HRV or ERV?

Choosing the right equipment for a balanced ventilation system

Posted on Jan 22 by Martin Holladay, GBA Advisor

Low-energy ventilation. The Venmar EKO 1.5 is a heat-recovery ventilator (HRV) equipped with two ECM blowers. At low speed, the energy-efficient appliance draws only 13.5 watts to deliver 40 to 80 cfm of fresh air.

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 the Venmar EKO 1.5 HRV (which draws 13.5 watts to deliver 40 to 80 cfm).

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.

Manufacturers of HRVs and ERVs
American Aldes Ventilation
HRVs and ERVs

Aprilaire
ERVs

Broan
HRVs and ERVs

Carrier Corp.
A relabeler of HRVs and ERVs manufactured by others

Fantech
HRVs and ERVs

Honeywell
HRVs and ERVs

Lennox International
A relabeler of HRVs and ERVs manufactured by others

Lifebreath
HRVs

RenewAire
ERVs

UltimateAir
ERVs

Venmar Ventilation
HRVs and ERVs

Last week's blog: "Home Dashboards Help Reduce Energy Use."

Tags: energy recovery, energy-recovery ventilator, enthalpy, ERV, heat recovery, heat-recovery ventilator, HRV, Humidity, indoor relative humidity, mechanical ventilation, relative humidity, ventilation

Image Credits:

Venmar Ventilation

Jan 22, 2010
8:34 AM EST

Heat Exchanger Cores
by Interested Onlooker

The North American HRVs seem to favour cross-flow heat exchangers rather than the counterflow heat exchangers which are more common in Europe. Why is this ?

Jan 22, 2010
6:50 PM EST

Poor installation
by Li Ling Young

Thanks for the patient, nuanced discussion. I wanted to make the case for simplifying. All the HRV's I've tested have shockingly low flow rates where it counts, and categorically low flow rates overall. To make matters worse, I've measured the power draw of the Venmar Eko 1.5 (with ECM) and found it to use twice as much electricity as rated, both at high and low speeds.
Both of these problems are symptoms of poor installation. They lead me to ask, what exactly are you getting for this $2000 doodad?
For nearly all homes I advocate an exhaust-only strategy. Because exhaust-only ventilation unbalances house pressures, this strategy can result in good distribution of ventilation air when randomly-scattered holes in the envelope leak inward. Exhaust ventilation is usually inexpensive to install, and has a high likelihood of good-enough installation such that 60-80% of rated flow is common. Finally, the motors use about 20% of the electricity of a typical residential HRV, making for low operating costs.
High flow rates, ok distribution, low operating cost: simple is beautiful.

Jan 22, 2010
7:59 PM EST

Exhaust-only ventilation systems
by Martin Holladay, GBA Advisor

Li Ling Young,
I agree that exhaust-only ventilation systems can work well, especially in small homes with simple, open floor plans. Their great strength is their simplicity and low energy use — especially if a Panasonic fan is chosen.

The downside to exhaust-only systems is uneven fresh air distribution. Whether this is a problem or not depends on the characteristics of the house where the system is installed. I don't believe that the use of passive air inlets, which are often overwhelmed by the stack effect and wind, solves the problem of uneven fresh air distribution in houses with exhaust-only ventilation systems. Nevertheless, in many homes exhaust-only systems work very well indeed.

You haven't really explained what you mean by the examples of "poor installation" that caused the low flow rates in the HRVs you tested. But I can imagine the problems included convoluted duct systems, undersized ducts, or clogged filters. Every ventilation system needs to be well designed, properly installed, and commissioned. Commissioning must include flow rate verification. If the systems were badly designed, poorly installed, and never commissioned, the problem is not the HRV. However, the examples you cite are arguments in favor of simplicity.

Jan 22, 2010
8:28 PM EST

Exhaust only
by Dan Kolbert

I worry about where you're sucking the air thru, esp. on a tight house. Why not just install some inlets if you want to avoid an ERV?

Jan 23, 2010
10:44 AM EST

Clarification and question
by Carl Seville, GBA Advisor

Great piece, Martin. This sentence confused me a little: "If the house has an ERV, some of the moisture from the incoming outdoor air is transferred to the dryer exhaust air." Are you referring to the clothes dryer exhaust air or the drier exhaust air in the house?

On this point "In Houston, a tight, well insulated house is more likely to have problems with high indoor humidity than an older leaky house.", it seems to me that it depends on the season. A tight house is less likely to have high indoor RH in the summer as the outdoor moisture won't work its way in through infiltration.

I agree with Joe L's point that 62.2 can be excessive. It seems to me that the standard was developed for worst case situations. If the house is well built, has minimal pollutants and toxins in construction materials, the occupants don't use nasty smelling cleaners and personal products, take their shoes off, and don't have pets, 62.2 is very likely overkill. If they smoke, have dogs, and the house is full of VOCs they may need more.

Finally, Broan has come out with an interesting new system (smart sense) that coordinates all the bath fans and the kitchen vent hood with an intake damper and turns fans on and off automatically to provide a set amount of ventilation. The switches communicate with each other and the damper control through the house wiring to manage the total ventilation CFM, making sure that it reaches the desired minimum. Looks good at first glance, I'd be interested in your take on it.

Jan 23, 2010
11:09 AM EST

Response to Carl
by Martin Holladay, GBA Advisor

Carl,
1. The intended meaning of the phrase, "the dryer exhaust air" was "the exhaust air, which happens to be more dry." Because my phrasing was confusing, I have corrected the text above. Thanks for pointing out the confusion.

2. Concerning moisture problems in tight houses in Houston: Armin Rudd of the Building Science Corporation did a lot of great research on this issue. I reported on his findings in the January 2003 issue of Energy Design Update. In that article, I wrote in part:

"For several years, Pulte Homes has been building new houses in Houston with a package of energy-efficiency features developed by the Building America program. Before Pulte’s involvement with Building America, their Houston homes had no noteworthy problems with high indoor humidity. But to the surprise of Building America consultants, Pulte’s first generation of Building America homes in Houston had unacceptably high levels of indoor humidity.

"To probe the origins of the humidity problem and to develop solutions, Building America sponsored a study examining dehumidification options for production builders in hot, humid climates like Houston’s. The study, directed by Armin Rudd, principal engineer at the Building Science Corporation, compared equipment designed to lower indoor humidity in 20 Pulte homes in Houston. Rudd presented his findings on October 10, 2002 at the Energy and Environmental Building Association (EEBA) conference in Phoenix, Arizona.

"Rudd’s data convinced him that the humidity problems in the Houston Building America houses were not due to the increased ventilation rate, but were an indirect result of the improvements in window performance and envelope tightness. These improvements lowered the buildings’ sensible cooling loads to the point that existing air conditioners are unable to handle the latent load. The research confirmed that in these houses, the only way to keep the indoor relative humidity dependably below 60% is to install a dehumidifier."

To learn more about of Rudd's findings, read his entire report, "Dehumidification Systems: Research Results," posted online here:
http://beta.buildingscience.com/documents/reports/rr-0215-dehumidificati...
or here:
http://web.archive.org/web/20060303154402/http://buildingscience.com/res... .

3. I agree about the Broan SmartSense fans. I reviewed the product in the April 2007 issue of EDU, when I wrote:

"The fans come with controls that are able to communicate with other fans in the home, keep track of the minutes of manual operation of all the home’s exhaust fans, calculate the amount of additional ventilation required each hour to meet ASHRAE 62.2 requirements, and automatically operate all of the home’s fans when necessary to meet ASHRAE 62.2 minimum ventilation rates. Moreover, the fans are quiet and energy-efficient.

"The fans are designed to be installed in homes requiring two or more exhaust fans. Each fan includes an integral control module. Using a table that comes with the installation instructions, the installer programs one fan -- the mas-ter unit -- with the desired ventilation rate for the entire home, according to the ASHRAE 62.2 formula; the factory default rate is 90 cfm.

"Like most bathroom exhaust fans, Broan SmartSense fans can be operated with a manual wall switch. Once each hour, the SmartSense control calculates the total manual run time of each fan during the previous hour. (The fans communicate with each other over the home’s power lines; no communication wiring is required.) Before initiating a cycle of automatic ventilation, the SmartSense control gives credit for any operation time resulting from use of the manual wall switches. Automatic ventilation run times are distributed evenly among all the fans in the house. If manual fan operation during the previous hour exceeds ASHRAE 62.2 requirements, no additional automatic ventilation occurs.

"For example, assume that a house has a four-fan system and an ASHRAE 62.2 ventilation requirement of 60 cfm (3,600 cubic feet per hour). The ventilation rate is programmed into the control when the system is commissioned. If none of the fans has been operated manually for a full hour, then the control will direct each of the fans to operate long enough to exhaust 900 cubic feet (one fourth of the ASHRAE 62.2 requirement). Ventilating from four fans provides a better distribution of fresh air than ventilating from a single fan.

"The control can be set to “vacation mode,” suspending all automatic ventilation until the next time the master fan is manually operated, at which point automatic ventilation resumes."

Jan 23, 2010
12:55 PM EST

More discussion of tight houses in Houston
by Martin Holladay, GBA Advisor

Carl,
Just to be clear: tight houses are always more energy efficient than leaky houses, even in Houston. I strongly believe that houses should be built as tight as possible.

Although I pointed out that tight houses in Houston often have high levels of indoor humidity, there's a simple solution to the problem: install a stand-alone dehumidifier. If you do that, your tight Houston home will be a low-energy home — and a dry one.

Jan 23, 2010
2:45 PM EST

Exhaust Only Systems
by Brett Moyer

I must take issue with Li LIng Young's stance on exhaust only systems; "For nearly all homes I advocate an exhaust-only strategy... Simple is beautiful" I understand that he clarified his advocacy of this type of ventilation strategy with "for nearly" but comments like these (intentionally or not) scare builders out of tight home construction.
They begin to think, "this ventilation stuff is too complicated" and its back to business as usual- building leaky, inefficient homes.
Or perhaps the builder decides to build a tight home, and puts serious effort into air sealing and tight construction. But because of his perception home ventilation, his ventilation strategy remains the same; exhaust only. Perhaps this strategy consists of 4 bathroom fans. But what about the other exhausting appliances in the home... clothes drier, 300 cfm range hood, and atmospheric-venting water heater... and why not throw in a wood-burning fireplace?
This really isn't all that atypical. Doesn't anyone see a problem in this?

Jan 23, 2010
10:36 PM EST

Great Article
by Allan Edwards

Thanks Martin for a great article and for references to Houston's climate.

Jan 24, 2010
3:43 PM EST

Errata
by Robert Riversong

Martin,

Good article, but some corrections are necessary.

You said twice that, with an ERV, no mixing of the two air streams occurs. While it's hard to find manufacturer's specs on cross-contamination, ASHRAE Standard 62-2004 allows 5%-10% ERV cross leakage (depending on whether it's exhausting from the kitchen or just bathrooms). I suspect there is some leakage in every unit.

You said that "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." Sure, but they evolved in those climates and didn't have expensive hardwood flooring or overly-sensitive mucous membranes. Low humidity not only causes wooden furniture to loosen up, but can cause respiratory problems and irritation of nose, throat and skin. Houses like RH between 20% and 40%, while humans like RH between 30% and 70%. So the appropriate indoor compromise is 30%-70% RH (at the low end in winter).

You say that "freeze-up problems were solved years ago by the development of controls with a defrost cycle." But that solution can involve an energy-intensive electric resistance defroster or dampers which recirculate indoor air, thereby bypassing the fresh air function. Those dampers are also likely to increase cross-contamination when they're closed.

You claim that " if we narrow our focus to new homes with tight envelopes...big homes with few occupants tend to be dry during the winter." I would like to see evidence of that. A big empty heated house might be dry, but almost any occupant load in a tight house (unless it's an enormous house) will drive moisture levels upwards. A big house with the same level of tightness has much smaller ACH because of the larger volume, and it's winter air exchange which dries out a house.

Don Fugler's commnet that “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” is largely irrelevant to the rest of us. Everywhere in the US, typical winter RH levels are higher (12% avg.) than in the summer (though absolute humidity is lower).

And I thought the most interesting finding of the Lawrence Berkeley study was "The use of an ERV did not change the humidity distribution in a hot, humid climate compared to a continuous exhaust system." Which suggests that we shouldn't simply gravitate to the latest technology without understanding what it really does.

Jan 24, 2010
3:46 PM EST

I disagree
by Robert Riversong

with Brett Moyer's comment about the elegant simplicity and high functionality of an exhaust-only system.

Brett said, "comments like these (intentionally or not) scare builders out of tight home construction."

On the contrary, if builders understood that they could build tight homes without the added expense of a centrally-ducted heat-recovery ventilation system, they would be more likely to concentrate on envelope improvements.

Jan 24, 2010
3:50 PM EST

Right On!
by Robert Riversong

And I'm right with Li Ling Young's advocacy of simple and functional exhaust-only ventilation. More complex technology invariably creates more potential failure modes and requires more maintenance for reliable operation, often underperforming from ratings or expectations.

There are many ways to mess up the design and installation of a central ventilation system.

Martin continues to base his concern about passive make-up air inlets on one flawed study which demonstrated stack effect exfiltration from inlets in leaky homes. In a reasonably tight home, passive fresh air inlets combined with efficient bath exhaust fans work beautifully. That's the only system I've ever used.

Jan 24, 2010
4:05 PM EST

HRV/ERV advice we recieved
by Travis Thompson

We just finished a tight house (0.6 nach) with no AC (solar assisted radiant floor heat only) and our testing trade contractor told us we would be wasting our money if we went with an ERV over an HRV. Their thinking was that without AC we would see no dehumidification benefit in the cooling season. I agree with this but I was more concerned with moderating humidity levels during the winter. The cost was negligible between the two and I felt like this was cheap insurance as I think homes this tight have a lot of unknowns as far as how they will behave with respect to humidity etc. I'd like to get others input on my thought that an ERV may help moderate humidity levels during the heating season. BTW we are in northwestern NC at an elevation of about 4,000 feet with 5480 HDD and 1033 CDD.

Jan 24, 2010
5:02 PM EST

About the "errata" noted by Robert Riversong
by Martin Holladay, GBA Advisor

Robert,
Thanks for your comments. Since you apparently agree with me on many points, I'm not sure why your comments refer to "errata."

1. ASHRAE 62.2 is a ventilation standard. The standard's maximum limits to air-stream cross-contamination are not measurements of actual cross-contamination. Any HRV or ERV manufacturers care to chime in with actual measurements?

2. You appear to agree with me about the health of people living in low-humidity climates. No argument there.

3. I never claimed that low indoor humidity was good for wooden furniture, so again, we are in agreement.

4. The defrost cycles with which I am familiar do not use electric resistance heat. They involve shutting down the ventilation function for a certain number of minutes to allow recirculation of indoor air through the core to raise the temperature of the core before resuming ventilation. Again, since I never described the mechanism of the defrost function, this fact is something we agree on and does not represent an error.

5. Although you assume that the experience of residents of Alberta and Saskatchewan is irrelevant, I always write with Canadian as well as U.S. readers in mind. Although the information I provided may not apply to your climate, it is still of interest to some readers. Obviously, the information I provided on Houston conditions similarly does not apply to you -- but it is of interest to Texans.

6. My article attempts to provide guidance to those trying to decide between an HRV and an ERV. I never derided exhaust-only ventilation systems or central-fan-integrated supply ventilation systems. For many homes, these two other ventilation methods are entirely appropriate and in fact preferred. On that fact I agree with you.

7. This is not the first time you have disagreed with my observation that wind and the stack effect often overwhelm passive air inlets, undermining their utility. It seems we have a clear disagreement on this one issue. Since we have debated it in the past, there's no need to reopen the issue here.

Jan 25, 2010
6:24 AM EST

Bypassing the HRV
by John Brooks

Martin,
Passivhaus mentions bypassing the HRV during "night flushing".
There are times when conditions outside are "better" than inside.
Precooling or preheating or "drying" with increased volume of outside air makes sense sometimes.
Of course occupants can open their windows.....
Are there mechanical systems that allow for bypass and high volume air change?

Also... I have noticed that the microclimate around my house can easily vary by 15 degrees (in the shade)depending on North or South

Do any HRV's take advantage by switching between warm and cool intakes?

Jan 25, 2010
7:58 AM EST

Night flushing
by Martin Holladay, GBA Advisor

John,
As far as I know, the best available equipment for night flushing -- air exchange at night for the purpose of cooling or drying interior air — is the NightBreeze developed by the Davis Energy Group. More info here:

http://www.davisenergy.com/technologies/nightbreeze.php

Apparent differences in microclimates on the sunny and shady sides of the house cannot be used to good effect for cooling. That's because the air temperature is the same on both sides of the house. As soon as you start sucking in outdoor air, you're basically mixing the air near the wall and pulling air from a large volume. If you are drawing 80 cfm of ventilation air, that's 4,800 cubic feet per hour. You're not just pulling the "shaded air" right near your siding — your pulling in outdoor air from the great outdoors.

That's why there are no energy savings that accrue from placing the outdoor unit of your air conditioner in the shade compared to placing the unit in the sun.

Jan 25, 2010
9:02 AM EST

for Travis Thompson
by John Semmelhack

Travis,

If you have a very tight house (I'm assuming you meant 0.6ACH50, not ACHn), the ERV will lead to higher indoor relative humidity (RH) levels during the heating season when compared to an HRV or an exhaust-only system.

In the heating season, your indoor air generally has a much higher moisture content than the outdoor air. The ERV will recover some of this moisture, leading to higher RH levels (when compared to an HRV or exhaust-only system).

Jan 25, 2010
9:03 AM EST

Cross-contamination in ERVs
by John Semmelhack

The cross contamination in the Ultimate Air 200DX is about 4%.

Jan 25, 2010
9:12 AM EST

For John Brooks
by John Semmelhack

Some ERVs and HRVs have an "economizer" function that allows heat recovery to be bypassed when outdoor conditions are favorable. I use the Ultimate Air 200DX in my house. It has an economizer that, when activated, turns off the energy recovery wheel when the outdoor air is in the range of 55-70F.

Jan 25, 2010
9:17 AM EST

Cross contamination
by Martin Holladay, GBA Advisor

John,
Thanks for the cross-contamination information on the RecoupAerator.

Jan 25, 2010
11:57 PM EST

4% Recoupaerator cross-leakage??
by Dave Brach

John, where did you get the 4%? The Home Ventilating Institute (hvi.org) has a certified products directory that lists the exhaust air transfer ratio (cross leakage) for each of its certified products. For the Ultimate Air Recoupaerator this ratio is .096, or 9.6%. Am I interpreting this incorrectly?

And to put this in perspective, ASHRAE standard 62-2004 states that up to 10% cross leakage is acceptable when exhaust air is from a restroom....

Jan 26, 2010
10:45 AM EST

RecoupAerator and more
by Jason Morosko

This is a great conversation happening here. I hope no one minds if I write a little... yes, we manufacture the RecoupAerator... I am a primary design engineer responsible for the RecoupAerator..... but I do try to stay neutral on facts. Cross leakage has been a passion (thorn) for me for the last 3 years or more. Mr. Brach is correct.. HVI lists our unit at 9.6%. I was at Bodycote (testing facility in Canada) for three weeks while they were doing all of the performance testing (including cross leak). The 9.6% was a number that was not repeatable... same day, time, test... the numbers were not consistant, so we settled with them on that number. I believe that the cross leak will vary depending on flow and duct pressures, anywhere from 2-10%... so i cannot tell you a firm number. The next question is.. does it matter? My opinion is... well, no. Reasons: First, most competitors will start recycling leaving stale air, back into incoming fresh air for defrost. This is 'cross leak'... and a lot! Second, I've never had major complaint concerning odors from leaving air getting back into incoming air.. Third... If you live in a house without mechanical ventilation, you re-breath the same air "all the time"... and I haven't heard of terrible results thereof... So, i'm just not sure there is a horrible consequence from cross leak. Further, the performance testing has penalized us for the leak... Meaning: our performance would be even higher without the leak. I had our unit testing in the EU, with some effort spent to limit the cross leak.. and with less cross leak, we had apparent sensibles of 98%+, because less air is leaking past the heat exchange medium when you have increased cross leak. OK.. enough on that one for now.

Next, yes, our unit has the night flush... "econocool", built in, standard. It has limitations.. you can read about it on our site. http://ultimateair.com/Ultimate_Air/pdfs/200DXG_Manual.pdf

The Venmar Eco... i've looked at this unit a few times... bravo for the EC motors (finally)... we've been using them since 1999... and the defrost is 28% recirc at 23 F, and 45% recirc below -17 F.
Performance for it and ours can be seen here, page 103 and 120 i think:
http://hvi.org/assets/pdfs/CPD/CPD_Sec3_01Jan10.pdf

The recoupAerator: our theory is that you wanted fresh air all the time.. or at least when you choose... in which case below 12 F, we need active defrost of some fashion. Basically, we have a pulse modulated duct heater... keeping air coming to the unit at 12 F if it is below.. or we have a water coil, which you can do a ground loop.. 50 F water... or what not... for defrost. When its cold outside.. there is a penalty.. no getting around it. Some recylce inside air.. some warm up the incoming air.. Both take energy, or a loss of fresh air. The only way around it is to move to a warmer climate.

OK.. i'm out of time right now.. Humidity and HRV vs ERV... my goodness.. i'll have to dedicate another novel to that discussion.. but later..

Jan 28, 2010
11:18 AM EST

More discussion of these issues
by Martin Holladay, GBA Advisor

Anyone interested in this discussion will probably also be interested in a parallel discussion inspired by this blog over at the JLC Web site:
http://forums.jlconline.com/forums/showthread.php?t=50913

Of course, if this topic interests our readers, I hope further comments to be made here at the GBA site so GBA readers can benefit from the discussion.

Jan 29, 2010
7:09 AM EST

for John Semmelhack
by Travis Thompson

John,
Thanks for your reply. Yes we are at 0.6 ACHn. It appears as if we made the right decision on the ERV over HRV.

HRV or ERV?

Choosing the right equipment for a balanced ventilation system

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HRV or ERV?

Choosing the right equipment for a balanced ventilation system

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