HYDRONICS
FROM THE FIELD Passive house, anyone? Part 2
BY DAN FOLEY CONTRIBUTING WRITER
very low infiltration and heat loss. The project profiled had a design heat loss of 24,000 Btu/hr. for a 4,600 square ft. structure. This month’s column will detail the mechanical system design and how we met the stringent Passive House Institute standards. The mechanical system design was formulated over
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multiple meetings with the architect, David Peabody. David brought in consultant Michael LeBeau, a Duluth, Minn., based contractor who has designed and installed several mechanical systems for passive houses. Together, we worked as a team to design the system. I studied the mechanical systems of several passive
houses completed in various parts of the U.S. Some used simple electric duct heaters or electric baseboard with no provisions for cooling other than the HRV/ERV. These houses were situated in cold climates. Some used ductless mini-splits for cooling but relied on only one or two wall mounted cassettes. These systems relied on the tight envelope and ventilation system for air and temperature distribution. Due to the hot, humid summers and fairly cold winters here in the D.C. metro area, I knew that a more elaborate mechanical system would be required for this house.
Due to the hot, humid summers and fairly cold winters in the D.C. metro area, a more elaborate mechanical system would be required for the O’Neill Development Passive House in Bethesda, Md.
with a ducted distribution system. All duct runs were sealed with mastic and insulated with R-8 foil-faced duct wrap. All duct runs were as short as possible. The mechanical equipment was chosen for efficiency, reliability and electrical draw. Every blower and pump motor uses efficient ECM variable speed technology. Every electrical component was scheduled and plugged into a spreadsheet required for passive house certification. Electrical draw had to be absolutely minimized. The heat source is a Lochinvar Knight WHN055 wall-
mounted condensing gas boiler (96% AFUE). This boiler has a low pressure drop fire tube heat exchanger and can modulate from 11–55 K Btu input. This was a perfect fit for this system. The low pressure drop also allowed the system to operate with only one Grundfos Alpha pump that only draws approximately 18 watts at design conditions. DHW is provided through a stainless steel dual coil
All duct runs were sealed with mastic and insulated with R-8 foil-faced duct wrap, and were as short as possible.
In order to guarantee comfort at all times of the year, I
felt it was necessary to design a ducted system with supply outlets in each room. The exceptions were the kitchen and baths, where the ventilation system would draw conditioned air into these spaces as air was continuously exhausted. The first several submissions were rejected as being too complicated. I revised and redesigned my system to meet the objections without compromising comfort. I finally came up with a design that met the acceptance of the Passive House Institute. This was a two-zone design
indirect water heater. The second coil allows for a future solar thermal connection. Insulated stainless steel solar lines were roughed-in up to the attic to facilitate this future connection. Space heating is provided by two hot deck coils installed above each air handler. Supply water temperature is controlled by the boiler’s integral outdoor reset control with a maximum temperature of 120 F at 5 F outdoor temperature. During space heat, the boiler will be in condensing mode at all times. Keep in mind that design heat loss is 24,000 Btu/hr. for the entire house. Since these houses are sealed extremely tightly (.6
AHH @ 50 Pascals), ventilation is key. We have two ERVs (energy recovery ventilators) that run continuously on low speed to exhaust stale air and bring in fresh air. We are exhausting air from the baths, kitchen and laundry room, while bringing in fresh air to the duct distribution
e Turn to FOLEY on p 50
ast month’s column covered my introduction to the Passive House concept. In summary, these houses are built super-tight to rigid standards and have
phc july 2011
www.phcnews.com
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