Solar heating is one of the renewable energy competency goals in the HVAC energy education. We cover flat-plate and evacuated tube collectors, the solar heating circuit with freeze protection, stagnation, the solar storage tank and how area and tank are sized to the hot water demand — as well as how solar heating works together with heat pump and district heating.
Introduction
A solar heating system converts solar radiation into heat in a fluid and uses it for hot water and possibly some space heating. Unlike solar cells that produce electricity, the solar collector produces heat directly — and that heat must be transported, stored and frost-protected. It is therefore a plumbing system and falls under renewable energy topics in the trade.
Solar collector types
Two main types dominate and the choice is a trade-off between price and performance at low outdoor temperatures:
- 01Flat plate collector (flat plate solar collector): an insulated box with a dark absorber plate behind a tempered glass cover. Robust and cheapest per m²; the most common type for domestic hot water in single-family homes.
- 02Vacuum tube solar collectors: the absorber sits in evacuated glass tubes that almost eliminate heat loss to the surroundings. It performs relatively better at low outdoor temperature and large temperature difference (e.g. in winter or at high flow temperatures), but is more expensive and sensitive to snow load.
The solar collector's performance is measured standardised to DS/EN ISO 9806, and products that are Solar Keymark-certified have documented performance curves that can be relied on. Use the certified data — not loose claims — when a system is to be sized.
Solar heating circuit
The solar collector is part of a closed circuit with its own fluid (solar fluid) that is never mixed with service water. The circuit typically consists of:
- 01Solar collector on roof/facade, connected to an insulated supply and return line.
- 02Solar heating station with circulation pump, safety valve, manometer, air separator and expansion tank.
- 03A differential thermostat/controller that starts the pump when the solar collector is a few degrees warmer than the bottom of the tank and stops it again when the sun can no longer contribute.
- 04A heat exchanger (typically a spiral at the bottom of the solar hot water tank) that transfers heat to the utility water.
Stagnation — when the sun shines but the heat is not removed
In summer, the solar thermal tank can become fully heated while the sun continues to shine. Then the controller stops the pump, but the solar collector continues to gather radiation — and the temperature in it can rise dramatically until the fluid evaporates. This is called stagnation, and a solar thermal system must be designed to withstand it: components and solar fluid must be able to tolerate the stagnation temperature, and the expansion tank is dimensioned so it can absorb vapour formation without the safety valve venting fluid. Repeated stagnations break down the glycol over time, so the solar fluid must be checked (pH/freeze protection) and changed according to the manufacturer's instructions.
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