From 1977 (when the Saskatchewan Conservation house was built) until 2004 (when the first U.S. Passivhaus was built), North American builders completed hundreds of superinsulated homes. In those days, anyone interested in rating the performance of these homes was probably interested in just one metric: annual energy use.
Over the last eight years, however, with the increasing attention paid to the Passivhaus standard, some builders of superinsulated homes are walking along a narrower path. Any builder interested in achieving the Passivhaus standard soon learns that a low energy bill is no longer sufficient to gain accolades.
This narrow Passivhaus path has several restrictions; I call them “unexamined Passivhaus postulates.” Like postulates in geometry, Passivhaus postulates need not be proven; they just are. Here are four of the postulates:
- It makes sense to deliver space heat through ventilation ductwork.
- It’s more important to achieve 15 kWh/m2•year and 0.6 ach50 that to calculate whether these goals are cost-effective.
- The output of PV system should not be considered in one’s annual energy calculations.
- Every house needs an HRV.
These Passivhaus postulates are not equally binding; for example, North American designers have chosen to ignore the postulate that affirms that space heat should be delivered through ventilation ductwork. (Although the principle is widely ignored, it is still prominently featured on the Passipedia page that establishes the definition of a “passive house.”) When I interviewed Dr. Feist in December 2007, he used the same definition for a Passivhaus that is enshrined on Passipedia: “As long as you build a house in a way that you can use the heat-recovery ventilation system — a system that you need anyway for indoor air requirements — to provide the heat and cooling, it can be considered a Passivhaus.”
Each of the four postulates listed above deserves to be examined more closely than it has up until now, because each of these postulates forces Passivhaus designers to follow a narrower path than the one followed by the North American designers of superinsulated homes who worked from 1977 to 2004.
In this article, I’ll address one of the unexamined Passivhaus postulates: the one holding that every house needs an HRV.
Does the Passivhaus standard require an HRV?
The question as to whether the Passivhaus standard requires an HRV is complicated. As far as I know, every Passivhaus in the U.S. includes an HRV or an ERV.
The requirement for an HRV is explained in a rule book published by the Passivhaus Institut in Darmstadt, “Certification Criteria for Residential Passive House Building.” The book lists “Documents necessary for Passive House certification,” a list which includes: “HRV commissioning report. The results must at least include the following: … name and address of the tester, time of adjustment, ventilation system manufacturer and type of device, adjusted volume flow rates per valve for normal operation, mass flow/volumetric flow balance for outdoor air and exhaust air (maximum disbalance of 10%).”
According to this document, it seems clear that an HRV is required, not optional.
As it turns out, however, the Passivhaus standard doesn’t require the use of an HRV. According to Floris Keverling Buisman, a certified Passive House consultant in New York City, “PHI [The Passivhaus Institut] does not require an HRV. … Why it is generally recommended is that it will be very hard to get your heating (or cooling) demand below 15kWh/m2•year without an HRV in most climates. The tricky part is that if you would like to get certified (voluntarily) you need to conform to all the Passivhaus criteria, which includes comfort — which is defined by ISO 7730 … If your supply air is more than 3.5°C (6.3°F) lower than the room temperature, your building in my understanding would no longer be comfortable and certifiable by PHI as a Certified Passive House. This is why you need an HRV with a high recovery rate in most climates.”
For most Passivhaus builders, the net result of these requirements is that it is very difficult or impossible to install an exhaust-only or supply-only ventilation system. In essence, the Passivhaus standard pushes builders in the direction of an HRV.
Other ventilation approaches are cheaper
While HRVs do an excellent job of ventilating a house, there are less expensive approaches: either an exhaust-only ventilation system or a supply-only ventilation system. Thousands of superinsulated homes successfully use one of these two approaches.
The main advantage of an HRV is that it recovers some of the heat that would otherwise leave the building with the exhaust air. However, an HRV is an expensive gadget, which raises the question: does this gadget recover enough energy to justify its high cost?
The answer depends on your climate. In a very cold climate, an HRV can recover enough heat to justify its high cost. In moderate climates, however, an HRV doesn’t make much sense.
This analysis raises an interesting question: why have Passivhaus proponents embraced one expensive gadget — the HRV — but rejected another expensive gadget — the PV module? Hint: the answer has nothing to do with cost-effectiveness. In most U.S. climates, an investment in PV modules provides a greater energy return than an equivalent investment in an HRV (or, for that matter, an investment in thick subslab foam).
Modeling HRV energy savings
John Semmelhack, an energy consultant and certified Passivhaus consultant in Charlottesville, Virginia, recently took a fresh look at the question of HRV cost-effectiveness. He reported his findings in a paper, “An energy and economic modeling study of exhaust ventilation systems compared to balanced ventilation systems with energy recovery,” which he presented on September 28, 2012 at the Seventh Annual North American Passive House conference in Denver.
Semmelhack is well aware of the disadvantages of HRVs: “These systems are relatively costly to install compared to other ventilation options, are somewhat complex for owners to operate and maintain, and require a significant amount of fan energy for operation (200-400 kWh/year, about 4-8% of total site energy for a modest-sized, all-electric Passive House). By contrast, well-designed exhaust ventilation systems are less expensive to install, are easier to maintain, and require significantly less fan energy for operation.”
Since he has consulted on several Passivhaus projects, Semmelhack knows exactly how much it costs to install an HRV. He wrote, “In North American Passive Houses, ventilation and space conditioning are often decoupled, requiring separate distribution systems and typically higher upfront cost. It is common for a stand-alone Passive House ventilation system to have an installed cost of $4,000 to $7,000 or more.”
Of course, HRVs recover heat that would otherwise be lost. Semmelhack’s energy modeling exercise showed that “both the ERV and HRV … save energy compared to an exhaust ventilation system in every case in the study.” Although this fact is well known, very few architects and builders have actually calculated how much energy is saved by an HRV compared to a simple exhaust-only ventilation system.
The answer, as it turns out, is not much — unless you live in a very cold climate. In mild climates, a PV array is a better investment than an HRV. Semmelhack wrote, “The amount of site energy saved by the ERV/HRV systems is much lower in the milder climates (330-600 kWh/year) than the colder climates (800-1,100 kWh/year). The lower energy savings in the milder climates leads to poor cost-effectiveness when compared to other energy-saving or energy-producing options such as extra insulation or a photovoltaic system.”
Details of Semmelhack’s modeling exercise
Semmelhack used the Passive House Planning Package (PHPP) to model a single-family three-bedroom house measuring 1,800 square feet. He assumed that the house was heated with an air-source heat pump (for example, a ductless minisplit system), and he assumed that the house achieved a Passivhaus level of airtightness (0.6 ach50).
He modeled the house in six different climates (San Francisco; Atlanta; Charlottesville, Va.; Portland, Or.; Chicago; and Burlington, Vt.) The mildest climate he looked at (San Francisco) has 3,200 heating degree days, while the coldest climate (Burlington, Vt.) has 7,300 heating degree days.
In each climate zone, he modeled the performance of the house with three different ventilation systems: an Ultimate Air 200DX ERV, a Zehnder Comfo 350 HRV, and a Panasonic FV-08VKS3 exhaust fan with variable airflow settings and passive air inlets. The electricity use of each appliance was assumed to be as follows: 0.58 W/cfm for the Ultimate Air ERV, 0.30 W/cfm for the Zehnder HRV, 0.12 W/cfm for the Panasonic exhaust fan. When installed in Chicago and Burlington, the HRV and ERV were assumed to have electric defrost systems.
The ventilation rate was assumed to be 56 cfm continuous for the HRV and ERV, and 64 cfm for the exhaust-only ventilation system “to account for occupant on-demand use of other exhaust appliances: other bath fans (50 cfm x 2 hours/day), range hood (100 cfm x 1 hour/day) and clothes dryer (125 cfm x 1 hour/day).”
By using PHPP software, Semmelhack was able to model aspects of HRV performance that aren’t captured by some other energy-modeling programs. He wrote, “The energy analysis included annual heating demand and latent cooling demand for ventilation and infiltration, ventilation fan energy use, defrost energy use, and space conditioning energy use for ventilation and infiltration.”
What do I get for my investment in an HRV?
To determine whether an HRV is cost-effective in a given climate, Semmelhack had to determine two numbers: the amount of energy saved, and the incremental cost of the equipment compared to an exhaust-only ventilation system.
Semmelhack assumed that the capital costs to install the three studied ventilation systems were as follows:
- The system with an Ultimate Air ERV cost $4,125 (including ERV, ductwork, fittings, registers, labor, and markup);
- The system with a Zehnder HRV cost $5,375 (including HRV, ductwork, fittings, registers, labor, and markup);
- The system with Panasonic fans cost $2,102 (including 3 bath fans, 1 kitchen range hood, ductwork, fittings, labor, and markup).
Some readers will probably note that an exhaust-only ventilation system could be installed for less than Semmelhack assumed; if so, this fact would only strengthen Semmelhack’s conclusions.
Using these figures, Semmelhack calculated that the Ultimate Air ERV represented a $2,023 upcharge from an exhaust-only system, while a Zehnder HRV represented a $3,273 upcharge. He assumed that the lifetime of the ventilation equipment was 20 years.
Using PHPP, Semmelhack calculated the total amount of energy saved over 20 years for the ERV option and the HRV option, compared to the baseline system (the Panasonic exhaust-only system). The energy savings had a cost, of course: the cost of the saved energy over 20 years was equal to the upcharge for the expensive equipment.
Semmelhack concluded, “The cost/kWh saved for the [Ultimate Air] 200DX ranged from as low as $0.12/kWh in Burlington to as high as $0.31/kWh in San Francisco. The cost/kWh saved for the [Zehnder] Comfo 350 ranged from as low as $0.17/kWh in Burlington to as high as $0.41/kWh in Atlanta.”
For purposes of comparison, Semmelhack calculated that the energy generated by a photovoltaic system in Charlottesville, Virginia, costs the owner $0.13/kWh, while the energy saved by an attic insulation upgrade in Charlottesville costs the owner $0.17/kWh. (The results of Semmelhack’s energy modeling are shown in greater detail in a bar graph and table, reproduced as Image #2 and Image #3, below).
Semmelhack concluded, “Based on this analysis, the ERV and HRV systems are not cost-effective (compared to the reference points) in terms of energy savings in the milder climates, and are only moderately cost-effective as the house ‘migrates’ to a cold climate (Burlington).”
Passivhaus blinders lead to irrational results
In his paper, Semmelhack takes the bull by the horns and questions why the Passivhaus standard requires the use of an HRV. He wrote, “Based on the poor cost-effectiveness of ERV/HRV systems in the milder climates, it seems irrational that these mechanical systems should be a ‘de facto’ requirement for meeting the annual heat demand requirement for [Passivhaus] certification.”
In his PowerPoint presentation at the Denver conference, Semmelhack was blunt. “Typical Passive House HRV/ERV systems do not appear to be particularly cost-effective in the milder climates,” he wrote. “We should remove our Passive House blinders and take a closer look at other ventilation options. … PHIUS certification metrics should not have a de-facto mandate for cost-ineffective mechanical systems.”
As PV systems continue to drop in price, the Passivhaus preference for HRVs rather than PV systems gets harder and harder to justify.
HRVs have several benefits
Defenders of HRVs will probably bristle at Semmelhack’s analysis, pointing out that an HRV provides better comfort and delivers fresh air more evenly than does an exhaust-only system. This is undeniable, and some homeowners may be happy to pay thousands of dollars for these benefits.
Others, however, are likely to find that a simple exhaust-only ventilation system meets all of their ventilation needs.
Unexamined postulates come at a cost
The energy-efficiency pioneers who built superinsulated homes from 1977 to 2004 were fairly nimble. They experimented with a variety of approaches to superinsulation, searching for innovative specifications that worked to lower energy consumption.
Compared to these nimble pioneers, Passivhaus designers are dragging around a ball and chain. (Note to Passivhaus designers: If you bend down and look closely at the device fastened to your ankle, you’ll see a small inscription: “Made in Germany.”) The requirement to include an HRV — which many designers interpret as a requirement for an expensive HRV from Europe — is making Passivhaus designers less nimble than other builders.
Years ago, Amory Lovins predicted that superinsulation techniques would allow builders to “tunnel through the cost barrier.” The concept was later championed by Wolfgang Feist, who predicted that Passivhaus buildings would require less expensive HVAC equipment than ordinary buildings.
As it turns out, most Passivhaus builders in the U.S. haven’t been able to tunnel through the HVAC cost barrier — in part because of the burden of unexamined Passivhaus postulates.
There is at least one builder in the U.S. who seems to have managed to tunnel through the HVAC cost barrier, however. I’m thinking of Massachusetts builder Carter Scott. Scott usually specifies simple exhaust-only ventilation systems.
Martin Holladay’s previous blog: “All About Wall Rot.”