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Wednesday, October 9, 2013

How It Works: Solar Heating

System Purpose.  Maximize sun as primary shoulder season (Apr and Oct) heat source and backup winter (Nov to Mar) heat source for home.

How It Works Summary.  Passive solar (windows) and solar hydronics (solar hot water) heat the air and concrete slab floor which then slowly releases its heat into home.    The combination works well since passive solar jump starts the heat in the morning, while solar hydronics continues drawing on excess solar hot water long into the evening. 

System Cost.  $4,000, including installation, to pump solar hot water into floor.  This excludes the cost of windows (which we had anyway), the solar hot water system (whose primary purpose is domestic hot water usage), and the concrete slab (we needed a floor anyway).

Shared System Components

Concrete Slab, heated both by passive solar and solar hydronics, slowly releases its heat via conduction (warming your feet), convection (warming the adjacent air) and radiation (heating you and all objects like direct sunlight).  Slab is insulated below to slow heat loss into underlying sand layer.   A day of sun can raise the slab from 70F to 78F.   During the shoulder season, when outside temperatures can fall to freezing, the heat stored in the slab from a couple sunny days keeps the house above 65F for several cloudy days.  During winter, when the wood-fired masonry heater is the primary heat source, the solar heating systems acts as a bonus.   On a bitter cold, yet sunny day, the stored heat in the slab allows us to open a window for extra fresh air and skip the morning fire.

Passive Solar System Components

South-facing windows convert sunlight into heat via the greenhouse effect (objects absorb solar energy and convert to heat whose wavelength is too long to escape through the glass).   South-facing windows are best for three reasons.    First, the sun spends most of its day in the south during the heating season.  Second, south facing windows take advantage of the most intense solar heating around noon.  Third, during the hot summer, sunlight can be effectively blocked from entering south-facing windows via an overhang.   Do not use low e glass for passive solar since they block the needed infrared light.  Passive solar is free heat, since it costs nothing to choose to orient windows south rather than north, east or west. 

Overhang blocks sunlight from entering windows during the high solar angle summer while allowing sunlight to slip beneath during the low solar-angle winter.    Our 2.5 foot overhang blocks all sunlight on the summer solstice.   On the spring and fall equinoxes, the sunlight reaches in 6 feet.  On the winter solstice, sunlight reaches in 19 feet, touching the far wall.    A warm September requires the addition of shade cloth to block unwanted sunlight.  While we used our roof and rain gutter as overhang, awnings could be used as well.

Solar Hydronics System Components

Solar Hot Water System (see How It Works:  Solar Hot Water for details).

In-floor Pump circulates water through a heat exchange element inside the 80 gallon solar hot water storage tank then out into pex tubing embedded within the 5” concrete slab.   Like any other in-floor pump, except our relies exclusively on “excess” solar hot water.

Programmable Thermostat turns In-floor pump on or off depending on the temperature of the water inside the solar hot water storage tank.  Since our solar hot water is primarily for domestic usage, the thermostat is set to only pull “excess” heat out of the tank.  We’ve discovered that “excess” is anything over 130F since even our well-insulated tank quickly loses its heat to the cellar at temperatures above 130F.  This excess temperature is only reached on sunny days. 

In-floor Pex Tubing, embedded like a winding plastic snake throughout the concrete slab, carries water whose heat is conducted into the slab.   As the slab absorbs the tubing’s heat, the water cools and is returned via the pump to be reheated by the solar hot water storage tank. 

System Maintenance.   Turn solar hydronics system on (via thermostat) at beginning of heating season and turn off at end. 
Report Card.   Winter = A.   Shoulder Season =  B.    (See Report Card for details).

Biggest Challenge.  The shoulder season house temperature can swing from a chilly 62F to a sweaty 80F.     We adapt to 62F with sweaters and blankets.  While we can keep the house below 80F by opening more windows, we often don’t.  On sunny days we allow the house to overheat so that the stored slab heat can keep the house warmer during the inevitable cloudy and cold days which follow.    We actually wanted (and designed) our house to experience the seasonal temperature fluctuations to feel more Land connected.

Biggest success factor.  Orienting most windows south.  Even the ancient Greeks practiced what we’ve largely forgotten.   In addition to the huge energy and cost savings, south facing windows maximize light during winter, brightening our sometimes darkened spirits.    And it costs us nothing to orient them south.  
The Connection.    What could be more Land connecting than sunlight streaming through a window, the bright patterns moving across the floor throughout the day?    And then there’s the absolute joy, the thrill, of experiencing  a beach-like 80F when its below zero outside.  


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  2. Solar energy is being utilized on a larger scale

  3. Thanks for the great information. I have solar hydronic heating installed at home and it is totally worth it!

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