Solar heat transfer fluid is a mix of water and monopropylene glycol whose concentration must not exceed 50 %, a ceiling set by NF DTU 65.12 P1-2. Fresh, it shows a pH of 9.0 to 10.5 at 20 °C and a reserve alkalinity of at least 20 ml of 0.1 n HCl on manufacturer data sheets, and it is that reserve, not the colour of the liquid, which protects copper and stainless steel. It must not be exposed to sustained temperatures above 170 °C, and above 200 °C propylene glycol decomposes slowly, which shows as a darkening of the fluid. Collector stagnation temperature, as defined in EN 12975-2, can exceed 200 °C. What matters on a service visit is therefore not the age of the fluid but what it has been through.
What the reference standard actually demands of the fluid
Material compatibility and potable water classification
NF DTU 65.12 P1-2 treats the heat transfer liquid as a component in its own right. The fluid has to be compatible with the materials of the collectors and of the whole hydraulic circuit in order to limit corrosion risk, and to meet the requirements of guide ISO/TR 10217 on the pairing of fluids and materials in aerated and non-aerated circuits. Where an indirect circuit produces domestic hot water through a single heat exchange, the product introduced, neat or diluted, must carry the national health authority approval for use in thermal treatment installations for water intended for human consumption. That approval is evidenced on paper: it is a supply acceptance criterion, not a marketing claim.
An antifreeze concentration capped at 50 %
The wording is explicit: antifreeze concentrations must not exceed 50 %. The standard gives the technical reason, and it is worth repeating to a client. The higher the glycol content, the lower the heat transfer capacity of the exchanger and the higher the pressure drops. Overdosing buys freeze protection nobody needs while paying for permanently degraded solar output. The antifreeze fluid and its additives must also remain stable at the temperatures reachable in the installation, which points at stagnation rather than at rated operation.
The concentration is therefore not chosen at the permitted maximum, it is chosen against the site's design temperature. The freezing points below apply to a monopropylene glycol and water mix, read on a refractometer by volume.
| Concentration by volume | Freezing point | What it means on site |
|---|---|---|
| 25 % | -10 °C | Floor for the inhibitors to stay effective, sheltered mild locations |
| 30 % | -13 °C | Mediterranean and Atlantic coastal exposure |
| 35 % | -17 °C | Most temperate inland locations |
| 40 % | -21 °C | Continental locations below altitude, the common reference value |
| 45 % | -26 °C | Altitude and continental east |
| 50 % | -32 °C | The NF DTU 65.12 ceiling, nothing is gained beyond it |
The floor matters as much as the ceiling: below 25 %, the corrosion inhibitors are no longer concentrated enough to protect copper and stainless steel, even where the freezing point still looks sufficient. An off-the-shelf ready mix sits around 45 %, so roughly -28 °C, and that is the figure you record at commissioning as the reference for every later visit.
The temperatures each component has to survive
This is the point commercial literature glosses over, and the one that explains most of the degraded fluid you find on site. The ranges in NF DTU 65.12 P1-2 are not uniform across the circuit.
| Component | Operating range applied | Watch point |
|---|---|---|
| Collector loop (pipework, fittings, pump, exchanger) | -10 °C to +120 °C | Maximum pressure tied to the safety valve setting |
| Expansion vessel membrane or bladder | 70 °C in general | Dictates vessel location and any upstream buffer volume |
| Air vent at collector level | -10 °C to at least 150 °C | 110 °C accepted where isolating valves are shut after venting |
| Collector in stagnation | Can exceed 200 °C | Value and working pressure (up to 10 bar) on the rating plate |
| EPDM type elastomeric insulation | Up to 150 °C | Mineral wool above, to the relevant insulation standard |
Recording the fluid on site: four values, one decision
Freeze point: refractometer or hydrometer, never an estimate
The sample is drawn from the circulating liquid, and its first purpose is to confirm that the freezing point has held. Read it on a hand refractometer or on a hydrometer calibrated for propylene glycol and water. The raw figure says nothing on its own: it is compared with the commissioning record. A freeze point that has risen points to water make-up, so to a leak or a botched vent, not to ageing.
pH and reserve alkalinity: what really protects the circuit
pH is a late indicator. What neutralises the acids of degradation is the reserve alkalinity, measured in millilitres of 0.1 n hydrochloric acid to ASTM D 1121. While it holds, pH stays within the fresh product range. Once pH drops, the reserve is already spent and the circuit is running unprotected. That is why a pH read well below the quoted range is not corrected by a top-up: it is dealt with by a full drain.
Colour and deposits: reading darkening as a thermal record
A fluid that has gone dark is not dirty, it has been cooked. Manufacturer data sheets tie darkening explicitly to the slow thermal decomposition of propylene glycol above 200 °C. A brown liquid, together with sticky deposits on the strainer and flow meter, tells you the installation has stagnated repeatedly. Replacing the fluid alone will fix nothing: the cause has to be dealt with on the expansion vessel, the controls or the dissipation strategy.
| Value | Fresh product figure | Method | Decision |
|---|---|---|---|
| Freeze point | Around -28 °C on a typical ready-to-use fluid | Refractometer or propylene glycol hydrometer | Drift from the commissioning record, look for a leak or water make-up |
| pH at 20 °C | 9.0 to 10.5 | Sample from circulation, pH meter | Below the fresh product range, full drain |
| Reserve alkalinity | At least 20 ml 0.1 n HCl (ASTM D 1121) | Laboratory analysis on a sample | The only measure that quantifies remaining corrosion protection |
| Antifreeze concentration | 50 % maximum | Refractometer | Above that, degraded transfer and higher pressure drops |
| Appearance | Clear, coloured by the tracer | Visual on the sample | Darkening, deal with the stagnation cause before refilling |
Replacing the fluid: the trigger is not an interval
Why no defensible "every X years" exists
No scheme sets a replacement interval for heat transfer fluid. The servicing rule bears on the maintenance of the fluid characteristics, verified by sampling, not on an age. Two installations of the same vintage can be in opposite states depending on whether they have stagnated. Announcing a fixed interval exposes you twice: you invoice a pointless drain on a healthy system, and you let a system that needed attention in its second season run on.
The three triggers that hold up with a client
A decision to drain is defended on measurements, not on a calendar. A freeze point that has drifted from the commissioning record, a pH outside the fresh product range, a darkened liquid in the sample. Each of those three findings can be photographed, dated and filed. That file is what turns a drain into a justified job rather than a contested line on a quotation.
The inspection cadence to work to
Sensible practice, and the cadence published by the French energy agency, is a professional inspection every two years, with a useful annual collector check covering in particular the condition of the heat transfer fluid, and a cylinder clean recommended every three years to remove scale. There is no legal servicing obligation on collectors or cylinder. The case for the visit is therefore yours to build, and it stands better on circuit protection than on an imaginary obligation.
The job itself, from sample to recommissioning
Preparation: collectors covered, circuit cold, fluid recovered
The job is set up with cold collectors, early in the morning or under cover, controls off. Spent fluid is recovered in a sealed container, with no discharge to drain. The standard requires a filling device and forbids any direct connection between the heat transfer loop and the mains water supply. Bring the charging pump, a gauge, a refractometer, sampling kit, and label the valves before anything is stripped.
Draining, flushing, filling: the mistakes that earn a callback
Unless the collector data sheet says otherwise, the circuit is flushed several times before filling, and filling is done cold. The circuit must stay closed: contact with atmospheric oxygen accelerates the consumption of inhibitors. Two points come back in service calls. Internally galvanised exchangers, cylinders or pipes are ruled out, because zinc is attacked by propylene glycol and water mixtures. And chloride flux residues from soft soldering have to be flushed out, or you invite pitting corrosion on stainless steel. For residual air, work to the logic set out for the automatic air vent.
Top-up: demineralised water or original fluid, never the other way round
The rule is simple and routinely inverted on site. A loss through evaporation is made up with demineralised water, because only water evaporated. A loss through leakage or removal is made up with the original fluid, like for like, because the whole mixture left. Topping up a leak with water dilutes the antifreeze and the inhibitors at the same time. Once filled, repressurising follows the static height and the vessel precharge, which is covered in checking solar circuit pressure.
Recording the work and defending it on the quotation
What goes on the service record
The standard places servicing with the building owner and the work itself with the installer, the contract setting out what is covered. Your service record therefore carries the date, the values recorded (freeze point, pH, concentration, cold pressure), the exact fluid reference used, volumes added, the reason for the top-up, the condition of the air vents, vessel and safety valve, and photographs of the sample. It is the document that makes the next visit quick, and the only one that makes drift readable over time.
Where your liability sits
Filling with a fluid that does not match the collector manual, exceeding 50 % concentration, or refitting an expansion vessel where the membrane sees more than its rated temperature are departures from the standard. They show up in an expert inspection, and they shift a claim from the manufacturer warranty onto your insurance. Keeping the fluid reference, its potable water classification where domestic hot water is produced through a single exchange, and the commissioning record is what protects you.
Justifying the price against a cheaper quote
A competitor quoting a flat-rate drain without a sample is not selling the same job. You invoice a measurement, a documented decision, an identified fluid and a calculated refill pressure; they invoice a fill. The argument fits in one sentence in front of the client: without a freeze point and a pH reading, nobody can say whether the fluid needed changing, or whether the system was stagnating. Across a portfolio, that traceability also drives the choice between simply treating the fluid and reworking the architecture, which is where an integrated or separate backup is decided. The quote still has to say so: Argile builds the quote from the services and equipment selected, and applies VAT line by line.
