HRV / ERV – Common Misconceptions

[Admin]

Here's a link to a great article regarding HRV's and ERV's,

http://www.greenbuildingadvisor.com/blogs/dept/musings/hrv-or-erv

After investigating various ventilation options, many residential designers conclude that they want either a heat-recovery ventilator (HRV) or an energy-recovery ventilator (ERV). They often remain confused, however, about which of the two devices to choose.

Every tight home needs a mechanical ventilation system. Most builders choose one of three ventilation options:
•An exhaust-only ventilation system;
•A central-fan-integrated supply ventilation system; or
•A balanced ventilation system using an HRV or an ERV.


A balanced ventilation system with an HRV or an ERV is the preferred ventilation system for a Passivhaus building. Although balanced ventilation systems are expensive to install, they have the lowest operating cost of any ventilation option — assuming, of course, that the designer or installer hasn't made any blunders. (Sadly, this can be an optimistic and risky assumption.)

The purpose of an HRV or an ERV is to deliver fresh air to a home's interior. Neither appliance is designed to provide makeup air for combustion appliances or kitchen exhaust fans. HRVs and ERVs are not space-heating devices, heat-delivery devices, or energy-saving devices. The more hours that an HRV or ERV operates, the more energy it uses — electrical energy to operate its fans, as well as heating or cooling energy to make up for the conditioned air that these devices expel from a home.

What they do and how they work
An HRV's fans pull fresh air into a home while simultaneously exhausting stale air from the home. In most installations, the fresh air is delivered to the living room and bedrooms, while the stale air is removed from bathrooms, laundry rooms, and sometimes the kitchen.

Both the fresh air stream and the stale air stream flow through the HRV. The core of the appliance allows some of the heat from the warmer air stream (the stale air in winter, the fresh air in summer) to be transferred to the cooler air stream. In winter, in other words, the appliance "recovers" some of the heat that would have otherwise been exhausted. This heat transfer occurs without any mixing of the two air streams.

An ERV does everything that an HRV does. In addition, an ERV allows some of the moisture in the more humid air stream (usually the stale air in winter and the fresh air in summer) to be transferred to the air stream which is dryer. This transfer of moisture — called enthalpy transfer — occurs with very little mixing of the two air streams. (The cross contamination rate for one well regarded ERV, the UltimateAir RecoupAerator, is 9.6%.)

Why ventilate a house?
Before we can clarify the choice between an HRV and an ERV, we have to consider the question, "Why should a house be ventilated?" As it turns out, the question has several answers, including:
•To provide enough fresh air to keep the occupants healthy;
•To remove odors;
•To dilute indoor pollutants; and
•To lower the indoor relative humidity.


Most of these goals are easy to understand. (Even so, establishing an optimal ventilation rate to achieve these goals is a contentious issue.) However, using ventilation to achieve the last of these four goals — lowering the indoor relative humidity — gets problematic.

To prevent moisture damage to a house, lower humidity levels are always preferable to higher humidity levels. In other words, dry is always better than damp. However, some people begin to complain if the indoor relative humidity is too dry — say, 20% or below. (Of course, people have lived healthy lives for thousands of years in climates where the relative humidity is often below 20%, so it's not at all clear that low humidity levels are unhealthy.)

Ventilation can only reduce the indoor relative humidity if the outdoor air is dryer than the indoor air. Since cold air can't hold as much moisture as warm air, ventilating a house helps lower the indoor relative humidity only when it's cold outside (or on dry days during the spring and fall). In most parts of the U.S., ventilation during hot weather actually introduces more moisture into the house — that is, it tends to raise rather than lower the indoor relative humidity.

What do manufacturers recommend?
Unfortunately, you can't depend on HRV and ERV manufacturers to tell you whether your home is better off with an HRV or an ERV. Many manufacturers' Web sites include misstatements:
•The Fantech Web site and Lennox advise readers that the only relevant criterion is climate.
•The Broan Web site falsely claims, "An HRV ... is used only in the cold months of the year to resolve high moisture problems in the home. An ERV ... can be used all year round to provide fresh air for your home."


Other myths
These myths — that the choice between an HRV and an ERV depends only on climate, and that HRVs can't be used during the summer — are only two of the many red herrings encountered by builders in search of accurate information on HRVs and ERVs. Other commonly repeated myths include:
•ERVs can't be used in cold climates because their cores will freeze.
•In a humid climate, an ERV can act as a dehumidifier or can help address high indoor humidity.


Both of the above statements are false. (Freeze-up problems were solved years ago by the development of controls with a defrost cycle.)

Ventilating in hot, humid climates
When an HRV or ERV ventilates an air-conditioned house during the summer, the cool exhaust stream absorbs heat from the incoming fresh air. In other words, the incoming outdoor air is cooled by the outgoing exhaust air. This is only possible in an air-conditioned house. If there's no air conditioning, the exhaust air won't be cool, so there is little opportunity for heat exchange to occur.

If the house has an ERV, some of the moisture from the incoming outdoor air is transferred to the exhaust air. This reduces, but does not eliminate, an undesirable moisture source. (In hot, humid climates, the increased moisture load caused by summer ventilation is an unavoidable drawback to any type of ventilation.)

In a hot, humid climate, it makes more sense to install an ERV than an HRV — but not for the reason that many people assume. "ERVs are not intended to reduce indoor relative humidity during the summer," explains Daniel Forest, the vice president of R&D for Venmar, a manufacturer of HRVs and ERVs.

Operating an ERV during the summer in Houston, Texas, doesn't lower the indoor relative humidity; rather, it makes the situation worse. The best that can be said is that, from a moisture-load perspective, operating an ERV is less bad than operating an HRV — assuming, of course, that the home is equipped with a dehumidifier.

The main reason to prefer ERVs over HRVs in Houston is that the additional moisture introduced by the ERV — a latent load that the air conditioner must contend with — is less than the moisture that would have been introduced by an HRV. While HRVs and ERVs both cause increased energy use, the energy attributable to ERV operation is less.

High humidity in tight houses
In Houston, a tight, well insulated house is more likely to have problems with high indoor humidity than an older leaky house. In a tight home, an air conditioner doesn't run anywhere near as often as in a leaky home, especially during the swing seasons (spring and fall). If the air conditioner is rarely on, there are fewer opportunities for the HVAC equipment to dehumidify the interior air. Of course, adding mechanical ventilation only makes the situation worse.

The solution to this problem is not an ERV. The solution is a stand-alone dehumidifier.

Two researchers from the Lawrence Berkeley National Laboratory, Iain Walker and Max Sherman, wrote a paper, "Humidity Implications for Meeting Residential Ventilation Requirements,"that includes a discussion of the effects of residential ventilation in Houston. They wrote, "The use of an ERV did not change the humidity distribution in a hot, humid climate compared to a continuous exhaust system."

Elaborating on this finding in an e-mail, Sherman wrote, "It is true our results show little value in ERVs in hot, humid climates, but it is important to understand why. ... Almost all hot, humid climates have hours when it is dryer outside than inside and then ERVs actually make the moisture problem worse. The net effect is that ERVs are about a wash for humidity control in those climates. ... On the other hand, if there were independent humidity control (such as ... a stand-alone dehumidifier) then ERVs pay big dividends in terms of energy savings in hot, humid climates. To say it another way, the whole idea of an ERV is not to change the indoor humidity (and temperature) with ventilation. So if the indoor humidity is good already, the ERV reduces energy costs. If it is bad already, the ERV is not likely to help."

What about cold climates?
Many people assume that HRVs make more sense than ERVs in cold climates. However, the situation isn't that simple.

Whether or not the interior of a cold-climate home is humid or dry during the winter depends on several factors, including:
•The leakiness of the building envelope;
•The ventilation rate of the ventilation equipment;
•The number of square feet per occupant; and
•The behavior of the occupants.


At one extreme would be a large, leaky, Victorian house occupied by two elderly people who rarely cook and have few houseplants. At the opposite extreme would be a small, tight home occupied by a six-person family that eats home-cooked meals and takes frequent showers.

Is the house dry or damp?
In most cases, an old leaky Victorian home doesn't need an HRV or an ERV. (It makes little sense to install a $2,000 ventilation system in a home that already has a high air-exchange rate.)

But even if we narrow our focus to new homes with tight envelopes, we find that winter humidity levels vary widely. Big homes with few occupants tend to be dry during the winter. Small homes with many occupants tend to be humid during the winter. These two types of homes may need different ventilation systems (or may need to be ventilated at different rates).

According to Don Fugler, a senior researcher at the Canada Mortgage and Housing Corporation, some Canadian houses can benefit from an ERV. "Although I have never promoted ERVs, we've started to see situations where an ERV may make more sense," said Fugler. "In a new, energy-efficient house with no major moisture sources in a very cold climate — a prairie climate — the code-required level of ventilation will dry out your house way too much."

Another building scientist, Terry Brennan of Camroden Associates in Westmoreland, New York, agrees with Fugler that the answer to the question, "HRV or ERV?" is, "It depends." Brennan said, "Where I have monitored indoor relative humidity — usually in houses that are typically 2,400 square feet or smaller — HRVs maintain 30% or 35% RH, so I would say they didn't need enthalpy. But in a bigger house with only two people, it might be different. Of course, it depends on how airtight the house is."

Other ways to address humidity and dryness
It's possible to overthink the choice between an HRV and an ERV. After all, there are other ways to address humidity problems in houses. In fact, these other factors tend to overwhelm performance differences between HRVs and ERVs.

For example, regardless of the type of ventilation equipment in your home, you can adjust your interior relative humidity during the winter by adjusting your ventilation rate. If your house is too humid — usually indicated by the presence of condensation or frost on your windows — just increase the ventilation rate. In other words, run your fan for more hours per day.

If your house is too dry — usually indicated by dry skin or static electricity problems — just reduce your ventilation rate. (Be careful, however — if you reduce the ventilation rate too much, you risk undermining other important ventilation functions like odor removal.)

If you live in a hot, humid climate, and you're worried about high indoor humidity during the summer, reduce your ventilation rate. (Building scientist Joseph Lstiburek has made the somewhat controversial recommendation that homeowners in hot, humid climates should ventilate at a lower rate than the level recommended by ASHRAE 62.2).

Finally, if your indoor relative humidity is too high during the summer, you probably need a dehumidifier. Ventilation won't solve this problem.

The bottom line
If you want to install an HRV or an ERV, which should you choose? Here are some guidelines:
•For a small, tight house in a cold climate — especially a house with a large family — choose an HRV.
•For a large house in a cold climate — especially a house with few occupants — choose an ERV.
•In a hot, humid, climate, an ERV will cost a little less to operate during the summer than an HRV.
•In mixed climates, choose either appliance.


However, the above guidelines aren't set in stone. For example, Paul Raymer, a ventilation expert and former member of the ASHRAE 62.2 committee, is skeptical of the idea that large homes with few occupants could benefit from an ERV. "For big homes, an ERV might have little or no impact [on indoor relative humidity] — unless it was a big ERV," said Raymer. "And you generally don't need as much ventilation air in a big house with few occupants."

In fact, the difference in performance between these two types of appliances is so slight that many builders ignore climate, house size, and occupancy, and instead make their choice based on energy efficiency.

If energy efficiency is your most important criterion — and I believe it should be — choose either the UltimateAir RecoupAerator 200DX ERV (which draws 40 watts to deliver 70 cfm, or 1.75 cfm/watt) or the Venmar EKO 1.5 HRV (which draws 24 watts to deliver 49 cfm, or 2.04 cfm/watt).

Remember, stupid installation details will undermine the efficiency of even the best equipment. It makes little sense to install a high-efficiency HRV or ERV by connecting it to the plenums of a furnace with an inefficient blower motor. If your furnace blower comes on every time your HRV operates, then the efficiency of the HRV motor is irrelevant. That's why anyone who goes to the expense of purchasing an HRV or ERV should insist on an installation with dedicated ventilation ductwork — not an installation that tries to use existing furnace ductwork.

Maybe you don't really need an HRV
Finally, it's worth mentioning that it's possible to have a well-ventilated home without an HRV or an ERV. It's much cheaper to install a central-fan-integrated supply ventilation system controlled by a FanCycler.

If you choose this route, be sure that your furnace has an energy-efficient ECM blower.
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