Drainback solar hot water heating systems

A drainback system is a hybrid that combines the best of open and closed loop designs. The part resembling a closed-loop system exists between the solar thermal collector and drainback tank, which is used to preheat water. The part of the drainback system between the preheat tank and the regular water tank resembles an open-loop system.

Drainback systems consist primarily of a water tank (11), a smaller tank that stores heat-transfer fluid (14), a solar panel array that collects heat from the Sun (3), and a heat transfer system that transfers heat to the water tank. A heat-transfer fluid only circulates through the system when the panels are collecting a usable amount of heat from the sun's rays. The heat transfer loop is only filled with the transfer fluid when the pump (13) operates. At other times such as at night, the pipe loop is full of air. When working properly, this configuration prevents the freezing of the heat transfer fluid.

strengths

Drainback systems can run years without owner intervention or maintenance. They "just work". Drainback systems are less likely to cause significant damage than open-loop draindown systems if the system fails to protect against freezing weather. Drainbacks can be turned off during high-heat days (such as when one goes on vacation) without damaging the system or causing it to lose expensive heat-transfer fluid. Also, the heat-transfer fluid needs changed less frequently than in a standard closed-loop system; about every seven to ten years instead of every 3 to 5 years.

weaknesses

Drainback systems require the feed and return pipes between the drainback tank and collectors to be installed so that the water will completely drain out when the pump turns off; Usually an incline of 0.25inch per horizontal foot is sufficient. Furthermore, the pipes need to be mounted so there are no dips or sagging points. Drainbacks heat water slightly less efficiently than direct open-loop systems (and better than closed-loop systems that use anti-freezing solutions as heat-transfer fluid).

summary

Drainback systems are an inexpensive, convenient way to reduce a household's energy consumption and CO2 footprint.

sizing systems

For best results, systems should be matched according to energy use patterns and local installation conditions. If you are sizing a system for a residence or residential building, and you have:

• a location (such as roof) to install panels that has a clear view of the sun as it moves across the sky from near sunrise to sunset
• a way for pipe between the drainback tank and the solar panels to have a slope of 0.25inch per horizontal foot or steeper

then, these recommendations are designed to optimize your savings for:

• A house, modular, manufactured, or mobile home:
  • KSDB-32 Drainback System with one KS-32 4x8 collector for 2 or 3 occupant residence. The KSDB-32 may not qualify for some tax credit programs.
  • KSDB-32-ST System a KSDB-32 and 80gal storage tank
  • KSDB-40 Drainback System with one KS-40 4x10 collector, for 3 or 4 occupant residence
  • KSDB-40-ST Drainback System a KSDB-40 and 80gal storage tank
• A duplex or 2 attached households and a shared water-heating system:
  • KSDB-64 Drainback System with two KS-32 4x8 collectors, for 4 to 6 occupant residence
  • KSDB-64-ST Drainback System a KSDB-64 and 80gal storage tank
  • KSDB-80 Drainback System with two KS-40 4x10 collectors, for 6 to 8 occupant residence
  • KSDB-80-ST Drainback System a KSDB-80 and 80gal storage tank
• A triplex or 3 attached households and a shared water-heating system:
  • KSDB-80 Drainback System with two KS-40 4x10 collectors, for 6 to 8 occupant residence
  • KSDB-80-ST Drainback System KSDB-80 and 80gal storage tank
  • KSDB-96 Drainback System with three KS-32 4x8 collectors, for 7 to 9 occupant residence
  • KSDB-96-ST Drainback System KSDB-96 and 80gal storage tank
• A quadplex or 4 attached households and a shared water-heating system:
  • KSDB-96 Drainback System with three KS-32 4x8 collectors, for 7 to 9 occupant residence
  • KSDB-96-ST Drainback System a KSDB-96 and 80gal storage tank
  • KSDB-120 Drainback System with three KS-40 4x10 collectors, for 9 to 12 occupant residence
  • KSDB-120-ST Drainback System a KSDB-120 and 120gal storage tank
• A 5-plex or 5 attached households and a shared water-heating system:
  • KSDB-120 Drainback System with three KS-40 4x10 collectors, for 9 to 12 occupant residence
  • KSDB-120-ST Drainback System a KSDB-120 and 120gal storage tank
  • KSDB-128 Drainback System with four KS-32 4x8 collectors, for 8 to 12 occupant residence
  • KSDB-128-ST Drainback System a KSDB-128 and 120gal storage tank
• A 6-plex or 6 attached households and a shared water-heating system:
  • KSDB-160 Drainback System with four KS-40 4x10 collectors, for 12 to 16 occupant residence
  • KSDB-160-ST Drainback System a KSDB-160 and 120gal storage tank
• A 7-plex or 7 attached households and a shared water-heating system:
  • KSDB-160 Drainback System with four KS-40 4x10 collectors, for 12 to 16 occupant residence
  • KSDB-160-ST Drainback System a KSDB-160 and 120gal storage tank
  
  The optimal system for a 7-plex can be the same as for a 6-plex, because more than 3 people are served by each of the solar panels in this size of system, and the average occupancy* is about 2 people per unit.

*You can have confidence in these recommendations knowing that we use data about water-heating usage from the US Energy Information Administration (EIA), performance measurements of independent laboratory certifications of system components, and our decades of experience with solar water-heating installations and maintenance. In some cases, you may be able to increase savings by adding an additional solar panel. (Note that these recommendations are also consistent with Table C. "Suggested Collector Sizes For Solar Water Heating Systems" published in Hot Water from the Sun: A Consumer Guide to Solar Water Heating by The Franklin Research Center, Philadelphia, PA; Beth McPherson author ©2005 University Press of the Pacific. Their recommendations are derived using a different methodology yet arrive at similar conclusions.

other drainback systems

For custom designed systems, King Solar offers these Drainback system modules:

• DBHX1062 Solar Drainback DBHX Module - Drainback system module, 10 gallon DB tank with heat exchanger
• DBHX1598 Solar Drainback DBHX Module - Drainback system module, 15 gallon DB tank with heat exchanger

Both system modules require at least one solar water-heating collector and a few other components in addition to the module. See drainback systems (above) for lists of other commonly used components.

For best results, systems should be matched according to hot-water use patterns and installation environmental conditions. Sizing a commercial solar system can be a challenging exercise, regardless of system type. The best thing to do is send us your requirements.

Tell us:

• how the solar hot water heating system will be used. For example, will it be used in a home or as part of a boiler system in a commercial environment
• how many gallons of hot water are used per day. We can approximate by knowing how many people live in the residence, or how much heat is created per day by a boiler etc.

Contact us for a price quote. We will reply within 48 hours with a suggested set of equipment specifications to meet your alternative energy needs.