Hej igen!
Jag kollade även med ChatGPT om att blanda etylen- och propylenglykol. Tydligen är det inte så bra att blanda vad gäller korrosion.
You said:
The ground source heat pump "brine" was ~600 l of standard antifreeze mixture for the time (commissioned in 1980), may have been ethylene glycol. Over the course of 45 years, the system may have been topped up by an estimated 100 l, in some cases with standard car antifreeze, quite possibly propylene glycol. Does that change anything?
ChatGPT said:
That’s a very good and technically relevant question — and yes, after 45 years and mixed antifreeze additions, there are several things worth checking.
Let’s go point by point:
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🔹 1. Chemical compatibility
Mixing ethylene glycol (EG) and propylene glycol (PG)–based brines is chemically compatible in the basic sense — they’ll mix without gelling or precipitating.
However:
• The corrosion inhibitor packages differ, and mixing them dilutes and destabilizes both.
• The result is usually reduced corrosion protection & possible sludge or film formation if O2 has been present over the years.
So: no catastrophic reaction, but the protective chemistry is probably long spent.
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🔹 2. Degradation over time
Even without mixing:
• Glycols slowly oxidize into organic acids, especially in systems with small air leaks or permeation.
• This lowers pH and increases the risk of corrosion in copper, brass, or steel components.
• After 45 years, even inhibited glycol that’s been topped up is chemically exhausted — inhibitors decompose and can no longer neutralize the acids.
So, after 40+ years, you can assume the brine is chemically “spent,” regardless of appearance.
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🔹 3. Physical condition
A few consequences you might have already (or soon) see:
• Higher viscosity → slightly higher circulation pump load and reduced flow.
• Deposits in heat exchangers or pipes if corrosion or degradation products have formed.
• Slight drop in freeze protection, since the actual glycol concentration drifts with top-ups.
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🔹 4. What to do now
If the collector circuit is still tight and functional (no major corrosion leaks), the sensible maintenance approach is:
1. Laboratory or field test sample for:
o pH (should be >7)
o Glycol type and concentration
o Inhibitor reserve alkalinity (or conductance / corrosion protection index)
2. Flush and refill if:
o pH < 7 or solution smells strongly acidic / burnt
o Heavy discoloration, sludge, or separated phases
o Unknown freeze point (below –8 °C margin from target)
3. Use a modern inhibited glycol, preferably propylene glycol with known inhibitor formulation (non-toxic, stable) such as DowFrost, Tyfocor LS, or Thermia’s recommended mix.
4. Record concentration and install a small air separator and fill point if not already present — that stabilizes chemistry for decades to come.
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🔹 5. In summary
Aspect Situation Recommendation
Chemical mixing (EG + PG) Compatible but inhibitors degraded Flush & refill
Age (~45 yrs) Far beyond inhibitor life Flush & refill
Top-ups (~100 L) Changed composition, uncertain freeze point Measure & adjust
System tightness If stable pressure → collector likely sound Reuse after clean fill
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✅ Bottom line:
No immediate danger — the collector itself is probably fine — but after 45 years and mixed glycol history, the brine chemistry is almost certainly exhausted. A complete flush and refill with modern inhibited PG brine would reset the system’s protection and efficiency for the next several decades.
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