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SDHW Design
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Its quite easy to design a system. 

  • Sizing the system you or your family needs.  Most people and families use roughly 15-25 gallons of hot water per day.  This is variable due to the temperature of the water you want, how many or the length of showers family members take, how often you run your dishwasher and wash cloths, etc.  This quantity determines how much storage volume is recommended or needed in either a water heater, solar storage tank or both.  The desired temperature of your water, the amount you use, and the your solar climate determines the size of the solar system. 
    • Gas water heater:  If you currently have a gas water heater, you must have a solar storage tank in series with it since gas water heaters heat from the bottom and thus don't allow any cooler water to be heated by your solar system. 
    • Electric water heater:  If, on the other hand, you have an electric water heater, then you can use it by itself as storage or add a solar storage tank to preheat this water heater.  Electric water heaters have two heating elements: one about 1/3 from the top of the tank and another about 1/3 from the bottom. 
      • Water heater by itself:  If you are just considering using a water heater by itself, it would be advised to control the bottom element with a time clock or one of our solar controls so it remains off whenever the solar can add heat to this tank.  In some areas that have sufficient solar resource most of the time, such as Hawaii, both top and bottom elements can be controlled by a time clock so the solar system can heat the whole tank during the day.  The clock can be set to supplement the water heater only as needed to have adequate hot water available throughout the day and evening.  If the water is already hot enough, which is typically the case most of the year, the electric element doesn't come on even if the clock comes on since the thermostat on the element still keeps it off.
      • Solar storage tank in series with the WH:  This is the more common approach since the solar storage tank can then be sized to the demand, the solar systems will heat this water as much as possible considering the weather, and then this preheats all water entering the water heater. 


  •  In the NW, sizing should be slightly higher to compensate for our overcast and inclement weather that we experience during half the year.  As with any solar system, the FAFCO systems will not perform well during these winter conditions.  However, during the occasional "sun breaks", or whenever the solar sensor is 10oF warmer than the water temperature sensor at the bottom of your water heater or storage tank, the system will operate and add heat.  It is always best to oversize the panel area when in doubt.  Panel square footage to gallon of storage ratio should be about 1.2-1.5 : 1.   Contact us for advise.
  • In Hawaii, systems should be sized about .8-1 : 1 (sq. ft. of panel per gallon of storage)
  • Face southerly direction.  Thankfully the panel area is small enough that finding a location that faces within 30o  of South is usually available.  This is especially important in states like Oregon that have tax credits and must comply to strict installation criteria to qualify.  Even the federal credits have similar criteria but less enforced.  IN any event, it is in your best interest to design the system for best performance.



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