propylene glycol vs water coolant
PG25 glycol vs water
How to pick between demineralised water and propylene glycol blends for AI hydronic loops — freeze, viscosity, biofouling and warranty.
9 min read Updated 2026-05-25 Field-verified
Water carries heat better than any glycol blend and costs nothing to buy. But one hard freeze on an outdoor pipe run destroys millions in equipment. PG25 is the insurance policy — with a real performance cost.
When to pick which
- Demineralised water. Indoor loops, warm-water systems, mild-climate sites with heated pipe chases.
- PG25 (25% propylene glycol). Any loop with an outdoor segment in a climate that drops below 0 °C.
- Ethylene glycol. Avoid — toxic, and hyperscaler safety programs ban it.
Trade-offs at a glance
| Property | Water | PG25 |
|---|---|---|
| Freeze point | 0 °C | -10 °C |
| Specific heat | 4.18 kJ/kg·K | 3.90 kJ/kg·K |
| Viscosity @ 20 °C | 1.0 cP | 2.4 cP |
| Pump power penalty | 0% | 10–15% |
| Heat transfer penalty | 0% | 6–8% |
| Biofouling risk | High if untreated | Lower (PG is mild biocide) |
| Environmental disposal | Trivial | Regulated waste |
Chemistry program
- Quarterly sample: pH (target 8.5–9.5), conductivity, ORP, inhibitor level.
- Annual full lab: metals, biological count, glycol degradation products.
- Top-up rules: pre-mixed only, never dilute in-situ.
- Replace on 5–7 year cadence or when inhibitor package can no longer be refreshed.
FAQ
Can I go higher than 25% PG for more freeze margin?
Yes, but every 10% adds 5–8% pump power and shaves heat-transfer performance. PG25 is the industry sweet spot — go higher only where genuine freeze risk justifies it.
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