Use an inhibited, high-temperature-stabilised propylene glycol made specifically for solar thermal systems, mixed with demineralised water at 30–50% concentration for frost protection between roughly -13 °C and -32 °C. Never use ethylene glycol or automotive antifreeze: ethylene glycol is toxic, and a single-wall heat exchanger is all that separates the solar fluid from your drinking water.
Why propylene glycol and not ethylene glycol?
Toxicity. Propylene glycol is classified as low-toxicity — food-grade variants exist as additives — so a pinhole leak in a cylinder coil contaminating the hot water is a nuisance, not a poisoning hazard. Ethylene glycol, the base of most automotive antifreeze, is acutely toxic, and most domestic solar cylinders use single-wall heat exchangers with no intermediate barrier. For that reason virtually all European solar systems serving drinking water specify propylene glycol, and the US Department of Energy makes the same distinction: ethylene glycol only in double-walled, closed configurations, propylene glycol everywhere hygiene matters. Car antifreeze fails on a second count too: its silicate-based inhibitor packages are formulated for engine metallurgy and moderate temperatures, not for solar stagnation.
Why must solar glycol be high-temperature stabilised?
Because a solar circuit regularly gets hotter than any heating circuit. When the pump stops on a sunny day — controller fault, power cut, full tank — the collector climbs toward its stagnation temperature, far beyond the boiling point of the fluid. Ordinary heating-system glycol degrades quickly under these excursions: it oxidises, turns acidic and forms brown sludge that coats absorbers and blocks strainers. Solar-grade fluids use temperature-stable inhibitor packages designed to survive repeated stagnation. Buy a fluid explicitly rated for solar collectors, and if your system stagnates often, fix the cause — see how to stop solar thermal overheating in summer — because no fluid survives chronic abuse indefinitely.
What concentration should I mix?
Match concentration to your site's design minimum temperature, then stop — more glycol is not better. Approximate frost protection for propylene glycol in demineralised water:
| Glycol share | Frost protection to approx. | Notes |
|---|---|---|
| 25% | -10 °C | Minimum useful concentration for inhibitor function |
| 30% | -13 °C | Mild-winter zones |
| 40% | -21 °C | Standard for most of central Europe |
| 50% | -32 °C | Cold-climate maximum; higher hurts performance |
Above ~50%, viscosity and pumping losses rise while specific heat capacity falls, so the circuit carries less energy per litre. Mix with demineralised or deionised water — hard tap water introduces calcium that reacts with inhibitors and scales the absorber. Many installers buy pre-mixed fluid to eliminate both errors. Full method details for the freeze-protection decision are in how do I keep a solar thermal system from freezing.
How do I check the fluid's condition?
Two annual measurements at the pump station tell you nearly everything. A refractometer (or hydrometer) reads the actual concentration — verifying frost protection hasn't drifted from top-ups or evaporation losses. pH strips read chemical health: fresh solar glycol is mildly alkaline, typically around pH 8–10 depending on product; as the fluid ages and oxidises the pH falls, and a reading drifting toward 7 or below means the inhibitors are exhausted and the now-acidic fluid has started attacking copper and steel. Dark colour and a burnt smell after a hot summer point the same way. Degraded fluid must be flushed and replaced, not topped up — the how and how-often, including typical replacement intervals, is covered in how often should solar glycol be replaced.
What do manufacturers actually fill with?
Factory and installer practice across Europe converges on the same specification: pre-mixed, inhibited propylene glycol rated for solar duty, concentration chosen per climate zone. Solimpeks specifies propylene glycol solar fluid for its Wunder flat-plate collectors, which hold Solar Keymark certification (e.g. licence 011-7S1941 F for the ALS series). Follow the collector manual's fluid specification — using a non-compliant fluid can void a collector warranty.
Frequently asked questions
Can I use car antifreeze in a solar thermal system?
No. Automotive antifreeze is usually toxic ethylene glycol, and its inhibitors are not designed for solar stagnation temperatures. It endangers drinking water through the cylinder coil and degrades rapidly into acidic sludge in the collector.
Can I top up my solar circuit with plain tap water?
Avoid it. Tap water dilutes the frost protection and adds hardness minerals that neutralise inhibitors and scale the absorber. Top up with the same pre-mixed solar fluid, and investigate why the system lost fluid in the first place.
How long does solar glycol last?
Typically several years in a well-behaved system, much less in one that stagnates every summer. Test concentration and pH annually and replace the fluid when the pH heads below about 7 or frost protection has drifted from the design value.
Is propylene glycol safe if it leaks into my hot water?
It is classified as low-toxicity, which is precisely why it is the standard for single-wall heat exchanger systems. A leak is still a fault to repair — you will lose circuit pressure and frost protection — but it is not a health emergency.