Installing a Zehnder ComfoAir 70

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Vermont remodeler and home performance contractor Jim Bradley installs a compact, stand-alone wall-mounted Zehnder ComfoAir 70 energy-recovery ventilator on an airtight remodeled home.

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Tight buildings need fresh air. But fresh air comes at a cost that can put a strain on budgets — especially in remodels, where you might have to demo existing ceilings in order to install the supply and return ductwork for a high-performance energy recovery ventilator (ERV). That's the problem German firm Zehnder is trying to solve with their latest U.S. product introduction, the ComfoAir 70. Zehnder's higher-capacity whole-house ComfoAir models already on the market (the ComfoAir 550, 350, 200, and 160), have large central air handlers installed in a mechanical room and require extensive ductwork. But the ComfoAir 70 is a one-piece unit that mounts through a side wall in the living space and needs no ducts; it draws in room air and supplies fresh air through small openings on the sides of the appliance.

Priced at $1,250 and sized to provide fresh air for about 600 square feet of living space, the ComfoAir is a good match for small spaces—and of course, the more units you install around the building, the more spacethey can serve. In June, JLC went on site with remodeler and home performance contractor Jim Bradley to see two ComfoAir 70 units installed for a deep energy retrofit on a country house in Vermont. These are the first two ComfoAir 70 units sold in the U.S., so Zehnder's U.S. rep, Norbert Wesely, was on the job to help out.

Setting a ComfoAir 70
Left: Zehnder America rep Norbert Wesely slides the ComfoAir 70 into its housing on a remodeling job in Vermont. Right, an exploded view of the device shows the heart of the system: an enthalpy exchanger core that transmits heat and moisture between incoming and outgoing air streams.
Left: Zehnder America rep Norbert Wesely slides the ComfoAir 70 into its housing on a remodeling job in Vermont. Right, an exploded view of the device shows the heart of the system: an enthalpy exchanger core that transmits heat and moisture between incoming and outgoing air streams.

The ComfoAir 70 competes with another European-made compact wall-mounted air exchanger, the Lunos e2, which is available in the United States from the New York firm 475 High Performance Building Supply, along with a smaller companion device, the Lunos eGO. The Zehnder and Lunos systems both mount quickly through a simple hole in a side wall, both offer balanced ventilation, and both have heat recovery. But they operate differently: The Lunos e2 system uses a matched pair of through-wall units with heat-storage cores, set up to operate in tandem on a 70-second cycle, so that one unit exhausts stale air while the other unit draws in fresh air. A thermal-mass core in the Lunos stores heat from the outgoing warm air and returns the heat to the incoming fresh air when the fan reverses. The ComfoAir 70, by contrast, does continuous intake and exhaust through just one unit, exchanging heat directly from the outgoing airstream to the incoming airstream through an "enthalpy" core, just like the larger central ComfoAir systems (and like most other ERV systems on the market).

The internal transfer core gives the Zehnder ComfoAir an advantage over the Lunos. Like its larger cousins, the ComfoAir transfers humidity as well as heat — that's why it's an "energy recovery ventilator" (ERV), not just a "heat recovery ventilator" (HRV). The ComfoAir also filters the incoming fresh air, and the enthalpy core can be removed and washed (in fact, the core should be cleaned on a regular basis if you want the unit to perform properly).

Jim Bradley's crew had already carried out an extensive reconstruction of the home's wall system, turning the existing underperforming fiberglass-insulated 2x6 stud wall into an airtight superinsulated assembly with an intelligent vapor barrier on the inside face, a plywood air control layer in the wall center, and a vapor-open exterior drainage plane membrane. To install the ComfoAir 70, they had to lay out and cut a circular hole through this multi-layer assembly, then install the plastic through-wall pipe and seal the pipe to the interior and exterior membranes with tape. An electrician hard-wired a Zehnder power transformer into the house wiring to supply the ComfoAir with 24-volt direct current, setting the transformer in a junction box. Then, the ComfoAir could be inserted through the wall pipe and set into its metal housing, attached to the wall.

Cutting Through the Superinsulated Wall
A carpenter cuts a hole through the superinsulated wall assembly's mid-wall sheathing (left), then through the interior layer of rock wool insulation (center). Finally, the ComfoAir tube is sleeved into the hole (right).
Exterior Details: ComfoAir 70 Install
If the standard through-wall sleeve for the ComfoAir 70 is too long for the wall thickness, the tube can be trimmed to length (left), as long as there is still room for the unit's fan, which is housed inside the tube. As a final step, the vent grille and cover are installed (right).
If the standard through-wall sleeve for the ComfoAir 70 is too long for the wall thickness, the tube can be trimmed to length (left), as long as there is still room for the unit's fan, which is housed inside the tube. As a final step, the vent grille and cover are installed (right).

The ComfoAir's through-wall pipe and insulated air tube are long enough to reach through a two-foot-thick wall. For thinner wall sections — as in this case — the pipe and tube can be cut to fit. But because the unit's fan is housed within the through-wall tube, there's a minimum limit to wall thickness. Even Jim Bradley's 11-inch-thick wall had to be packed out to 12.5 inches before the ComfoAir tube would fit. In a standard U.S. 2x6 wall, leaving room for the fan would require the tube to project past the wall surface by almost 6 inches, requiring the wall to be built out either on the inside or on the outside to accommodate the appliance.

The ComfoAir 70 doesn't require ducts to operate. But you can hook short runs of ductwork up to it. That's handy in certain cases: For instance, you could set the unit up to draw stale air from a kitchen or bath, and deliver fresh incoming air to a living room or bedroom suite.