A few months ago, we interviewed Bronwyn Barry of Quantum Builders on the Passive House standard. When I called her again to ask about ventilation, she recommended Zehnder, a Swiss company, because their products are Passive House certified and if there’s anyone who knows about ventilation, it’s the Passive House folks. Passive Houses need state-of-the-art ventilation because they rely so heavily on an airtight building envelope, and their stringent energy budgets also mandate the most energy-efficient motors available.
I spoke with Barry Stephens of Zehnder America, the U.S. subsidiary. Zehnder has over 100 projects all over the U.S., including some larger residential complexes. Two-thirds of their projects are Passive House projects, but the company also works with other types of energy retrofits. You can see a great animation of their system here. Barry then referred me to his brother Charlie Stephens of the Northwest Energy Efficiency Alliance, who’s an energy research expert.
We asked both of them some general questions about ventilation principles, along with specific questions for Barry about the Zehnder product line. Most of the discussions here reference the Passive House standard, because it’s so far ahead on ventilation. In fact, if you’re looking for a good ventilation consultant, finding someone with Passive House certification wouldn’t be a bad place to start.
WHY does a home need a ventilation system?
In other words, why is “just opening a window” or “just running the bathroom fan” not enough?
There’s a definite knowledge gap in the residential design and build communities about the need for mechanical ventilation systems. The newest version of California’s energy code mandates whole-house ventilation for new construction based on ASHRAE Standard 62.2, “Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings”. This standard includes some specifics on output, such as the number of cubic feet per minute of air moved. However, Title 24 gives little direction on ventilation system design, installation, or operation. The old theory was that a simple exhaust fan in the bathroom would create negative pressure inside the home which would, in turn, draw fresh air in directly through the walls of the home. In older times, homes were more air-permeable (i.e., drafty) and this was a simple, low-tech solution. Today, however, it’s no longer enough.
The main reason to ventilate include comfort and health – prevent stale air from building up, and also avoid bringing in outside pollutants such as dust, mold, or pollen. But why the sudden focus on ventilation? Several reasons, among them the following:
- Homes are far more airtight nowadays, leading to a new emphasis on the need for indoor air quality.
- Poor circulation within a home means fresh air doesn’t get to where it’s most needed, like bedrooms. If you’re relying solely on exhaust vented from a bathroom, what happens when doors are closed?
- Exhaust-only ventilation causes pressure imbalance inside the home, not always desirable.
- Air coming in through any old crack in the house means you’re not controlling the source very well. You could be bringing in all sorts of pollutants and dust. Better to choose your intake points and add filtering.
- Drawing air in through the walls introduces the risk of moisture condensation inside the walls, especially if there’s a big temperature differential between outside and inside air.
- Ventilation systems now include heat recovery ventilation units (HRV) so that you’re not continually bringing cold air into the house and then using extra energy to heat it. Instead, heat from the inside air that’s being exhausted outward is transferred directly to the intake air. Essentially, you’re recycling your own heat without sacrificing your fresh air supply.
- Additional features can remove humidity from intake air in climates where that is a concern. One thing to note is that humid air takes more energy to heat or cool than dry air, so managing humidity can save energy, up to 15% on heating in cold climates and up to 45% on cooling in hot, humid climates.
Carbon dioxide buildup creates stuffy rooms and foggy brains
One problem with a poorly ventilated, airtight building is carbon dioxide buildup – stale air. Too much CO2 in the air can cause people to feel sleepy, or worse. This problem doesn’t get as much attention as carbon monoxide poisoning, but it’s still a concern. Carbon dioxide is, of course, a naturally occurring by-product of human respiration. Carbon dioxide levels in fresh air measured in parts per million varies between 0.036% (360 ppm) and 0.039% (390 ppm). It can go higher indoors, perhaps 600-800ppm. Some literature implies that a stuffy auditorium could go as high as 10,000 ppm, which is high enough to cause noticeable drowsiness.
The length of exposure is a factor as well. Barry Stephens mentioned a study he saw presented at a Passive House conference at Brandeis University showing carbon dioxide levels throughout the night inside the bedroom of a modern, airtight home as measured in parts per million. In some of these homes, the CO2 level would rise to 3,000 or even 4,000 PPM by morning, causing occupants to wake up feeling tired.
OSHA’s safety threshold for CO2 level is 5,000 PPM (0.5%) over 8 hours of exposure. At levels over 10,000 or 20,000 ppm more serious symptoms of carbon dioxide poisoning start to appear: headache, lethargy, mental fogginess, confusion, irregular heartbeat, anxiety. Carbon dioxide levels of 100,000 ppm can cause loss of consciousness or even death.
“The correlation between CO2 levels in buildings and health systems in living organisms isn’t well established at this point,” cautioned Charlie Stephens. “[But] it’s the reason why many people have noticed that they sleep better with a window open.”
How do you know if you need a fully pressure-balanced whole-house ventilation system?
Is there some sort of test or threshold for determining whether a home needs the full ventilation treatment? Not yet. A house that passes the California blower door test could potentially benefit from whole-house ventilation. If it is extremely airtight, enough to have under 0.6 air changes per hour (that’s the Passive House standard) it should definitely have one. This still leaves a large gray area of uncertainty.
Building and energy codes are mandating tighter buildings, increasing the importance of ventilation. Building air infiltration, i.e., air leakage, is usually measured using a blower door test and is often expressed in terms of number of air changes per hour (ACH). So how many ACH does it take to change a ventilation requirement? Well… the U.S. Department of Energy is proposing 5 ACH as the threshold for requiring mechanical ventilation, but since an “average” new home might now be 3 ACH and an “efficient” home might be around 1.5 ACH, all we can say is it’s somewhere in that range.
“There is no defined ACH level that I know of at which ventilation is mandated, or even recommended at this point, because it hasn’t been much of an issue until recently,” said Charlie. “What they would be saying is that for any ACH level below this number, you would be required to have an effective ventilation system, but almost no one knows what an ‘effective’ ventilation system is, except in the case of a properly designed and installed HRV or ERV system.”
And what does ACH measure, anyway?
Air changes per hour is one way to measure the permeability of a building envelope, based on the known physical behavior of gases such as our own air and atmosphere. As we know, gases expand to fill all available space, and this expansiveness creates pressure – air pressure. The more space it has to expand within, the lower the air’s pressure becomes. If there are different air pressures inside vs. outside, air will automatically begin to seep from the higher pressure enclosure into the lower pressure surrounding areas. The higher this differential, the faster the seepage. “The measurements for ACH levels of air leakage are taken at a differential pressure of 50 pascals (0.2 inches water gauge) between the indoors and outdoors,” explained Charlie.
(The measure’s full name is ACH50 – we’re abbreviating.)
So what ACH level would be considered “normal”?
Charlie stressed that the proposed 2012 IECC Residential Energy Code upgrades include a provision for a maximum ACH level of 3.0. (Title 24 uses a different calculation, not ACH, and the building must fall within a range and not be TOO tight.) But what is “normal” for ACH as of today? Here are a few random statisics from Northouse and Green Building Advisor:
- An average home’s ACH is 3-3.5 in Alberta and 3.9 in Wisconsin
- The average for new construction in Minnesota is 2.5 ACH
- Older homes might be around 4-9 ACH
- An “efficient” home is around 1.5 ACH
Energy researchers all over the country are wrestling with this question even as we speak, including the Northwest Energy Efficiency Alliance in Portland, OR – Charlie Stephen’s group. “The NEEA ventilation field research will be conducted on recently built homes of recent and reasonably airtight construction during 2011 and 2012,” Charlie informed me. “The monitoring of CO2 levels isn’t so much an end as it is a means to understand how air is moving in a home. It’s essentially a cheap tracer gas.” Even though it’s not the primary purpose of the study, it may help to begin associating CO2 levels with specific ACH numbers.
“We’re also measuring the performance of the mechanical ventilation components (such as fans) installed in the homes, and trying to understand how they’re operated, and whether or not they are effective at providing sufficient ventilation air for the occupants of the home. We’re only looking at homes we consider to be quite airtight – probably 3.0 ACH or lower. We don’t know how many of those we’ll find in our sample set, which is a 4-state sample of existing homes. We’re only looking at 30-50 homes initially. We may look at more after the first phase of the research, depending on what we find. As you suggest, all of our sample homes will have to be blower door-tested to start with.”
But you don’t really know the ACH until you do a blower door test, and you can’t do this until the house is already built.
We have been encouraging our Title 24 clients to plan ahead in the early design stages for things like systems, but how can you plan ahead for something that’s got to be measured in the field after being built? Charlie had an answer, although it still involves measurements taken during construction.
“Blower door tests can be conducted before the home is finished, but not before the air barrier is fully in place. This tends to be after sheetrock, but before interior finish, if the home is new. Many of the sites of air leakage can be accessed from inside the house in the crawlspace, basement, or attic, and so some corrections are possible even in remodels. But it’s a lot easier in new homes to pay attention to the air barrier for the envelope and to be meticulous about air-sealing as the home is built. Lots of homes are achieving air leakage results under 1.0 ACH50, so it’s not impossible, or even difficult in many cases.”
Do HRV systems make sense in warmer climates, too?
Heat Recovery Ventilation systems make sense in cold climates, but what about hot climates? Actually yes, although in places like Florida you might want an Energy Recovery Ventilation system instead. An ERV will remove some of the humidity from the intake air. Why bother with that? Because it takes more energy to change the temperature of humid air than it does to condition dry air. (It takes more energy to change water temperature than air, and the presence of water in the air changes the air’s thermodynamic properties.)
The difference between the mechanical units is that an ERV uses a membrane that transfers both moisture and heat, while an HRV transfers heat through layers of plastic sheets. As to where exactly to change from one to the other, that’s still up in the air. But you can if you want to; Zehnder units have swappable cores to change from HRV to ERV.
How does the Zehnder ventilation system work in a retrofit?
The Zehnder ventilation system uses smaller, flexible ducts that can be fed through the walls, rather than large metal ducting that requires extensive use of sealing and mastic. Half of Zehnder’s projects are retrofits, usually including other energy features besides heat-recovery ventilation.
One project Barry mentioned was a homeowner named Tad Everhart in Portland. He’s now a Passive House consultant. He did a full Passive House retrofit on his 2,100 SF home. For a Passive House, you need a fully sealed ventilation system with heat recovery, that meets stringent requirements for both energy efficiency and air movement. As Barry Stephens tells it, Everhart was intrigued by Zehnder’s ducting system, and found that he could feed it through the walls just by cutting a small hole. He did the downstairs first using 2 6-tube manifolds, and later did the upstairs the same.
Another set of projects that Barry mentioned include a series of gut rehabs of Brooklyn brownstones, where the owners are retrofitting to the Passive House standard. There’s a whole core of Passive House consultants, mainly architects, in the New York area, including Ken Levenson, Jeremy Shannon of Prospect Architecture, and David White of Right Environments. Their work includes both new designs and retrofits.
Tell me more about deep energy retrofits.
“A lot of people start by thinking about the envelope,” said Barry. “They seal it up tight and add insulation, upgrade the windows, maybe re-do their ductwork and heating system. Then they do a blower door test and say, ‘Oh yeah… I guess we have to add ventilation.’”
Barry noted another thing about deep energy retrofits. When you tighten and insulate a building, the heating and cooling loads go way down. In a recent interview, Green Builder Jeff King told us that many HVAC systems are already grossly oversized to begin with. After a retrofit, those systems are REALLY oversized.
Can you integrate whole-house heat recovery ventilation with a conventional forced air heating system?
Yes, you can integrate HRV into existing forced-air systems. The existing forced-air heating system does not make whole-house HRV redundant, because the forced-air heating system by itself is just moving air around inside the home, unless you add an extra intake by the furnace to add fresh air. To add a heat-recovery ventilation system in a building with existing forced-air heating, you can install the HRV intake to feed into the furnace return plenum on one end, and to draw exhaust out on the other. This would reduce load on the furnace because it’s no longer having to heat outside air directly. If it’s 20 degrees outside and you add the HRV, then the furnace no longer has to heat up 20-degree air. Instead, it’s getting air that’s already around 65 degrees.
Charlie explained the concept of a hybrid ventilation system that is integrated with an existing forced-air heating system. In this scenario, fresh air would be supplied just before the air handler, in the return duct, while exhaust is through typical bathroom fans. Since this would potentially lead to a pressure imbalance, an additional step is needed. “To create a balanced ventilation system, you’d measure pressure inside the house relative to outdoors, and adjust the capacity of the exhaust fan(s) to match the supply air intake rate at the air handler return. Obviously, the fans would have to be controlled to operate at the same time. And most exhaust fans and air handlers do not have adjustable capacity, so controlling flow is done with dampers – not a particularly efficient way to manage air flow.”
What is the cost of a Zehnder ventilation system, including design, equipment, installation?
For a Zehnder ventilation system, an average 2,500 SF house with heat-recovery, ducting, silencers (a noise abatement option), controls, and installation service would be $6,500 – $7,500. The equipment is around $5,500 and installation another $2,000. Installation takes half the time compared with conventional metal ducting, because it’s smaller, and the connections are simple O-rings and clips rather than requiring extensive use of mastic, sealing, etc. It’s a lot lighter too – one person can carry all the tubing. Barry estimates a typical house project would take 2 people and 2 days.
Note, however, that HVAC subcontractors have bid higher, sometimes $6,000 because they’re unfamiliar with the new technology and that’s how long it might take for them to install a more conventional HVAC system. Barry suggested that you might have to bypass the HVAC guys altogether, or push back to get them down to a more reasonable installation figure.
Can a regular builder or HVAC contractor install a Zehnder ventilation system?
The system isn’t hard to install, in fact it’s lighter and takes less time to install than a conventional forced-air heating system with metal ductwork. Zehnder has been providing the system specifications and design for projects, so all the builder has to do is follow the instructions for installation and commissioning. Builders don’t need to be Zehnder-certified. However, because conventional HVAC contractors have been over-bidding on installation, Barry notes that on many projects it’s the carpenters who’ve been installing the ventilation and ducting, and it’s working fine.
The installers don’t have to design or spec the system. Zehnder has been doing that – taking the plans, doing the layout and specifying the equipment needed.
What does the architect have to do during the design phase to account for a whole-house ventilation system?
Barry/Zehnder: Architects need to plan for the chases. “We can’t just let the HVAC guys figure it out afterwards,” said Barry. Zehnder’s ducts are 3 inches in diameter. For example, consider 5 supplies and 5 returns with 3″ ducts, plus a main intake and a main exhaust going to the outside, each around 6 or 7 inches. You can put the HRV unit in the basement, but don’t forget to plan for chases going to other floors. For example, consider fitting them into stairwells or the back of a closet.
Barry/Zehnder: One important point is to locate the HRV unit close to an outside wall, to minimize duct length between the air intake and the HRV unit. The 6″ intake duct is surface area that’s exposed to the outside, and although it’s protected, it’s not as well insulated as a solid wall would be. The Passive House standard actually penalizes you for long intake and exhaust runs because they waste energy.
Barry noted a few “gotchas” with new structural systems were noted, particularly laminated veneer lumber beams. You can’t drill through them or thread through them the way you can with a regular truss. This issue comes up sometimes with other structural systems as well, with some materials requiring pre-drilling or fabrication because they can’t be easily modified on site to accommodate last-minute decisions.
Does every room need its own intake and exhaust?
No. You can have intake in some rooms and exhaust from others. Barry’s strategy is as follows: get fresh air IN to the bedrooms and living spaces, and get exhaust OUT of bathrooms and kitchens. You want to have constant low-level air movement. This distributes the air quickly and evenly throughout the home.
I’m guessing that the exhaust from the bathroom doesn’t go straight out the window.
The bathroom’s exhaust air doesn’t go straight out the window as with an exhaust fan, but instead that it is drawn back through the central HRV unit, where its heat energy is removed. Only then is the air expelled from the home.
Charlie: All of the air being exhausted by the HRV or ERV leaves the house through the main unit’s HRV or ERV ducting, but almost all of the energy in the air being exhausted is transferred back into the incoming air stream. Thus, the fresh air from the supply intake is only a couple of degrees Fahrenheit different in temperature than the indoor air being exhausted, regardless of the temperature outside. Unlike a conventional heating/cooling system, the air won’t be returning to be recycled inside the home. Instead, it’s being sent outside – exhausted, along with all of the moisture, odors and contaminants in the air.
Is it important to have ducts placed high or low on the wall?
Barry/Zehnder: Where we use register boxes, the supply tends to be low and the return is high. Diffusers can be either.
What’s the maximum recommended duct length for a Zehnder ventilation system?
“I’d say 50 feet, but the number of bends is actually a lot more important. You want to minimize bends. You might be OK with a straight duct that’s as long as 100 feet, but if it’s got several 90-degree bends, the max length might be more like 20 feet,” said Barry.
How many Zehnder HRV units would you need for a single home?
For Zehnder products in a typical single-family residence, usually just one. It’s based on the total conditioned air volume inside the home, the desired amount of air changes per hour, and the number of people. Sometimes even family pets get counted. Here’s a little known fact – did you know that cats and dogs use more oxygen for their body weight than people? It’s because they breathe faster. If you have a bedroom with two people and two large dogs all sleeping in there, it’s going to collect a lot more CO2.
Can you retrofit a Zehnder ventilation system in a single apartment?
Yes. You can get a smaller unit installed on the wall with vents to the outside. A lot of apartments in Europe are doing this. For an apartment, you don’t need chases in every room, maybe just the main room and the bathroom.
What about maintenance and troubleshooting?
For Zehnder systems, the filters should be changed once a year, and they are externally accessible so the homeowners can do it themselves. Some filters can actually be rinsed with water and reused, but still should be replaced annually. For the HRV unit, you can wash the core with water – you need only a screwdriver to take it off. Ducts can be cleaned, either with a special “roto” type brush or a vacuum. The ducts are smooth with no connectors so it’s easy to brush them out.
What are the options for filtering?
Zehnder products fall into two main levels, measured using the Minimum Efficiency Reporting Value system, or MERV. MERV ratings are based on the size of the molecules that are filtered out. The higher the rating, the smaller the molecules that filter can catch. Zehnder filter options are twofold:
- The G4, which is MERV 7-8 – fine for eliminating pollen.
- The F7, which is MERV 13 – best residential dust control; almost good enough for an industrial “clean room”
What about filtering for smog?
Zehnder’s post-filtering solutions include an optional filter box that can go up to MERV 15 and includes charcoal filters. Charcoal is what you want for smog gases such as carbon monoxide or industrial fumes. This box is big, to keep air flowing freely. The filter box is 15-24″ long x 8″ x 10″. Costs $400. Barry noted that filtering outside air won’t necessarily help with dust mites or mold, because those air contaminants are already IN the house. What the filters do is remove outside particulates like pollen, street dust, or soot.
Do you recommend installing CO2 sensors too?
Barry/Zehnder: We offer sensors for CO2 levels and relative humidity. They can be tied to our controls so the system automatically adjusts. A karate studio, for example, might have a sensor so that when there are only 2 people in the studio, the system can stay on low – but if there are 25 people working out hard and raising the CO2 level, the sensor automatically kicks the system up to a higher level.
All the Zehnder units have a summer bypass feature, too. In the summer, it might be hot in the day but cooler at night. So at night you might want to bypass the HRV to help keep the house cooler. Charlie adds, “the ‘summer bypass’ feature is an automatic one – you push the button to engage the feature, then the HRV decides when to bypass the heat recovery core.”
Does Zehnder provide a commissioning protocol for use by installers?
There is a document on how to test whether the system is properly installed and functioning. Barry recommends using an energy auditor for this, someone who’s got flow hoods, and to have them test room by room for airflow and pressure balance. They can then adjust the air flow at various registers, using what Charlie described as “an ingenious bundle of tubes and washers”.
A Doubting Thomas Chimes In…
Many experienced residential architects will be questioning the utility of requiring yet another system to be incorporated into their home designs. Here we’re posing some questions from the skeptic’s gallery. Readers will have to make up their own minds – we hope we’ve given them enough food for thought, and maybe a few yardsticks, too. The designation of something like “stale air” is pretty vague, but we’ve all been in rooms that were far too stuffy – and if you had the choice, and you’re already spending the money to customize a home, wouldn’t you prefer to be breathing clean air instead?
What about having a bathroom fan with ceiling fans in every room? Won’t that move enough air around?
You’re still dealing with a pressure imbalance, uncontrolled air intake, and if doors between rooms are closed, you’ll still have pockets of stale air in the bedrooms at night – the last place you want it. In homes with radiant heating systems, there isn’t even the air circulation within the home that you would get from a conventional forced-air heating system.
Charlie adds, “in a very airtight house, with all of the windows and doors closed, there’s no guarantee of how much air will come in when the exhaust fan is on, and air that didn’t come in can’t be exhausted out. We suspect that exhaust fan flows in tight houses may be much lower than the fan’s rated capacity, and so total ventilation may be much lower than people think. That’s one reason why we’re doing the field research.”
But that’s ridiculous. I’ve been building efficient homes for 20 years and I think whole-house ventilation adds needless cost.
It does add somewhat to the cost, but it’s still something to consider if you’re already doing a renovation. There’s an opportunity to add in the chases while the walls and floors are already open.
Charlie adds, “People who have been building ‘efficient homes’ for years have almost never been building airtight efficient homes -until recently. Any home that leaks air at more than about 5 ACH is probably leaky enough to keep the builder and the occupants out of serious trouble regarding CO2 buildup, but that doesn’t mean the indoor environment isn’t laden with other contaminants or excessive moisture – and CO2 at times. A properly designed and installed balanced heat recovery ventilation system solves multiple problems and delivers multiple benefits in any home that is substantially airtight.”
OK, so mold is a problem in Florida. Would a house that passes a blower door test but relied only on a bathroom exhaust fan for ventilation still be at risk for mold in a drier climate like California? Why can’t we just use vapor barriers to manage this?
Charlie explained why vapor barriers aren’t enough to manage mold. Mold can exist wherever it can find three things: food, cool temperatures, and high relative humidity or bulk moisture. In a lightly or poorly insulated home, or one where there are thermal bypasses in the insulation system (e.g. where framing members pass all the way from the interior to the exterior), mold can grow on damp surfaces with a food supply such as cellulose.
“An HRV in this circumstance removes excess moisture from the home. When fresh air comes in, its temperature is elevated in the heat recovery core to within a couple of degrees Fahrenheit of the indoor temperature, which significantly lowers its relative humidity, helping to maintain a lower relative humidity level in the home, in spite of elevated levels of cooking or showering activity. There are lots of climate zones in California where mold is an issue. And if indoor relative humidity levels are consistently elevated, for whatever reason, mold can even be a problem in the desert.”
About the author
Rebecca Firestone has been working in the Bay Area since 1998 as a technical writer, business content developer, architectural filing lady, marketing director, and sorcerer’s apprentice.