Differential Controls

Differential controls are used in virtually all active systems that use liquid collectors except photovoltaic

powered designs. A differential controller uses an electronic comparator circuit which constantly measurers the output of two thermistors which are called sensors. The sensors change their resistance upon a change in temperature and the controller turns equipment ( normally pumps and blowers) on and off depending on the difference in temperature between the two sensors. This type of control maximizes the efficiency of hot water systems by always transferring energy from the collectors whenever even slight gains are available. Controllers normally turn pumps on when the collector(s) is between 5 and 20 degrees F. above the storage cold water. When the collector temperature falls to between 5 to 10 degrees above the tank cold water temperature the pump turns off. Systems with heat exchangers normally have differentials of about 15 to 20 "on" and 5 to 10 "off ". Direct pump systems have lower differentials since there is no loss through a heat exchanger.

Controllers made today also have a storage high limit of 160 to 180 degrees which is field setable. Many

controls also allow for a thermostat to be connected and also have options for digital readouts of collector and storage temperatures.

Swimming pool differentials have very low turn on and turn off differentials and high limits. Typically a

3 to 5 degree turn on and 1 to 2 degree turn off.

A MULTI-METER IS REQUIRED.

Resistance in K (1000) Ohms

NOTE: Resistance declines as temperature

rises - all new controls use 10 K sensors

TEMP   10K   3K            TEMP   10K   3K

27           39.8   11.9            107       5.0      1.5

32           32.6     9.8            112       4.5      1.4

37           28.3     8.5            117       4.0      1.2

42           24.7     7.4            122       3.6      1.1

47           21.5     6.5            127       3.2      1.0

52           18.9    5.7             132       2.9      0.9

57           16.5    5.0             137       2.6      0.8

62           14.5    4.4             142       2.4      0.7

67           12.8    3.8             147       2.2      0.6

72           11.3    3.4             152       2.0      0.6

77           10.0    3.0             157       1.8      0.5

82             8.8    2.7             162       1.6       —

87             7.9    2.4             167       1.5      0.4

92             7.0    2.1             172       1.4       —

97             6.3    1.9             177       1.2       —

102           5.6    1.7             182       1.1       0.3

Goldline GL-30 Differential Control has become an industry staple due to it's simplicity and reliability and can be purchased HERE.

SENSOR RESISTANCE AT DEGREES F.

Batch or Breadbox Solar Water Heaters

Batch water heaters are the simplest of all solar water heaters. These are also known as integral tank and collector heaters. Batch water heaters use tanks between 30-80 gallons. The tank is placed within a weatherproof, insulated enclosure with one side having a transparent or translucent glazing(s), normally two layers of glass. The side of the tank facing the glazing is painted black to absorb the solar energy which is conducted directly to the water in the tank.

Batch solar water heaters work great in the tropics. Due to nightime heat losses we do not reccomend them in climates above approximately 35 degrees latitude. The only thing on one side of the collector between the hot tank and the cold air outside is 2 layers of glass. In climates harsher than this, throughout the winter months the heater tends to lose at night most of the daytime energy gained. Batch water heaters are simple, reliable, easy to install and are tipically low cost. They can provide 50-90% of hot water needs depending on usuage and climate. The piping to and from the heater is the weak link in the chain. It should be well insullated and heat tape should be installed in harsh climates.

Sunbather batch water heater manufactured by AAA Solar Supply, Inc. in Albuquerque, NM ready to be installed. This 40 gallon batch water heater retails for $1,935 and can be purchased by calling 1(800)-245-0311.