What parts are in a solar thermal system.

What is solar thermal?

Solar thermal is a way of heating hot water from the sun. Have you ever wondered what parts are in a solar thermal system?

Fluid is pumped through a panel exposed to the sun. The fluid heats up in sun. The hot fluid is then pumped back to the hot water tank where it transfers it heat.

The are two main types of solar thermal:

Sealed system
Drain back system

Solar pumping station

The solar fluid is pumped to the solar panel then back to the hot water tank by the solar pumping station.

solar thermal pumping station — Solar thermal pumping station

A solar pumping station generally comes as one unit. With all the required plumbing fittings for the system to work.

Solar thermal controller

The solar thermal controller is the brains of the solar system. It decides when the solar system can activate to heat the hot water tank.

Solar panels

The solar thermal panels (also called collector) are designed to collect the heat from the sun. This is then transferred to the solar fluid.

There are two types of solar thermal panel:

Flat panel
Evacuated tubes

Flat panel solar collector

A flat panel solar collector is the simplest type of solar panel.

It is a flat panel built into the roof of the property. This has pipes or channels to carry the solar fluid covering the whole surface area.

The panels are dark which helps to absorb the heat from the sun.

solar thermal panel — Solar thermal flat panel.

Solar thermal evacuated tube.

Evacuated tube solar thermal is a more efficient but much more complicated system.

Each tube collects heat when exposed to the sun. The heat is transferred via an internal heat pipe to the solar fluid.

The collector is in the glass tube. The glass tubes are in a vacuum so any heat that enters can’t escape. This makes them 30 to 50% more efficient than a flat panel collector,

Pressure gauge

Sealed and pressurised solar thermal systems are the most common. Some brands have a system where the panel drains back into a holding tank after use. You can read more about them here.

A sealed system raises the boiling point of the solar fluid. This reduces the number of times the fluid boils in the panel during stagnation.

Read more about this in our guide to stagnation in solar systems

The pressure should be close to 2bar.

Like all sealed systems the pressure will go up when it is hot.

solar thermal pressure gauge — Solar thermal pressure gauge.

Pressure relief valve

Designed to open if the pressure rises too high in the solar system.

Flow meter and adjustment

The solar fluid should be pumped around the solar system at a low flow rate.

This is to give the fluid enough time to collect the maximum amount of energy from the sun.

solar thermal flow meter and flow adjustment

The flow rate is given in l/min and should be set at either:

2l/min for each panel on the roof
The flow rate in the manufacturers instructions

On this pumping station the flow is adjusted by turning the Allen key grub screw.

Circulating pump

The circulating pump moves the solar fluid around the circuit. It’s a continuous loop through the panel then back to the tank.

The pumps are specially made to handle the combination of solar fluid and high temperatures.

Isolating valves with thermometer

Solar pumping stations always have two Isolating valves with thermometers.

They are used during flushing and filling the system.

They have several jobs:

Isolate the flow and return pipework
Show the temperature of the flow and return pipework.
They have integral non return valves that stop reverse circulation.

Expansion vessel

Every sealed heating or hot water system has to have somewhere for the fluid to expand into.

The most common is this 18l specialist solar expansion vessel.

They generally have a pre charge of 2.5bar.

Solar expansion vessels must be installed with the pipe coming off the top. (the expansion vessel in the picture is upside down). This keeps any bubbles of steam away from the diaphragm in the vessel.

Discharge container

In a situation where the pressure in the solar system gets too high a safety valve opens. Solar fluid is discharged from the system to reduce the pressure.

The fluid is a mixture of glycol and water and is an environmental pollutant. This means it should be collected instead of being sent into a drain.

A plastic container is required in the discharge pipework to catch any fluid that is discharged.

Filling and drain valves

A filling machine must be used to fill and drain the system.

The machine is attached to the filling and drain valves.

solar thermal flow gauge and filling valve

If you found this confusing:

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