Solar Water Heating

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Solar Water Heating

• Tony Stephenson
• Solar Texas

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Two Basic Types

• Open Loop Household water is circulated through
  the collectors and gains heat directly from the
  sun.
• Subject to damage from freezing
• Closed Loop Heat transfer fluid/heat exchanger
• More complex
• Good freeze protection

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Solar Water Heating in Texas

• Open Loop

• Good performance
• Usually simple
• Budget cost
• WILL FREEZE

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Solar Water Heating in Texas

• Solar Texas recommends Closed Loop systems
  throughout Texas.

• Closed Loop systems use a heat exchange fluid to
  transfer heat from the sun to a separate water
  storage system.
• More complicated, but
• Excellent freeze tolerance

Solar Loop
Heat Exchanger
Household Water
Mains in
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Closed Loop Passive
Insulate well ¾ Cu min.
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Closed Loop Passive
gt10 slope lt 45 deg.
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Roof Bracing
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Roof Bracing
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Closed Loop Active
Heat exchange fluid is pumped to the collectors
when required. When no more additional heat is to
be gained, pump shuts off and fluid drains back
out of the collectors to a small reservoir in
the house.
Drainback
Collectors and plumbing lines are filled with an
anti-freeze solution that is circulated by a pump.
Pressurized
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Active Drainback
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Active Drainback
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Active Drainback
Temperature sensor on outlet pipe
Slope all piping/collectors for drainback
Various methods of attaching to roof
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Drainback with HE Tank
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Active Drainback
Retrofit
Cold domestic inlet Td to tank drain
Hot heat exchanger return to modified Cold inlet
Dip tube reduced in length
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Active Drainback

• Requires care in installation
• Takes up a little more space than most SHWs
• Owner can provide basic maintenance

• Single pump system shown with HE tank

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Drainback

• High temperature limited.
• The system cant be damaged if the pump doesnt
  operate
• Cant reverse thermosiphon at night
• The heat exchange fluid (usually water) never
  needs to be changed
• Collectors can be oversized
• Easiest closed loop system to install and
  maintain
• No check valves, air vents, pressure gauges or
  expansion tanks

Advantages
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Pressurized Systems

• Pressurized systems are a little more flexible in
  system location than drainback systems.
• Can utilize DC pumps with PV (not recommended for
  evacuated tube collectors)
• Max. operating temp. of propylene glycol 325
  degrees
• Stagnation can have collectors between 330 and
  400 degrees
• Propylene glycol turns acidic when exposed to
  stagnation
• Smaller pump/s than drainback systems

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Active Pressurized
Low pressure pump
Avoid pipe joins in attic/wall spaces
Air vent at highest point
Vacation bypass
DC and/or AC pump/s
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Active Pressurized
Fluid and expansion tank need to be changed every
10 years
Fluid needs to be changed asap after stagnation
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Active Pressurized

• 2 ways to charge system
• specialized charging tank
• Charge from tank drain with backflow preventer

HE tank shown
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Pressurizing the System
Why? To prevent the system pulling a vacuum
leading to vapor lock.
Rule of Thumb Add atmospheric pressure (15 psi)
plus 1 psi for every 2.31ft of height of the
collectors above the lowest point in the system.
In reality, double the atmospheric pressure to 30
psi. Ideal between 35 psi and 65 psi.
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Pressurizing the System
The 2-way ball valve configuration with boiler
drains and a 100 psi air vent at the highest
point (3 pipe run above a T) is the most
successful arrangement for purging air from
pressurized glycol systems. Ground mounted
collectors or collectors positioned lower than
the storage tank may require additional vents.
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System Sizing
In San Antonio 1 sq. ft. of collector per
1.50 to 2.0 gallons of water storage Storage
size 20 gallons for first two people and 15
gallons for each additional person Other
applications RETSCREEN
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Collector Mounting

• Try to face the collectors
• South
• To the southwest before the southeast
• Within 45 degrees of south
• To the west before the east
• Consider tilting

Pitch is not as important, but check
manufacturers instructions.
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Collector Mounting
Sloped for Drainback
Serpentine collectors Not for drainback
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Startup and Maintenance
Keep the collectors covered for installation and
maintenance until ready to use!
Test systems with air at 50-60psi. Watch for at
least 30 minutes and preferably
overnight Caution some drainback reservoirs have
low air pressure limits and must be bypassed
during testing. Passive systems may only need 15
psi.
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Startup and Maintenance
Then clean system with 1 cup trisodium phosphate
/gallon of hot water, run pump for 30 minutes and
drain. Fill with water, run pump and drain.
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Things I Look for in Quoting

• Roof type
• Direction and pitch
• Location of storage tank
• Anticipated pipe runs
• Anticipated collector mount
• Electrical outlets
• Roof truss construction
• Attic access and egress
• Obstructions

• Space for storage tank and auxiliary equip.

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Necessities

• Mixing valve or anti-scald valve
• ¾ L type soft Cu pipe to the mixing valve
• Avoid pipe joins in the attic or in walls
• Good roof flashing
• Elastomeric insulation rated to 230 degrees

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Solar Texas
P.O. Box 840 Helotes, TX 78023 Ph. (210)
669-2504 Web www.soltx.com Email info_at_soltx.com