Solar Water Heater Not Heating

Solar Water Heater Not Heating: Complete Troubleshooting, Diagnosis, and Repair Planning Guide

The Direct Answer

When a solar water heater stops heating, the cause is usually not the collector alone. Most failures come from one of five areas: no solar circulation because the pump or controller is not operating, poor heat transfer because of air locks, degraded glycol, scaling, or blocked flow, incorrect sensor or differential settings that prevent the pump from starting, shading or orientation problems that reduce collector output, or a backup system that is misconfigured so the tank never reaches the desired temperature. A structured checklist can separate "collector is fine but water is not moving" from "collector is not collecting enough energy" and from "tank is losing heat faster than solar can replace it."

First Safety Checks Before Any Diagnosis

Before opening panels, pumps, or electrical enclosures, complete these safety steps:

• Turn off solar pump power at the dedicated breaker or controller disconnect.
• For systems with electric backup, isolate the backup circuit before working on tanks, elements, or thermostats.
• For gas backup, shut the gas supply and use a qualified technician for burner, pilot, or gas-control work.
• Do not open pressure relief valves, expansion vessels, or glycol fill points while the loop is hot or pressurized.
• Do not bleed air from a hot closed loop without following the manufacturer's cold-pressure procedure.
• If there is any smell of burning, melted wiring, water on electrical parts, gas odor, or suspected freeze rupture, stop and call a licensed solar thermal, plumbing, or electrical contractor.

Safety is part of diagnosis. Many "not heating" calls are made worse by uncontrolled draining, incorrect refilling, or electrical testing on live controllers.

Symptom-Based Diagnostic Table

Use the symptom first, then move to the subsystem. This prevents random parts replacement.

Active Systems: Pump, Controller, and Differential Logic

Most modern domestic solar water heaters use an active loop with a controller, sensors, pump, heat exchanger, and storage tank. If the pump never runs, solar heat stays in the collector and never reaches the tank.

Check the controller display first. A blank display usually points to power loss, tripped breaker, blown fuse, damaged transformer, or failed controller supply. A lit display with wrong temperatures points to sensor or wiring problems. A display that shows plausible temperatures but no pump output may indicate relay failure, pump seizure, incorrect differential, or wiring fault.

Differential control is the core logic. The controller compares collector sensor temperature with tank sensor temperature. Typical residential settings use a turn-on differential of about 5 to 10 degrees Celsius and a turn-off differential of about 2 to 4 degrees Celsius . If turn-on is set too high, the pump may never start on marginal days. If turn-off is set too high, the pump may short-cycle or stop before the tank gains useful heat. If the differential is reversed or sensors are swapped, the controller may run the pump at night, run continuously, or never run.

Diagnostic steps for active non-heating:

1. Confirm controller has power and no fault code.
2. Read collector temperature and tank temperature at midday under sun.
3. If collector is much hotter than tank but pump is idle, check differential setting, sensor assignment, pump power, relay output, and wiring.
4. If pump runs but tank does not warm, check flow, air locks, check valves, heat exchanger condition, and whether the collector sensor is mounted correctly.
5. Verify the tank sensor is in a representative location, not near the backup element only or outside the thermal zone.
6. Verify the collector sensor contacts the absorber, header, or manufacturer-specified point with thermal coupling, not loose in air space .
7. Test pump operation manually if the controller has service mode; if the pump still does not move fluid, inspect impeller, capacitor, motor, wiring, and isolation valves.

Circulation Failures: Air, Flow, Valves, and Pressure

A pump can run while no heat moves. This is common in closed glycol systems and in improperly filled direct systems.

Air lock symptoms include gurgling, uneven pipe temperatures, low flow, and a pump that feels warm but does not deliver heat. Closed-loop glycol systems should be properly pressurized; many residential loops are checked around 20 to 30 psi cold, but the correct value depends on system height, fluid, and manufacturer specification . Low pressure may indicate a leak, air ingestion, or loss of expansion-vessel precharge. High pressure may indicate overheating, closed expansion path, or faulty relief setting.

Bleed air from high points, auto vents, and pump housings using the approved procedure. After venting, recheck pressure and observe a full heating cycle. If pressure keeps dropping, do not keep topping up without finding the leak.

Check valves prevent reverse thermosiphon at night, when warm tank water could flow backward through cold collectors and lose heat. A stuck-open check valve can cause morning temperature loss even if daytime solar works . A stuck-closed check valve can block circulation entirely. Inspect valve orientation, debris, spring condition, and seating; replace if damaged.

Flow blockage causes include sludge, scale, solder debris in new systems, biological contamination in neglected loops, collapsed insulation entering piping, and frozen sections. Filters and strainers should be cleaned. Heat exchangers should be checked for scaling, especially in hard-water indirect systems where potable side fouling reduces transfer even if the solar loop is healthy.

Residential active solar pump flow guidelines vary, but many systems are sized in the region of 3 to 10 gallons per minute depending on collector area, fluid, and design temperature rise . Confirm design flow from the system schematic rather than guessing by feel alone.

Passive Systems: Thermosyphon and Batch

Passive systems have no pump or controller, so "not heating" usually comes from installation geometry, freezing, scaling, or tank position.

Thermosyphon rules:

• The tank must be above the collector top in most configurations. If the tank is lowered, relocated, or if air enters the highest point, natural circulation fails.
• Collector tilt should match local solar geometry. FSEC guidance for troubleshooting uses tilt near local latitude, adjustable by about plus or minus 15 degrees depending on season and performance goal .
• The collector must be lower than the tank inlet/outlet path appropriate to the design; supply and return lines should rise continuously toward the tank without traps.
• Shading during morning or afternoon sharply reduces passive output because there is no forced circulation to compensate.
• Sediment or scale inside risers restricts flow; passive systems are sensitive to small losses.
• Freezing in direct thermosyphon systems can rupture risers or headers; indirect passive designs or drain provisions are required in freeze climates.

Batch or integral-collector-storage systems often lose more heat overnight. If morning water is cold, check collector orientation, insulation, glazing integrity, and whether demand exceeds thermal mass. Batch systems are also strongly affected by shading and by very low ambient temperatures.

Collectors: Shading, Orientation, Soiling, and Damage

Even a perfect hydronic system will not heat if the collector cannot collect.

Orientation: In the Northern Hemisphere, residential collectors usually perform best when oriented toward the equator, commonly south within a tolerable deviation; east or west orientations reduce peak performance and shift timing . Western orientation may help afternoon demand but reduces morning solar preheat; eastern orientation helps morning demand but reduces afternoon gain.

Tilt: For general annual performance, tilt near local latitude is a common starting point. For improved winter gain in cold climates, steeper tilt may help; for improved summer gain, lower tilt may help. FSEC troubleshooting uses latitude plus or minus about 15 degrees as a checking range .

Shading: Trees, neighboring buildings, chimneys, satellite dishes, roof equipment, and even seasonal shadows reduce output. Because solar water heating depends on cumulative radiation, partial shading during midday is more damaging than brief early or late shading. Trim vegetation, relocate obstacles if structurally possible, or redesign array layout.

Soiling: Dust, pollen, bird droppings, leaves, and construction residue reduce transmission. Clean glazing only when cool using mild detergent and water. Do not walk on collectors, use abrasive pads, or clean under hot stagnation conditions.

Damage: Cracked glazing, broken flat-plate enclosures, ruptured risers, and lost-evacuation tubes reduce or eliminate output. Evacuated tubes with white fog, silver coating loss, or apparent vacuum loss should be tested and replaced per manufacturer procedure. A single damaged tube may reduce output; multiple damaged tubes may make the system appear "not heating." Check manifolds, gaskets, and header connections at the same time.

Indirect Glycol Systems: Fluid Quality and Freeze Interaction

Indirect systems use a closed loop with propylene glycol or another solar-rated heat-transfer fluid. "Not heating" may actually be "fluid cannot move or transfer heat."

Failure signs:

• Pump runs but collector-to-tank delta is small.
• Loop pressure slowly drops.
• Fluid appears dark, cloudy, acidic, or has odor.
• System overheated repeatedly in previous seasons.
• Winter performance collapsed after a cold snap.

Glycol degrades with time, heat, oxygen ingress, and stagnation. Many residential solar glycol systems are serviced or replaced around every 3 to 5 years, but interval depends on fluid type, temperature exposure, and analysis . Some systems need earlier replacement after frequent stagnation; others last longer with proper expansion control .

Check freeze protection rating with a refractometer or laboratory test, not by color. Verify pH and inhibitor reserve. Never top up an unknown glycol mix with arbitrary automotive antifreeze; use solar-rated inhibited propylene glycol or the fluid specified by the system designer. Plain water dilution reduces freeze protection and corrodes components.

If glycol is badly degraded, it can foul the heat exchanger, increase viscosity at low temperature, reduce pump flow, and lower collector efficiency. Replacement includes draining, flushing, filling with correct concentration, venting air, and verifying expansion-vessel precharge.

Scaling and Water Quality

Hard water causes scaling on the potable side of indirect systems and, in direct systems, inside collectors and piping. Scale acts as insulation, reduces heat-exchanger area, restricts flow, and forces backup heating to do more work.

Symptoms of scaling:

• Gradual decline in performance over months or years.
• High collector temperature but low tank temperature rise.
• Reduced flow from fixtures even when supply pressure is normal.
• Sediment in tank drain samples.
• Premature failure of elements, thermostats, or temperature sensors coated by mineral deposits.

Treatment options:

• Descale potable heat exchanger or tank according to manufacturer instructions.
• Install water softening or conditioning where hardness is high.
• Use indirect glycol systems in hard-water regions to keep potable scale away from collectors.
• Inspect anode rods where used; replace according to tank specification.
• Flush sediment from tank bottoms annually or as water quality requires.

Do not use aggressive chemical descalers without confirming material compatibility with copper, stainless, gaskets, and tank lining.

Sensors, Wiring, and Controller Settings

Incorrect sensor data is one of the most common hidden causes of "not heating."

Common sensor problems:

• Collector sensor loose, poorly bonded, or placed in air instead of on absorber/header.
• Tank sensor placed too close to backup element, too high, too low, or outside the solar heated zone.
• Sensor cables swapped at controller terminals so collector and tank inputs are reversed.
• Open or shorted sensor wiring causing false high or low readings.
• Drifting thermistor or RTD producing small but progressive errors.
• Water-damaged connectors, rodent damage, UV-cracked insulation.

Controller checks:

• Confirm power supply voltage matches the controller and pump requirements.
• Confirm sensor type matches the controller; NTC, PT100, PT1000, and other types are not interchangeable without proper configuration .
• Verify differential start and stop values.
• Verify maximum tank temperature, backup enable temperature, anti-legionella cycle, and any timer or vacation mode.
• Check manual or service mode; if the pump runs in manual but not in auto, suspect sensor, setting, or control logic.
• After power loss, some controllers reset to default or standby mode; re-enter solar mode, correct differentials, and correct sensor assignment .

If the display shows plausible temperatures but behavior is wrong, compare displayed values with independent thermometers on the collector header and tank. A controller can be electrically fine while reporting garbage temperatures from a bad probe.

Storage Tank and Backup Integration

A solar heater can be technically working yet still feel "not heating" because of tank and backup issues.

Tank problems:

• Undersized storage for collector area and demand causes rapid depletion.
• Oversized storage may take too long to reach setpoint and appear ineffective on cloudy days.
• Poor tank insulation increases standby loss, especially in unconditioned spaces.
• Stratification issues cause the top outlet to be cool while lower layers are warm.
• Internal leak between solar coil and potable side, or between potable and backup zone, changes temperatures and pressures.
• Failed anode, corroded heat exchanger, or sediment layer reduces effective volume.

Backup problems:

• Electric element burned out, thermostat set too low, breaker tripped, or wiring open.
• Gas backup with failed pilot, blocked burner, faulty gas valve, or incorrect aquastat.
• Backup setpoint higher than solar setpoint, causing backup to dominate and mask solar problems.
• Backup disabled after power outage, controller reset, or timer misprogramming.
• Anti-legionella routine set too high or too frequent, increasing energy use and giving impression of "only electric works."

Best practice is solar priority: solar loop heats the tank toward the main setpoint, backup boosts only when solar cannot reach required temperature. For many domestic systems, storage around 55 to 60 degrees Celsius supports efficiency and safety, with thermostatic mixing at outlets to reduce scald risk . Backup thermostat should be set below or coordinated with solar target so the burner or element does not displace solar heat.

Nighttime Loss and Reverse Circulation

If water is hot at sunset but cold next morning despite no demand, suspect nighttime heat loss through the collector loop.

Causes:

• Pump running at night due to wrong differential, faulty sensor, reversed sensor, or controller fault.
• Check valve stuck open allowing reverse thermosiphon.
• Collector loop piping uninsulated or poorly insulated outdoors.
• Drain-back system not fully draining because of incorrect slope, trap, or undersized reservoir.
• Passive system with incorrect height relationship causing reverse flow.

Test by feeling collector supply and return early in the morning before pump start. If the return from collector to tank is warm after a cold night with no solar gain, reverse circulation or parasitic loss is likely.

Freeze-Related No-Heat After Cold Weather

If the system stopped heating after freezing temperatures, do not immediately restart the pump.

Inspect for:

• Cracked evacuated tubes, ruptured flat-plate risers, split manifolds.
• Burst outdoor piping, especially low spots and poorly insulated runs.
• Frozen glycol loop caused by incorrect concentration, low pressure, closed valve, or failed pump.
• Drain-back lines with insufficient slope that retained water.
• Heat exchanger or pump station damage from expansion.

Confirm glycol freeze point against local minimum design temperature. Verify all exposed piping is insulated with weatherproof, UV-resistant covering. After any freeze event, pressure-test before returning the system to automatic operation.

Commercial and Multi-Unit Diagnostics

Large systems need the same logic but with zone isolation.

• Review central controller trends: pump run hours, collector temperatures, tank temperatures, loop pressures, backup energy use.
• Isolate arrays zone by zone to find a single faulty pump, air-bound circuit, or sensor group.
• Check balancing valves; a closed or mis-set balance valve can starve one array while others work.
• Verify heat interface units in apartment systems; a closed solenoid, stuck valve, or incorrect room controller can make one dwelling "no hot water" while the plant works normally .
• Check commercial pump stations for multiple pumps, standby status, variable-speed settings, and alarm logs.
• For high-demand facilities, model morning and evening peaks separately; "not heating" may be correct solar output overwhelmed by simultaneous showers, laundry, and kitchen use.

Maintenance Schedule to Prevent No-Heat Calls

Step-by-Step Homeowner Diagnostic Sequence

Use this order for a standard active residential system:

1. Check the controller display. No display means power, breaker, fuse, or controller supply issue. Display with error means sensor, overheat, low-pressure, or system fault per the manual.
2. On a clear midday hour, read collector and tank temperatures. Collector should be clearly hotter than tank when solar is available.
3. If collector is hot and tank is cold, go to circulation: pump running, flow, air, check valve, heat exchanger.
4. If collector and tank temperatures are nearly equal and pump never runs, check differential, sensor placement, sensor assignment, controller output, and pump power.
5. If pump runs but pipes are uneven—hot in one place, cold in another—bleed air and verify pressure.
6. If performance is slowly worsening, test glycol, descale exchanger, replace sensors, and service pump.
7. If only backup produces hot water, review collector size, shading, tilt, and backup setpoint; solar may be undersized or disabled by control settings.
8. If problem began after freezing, inspect for rupture before restarting.
9. If problem began after electrical storm or power loss, reset controller correctly, verify sensor type, and test pump output.
10. If unsure about pressurized glycol, gas backup, or mains electrical parts, stop and schedule qualified service.

Frequently Asked Questions

Q1: Why is my solar collector very hot but my water is cold?

This usually means heat is not moving from collector to tank. The most common causes are no pump operation, air lock, stuck check valve, failed sensor, reversed sensor wiring, or a fouled heat exchanger. Feel the supply and return pipes; if one is hot and the other cold while the pump should run, circulation is restricted or absent.

Q2: How do I know if the controller is bad or the sensor is bad?

Compare controller temperatures with independent thermometers on the collector header and tank sensor well. If displayed values are obviously wrong, suspect the sensor or wiring. If sensors read correctly but the pump never receives command, suspect the controller relay, power supply, or programming. Swapping sensor cables at the terminal is a quick test only when the sensor types are identical and the manual allows it; otherwise misassignment can create new faults.

Q3: What differential setting should I use?

Many residential controllers use 5 to 10 degrees Celsius turn-on and 2 to 4 degrees Celsius turn-off . Very cold climates may need wider differentials to avoid short cycling; very mild climates may use narrower bands for faster response. Follow the system design and collector type rather than a universal number.

Q4: Can air in the solar loop stop all heating?

Yes. Air pockets prevent fluid movement, create false temperatures, and cause gurgling. Closed glycol loops should be vented from high points and stabilized at design pressure. Repeated air problems indicate a leak, faulty auto vent, incorrect fill procedure, or expansion-vessel issue.

Q5: How often should glycol be changed if the system is not heating well?

Many systems are checked annually and fluid is replaced around every 3 to 5 years, but heavy stagnation, contamination, or acidic fluid requires earlier service . Test freeze point, pH, inhibitor, and appearance rather than guessing by time alone.

Q6: Why does the system work in summer but fail in winter?

Winter issues often involve lower sun angle, shorter days, snow coverage, incorrect tilt, weak glycol concentration, pump difficulty moving cold viscous fluid, and higher domestic demand. Steeper tilt, proper antifreeze, clean collectors, and adequate collector area improve winter performance. Backup should be sized so comfort is maintained even when solar fraction drops.

Q7: Could hard water make solar heating fail?

Yes, especially in direct systems or on the potable side of indirect coils. Scale reduces heat transfer and flow. Use indirect glycol designs, desalting or conditioning where appropriate, periodic descaling, and proper heat-exchanger service.

Q8: My electric backup works but solar never heats. What should I check first?

Check collector shading and tilt, controller power and differential, pump operation, sensor placement, loop pressure, air locks, and whether the backup setpoint is so high that it masks solar contribution. Also confirm the system is in solar mode after any reset, not backup-only or timer-only mode.

Q9: Is it normal for the pump to run at night?

No. Nighttime operation usually indicates reversed or faulty sensors, incorrect differential, stuck relay, or wrong controller programming. It wastes energy and can cool the tank through the collector. Investigate before continued automatic operation.

Q10: What if the system stopped heating after a freeze?

Do not force the pump. Inspect collectors, manifolds, piping, pump station, and expansion components for cracks or rupture. Verify glycol concentration and loop pressure after repairs. If any component is split or leaking, isolate the loop and use qualified solar thermal service.

Service and Procurement Checklist for No-Heat Resolution

Before requesting service or specifying repairs, prepare:

• [ ] Controller brand/model, fault codes, and display readings at time of failure.
• [ ] Collector type: flat plate, evacuated tube, heat pipe, batch, thermosyphon.
• [ ] System type: direct, indirect glycol, drain-back, passive, hybrid with electric or gas backup.
• [ ] Collector orientation, tilt, shading map, and local climate data.
• [ ] Installed collector area and tank volume versus number of occupants or commercial demand.
• [ ] Pump model, rated flow, power supply, and whether it runs during fault.
• [ ] Sensor positions, sensor type, and controller differential settings.
• [ ] Glycol type, last test date, freeze point, pH, color, and replacement history.
• [ ] Loop pressure, expansion vessel precharge, relief valve setting, and any leaks.
• [ ] Heat-exchanger access, descaling history, water hardness report.
• [ ] Backup heater type, setpoint, breaker or gas supply status, and anti-legionella schedule.
• [ ] Recent events: power outage, freeze, storm, relocation, remodeling, water-quality change.
• [ ] Maintenance records for pump, sensors, fluid, anode, and insulation.

A solar water heater that is "not heating" is rarely a single mystery part. The fastest correct diagnosis comes from temperature evidence at the collector and tank, pressure and flow evidence in the loop, and control evidence from sensors and controller logic. Collect those values before replacing pumps, controllers, or collectors, and the system can usually be returned to reliable solar-first operation with minimal unnecessary cost.