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Part III: Solar Space HeatingChapter 5. Water-Based Space Heating and Storage

Pumps, Heat Exchangers, and Controllers Explained

The parts that move heat between a collector, a tank, and the house, in plain language.

Difficulty
Intermediate
Cost
Free
published

A pump moves the fluid. A heat exchanger hands the heat to a second fluid when the two must not mix. A controller decides when the pump is worth running. Get those three wrong and a well-built collector sits full of hot water that never reaches the tank, or it runs all night and cools the tank.

This is the vocabulary for the combined system and for the layout choice. Pipe size, which decides whether the pump is big enough, is the next guide.

Difficulty

Level
Intermediate
Time
An evening with a pump curve and a control manual
Skills
Reading head and temperature difference without skipping the failure mode

Cost estimate

Free

Learning the parts is free. A solar pump, a heat exchanger, and a differential control are often several hundred dollars together, and the electrician is extra if you need a new circuit.

Materials

ItemQtyNotes
A pump curve1Flow on one axis, head on the other. The duty point is the flow you still get at the head you actually have.
A heat exchanger, if the fluids must stay apart1Double wall when code requires it between a chemical and drinking water.
A differential control and two sensors1 setCollector sensor and tank sensor. High limit included.

Pumps

A centrifugal pump's flow drops as the resistance, called head, rises. Head includes friction in the pipe and, for drainback, the vertical lift from the water surface in the tank to the top of the collector. A small circulator sold for a boiler loop is often sized to stir an already-full loop. It may not lift a drainback collector at all. Read the curve at the lift you measured, not at the pump's best brochure point.

The pump should sit where the manufacturer says it can, often low, flooded, and serviceable. It should not be the highest point in a loop that is supposed to drain. Valves on both sides let you replace it without draining the house. A strainer is useful if the collector can shed solder or dirt, and it is one more thing that can clog and stop the flow.

Power failures stop pumps. Any freeze protection that depends on the pump running is not protection during an outage. That is the part-time home problem.

Heat exchangers

Heat moves from the hotter fluid to the cooler one across metal. More area and a larger temperature difference move more heat. A coil in a tank is simple and can be awkward to service. An external plate exchanger is compact and needs two pumps, one on each side. Either one has to be large enough that the collectors are not forced to run extremely hot to push heat across a tiny surface.

If one side is drinking water and the other is glycol or non-potable tank water, the plumbing code often wants a double-wall exchanger or another approved separation. Ask. A single-wall coil is not a universal answer. The exchanger does not replace a mixing valve on the tap. It also does not replace a pressure-relief valve on a pressurized loop.

Controllers

A differential control turns the pump on when the collector is hotter than the tank by a set margin, and off when the difference shrinks, so the pump does not chatter or run at night. The margin is whatever the control's instructions call for. A high limit stops the pump when the tank is as hot as you allow, which protects the tank and sends the collector toward stagnation. That high limit is a last stop, not a complete summer strategy.

Put the collector sensor on the absorber or on the outlet pipe, insulated from stray air, where it sees the heat you care about. Put the tank sensor at the height that represents the water you are heating. A sensor lying on the floor of the mechanical room will run the pump at the wrong times.

The control's output is often line voltage. That is a listed device on a proper circuit, or it is an electrician's job. Do not power a pump from a homemade sensor circuit.

Pump, exchanger, and sensorsA pump moves the collector loop through a heat exchanger. A second loop moves tank water. Sensors on the collector and the tank tell the controller when to run.CollectorPumpHXTankSensor on the collector. Sensor in the tank. Controller in between.A drainback pump has to lift the water to the top of the loop.
The pump is in the collector loop. The exchanger separates that loop from the tank loop. Sensors on the collector and the tank tell the controller when the pump should run.

  1. Write the head the pump must overcome

    Friction plus, for drainback, the measured vertical lift. Then read the curve.

  2. Choose the exchanger the code allows

    Area enough to move the heat, and a double wall if the fluids are not allowed to mix.

  3. Place the two sensors in the fluids, not in the room air

    Collector and tank. High limit on.

  4. Decide what happens when the power fails

    If the answer is "the collectors freeze," change the design before you buy the pump.

Further reading

These links go to outside sources. This book cites them by link and does not copy their words or pictures.

General education, not a permit or a stamped design. Follow the safety note in the guide, read the safety disclaimer, and hire a licensed trade when the work requires one.