Five-Year Service Call: Degraded Glycol and a Lowered Setpoint

1. pH 6.1 - urgent. New propylene glycol is alkaline (roughly 9–10.5) to inhibit corrosion. At 6.1 the fluid is acidic and the inhibitors are spent; corrosion, sludge, and hydrogen gas generation follow. Most manufacturers condemn fluid below about pH 7.0, and pH 6.1 means replacement, not topping up.

2. Freeze point −10 °C vs. original −30 °C - urgent before next winter. This indicates dilution with water, typically from a leak, repeated make-up, or an air vent that has been passing fluid. A freeze event can split collectors and headers.

3. Cold pressure 6 psi vs. 20 psi - urgent. A 14 psi loss is a leak or a failed expansion tank bladder. Locate it before adding fluid; adding water to a glycol loop is how the freeze point got to −10 °C in the first place.

4. Air vent crust - the vent has been weeping or passing fluid for a long time. Replace it; do not clean and reuse.

5. Hazy glazing - likely permanent degradation (UV/heat), reducing output; note it and set expectations rather than promising a cleaning will restore performance.

6. No service record - the root cause of all of the above.

Lowering a solar storage tank to 45 °C does not save meaningful energy; it reduces the solar resource's usefulness and creates a health issue.

• Solar collection efficiency depends on the tank being cooler than the collectors, so a lower tank can slightly improve collector efficiency - but the tank then cannot store enough energy per gallon, so the backup element runs longer, not shorter. The homeowner's "savings" are an illusion. • Legionella risk: potable water stored at 45 °C sits in the growth range. Storage should be kept at 60 °C or higher, with a thermostatic mixing valve tempering at the fixture to a safe delivery temperature (commonly ≤ 49 °C / 120 °F). • The mixing valve may also be scaled or failed; "never really hot" can be a valve problem, not only a setpoint problem. Test hot-water delivery at a tap and compare with the valve's setpoint.

Correct strategy: store at 60 °C, temper at the valve, and use the backup element on a timer only as a top-up. If the tank cannot hold 60 °C because the array is underperforming, fix the array - do not lower the setpoint.

Corrective work order:

1. Pressure-test the loop, locate the leak, and repair it before adding fluid.
2. Drain and flush the old glycol; do not mix new fluid with acidic fluid. Capture and dispose of it per local rules.
3. Replace the automatic air vent and any weeping fittings; verify the vacuum breaker.
4. Replace or recharge the expansion tank; set precharge to the correct cold fill pressure for the system height, and confirm tank volume against loop volume and maximum temperature.
5. Refill with propylene glycol at the concentration needed for the site's design low temperature; record the freeze point and pH on a tag.
6. Reset the tank to 60 °C, test and replace the mixing valve as needed, and verify 49 °C or less at the tap.
7. Clean the collectors if the haze is surface-only; if the glazing is degraded, document output loss.
8. Re-commission: flow, ΔT, pump cycling, high-limit, and pressure.

Maintenance schedule: • Annually: glycol pH and freeze point, system pressure, visible leaks, air vent operation, expansion tank precharge, mixing-valve outlet temperature, collector glazing and mounting, insulation and UV jacketing. • Every 3–5 years or when test values drift: full fluid replacement. • Keep a log with date, pH, freeze point, pressure, and any work performed. Give the homeowner a copy and post one at the mechanical room.