A ‘Magic Box’ For Your Passivhaus
Can Americans buy a combination appliance incorporating an HRV, heat pump, and water heater?
By designing a tight envelope with thick insulation, Passivhaus designers work hard to whittle a home’s space heating load to a bare minimum. Many European designers strive to get the heating load so low that all space heat can be provided by raising the temperature of the ventilation air.
In a home with an area of 1,600 square feet and a ventilation rate of 0.3 ac/h, ventilation air flow is only 64 cfm. Since Passivhaus designers try to keep the temperature of the ventilation air below 122°F (or, according to some sources, 131°F), it’s hard to pack much heat into the small volume of air that flows through typical ventilation ducts. That’s why it’s such a challenge to insulate a building’s shell well enough to deliver all of a home’s heat through its ventilation system.
In central Europe, where winter temperatures are much milder than they are in Minnesota or Maine, some designers have succeeded in supplying all of the space heat needed for a Passivhaus through ventilation ducts. In most cases, these homes are equipped with a “magic box” — the nickname for a combination appliance that includes a heat-recovery ventilator (HRV) and an air-source heat pump. Typically, the heat pump’s evaporator coil is located in the ventilation exhaust duct, downstream from the HRV, where the coil can scavenge heat from the exhaust air before it leaves the building.
Many (but not all) of these magic-box appliances also include a hot water tank. Such appliances use the air-source heat pump to heat domestic hot water; in most cases, the tank includes electric-resistance backup. Some manufacturers (including Zehnder) design their magic-box appliance around a ground-source heat pump instead of an air-source heat pump.
Magic boxes in North America
I’ve heard rumors that magic-box appliances are beginning to become available in North America. Katrin Klingenberg, the director of the Passive House Institute U.S., informed me that “the Drexel & Weiss unit is already or is scheduled to be available on the Canadian market.” Unfortunately, the rumor isn’t true.
Drexel und Weiss is a manufacturer with headquarters in Wolfurt, Austria. One of their magic-box appliances — combining an HRV, air-source heat pump, water heater, and hydronic space-heat distribution pump — was installed in the Austria House, a Passivhaus building in Whistler, British Columbia, that was used by the Austrian Olympic team.
I called Matheo Durfeld, one of the builders of the house in Whistler. He told me that Drexel und Weiss has no plans to distribute its magic-box appliances in Canada or the U.S.
The next rumor I tracked down concerned the Zehnder ComfoBox, a combination appliance that includes a ground-source heat pump, hydronic heat distribution system, domestic water heater, and HRV.
Zehnder’s U.S. representative is Barry Stephens. For several months, Stephens has been making the rounds of U.S. trade shows, where he displays Zehnder’s HRVs and duct fittings. Unfortunately, Zehnder has no plans to sell its magic-box appliance in North America — at least not for several years. “The only ComfoBox in North America is in my home in Maine,” Stephens told me.
The Nilan appliance is almost a magic box
The only appliance resembling a magic box that is available in North America is made by a Danish manufacturer, Nilan. The Nilan appliance — a combination HRV and air-source heat-pump — is distributed by Solution Nilan of Chambly, Quebec. The appliance provides a limited amount of space heating and space cooling, but does not include a water heater.
Two sizes of the Nilan appliance are available: the VPL-15 and the VPL-25. The VPL-15 provides 6,500 Btu/h of space heating and 5,500 Btu/h of cooling (about half a ton). The larger unit, the VPL-25, provides 13,500 Btu/h of heating and 11,500 Btu/h of cooling (about a ton). The units have maximum air flow rates of 190 cfm (for the VPL-15) and 500 cfm (VPL-25) — considerably higher air flows than required to ventilate a typical home.
These high air flow rates raise an important question: is the unit’s maximum air flow (500 cfm for the VPL-25) necessary to extract the maximum rated heat and cooling output? If so, it seems clear that the Nilan unit needs to be installed with a duct system that allows partial air flow recirculation — because 500 cfm is too much ventilation air for a house.
If you’ve got a very big house, you may want to consider another combination appliance: the Matrix boiler manufactured by a Canadian manufacturer, NY Thermal of Sussex, New Brunswick. The Matrix performs the functions of a boiler, furnace, water heater, and HRV.
Unlike European magic-box appliances, the Matrix is a fuel-burning appliance. It does not include a heat pump. The heart of the Matrix is a natural-gas burning condensing boiler with an output of 90,000 Btu/h. Hot water from the boiler circulates through two coils — a heat-exchange coil in the domestic hot water tank, and a space-heating coil in the air handler plenum.
The Matrix was not designed for Passivhaus buildings with a small heating load. “If someone was going to do their entire home with hydronic heat distribution, this would be the wrong appliance,” admitted NY Thermal representative Rob Alexander. “It would be more economical to go with a boiler and a stand-alone HRV. This is more for the guys who have forced-air heat but maybe have a basement slab or a garage floor they want to keep warm with in-floor radiant.”
The Japanese offer a different type of combination appliance: one without an HRV. The Daikin Altherma is an air-source heat pump that provides space heating, space cooling, and domestic hot water. Some Passivhaus designers are pondering the feasibility of a system that circulates hot water from a Daikin Altherma through a hydronic coil in the ventilation ductwork, downstream from an HRV.
The first Daikin Altherma unit in the U.S. was installed in 2009 in a Portland, Oregon home designed by architect Brent Hinrichs. The 1,800-square-foot house has a design heat loss of about 28,000 Btu/h and a hydronic heat distribution system (some in-floor tubing supplemented by wall-mounted radiators). According to Hinrich, the electricity bills have been "surprisingly low."
To learn more about the Altherma unit, check out the Daikin Altherma brochure.
Although some Passivhaus designers long for the day when more European magic-box appliances become available in North America, those who have glanced at the appliances’ price tags usually swallow hard — and then conclude that they’re perfectly satisfied with available North American appliances.
According to Barry Stephens, a Zehnder ComfoBox costs about $18,000 in Europe — not including the ground loop for the ground-source heat pump, which can easily cost $10,000 or more.
A magic-box appliance from Drexel und Weiss costs about $25,000 in Austria. One of the company’s owners, Reinhard Weiss, explained the price tag to Matheo Durfeld this way: “We’re not selling equipment; we’re selling knowledge.”
Since the Nilan HRV doesn’t include a water heater, its price is significantly lower than magic boxes from Zehnder or Drexel und Weiss. The Nilan VPL-15 sells for $4,950, while the VPT-25 costs $6,450. (Both prices are in Canadian dollars). So, if you are able to whittle down your heating load to 13,500 Btu/h — thereby avoiding the cost of space-heating equipment — you might consider buying a Nilan VPL-25.
Some Passivhaus designers are drooling over Nilan’s VP-18 model — a true magic-box appliance that includes a hot water tank. Unfortunately, the Nilan VP-18 is unavailable in North America; European distributors sell the VP-18 for 6,817 euros ($9,288).
Daikin declined to provide specific pricing information on the Altherma; according to a company statement, the cost of installed systems ranges from $10,000 to $20,000.
The Matrix combination appliance costs between $7,000 and $8,000.
Pluses and minuses
In light of the relatively high cost of magic-box appliances, why would anyone consider buying one? Here are the arguments in favor of a magic box:
- The best way to capture the heat contained in escaping ventilation exhaust is to locate the evaporator coil of an air-source heat pump in the exhaust air stream.
- To optimize the efficiency of a home’s HVAC equipment, all of the HVAC system’s elements should be combined into a single appliance.
- When appliance integration occurs at a manufacturing plant instead of a job site, there are fewer opportunities for set-up and commissioning errors.
On the other hand, here are the arguments against the magic-box concept:
- The more components that are crammed into a single appliance, the greater the chance that a failed component will disable the entire unit, and the more complicated any repairs. (That’s one reason that few people buy a combination television/DVD player/VCR anymore.)
- The cost of these appliances is so high that it’s more economical to buy three separate appliances: an HRV, a water heater, and a ductless minisplit heat-pump.
- Ventilation requires small air flows, while the delivery of space heat and cooling usually requires larger air flows — making the integration of these two functions illogical. In hopes of achieving a particular heat-delivery goal, a magic-box designer can be tempted to solve this technical challenge by overventilating. It makes more sense to optimize ventilation ducts for fresh air delivery without consideration of heating or cooling loads; combining these functions often results in technical compromises.
Because of this last point, I predict that the long-touted Passivhaus recommendation to deliver space heat through ventilation ducts will eventually fall by the wayside — if not in Europe, then certainly in North America. Although Dr. Wolfgang Feist has been a strong proponent of this method of heat delivery, it has never been a requirement of the Passivhaus standard, and the technical challenges facing anyone striving to achieve the goal are hardly worth overcoming.
One thing is clear: the very high price of magic-box appliances undermines the hope that the Passivhaus standard will help builders (in Amory Lovins’ words) “tunnel through the cost barrier.” That’s why North American Passivhaus builders will probably continue installing separate HRVs with well-designed dedicated ductwork — independent of the heat-delivery system.
Daikin AC, 1645 Wallace Drive, Suite 110, Carrollton, TX 75006. Tel: 972-245-1510; Web site: www.daikinac.com.
NY Thermal (NTI), 30 Stonegate Drive, Saint John, NB E2H 0A4, Canada. Tel: 506-657-6000 or 800-688-2575; E-mail:email@example.com; Web site: www.nythermal.com.
Solution Nilan, 45 Georges-Pepin, Chambly, Quebec J3I 4Y8, Canada. Tel: 514-990-3604 or 800-808-0496; Web site: www.nilan.ca/indexen.htm.
Last week’s blog: “Airtight Wall and Roof Sheathing”
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