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Wednesday, May 29, 2013

How It Works: Solar Hot Water

System Purpose.  Maximize sun as heat source for hot water.

How It Works Summary.  Like a fancy solar camp shower, solar hot water panels convert sunlight into heat which is stored in hot water tank until used.  Gas-powered tankless backup.

System Cost.  $9,000 including installation.  Also received large tax credit.    Long system life reduces major future replacement cost.    LP gas for backup estimated at $20 annually. 

System Components

Pump circulates a few liters of glycol through solar hot water panels and---when the glycol is warm enough---through a heat exchange coil in the hot water tank, thereby heating the tank’s water without contaminating it with glycol.  Glycol prevents freeze-up during our long winter.    An in-line expansion tank prevents explosion from boiling glycol, a frequent occurrence on sunny, hot days.  

3 Solar Hot Water  Panels, interconnected by tubes filled with circulating glycol, warm the glycol with sunlight, much like a sun-penetrated windshield warms the inside of a vehicle.   Amount of heat generated depends upon amount of sunlight which is a function of solar veiling (trees, structures, clouds, fog, snow, rain, ice..) and solar angle.  No trees and structures veil our panels.  Angle impact changes throughout the day, peaking at noon and falling to zero  between sunrise and sunset.   Angle impact changes throughout the year,  maximized on summer solstice and minimized on winter solstice.   Unlike solar electric, the actual heat delivered to the hot water tank is significantly less than the amount generated due to thermal losses (conduction, convection, radiation).  Though the circulating pipes are well-insulated, the heat loss is substantial on cold days and (sadly) becomes larger as the panels produce more heat.   Unlike the “instant-on” of solar electric, the pump will not turn on---and thereby heat the tank--- until the glycol in the solar hot water panels is significantly warmer than the water in the tank.    

Hot Water Tank capable of storing 80 gallons of water.  The well-insulated tank is fed by cold water (from our rainwater harvest cistern).  This cold water is then heated, via a heat exchange coil, by the circulating hot glycol.  Sunny days heat the tank well over 130F, as high as 160F or more.  The programmable maximum temperature is kept at 170F in winter and 130F in summer.    The lower summer temperature prevents the tank from heating the basement via thermal loss.  Tank temperature drops after each usage (shower, dish-washing…), since incoming water is cold.

Backup gas water heater provides a boost  if the water temperature in tank drops below 100F, else the backup remains off, allowing a “solar-only” hot water experience.   Our backup, the in-famous Bosch LP tankless, is one the few on the market which accept pre-heated water, essential for our solar hot water application.  In reality, all hot water passes through the backup.  What varies is whether the backup heats the water and by how much.  The lower the incoming water temperature the greater the boost required.  

System Maintenance.  Change maximum hot water tank temperature (when both summer and winter begin).    Replace glycol in loop (every 5 years).  Scrape snow off solar panels (as needed). 

Report Card.   Warm Season = A.   Cold Season =  B-.    (See blog post for details).

 Biggest Challenge.  The inability of the Bosch tankless heater to preheat water above 100F causes cold-shower when tank temperature is between 100F and 105F.  (See blog post for details).

Biggest success factor.  Right-sizing of hot water tank versus number of solar panels.  Tank too big will not achieve desired heat.  Tank too small will achieve maximum temperature too quickly.   Either way, backup is used too much.  For 3 solar panels, our 80 gallon tank seems just right.  A bigger system requires both a bigger tank and more solar panels.

The Connection.    Behaving like our ponds, solar panels make transparent the amazing power of the sun.  Like the Land, off-grid  solar hot water follows the seasonal cycle of scarcity and abundance, requiring extra sources of heat in winter and avoiding the excess heat in summer. 


  1. This comment has been removed by a blog administrator.

  2. Quite an interesting blog I must say...really impressive..installers

  3. I was looking for something like this...thanks a lot for that

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  4. I appreciate everything you have added to my knowledge base.Admiring the time and effort you put into your blog and detailed information you offer.Thanks.

  5. I live you post very much! How does it work? These heaters use propane gas to raise the water temperature. As the price of electricity continues to grow, more and more homeowners are switching to Tankless Water Heater.

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  7. It makes sense that you'd want a backup water heater. There are days where it's cloudy or rainy and you want to be able to take a warm shower and other things on those days. We have been talking about putting in solar panels, but this is a different direction than I'd originally thought. The fact that the system has a long life cycle is a big bonus for me. I'll have to research this a little more.

  8.  I want say that this article is very nice and very informative article.I will make sure to be reading your blog more. 
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