Heat and Energy Recovery Ventilators (HRV and ERV)


This post follows up on the last post, covering how to get rid of smells in your home.

One of the best ways to get rid of smells or “staleness” in your house is to bring in fresh air. We all do this during these wonderful autumn days when we open the windows and let the fresh air flush out our home. Wouldn’t it be great to be able to have fresh air year round?

Some people try this by cracking the windows, but this has its downsides. During the winter, the outside air is cold and dry. When you bring in that air, you have to spend energy heating it. Plus, it’s so dry that it sucks the moisture out of the air, leading to uncomfortable conditions in the house.

During the much of the year, the outside air is very humid (in some climates, like mine in eastern Pennsylvania). So drawing that in can also be a problem. High humidity can lead to uncomfortable and moldy homes. It also puts extra strain on your air conditioner because of the warm, humid air.

Because of these issues, people typically close up their homes whenever they’re heating or air conditioning. As home become tighter and more energy efficient, this leads to stale and potentially unhealthy air. So what to do?

How do HRVs and ERVs work?

This is where heat recovery ventilators (HRVs) and energy recovery ventilators (ERVs) come to the rescue! As shown in the above diagram, ventilators bring in fresh air and flush out stale air. But they do more then that. They transfer some of the heat and/or moisture between the incoming and outgoing air, making them much more energy efficient.

During the winter, when you bring cold fresh air in, and flush warm, stale air out, the warm air transfers some of its heat to the cold air so that when the fresh air enters your home, it’s not so cold. Both HRVs and ERVs do this. An ERV adds a feature that allows some of the moisture to transfer between air streams. During the winter, the cold DRY air comes in, and the warm HUMID air goes out. As it does, the ERV transfers heat and humidity to the incoming air, so that it’s both warmer and more humid.

In the summer, the ERV does the opposite. It removes some of the heat and moisture from the incoming air and transfers it to the outgoing air, allowing your home to remain comfortable and energy efficient.

Once you’ve lived with an HRV or ERV, you’ll never want to go back. Fresh air year round is a real joy. You may even find that your health improves because of more consistent fresh air and humidity levels.

But there’s a problem…

These systems can work really well but they’re often installed in a way that makes them not work as intended. Here’s the problem…

Most homes these days include central air conditioners or furnaces. This is really convenient because there’s already ductwork in place. So ventilators are often installed so that they supply fresh air directly into the air stream of the central ductwork. They also pull stale air from the ductwork.

This causes a serious problem. The “return” air part of the duct system is under a vacuum – it sucks air from your house and circulates it through the duct work. When you connect that strong suction to the ventilator, it sucks air through the ventilator even when the ventilator isn’t operating. This is no different than leaving the windows open because if the ventilator isn’t running, there’s no heat or energy recovery. You’re just connecting your return air to the outdoors. This is basic physics, not opinion.

I’ve seen more than one installation (and read about many) where the house was more hot and humid in the summer with the ventilator in place than before it was installed! Needless to say, the home-owners were not pleased. In the winter, the house was extremely dry because the furnace was pulling in that cold dry air whenever it ran. So not only was the house uncomfortable it was really energy inefficient. Imagine leaving your window open year round.

In addition, the air flow through the ventilator has to be balanced in order for it to work right. When you connect it to an existing duct system, it is impossible for it to work as the manufacturer has designed it to because it has a huge fan blowing air through the same ducts that the ventilator is trying to use. Again, this is basic physics, not opinion. It is my opinion that manufacturers that allow such installations are doing their customers a great disservice because this configuration can result in poor performance.

To summarize – it’s a HUGE MISTAKE to duct a ventilator using the same ducts as a central air conditioner or furnace. There is no possible way for it to work correctly (based on the physics of air movement). Ducting this way will be even more inefficient than just opening the windows. So if you’re considering doing it this way, save yourself a couple thousand dollars and just open the windows to bring in fresh air.

The right way

The right way to do it is really simple – just give the ventilator its own ducts. You don’t need ducts everywhere like you do with your central system. You just need to pull stale air from critical areas of your home and supply fresh air to some point where it will naturally flush through the house. In my own home, I simply supply the fresh air to a central hallway, right next to an air intake for my air conditioner/heat pump. Since it’s not physically connected, it doesn’t suck air through the ventilator, but it does draw in the fresh air and distribute it throughout the house when it runs.

Think about your home when you open some windows. You don’t need to open every window and door in the house to bring in fresh air. Usually you open a few and the air flows through the house. It may not flush out every single room perfectly, but it’s a lot better than the alternative!

While it may seem logical to use your central ducts because they run everywhere, don’t do it. Go back and read the preceding section until you’re convinced that this is a bad idea.

Addendum

You could improve (but not entirely fix) the situation with ventilators that are ducted together with the main system by using electro-mechanical air dampers. These would close whenever the ventilator is off and open when it is running. On paper, this sounds like a great idea…

The problem with this is, these dampers are usually just cheap sheet metal flappers. They are not air tight. They simply reduce the amount of air flowing. So even if you go through the trouble of installing these dampers, you’ll find that many of the problems remain.

In addition, this still doesn’t fix the problem of the main air handler working against the small blowers in the ventilator. So you’re still best off with a separate duct system. No matter what you do to put a band aid on the problem, it’s just that, a band aid, not a proper solution based on sound engineering practices.

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3 thoughts on “Heat and Energy Recovery Ventilators (HRV and ERV)

  1. Pingback: Why Does My House Smell? | Ted's Energy Tips

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