Hot water not hot enough in winter? Here's why

When the temperature drops, plenty of homes suddenly find the hot water does not go as far as it used to. The shower runs cool sooner, or never gets as hot. It is one of the most common calls Terry gets over June and July, and there are a few separate reasons behind it, some normal and some worth fixing.

Colder water coming in

The biggest factor is simple physics. In summer, the water arriving from the mains might already be 20 degrees or more. In winter it can drop closer to 10 degrees. Your system still has to lift that water to around 60 degrees in the tank, so it has a much bigger gap to close. That means it takes longer to recover between showers, and a tank can run out sooner because more cold water is being mixed in at the tap to get a comfortable temperature. On a continuous flow unit, the same effect can drop the flow rate slightly, because the burner has more work to do per litre.

This part is normal. It affects every system to some degree. But it does expose any system that was already borderline.

Heat pumps work harder in the cold

Heat pumps are the most affected by winter, and it is worth understanding why. A heat pump does not make heat directly, it moves warmth out of the surrounding air into the water, using about a third of the power of an electric element. That is what makes it so cheap to run. The catch is that when the outside air is cold, there is less warmth to draw on, so the unit works harder and recovers more slowly. A good quality heat pump is built for this and still performs well through a Sydney winter, but a cheaper unit, or one sited in a cold, shaded spot, can struggle on the frostiest mornings. Most run a small electric booster for exactly these conditions.

An undersized system gets found out

Winter is when a system that is slightly too small finally shows it. In summer, with warm inlet water and shorter showers, an undersized tank or a low flow rate unit just about keeps up. Add colder water, longer hotter showers and everyone home more often, and demand climbs while the system's output falls. Suddenly it runs out. If your hot water only ever struggles in winter, and the family has grown or your habits have changed, the honest answer may be that the system is undersized for the household rather than faulty.

As a rough guide, a continuous flow unit wants around 16 to 20 litres a minute for one bathroom, 24 to 26 for two, and 32 for three or more. Storage tanks scale the same way, from 125 to 170 litres for one or two people up to 315 litres and beyond for a larger family.

Thermostat and tempering valve faults

Not everything is down to the weather. If the water has genuinely gone lukewarm rather than just running out faster, a fault may be to blame:

• A failing thermostat on an electric tank can stop it heating to the right temperature.
• A tempering valve, which is fitted to deliver a safe 50 degrees to your bathroom taps, can stick and either scald or run cool.
• On a gas unit, a partly blocked burner, or an ageing element on an electric one, will cut output.

These are repairs, not weather, and they are worth diagnosing properly rather than living with. Fixing them is licensed work, so it is not a DIY job.

Terry can quickly tell whether your winter hot water problem is just cold inlet water, a system that is now too small for the house, or an actual fault that needs a repair. If your hot water is letting you down this winter, call Terry on 0415 274 542 for a straight answer.