Solar Thermal Domestic Hot Water System Monthly Glycol, Pressure & Collector Inspection

ContextSample valves are positioned to capture fluid circulating in the active loop, not stagnant glycol pooled at the bottom of a drain leg where degraded fluid tends to accumulate. Draw into a clean glass or clear plastic vial. Flush the first 5 ml to waste so you are genuinely sampling active loop fluid, not residue sitting in the valve body itself. Label the vial with the date and port location. If your system lacks a dedicated sample point, budget for one at the next service — a plumber can install it in about 20 minutes for £40–80 in parts. Do not sample from the expansion vessel; that fluid is frequently older and more degraded than the circulating loop.

ContextA propylene glycol solar system should maintain freeze protection to at least −15°C for UK and Northern European climates, or to the lowest ambient temperature recorded at the site plus a 5°C safety margin. Read the refractometer at eye level in good natural light with the sample temperature between 10–25°C — temperature-compensating models are worth the additional £15 over a basic unit. If freeze protection has risen above −10°C (meaning concentration has dropped due to water top-ups or a small unnoticed leak), ice damage to collectors and pipework during a hard frost becomes a real and specific risk. Diluted concentration also means the inhibitor package is proportionally diluted, accelerating corrosion of internal metal surfaces.

ContextFresh propylene glycol solar fluid registers between pH 7.5 and 8.5. As the fluid degrades under heat cycling, organic acids form and drive pH steadily downward. Below pH 7.0, the fluid is mildly acidic and begins corroding aluminium absorber plates, copper pipes, and pump seals at an accelerating rate. Below pH 6.5, the fluid is actively corrosive and replacement is urgent — continued operation risks pump seal failure, pinhole leaks in the pipework, and absorber plate perforation. Standard litmus paper is unreliable for glycol solutions — use strips specifically rated for glycol chemistry, or a digital meter calibrated with two-point buffer solution. Record the reading to one decimal place, never as 'okay' or 'borderline'.

ContextFresh propylene glycol solar fluid is colourless to pale yellow. Light amber is acceptable and expected after one to two years of normal service. Orange-brown or dark brown fluid has experienced significant thermal stress, typically from repeated stagnation events where collector temperatures exceeded 150°C. Black or very dark fluid is heavily degraded and contains carbonised inhibitor residue — colour alone at this stage justifies a flush regardless of pH value, because the residue will progressively block heat exchanger passages and damage pump bearings. Cloudy or milky fluid suggests contamination with domestic water from a heat exchanger failure — an urgent finding requiring immediate system isolation and investigation.

ContextFresh propylene glycol is nearly odourless or carries a faint, clean sweet smell. A sharp, sour, or vinegary odour indicates organic acid formation from thermal breakdown — the olfactory equivalent of a low pH reading, and a valuable cross-check when test strips are old or the reading is right on the borderline. A burnt caramel or acrid smell, particularly in combination with a dark colour, is strong evidence of stagnation damage to the fluid. Record qualitatively: 'sharp sour', 'normal sweet', 'burnt', 'no odour detectable'. This observation costs nothing and takes five seconds, yet it provides a corroborating data point that purely numerical testing can miss when samples sit on the margin.

ContextSystem commissioning documentation should specify a target glycol concentration — typically 25–40% propylene glycol by volume for most European climates. Record the refractometer reading as a percentage and compare it against both the minimum freeze-protection threshold and the maximum performance threshold. Above approximately 50% concentration, thermal performance begins to fall because glycol transfers heat less efficiently than water, meaning an overly concentrated system runs hotter than necessary and delivers less useful energy to the cylinder per unit of solar irradiance. If top-ups have consistently used neat undiluted glycol rather than a pre-mixed solution, concentration can creep upward over years without triggering any alarm.

ContextTake this reading when the system is genuinely cold — fluid temperature within 5°C of incoming mains water temperature, ideally first thing in the morning before the sun reaches the collectors. This is your most directly comparable reading month to month, and any deviation from this protocol must be noted. Most domestic solar thermal systems operate between 1.5 and 3.0 bar cold fill pressure, with the correct value for a given installation determined by system height: approximately 0.1 bar per metre of static head from the lowest point to the highest collector outlet, plus a 0.5 bar base pressure. Record the exact value to two decimal places — not 'normal' — so you can track drift with precision. A reading 0.1 bar lower than the same time last month is not a crisis in isolation; a reading 0.1 bar lower every single month is a slow leak.

ContextThe expansion vessel absorbs the volume increase of glycol as it heats up, maintaining stable system pressure across the temperature range. The pre-charge (the nitrogen charge on the gas side of the bladder) must equal the cold fill pressure — a system filled to 2.0 bar requires a vessel pre-charged to exactly 2.0 bar. To check correctly: isolate the vessel from the system, depressurise the circuit side to zero, and use a standard tyre gauge on the Schrader valve. If the pre-charge is significantly below the fill pressure, the bladder cannot fully absorb expansion, leading to pressure relief valve operation at temperature — which homeowners frequently misdiagnose as a separate system leak. Pre-charge can be topped up with a foot pump or nitrogen cylinder. A bladder that will not hold charge means full vessel replacement: £80–250 depending on size and make.

ContextCorrosion on the vessel body — particularly at welds or the base plate — indicates either condensation in a poorly ventilated plant room or a slow weep from the circuit-side connection. Run a dry finger around both the top and bottom fittings, and look for green or white mineral deposits that water leaves behind as it evaporates from a metal surface. A vessel that has been seeping will often show a faint stain ring on the floor or wall directly beneath the connection. Physical dents or deformation on the vessel body are uncommon but indicate the vessel was overpressurised at some point — this can compromise the internal bladder and cause premature failure without further warning.

ContextThe PRV is the circuit's last defence against dangerous overpressure. It should be completely dry and clean between monthly inspections. Lift the test lever briefly (if fitted and safe to do so) to confirm the seat has not seized — a PRV that has never been operated in ten years may have a stuck seat, which entirely defeats its safety function. Look for mineral deposits, amber glycol residue, or moisture at the valve body or at the end of the discharge pipe. A PRV that operates regularly during warm weather is not 'doing its job' — it signals an underlying fault such as a waterlogged expansion vessel, incorrect fill pressure, or the absence of anti-stagnation measures. Repeated PRV operation also discharges glycol, causing circuit concentration to fall each summer.

ContextCheck the gauge reading at around midday on a clear day with the pump running normally. A correctly sized, properly filled system should not exceed 3.5–4.0 bar at the gauge even at maximum collector output. If operating pressure regularly approaches the PRV set point (typically 6 bar on most domestic installations), this points to an undersized expansion vessel, an incorrect pre-charge, or frequent stagnation events that the vessel is not sized to absorb. Record the ambient temperature and approximate sky conditions alongside the hot-system pressure reading, so the month-to-month comparison is meaningful and context-appropriate.

ContextUse binoculars from ground level if roof access is not safe — most glazing damage is visible from below with a good pair. Hairline cracks in toughened safety glass are common after hail or falling debris and may not immediately reduce measured output, but they allow moisture to enter the air gap between glazing and absorber, causing gradual selective coating deterioration that remains invisible until output has already fallen noticeably. On evacuated tube collectors, coating delamination appears as a white or silvery bloom inside the tube — any affected tube produces significantly less heat and the problem tends to spread to adjacent tubes. A shattered flat-plate glazing panel must be replaced before the next expected frost, as an unglazed absorber can freeze and split even in mild sub-zero temperatures. Replacement glazing costs £120–350 per panel depending on make.

ContextCheck every visible fixing — coach bolts, clamps, and rail brackets — for rust streaking, which indicates the protective coating has been breached by dissimilar-metal contact or mechanical abrasion. Grip individual collector frames and apply a gentle lateral movement; more than a few millimetres of travel suggests a loose bracket or an opened frame corner joint. Wind loading on a standard 2 m² flat-plate collector can exceed 150 kg during storm events, so mechanical looseness is a structural concern, not a cosmetic observation. Also check that mounting rails are not lifting adjacent roof tiles or flashing at their edges — any visible substrate distortion around fixings warrants a roofer's assessment before the next monthly inspection.

ContextPartial shading on even a small area of a flat-plate collector can disproportionately reduce output if the absorber is a series-connected single-plate design. Use a sun-path tool app to assess whether obstructions have grown or appeared since last month. Bird nesting under or between collectors is a recurring maintenance issue: nesting material creates direct shading and birds damage pipe insulation with beaks and claws during nest-building. Fit bird-proofing wire mesh if nesting activity is evident — roofing suppliers stock appropriate products for £30–80 for a typical domestic array. Clear any leaf litter or debris from the collector face after autumn storms, as partial coverage during low winter sun angles has a disproportionate impact on output.

ContextInspect the conduit boot, lead flashing, and any mastic sealant around the collector frame perimeter. Look for lifted lead, cracked sealant, or gaps between the collector bottom edge and the roof surface. A slow ingress at a flashing is typically undetected until ceiling staining appears indoors — sometimes 12–18 months after the lead first lifted. Pay particular attention to the downslope edge of the collector and around pipe entry points, which collect rainwater runoff and face maximum exposure. Press the flashing against the roof substrate with a gloved hand — a hollow sound or spring-back indicates the lead has separated from the underlying felt or boarding and needs re-bedding by a roofer.

ContextThe insulation on short exposed pipe runs between the collector manifold and the conduit entry is typically Armaflex or similar closed-cell foam. UV exposure causes the outer surface to harden, crack, and crumble over three to five years unless a UV-resistant protective paint or aluminium wrap is applied at installation or the first service. Compromised insulation causes measurable heat loss in cold weather and, in a severe frost with already-diluted glycol, can allow localised temperature drops sufficient to cause fluid gelling in the worst-exposed section. Replacing a 1-metre run takes 30 minutes and costs under £15 in materials; waiting until the insulation has disintegrated typically means replacing the entire outdoor section along with its associated fittings and connectors.

ContextDuring a sunny period with the pump running and the controller actively calling for heat transfer, the temperature difference across the collector array (outlet minus inlet, read from the controller display or clamp-on thermometers) should typically fall between 5°C and 15°C depending on flow rate and solar irradiance intensity. A differential consistently above 20°C during moderate sun suggests flow rate is too low — possibly a partially closed isolation valve, a failing pump, or an air lock restricting flow through the collector headers. A differential below 3°C during strong direct sun suggests flow rate is too high for the collector area, the absorber selective coating is deteriorating, or a sensor is reading incorrectly. This single figure is one of the most sensitive real-time indicators of overall system efficiency.

ContextMost solar differential controllers (Resol, Steca, Sorel, and equivalents) log cumulative operating hours, maximum system temperatures reached in the current period, and recent fault events in a dedicated statistics screen. Navigate to this menu and record the monthly operating hours and the maximum collector temperature logged since the last inspection. Compare operating hours to the equivalent month in the previous year — a 30% reduction during a period of similar weather strongly suggests a system fault rather than a weather explanation. Repeated maximum temperature entries above 185°C confirm regular stagnation events, which degrade glycol significantly faster than normal heating cycles. Note all fault codes with the dates they occurred, cross-referenced against the controller manual.

ContextCollector sensors typically sit in a pocket welded to the absorber plate or manifold. If a sensor has migrated out of the pocket due to repeated thermal expansion cycling, the controller receives an artificially low reading and under-runs the pump — the collector then overheats unnecessarily, degrading glycol faster than normal operation would. Twist the sensor cable gently: if the sensor slides freely in the pocket without resistance, re-seat it and secure with heat-resistant cable ties or sensor clips. Compare the displayed controller reading to a contact thermometer placed against the adjacent pipe — they should agree within 3–5°C. A discrepancy above 10°C warrants sensor replacement, typically £15–40 depending on the controller brand.

ContextAir locks in the collector circuit are most common in spring after winter low-usage periods or following maintenance drainage. Symptoms include: the pump running at rated speed but outlet temperature failing to rise as expected; an audible gurgling or trickling sound from the high point of the circuit; or the controller triggering stagnation protection even though the pump appears to be operating. Confirm by reading the flow meter if one is fitted — actual flow will be substantially below rated flow even at full pump speed. Minor air locks can often be cleared by running the pump at maximum speed for 30–60 minutes with the automatic air vent at the collector manifold opened one quarter turn. Persistent air locking that does not clear with this method requires system re-commissioning with a differential pressure pump unit.

ContextThe cylinder sensor reading and the actual measured tap temperature should be within 3–5°C of each other. A discrepancy above 10°C indicates a sensor fault, an incorrectly calibrated thermostatic mixing valve, or significant heat loss between the cylinder and the tap due to failed or absent pipe insulation on the secondary circuit. Record both values in the log — you need the cylinder temperature to assess solar contribution and thermal store performance across months, and the tap temperature to assess legionella risk for building occupants. In regularly occupied properties, stored water should be at 60°C or above to suppress legionella proliferation, or a scheduled pasteurisation cycle must operate.

ContextSolar thermal systems can fail to reach 60°C on consecutive cloudy winter weeks, leaving the lower cylinder zone in the legionella danger zone of 20–45°C for extended periods. Most MCS-compliant installations include an immersion heater or boiler top-up circuit specifically programmed to run a weekly pasteurisation cycle. Verify via the controller log or mechanical timer that the cycle ran within the past seven days and that it recorded a stored temperature exceeding 60°C sustained for at least one hour. If the system relies solely on solar gain for pasteurisation, flag any week between October and March where the log shows the stored temperature did not reach 60°C. Under HSE guidance, this is not an efficiency matter — it carries health and legal implications for properties with multiple occupants.

ContextThe heat exchanger coil connections — typically compression fittings or brazed joints where the solar circuit enters and exits the cylinder — are a common site for slow weeps in cylinders that have experienced many thousands of thermal expansion cycles over years of service. Dry the area around both connections with a cloth, wait five minutes, then run a dry finger across each joint. Glycol weeps feel slightly oily and leave a characteristic brownish-amber residue on copper. Any fresh moisture at these joints must be properly investigated — do not simply re-tighten a compression fitting as the first response, because over-tightening a used compression fitting frequently splits the olive and turns a slow weep into an active leak requiring full joint replacement.

ContextWith the sun on the collectors and the controller actively calling for heat transfer, lightly rest a finger against the pump body — you should feel a steady, moderate vibration from the motor and impeller rotating normally. No vibration whatsoever means the pump is not running; check the controller output signal, the wiring connections, and the pump fuse or circuit breaker before concluding the motor has failed. Excessive vibration or a rattle suggests bearing wear. A correctly operating pump is nearly silent; a high-pitched whine indicates bearing damage; a churning or intermittent gurgling indicates cavitation, often caused by insufficient system pressure, an upstream air lock, or a partially closed isolation valve. Replacement of a typical domestic solar pump costs £120–280 for the pump unit alone, or approximately £300–500 fully installed including system re-pressurisation.

ContextVariable-speed pumps allow flow rate and collector temperature differential to be optimised for the specific collector area and cylinder volume. A pump speed that has drifted higher than the commissioned value — perhaps increased informally by a previous technician to 'improve output' — can cause too-high a flow rate through the collector, reducing the temperature differential and paradoxically delivering less usable heat to the cylinder per unit of solar energy collected. A setting lower than commissioned value permits the collector to overheat and stagnate unnecessarily. Note the pump speed percentage or dial position in the log each month. Any unexplained change from last month should prompt investigation of the controller output signal and the pump's control wiring.

ContextSolar circuit pumps are routinely installed in utility cupboards and plant rooms where ambient humidity varies significantly. External moisture on the motor housing is typically either condensation during cold periods when the pump body is below dew point, or a weep from a pipe connection located directly above the motor. Condensation in isolation is generally not immediately harmful but indicates poor ventilation in the enclosure — a small passive vent or a desiccant pack will help. A weep dripping onto the motor body is a short-circuit risk and must be repaired urgently before the next heating cycle. Check the terminal box cover — which seals the electrical connections — for rust staining that indicates repeated moisture ingress and potential long-term damage to motor insulation.

ContextPerform this check with the system under normal operating pressure, and ideally after a recent heating cycle when thermal expansion has placed stress on all connections. A dry white cloth or kitchen paper pressed firmly against each joint will reveal a glycol weep even if the active drip has since dried — glycol residue feels slightly sticky and oily and leaves a distinct amber-brown smear on white absorbent material. Inspect particularly at pipe bends where thermal movement concentrates mechanical stress, at pipe-through-wall and pipe-through-floor points which act as pivot locations during expansion, and at any union or compression fitting disturbed during the last service or maintenance activity. A joint showing residue this month that was clean last month has been weeping between inspections and will not self-resolve.

ContextGate valves and ball valves that remain in a fixed position for extended periods accumulate mineral and glycol deposits on the stem and ball seat, progressively seizing until they cannot be operated at all when needed in an emergency. Monthly operation through a complete cycle — fully open to fully closed and back — keeps the seating flushed with system fluid and confirms operability before you actually need it. A ball valve should rotate smoothly through 90° with moderate, consistent resistance. Stiffness that has increased compared to last month, or a grinding sensation in the turn, signals fouling that will worsen. Valve replacement costs £20–60 in parts plus approximately 30–60 minutes of labour to drain the affected pipe section, replace the valve, and restore system pressure.

ContextNumbers in isolation are data. Numbers in sequence are intelligence. Before closing this month's log, write a single trend sentence beneath each parameter group — 'Pressure stable at 2.0 bar for four consecutive months, no trend observed', 'pH has fallen from 7.8 to 7.2 over three months, schedule glycol flush within 30 days', 'Pump operating hours this month 12% lower than the same month last year in comparable weather conditions — investigate controller statistics and check flow rate'. These trend sentences are what a service engineer needs to diagnose a developing fault in minutes. If you maintain logs digitally, a simple spreadsheet with monthly columns makes these patterns immediately visible across a row without any manual calculation.

ContextNot every finding requires the same urgency of response, and treating every borderline reading as an emergency is as counterproductive as ignoring findings entirely. A glycol pH of 6.9 in an otherwise healthy, stable system warrants a re-test in two weeks, not an emergency call-out. A pH of 6.3 combined with dark brown fluid, a pressure 0.4 bar below specification, and a collector ΔT showing near-zero differential during sunny weather is an Immediate priority — the system should be shut down and an engineer called before the next significant solar gain period. Build a consistent three-tier system: Immediate means action this week; This Month means schedule within 30 days; Next Service means note it in the annual service file. Record the tier alongside the finding, not just the raw number.

ContextA maintenance log is only valuable if the next person to work on the system can locate and read it. Store a physical copy in a waterproof sleeve inside the airing cupboard adjacent to the hot water cylinder, and maintain a digital backup in a named cloud folder associated with the property address. If the property changes hands, include the complete log file alongside warranty documentation, the system commissioning certificate, and installer contact details in the legal handover pack. A thorough and consistent log provides a prospective buyer's surveyor with tangible evidence of system health, which can positively influence the survey outcome and support the asking price during negotiation.

ContextThe most common reason monthly inspection schedules slip is the absence of a committed next date. Before closing this log entry, write the next date — four to five weeks from today — in your calendar and set a preparatory reminder two days in advance so you have time to confirm you have fresh glycol test strips, clean refractometer solution, current buffer solutions for a digital pH meter, and the previous log to hand. If this inspection identified any 'This Month' priority findings, schedule a separate focused follow-up within two weeks — do not absorb those actions into the next routine monthly check. Two weeks of deferred action on a declining pH value, or a pressure trend that appeared this session, is categorically not the same as four weeks.