: heat-recovery ventilators (HRVs) and energy-recovery ventilators (ERVs). HRVs and ERVs are similar devices in that both supply fresh air and exhaust stale air, while recovering heat. The primary difference between the two is that an HRV transfers only heat, while an ERV transfers both heat and moisture.
Both HRVs and ERVs include a heat exchanger, one or more fans, and controls. These systems includes two fans, one to exhaust stale indoor air and one to bring in fresh outdoor air. There are also filters for both air streams.
In the summer, an energy-recovery ventilator may help to control humidity in the house. There is, however, some controversy about using these ventilation systems during humid, but not overly hot, summer weather. Some experts suggest that it is better to turn the system off in very humid weather to keep indoor humidity levels low.
HRVs and ERVs are more expensive than exhaust, supply and balanced ventilation systems. HRVS and ERVs are most cost-effective in climates with extreme winters or summers, and where the cost of fuel costs is high. In milder climates, you probably won’t get enough cost savings to justify purchasing and running the ventilation systems. In other words, for most houses, attempting to recover all of the energy in the exhaust air will probably not be worth the additional cost of an ERV or HRV.
Energy recovery ventilation systems require more maintenance than other ventilation systems. They need to be cleaned regularly to preserve efficiency and to prevent mold and bacteria on heat exchanger surfaces.
There is so much more information that you can read about ventilation systems. I just wanted to give you a brief overview in this week's mini lesson. Let’s go over a few key points with a couple of quiz questions.
1. Which of following statements are true about the 4 types of whole house ventilation systems?
A. Exhaust ventilation systems work by removing air from the house.
B. Supply ventilation systems work by adding air to the house.
C. Balanced ventilation systems add and remove equal amounts of fresh outside air and stale inside air.
D. Energy recovery ventilation systems (ERVs) and Heat recovery ventilation systems (HRVs) provide controlled ventilation while minimizing heat loss.
E. All of the above
The answer is E, all of the above
2. Which of the following statements is false?
A. Exhaust ventilation systems are best for cold climates.
B. Supply ventilation systems work best in hot or mixed climates.
C. Balanced ventilation systems are appropriate for all climates.
D. HRVS and ERVs are most cost-effective in climates with extreme winters or summers, and where the cost of fuel costs is high.
E. Both HRVs and ERVs transfer heat and moisture.
The answer is E. E is false. ERVs transfer both heat and moisture and HRVs transfer only heat.
That’s it for this week. I hope you learned as much as I did and I hope you’ll join me next week for another episode of Build Your House Yourself University--BYHYU.
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