Tuesday, September 3, 2013

Solar attic heating

Richard asks a $20 question :-)

>I live in Kutztown, PA in a 3,000 SQ FT detached 2 story "colonial" - I've had the idea that there must be some way to recover the attic's heat in winter for assistance in living space heat...

You can recover some heat from the attic with a blower in series with a heating thermostat in the house and a cooling thermostat in the attic, eg 2 of these $20 line voltage thermostats: http://www.zorotools.com/g/Line%20Voltage%20Thermostats/00053175/

Kutztown has a 40.5 degree north latitude, so the max sun elevation at noon on 12/26 is 90-40.5-23.5 = 26 degrees. A 4/12 south roof with an 18.4 degree elevation would receive about 250cos(90-18.4-26) = 150 Btu/h/ft^2 in full beam sun at noon, which could raise a dark shingle temp by 150/2 = 75 F on a calm day with a 2 Btu/h-F-ft^2 still airfilm conductance and no radiation loss.

Solar Attic, Inc of Minneapolis claims a 30% space energy savings using their system, which seems optimistic. IIRC, their system sold for $1400 until the 40% tax credit disappeared in 1984, when the price dropped about 40% to $900. These days, they mostly focus on swimming pool heating with air-to-water heat exchangers (like car radiators) in attics.

If the house is cooler than (say) 70 F and the attic is warmer than 80 F, the thermostats would allow the blower to circulate attic air through the house through supply and return air paths. One of the paths could have a one-way plastic film damper to avoid warm house air flowing up into the attic at night. The blower would draw down warm air from the highest point in the attic with the ridge vent (if any) blocked and gable vent doors closed. A vertical east-west plastic film partition could confine warm air to the space under the south roof. This can work a lot better with a steep transparent south roof, as described at: http://www.ece.vill.edu/~nick/Soldier...On.pdf

You could also add winter heat to the house with one of Gary Reysa's simple thermosyphoning air heaters over an insulated south house wall, described at: http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm

NREL says 800 Btu/ft^2 of sun falls on a south wall on an average 31.8 F December day with a 39.2 high and an average 35.5 F daytime temp in Allentown, PA, so an average 0.9x8ft^2x800/6h = 960 Btu/h of heat would enter a 1 ft wide x 8' tall strip of R1 polycarbonate air heater south glazing with 90% solar transmission.

One empirical chimney formula says C = 16.6Asqrt(HdT) cubic feet per minute of air will naturally flow up through a warm vertical H' tall duct with A ft^2 of cross-sectional area and a dT (F) temp diff between the top and the bottom, with CdT Btu/h of heatflow. If the solar heat that flows into the 8' strip equals the heat that flows into the room plus the heat that flows from the warm glazing to the outdoors, with a 3% vent area and A = 0.03x8 = 0.24 ft^2, 16.6x0.24sqrt(8)dT^1.5+(70+dT/2-35.5)8ft^2/R1 = 960 Btu/h makes dT = (60-0.355dT)^(2/3). Plugging in dT = 20 on the right makes dT = 14.1 on the left. Repeating makes dT = 14.4, then 14.4, with a T = 84.4 F air heater outlet temp and C = 43 cfm and a 43x14.4 = 616 Btu/h useful heat output for the 8' strip and a 100x616/800 = 77% solar collection efficiency and a 616x6h/8ft^2 = 462 Btu/ft^2 air heater gain on an average December day. A 4'x8' air heater would collect the heat equivalent of 4.6 therms of natural gas during the month of December.

Thanks for asking a question. Please let me know if you'd like further clarification.  


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