Sunday, December 4, 2011
An intriguing new approach to heat-recovery ventilation
Lots of people like the performance of heat-recovery ventilators (HRVs), but not many people like the high cost of purchasing and installing the equipment. It’s complicated to run ventilation supply and exhaust ducts to seven or eight rooms, so the typical installed cost of an HRV is $2,000 to $3,000.
Is there a simpler way to provide ventilation that includes heat recovery? Perhaps. Four Seven Five will soon being distributing a small ventilation fan, the Lunos e2, manufactured by Lunos Lueftungstechnik of Berlin, Germany. The cylindrical Lunos e2 has a diameter of about 6 in. It is designed to be installed inside a wall, but the wall must be fairly thick, with a minimum thickness of 11 7/8 inch. Lunos e2 fans are fairly unobtrusive; the indoor cover plate measures 7 in. by 7 in.
Inside each Lunos e2 unit is a small 12-volt fan (powered by a transformer) and a perforated ceramic heat-storage disk. As air flows through the disk, the disk absorbs or gives up its heat.
Here’s the ingenious part of the Lunos e2 design: the fans are reversible. A Lunos fan is programmed to operate as a supply fan for 70 seconds, and then reverse itself and operate as an exhaust fan for 70 seconds. Fifty-one times per hour, the air flow reverses.
During the winter, the exhaust air stream heats up the ceramic disk. When the air flow reverses, the ceramic disk gives up its stored heat to the supply air stream. One minute later, the air flow reverses again, and the exhaust air streams heats the ceramic disk back up. The net result: The Lunos e2 is able to recover heat that would otherwise escape with the exhaust air stream.
The Lunos e2 fans are designed to be installed in pairs. A controller synchronizes the functions of the two fans: when one fan is operating as an exhaust fan, the other fan, which can be located some distance away, is operating as a supply fan.
Lunos e2 fans are very energy efficient. They draw only 2.8 watts at high speed (17.6 cfm). That’s an efficiency of 6.3 cfm/watt, about the same as a Panasonic FV-08VKS1 fan, which supplies 80 cfm at 11.3 watts (7 cfm/watt).
The Lunos e2 fan is rated at only 17.6 cfm. That’s not much.
How many Lunos e2 fans do you need for a small house? Let’s take an example: a 1,200-square-foot house with three occupants requires 35 cfm of continuous mechanical ventilation, according to the ASHRAE 62.2 formula of 7.5 cfm per occupant plus 1 cfm for every 100 square feet of occupiable floor area. So two pairs of 17.6-cfm Lunos fans would be adequate for such a house. (At any one time, two of the four fans would be operating as supply fans, and two as exhaust fans.)
The main disadvantage to the Lunos e2 fan is its high price. Four Seven Five plans to sell the fans for $1,200 a pair, or two pairs (the minimum number of fans necessary to ventilate a small house) for $2,250. That’s steep. So, to return to the question we started with — “is there a simpler way to provide ventilation that includes heat recovery?” — the answer is yes. Unfortunately, the simpler way is just as expensive as installing an HRV.