Solar glycol should be tested every 2 years and replaced every 5–8 years, or immediately if its pH falls below 7. Two instruments decide it: a refractometer for frost protection and a pH strip for acidity. Fresh solar-grade propylene glycol leaves the factory at pH 8.5–9.5; once it turns acidic it attacks copper and steel.
How long does solar glycol actually last?
Solar glycol lasts 5–8 years in a well-designed system and 2–3 years in one that stagnates every summer. The fluid does not wear out mechanically — it oxidises. Above roughly 170 °C, the temperature a flat plate reaches during stagnation, propylene glycol breaks down into glycolic, formic and acetic acids. The corrosion inhibitor package neutralises those acids until its reserve alkalinity is spent. After that the fluid turns acidic and starts dissolving copper and steel.
That is why "how often" is the wrong question. The right one is "what does the test say".
How do you test whether the glycol is still good?
Two instruments answer it in five minutes, on a sample drawn from the pump station drain point with the circuit cold.
| Test | Tool | Healthy | Replace when |
|---|---|---|---|
| Frost protection | Refractometer, glycol scale | At or below the site design frost point | Frost point has risen above design |
| Acidity | pH strip or pen | pH 7.5–9.5 | pH below 7 — no exceptions |
| Appearance | Eye | Clear, dye colour intact | Dark brown, cloudy, particles present |
| Odour | Nose | Faintly sweet | Sharp, burnt or caramelised |
pH is the decisive test. Frost protection can still read −25 °C on fluid that has already turned acidic, because water loss concentrates the glycol while the inhibitor is exhausted — a good number hiding a dead fluid. Test every 2 years, and always after a summer in which the system stood unused.
How do you change solar fluid, step by step?
- Work at dawn, under cloud, or with the collectors covered. Never drain a hot array.
- Isolate the collector circuit and connect a filling station or drain hose at the solar pump station.
- Drain the old fluid into a sealed container. Propylene glycol is non-toxic but goes to licensed disposal, not the drain.
- Flush with clean water — twice if the old fluid was dark — and drain fully each time.
- Refill with pre-mixed solar-grade fluid using a filling pump. Do not mix concentrate on site by eye.
- Purge air through every high point and the air vent until flow reads steady, then set the cold working pressure and record it.
- Label the pump station with the fill date, mixture and pressure.
Steps 6 and 7 are where do-it-yourself changes fail. Trapped air stops circulation on the first hot day, and an unlabelled system leaves the next engineer guessing at the concentration.
What happens if you leave degraded glycol in the system?
Acidic fluid attacks the softest metal first — usually the copper absorber tubing or the tank's solar coil — and the corrosion products then foul the pump and flow meter. The sequence is predictable: falling yield, an intermittently noisy pump, then a pinhole leak. Replacing fluid costs a fraction of replacing a coil, which makes the two-yearly check the highest-return task in the whole maintenance schedule.
Degraded fluid also raises the stakes on every subsequent stagnation event, because acids already present accelerate once temperatures climb again. A well-built collector stays in service for 25 years or more; over that life its fluid should be changed three or four times, which is exactly how it should be.
Can you top up instead of replacing?
Top up only to correct a small pressure loss, and only with the same fluid at the same concentration. Topping up restores volume but not the inhibitor package, so it is a stopgap between services rather than an alternative to a fluid change.
If a system needs topping up more than once a year, the real fault is a leak or a failed expansion vessel. Find that first: repeatedly adding fluid to a leaking circuit dilutes frost protection and hides the problem until the first hard frost finds it.
Frequently asked questions
What pH should solar glycol be?
Fresh solar fluid sits at pH 8.5–9.5 and stays serviceable down to about 7.5. Anything below pH 7 is acidic and must be changed immediately, regardless of how good the frost protection reading looks.
Can I use car antifreeze in a solar thermal system?
No. Automotive antifreeze is usually toxic ethylene glycol, and its inhibitor package is formulated for engine temperatures, not the 170 °C-plus a stagnating collector reaches. Use only solar-grade inhibited propylene glycol rated for stagnation service.
How much fluid does a domestic solar system hold?
A typical two-collector domestic system holds 15–30 litres, including the collectors, the primary pipework and the heat exchanger. Always buy a little more than the calculated volume so the refill and purge can be completed in one visit.
Do drain-back systems need glycol?
No. A drain-back circuit empties into its reservoir whenever the pump stops, so plain water is normally used and there is no fluid to degrade or replace. That is one of the design's biggest maintenance advantages.
Can I test solar glycol myself?
Yes. A glycol refractometer and pH strips are inexpensive and take five minutes on a cold-circuit sample. Draining and refilling is the part best left to an installer, because purging air and resetting the working pressure decide whether the system actually circulates afterwards.