Solar Geyser Losing Pressure? Your Glycol Loop May Be Failing

Two kinds of solar geyser, and only one has glycol

If you have a direct (thermosiphon) solar geyser — the tank sits on the roof above the collector — there's no glycol. Potable water flows straight through the collector. Skip this article; your concerns are different.

If you have an indirect (pumped, split) system — tank inside the roof or ceiling, collector on the roof, a small circulating pump and an expansion tank — you have a glycol-water mixture circulating in a closed loop. That fluid is the most overlooked maintenance item on any solar installation in South Africa.

Why glycol degrades

Propylene glycol (the food-safe kind used in domestic solar) breaks down under sustained high heat. When a SANS 10106-compliant flat-plate collector stagnates on a 35°C Highveld summer day with the pump off, the collector can hit 180°C. Each stagnation event nudges the glycol toward acidity. After 5–7 years the fluid is brown, the pH has dropped below 7, and it's now actively corroding the copper risers inside the collector.

Symptoms of a tired glycol loop

• Pressure in the loop slowly drops — the small pressure gauge near the pump station reads under 1 bar where it used to sit at 1.5 – 2 bar.
• The expansion tank feels heavy and sloshy (waterlogged bladder).
• Glycol smell or pink/brown residue under the pump station.
• Hot water lukewarm even on a clear sunny day — pump is running but heat transfer has collapsed because air has entered the loop.
• A black, sooty deposit visible through a transparent section of pipe (degraded glycol).

What needs to happen — and how often

• Glycol change-out every 5 years (every 3 years in the Highveld where stagnation temperatures are worst). Drain, flush with demineralised water, refill with fresh propylene glycol at the manufacturer's ratio — typically 40/60 glycol/water.
• Expansion tank pre-charge check at the same interval. The bladder loses nitrogen pressure; a deflated tank is the #1 cause of pressure-relief weeping.
• Pump station strainer clean annually.

Why DIY here is risky

Solar loops run hot, pressurised, and contain a fluid you should not pour down the drain (most municipal by-laws classify spent glycol as trade waste). Plus, bleeding air out of a split system requires the pump in manual mode and a method most homeowners don't have. Pay a solar-trained plumber R1 500 – R2 500 and be done with it.

When the collector itself is dead

If the glycol comes out coal-black and there's no pressure at all, the copper risers inside the collector may already be perforated. A new flat-plate collector runs R6 000 – R12 000 plus install — but a fresh collector on a 12-year-old tank is throwing good money after bad. Get the tank's anode and element checked at the same time.