Why Is My Solar Hot Water Cold in Winter? Troubleshooting Guide

If you have noticed your solar hot water system struggling to keep up during the colder months, you are certainly not the only one.

We field calls every week from Perth residents wondering why their showers have suddenly turned tepid.

This drop in temperature is rarely a sign of a catastrophic failure.

It is usually a simple calibration issue between your system setup and the shift in seasons.

Let’s look at the data behind this seasonal dip and the practical steps you can take to fix it.

Understanding Solar Hot Water in Winter

Solar hot water systems rely heavily on direct sunlight to heat water efficiently.

While Perth is one of the sunniest cities in Australia, winter changes the equation significantly.

The Bureau of Meteorology data shows that Perth’s mean daily sunshine hours drop from over 11 hours in January to roughly 5 or 6 hours in July.

That is a 50% reduction in available energy.

On top of shorter days, the sun sits much lower in the sky during winter.

Solar collectors work best when the sun hits them at a direct angle.

Because of this lower angle and reduced exposure time, your panels generate significantly less thermal energy than they do in summer.

A functional booster system is not just an optional add-on.

It is a mandatory requirement for maintaining consistent hot water temperatures during these months.

Common Causes of Cold Solar Hot Water in Winter

1. Booster System Not Functioning

This is the single most frequent reason we attend service calls in winter.

Every compliant solar hot water system installed in Australia must have a booster unit.

This booster, which is either electric or gas, bridges the gap when solar energy falls short.

We often find that homeowners simply forget to switch this isolator on because the solar panels handle 100% of the load in summer.

Signs of booster issues:

• You had endless hot water in January but it runs cold in July.
• Water gets lukewarm (around 30-35°C) but never reaches a hot steaming temperature.
• The dedicated circuit breaker in your meter box has tripped.

What to check:

• Locate the “Hot Water” or “HWS” isolator switch in your electrical meter box and ensure it is ON.
• Find the manual override switch if you have an electric storage tank.
• Verify the pilot light is active on gas continuous flow boosters.

2. Booster Timer Settings

Timers are excellent for saving money, but they can work against you in cooler weather.

Many systems are set to boost only during off-peak windows to utilize cheaper electricity rates.

However, a two-hour boost window might be sufficient in summer but inadequate when the inlet water temperature drops to 10°C in winter.

Comparison: Timer Strategies

The fix: Expand your timer window to allow for 4-6 hours of boosting during winter months.

You can revert to a stricter schedule once spring returns.

3. Thermostat Set Too Low

Both your solar controller and your booster thermostat play a role in final water temperature.

Australian Standard AS/NZS 3500.4 requires hot water storage tanks to act as a barrier against Legionella bacteria.

To achieve this, the water inside the tank must be kept at a minimum of 60°C.

We often see thermostats dialed down to 50°C in an attempt to save energy.

This is dangerous and prevents the water from feeling hot after it passes through the tempering valve.

Recommended settings:

• Storage tank: 60°C minimum (non-negotiable for safety).
• Booster thermostat: 60-70°C.
• Tempering valve: This is a mixing valve that ensures water hits your tap at a safe 50°C to prevent scalding.

4. Insufficient Solar Collection

Physical obstructions often go unnoticed until the sun angle changes.

Since the winter sun in Perth sits at an angle of approximately 36° (compared to 83° in summer), shadows stretch much further.

A neighbour’s roofline or a tree that poses no issue in December might completely shade your panels in June.

Shading and debris checklist:

• Vegetation: Inspect trees to see if winter growth is casting new shadows.
• Structures: Check if nearby renovations or fences are blocking low-angle sun.
• Collector surface: Dust, bird droppings, and eucalyptus sap can reduce efficiency by 10-20%.

5. Circulation Problems

Split systems, where the tank is on the ground and panels are on the roof, rely on a small circulator pump.

This pump pushes cold water up to the roof to be heated and returns hot water to the tank.

If this pump fails, the water in your panels will boil while the water in your tank stays stone cold.

Signs of circulation problems:

• The pipe leading down from the roof is cold to the touch during the day.
• You hear a grinding noise coming from the pump unit.
• The controller displays an error code (often flashing red).

6. Heat Loss from Pipework

Your pipes work harder in winter due to the temperature differential.

Exposed copper pipes lose heat rapidly when the ambient air temperature drops below 10°C.

We consistently find that standard foam insulation (lagging) degrades after a few years of UV exposure.

If you see black foam crumbling off your pipes, your system is bleeding heat before the water even reaches your tap.

The solution: Replace damaged lagging with UV-stabilized solar pipe insulation.

7. Tank Issues

The storage tank itself acts as a thermal battery.

If that battery cannot hold a charge, your heating method does not matter.

Common tank issues:

• Sediment buildup: Minerals settle at the bottom of the tank, covering the heating element or heat exchanger.
• Failed anode rod: This sacrificial rod prevents rust; once it dissolves, the tank walls begin to corrode.
• Broken dip tube: This internal component directs incoming cold water to the bottom of the tank. If it breaks, cold water mixes directly with the hot water at the top.

8. Increased Hot Water Demand

Winter usage patterns put a massive strain on system capacity.

Inlet water temperature in Perth drops significantly in winter, meaning your system has to work twice as hard to raise water from 12°C to 60°C compared to summer inlet temperatures of 25°C.

Simultaneously, household occupants tend to take longer showers.

If your system was borderline adequate in summer, this increased “thermal load” will overwhelm it in winter.

Troubleshooting Steps

Step 1: Check the Booster

Start with the electrical basics before climbing on the roof.

1. Go to your electrical switchboard.
2. Locate the circuit breaker labeled “Hot Water” or “Solar.”
3. Ensure it is in the ON position and has not tripped.
4. If you have a gas booster, check the unit for an error code or confirm the pilot flame is visible.

Step 2: Review Timer Settings

We recommend temporarily bypassing the timer to rule it out.

1. Locate your timer switch (usually in the meter box or near the tank).
2. Switch it to “Continuous” or “24H” mode for 48 hours.
3. If your hot water returns, your previous timer window was too short.

Step 3: Inspect Collectors

You can do this safely from the ground level.

1. Stand back and observe your roof at 10:00 AM and 2:00 PM.
2. Look for shadows cast by trees or flues across the glass panels.
3. Check for heavy dust or debris buildup that dulls the glass reflection.

Step 4: Check for Circulation

This applies only to split systems with a ground-mounted tank.

1. Place your hand on the “flow” pipe (usually insulated) leading into the tank during a sunny day.
2. It should feel significantly warmer than the ambient air.
3. Listen near the pump; a soft hum is normal, but silence or loud rattling indicates a fault.

Step 5: Test Water Temperature

Accurate data helps us diagnose the issue faster.

1. Run the hot tap in your kitchen for 60 seconds to clear the pipes.
2. Fill a mug and place a cooking thermometer in the water.
3. It should read at least 50°C.
4. If it is below 40°C, the booster is likely not engaging.

When to Call a Professional

Some troubleshooting steps are simply too dangerous for homeowners to attempt.

Call immediately if:

• You smell gas near the unit.
• Water is physically leaking from the tank body or pressure valve.
• You see scorch marks or smell burning plastic near the electrical cover.
• Water is coming out scalding hot (above 55°C at the tap), indicating a tempering valve failure.

Schedule a service for:

• A pump that refuses to run.
• Solar controllers displaying error codes.
• Systems that have not been serviced in more than 5 years.

Preventing Winter Performance Issues

Regular Maintenance

Manufacturers recommend a major service every 5 years to extend the life of your unit.

Routine maintenance prevents the “winter shock” by catching degradation early.

A standard service includes:

• Replacing the sacrificial anode to stop tank corrosion.
• Testing the Pressure Temperature Relief (PTR) valve.
• Cleaning collector glass.
• Checking the glycol level (for frost-protected systems).

System Sizing

If your household has grown, your system might simply be undersized.

A standard 300L tank is generally sufficient for a 3-4 person family.

However, if you have teenagers taking long showers or guests staying over, you may need to upgrade to a system with a higher recovery rate or larger collector area.

Booster Considerations

Ensure your backup heating method matches your lifestyle.

Gas boosters are generally superior for large families because they provide “endless” hot water on demand.

Electric storage boosters are effective but have a slow recovery time; once the tank is empty, you must wait hours for hot water.

The Perth Winter Reality

Perth winters are mild by global standards, but they still impact solar thermal physics.

With average overnight lows of 8°C, the starting temperature of the water entering your home is much colder than in summer.

A well-maintained system will handle this shift without issue.

If your system struggles, it is not because “solar doesn’t work in winter.”

It is because a specific component or setting needs adjustment.

Get Your System Winter-Ready

We know how frustrating it is to start your day with a cold shower.

At Hot Water System Perth, our technicians specialize in diagnosing the specific efficiency gaps that occur during the seasonal shift.

We can adjust your controller settings, replace worn insulation, and ensure your booster is firing correctly.

Don’t wait for the coldest morning of the year to find out your system isn’t ready.

Contact our team today to schedule a performance check and secure reliable hot water for the season ahead.