Differential Thermostat

The single, AC powered, Grundfos circulating pump.

• The liquid-to-liquid heat exchangers. These transfer the sun's heat into your domestic hot water system. In my setup, one heat exchanger extracts the heat from the water/glycol mixture in the collection loop and transfers it to the water in the storage tank. The other heat exchanger warms the city water in the delivery system by transferring the heat from the water in the storage tank to the hot water we use.

• The plumbing system. It moves the heat, captured in a fluid mix of water and nontoxic glycol, from the collectors to a heat exchanger.

• The differential thermostat/controller. This regulates the operation of the circulation pump.

How Do the Panels Work?

When you open a car door on a warm summer day, you feel a blast of hot air, and the seats are too hot to sit in comfortably. The car is basically a box of metal with openings covered by glass. The sun's rays pass through the glass and heat up the inside of the car. The car acts as a solar collector.

The flat plate solar collector works similarly, but better. The sun's rays pass through the glass cover and hit the dark metal absorber plate below. The temperature on the metal surface may reach 250°F (121°C). The water/glycol mixture circulating through the absorber plate picks up heat. The insulated, glass-covered collector helps contain the heat. If you turn off the circulation pump in an SDHW system on a sunny day, the water/glycol mixture in a flat plate collector can easily reach boiling temperatures.

With most common collectors, many thin water pipes are sandwiched between two sheets of metal absorber plate, usually running vertically. In my system, a small, Grundfos UPS 15-42, circulating pump pushes the antifreeze mixture of distilled water and nontoxic propylene glycol very slowly through the pipes. As the fluid exits the top of the solar collectors, it has picked up quite a bit of heat from the metal sheets.

The liquid continues from the solar collectors outside my house to the liquid-to-liquid heat exchanger at the bottom of the water storage tank located in my basement. As the hot liquid passes through the heat exchanger, some of the heat circulating through it is transferred to the water surrounding it in the water storage tank.

The fluid is cooler after it has passed through my homemade heat exchanger, and is then pumped back to the solar collectors. If the sun is still shining, the cycle starts all over again, continuing to heat the water in the storage tank. All of this happens in a closed loop where the water/glycol mix runs in a circle, and never mixes with the water in the storage tank. This part of the system is often called the collection or solar loop.

Differential Thermostat

The controller or brain of our system is a Kera Tech DSD-2 differential thermostat (DT). The DT is used to turn the circulation pump on and off at the appropriate temperatures.

A differential thermostat is not like the household thermostats you may be familiar with. It measures the difference in temperature between two points, based on readings from two heat sensors. The two points are the temperature at the outlet of one of our solar collectors,

A Kera differential thermostat controls operation.

A Kera differential thermostat controls operation.

Differential Thermostat

Air Vent & Pressure Relief Valve:

Temperature Sensor

Roy Tonne/sen*/ Solar DHW System

Differential Thermostat:

Kera Tech, DSD-2

Air Vent & Pressure Relief Valve:

Temperature Sensor

Differential Thermostat:

Kera Tech, DSD-2

Glycol Heater Relief Valve

Check Valve

Solar Coils:

Six coils (two shown), 10 ft. each, 1/4 in. soft copper, bottom of tank

Backup Water Heater:

Natural gas, 40 gallon

Check Valve

Solar Coils:

Six coils (two shown), 10 ft. each, 1/4 in. soft copper, bottom of tank

DHW Coils:

Three coils (one shown), 60 ft. each, 1/4 in. soft Boiler Drain: copper, top of tank Lowest point inside

Backup Water Heater:

Natural gas, 40 gallon and the temperature at the top of the solar hot water storage tank. If I set the DT at 19°F (11°C), it will search to see if the temperature in the solar collector is higher than the temperature in the storage tank. When the water/glycol mixture in the collector gets to be 19°F warmer than the water in the storage tank, the DT will start the circulation pump.

The DT will continue to register the difference in temperatures. The pump will be turned off when the temperature in the solar collector is down to 6°F (3°C) warmer than in the storage tank. Our DT allows us to select any temperature differential from 3°F to 22°F (2-12°C) to start the pump. We can choose to let the circulation pump be turned off when the temperature difference is at any point between 2°F (1°C) and 12°F (7°C).

Our unit has two digital displays, which alternate to show four different readings. At a glance, I can see the temperature of the water/glycol mixture in the collectors, the water temperature in the storage tank, and the two temperature differentials I have selected. On my DT, a light comes on when the circulation pump is running. The pump is so quiet that I usually cannot hear it.

Tank & Heat Exchanger Details

An expansion tank automatically absorbs the changes in liquid volume and pressure. An air vent is installed at the highest point of the loop at the collector outlet. A check valve in the solar loop prevents reverse thermo-syphoning of the antifreeze mixture at night when the collectors become cooler than the storage tank. This swing check valve does not have much restriction to flow, and opens easily. It must be mounted in a vertical position so it is normally closed by gravity.

A nonpressurized solar hot water storage tank houses the two heat exchangers. We used a galvanized steel cattle watering tank. It is 5 by 2 by 2 feet (1.5 x 0.6 x 0.6 m), and holds roughly 120 gallons (455 l) of water. For most of the year, when I turn off the gas water heater,

Tonnessen SDHW Costs

Item

Cost (US$)

3 Yazaki SC-201S panels, incl. plumbing, insulation, etc. (used)

$259

Kera DSD-2 differential thermostat with sensors

0 0

Responses

  • kelley peterson
    How it works differential thermostat?
    7 years ago

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