Hot water in the cold water tank – why?

While the cold water cistern is designed to accommodate modest levels of hot water entering via the expansion pipe, things can go wrong, and in some cases, there may be hot water in the cold water tank. In such circumstances, the temperature of the water can range from unusually warm to scalding hot. Let’s take a look at the possible reasons why.

Hot water in the cold water tank

The cistern is not lagged

The water may not necessarily be hot, but warm water coming from the cold water cistern is still undesirable. In the warmer months, the cause may be the absence of lagging. Not only does lagging prevent the cistern from freezing, it also keeps the water cool. During the summer months, the temperature in a loft can easily reach 100° F / 38° C. Water in the cold water cistern ideally should be kept below 68° F / 20° C in order to minimise the risk of bacterial growth.

A mixer tap or mixer shower is displacing hot water

If the problem correlates with the installation of a mixer tap or a mixer shower, then that is very likely to be the cause of the problem, especially if the cistern overflows when a mixer valve is open, such as while someone in the household is taking a shower.

The cause is usually the result of a mixer valve which is supplied with mains cold water, and hot water from a hot water cylinder. Mains cold water is at a much higher pressure than hot water from the cylinder, which is delivered via gravity. When the two waters are allowed to mix, the mains cold water displaces the hot water and pushes it back the way it came: back into the hot water cylinder, up the supply pipe into the loft, and into the cistern via the outlet. The supply pipe at the base of the cistern may feel warm when the mixer tap in question is open, and it may be possible to see or feel a current of water entering the cistern.

The expansion pipe is dipping into the water in the cistern

The expansion pipe must not dip into the water in the cold water cistern, as this may prime the expansion pipe with water, and create a thermal convection circuit with the hot water cylinder. Once heated in the hot water cylinder, hot water will rise up the expansion pipe and into the cold cistern – the cold cistern will effectively become one with the hot water cylinder.

The immersion heated thermostat has failed

This is the most concerning cause of hot water in the cold water cistern, and the consequences can be disastrous if left unfixed. Older pattern immersion heater thermostats may fail in the ‘on’ position and heat the water in the hot water cylinder indefinitely. The water rises up the expansion pipe and discharges into the cistern. Bearing in mind that the cistern is also the hot water cylinder’s source of water, it is only a matter of time before the cold water cistern is full of scalding hot water. There have been fatal incidents in these circumstances in which improperly supported plastic cisterns collapse, dumping hundreds of litres of scalding hot water onto a resident in the bedroom below.

As explained by the Health & Safety Executive, signs of a failed immersion heater thermostat include:

  • Excessively hot water coming from hot water taps and outlets
  • Bubbling or fizzing noises from the hot water cylinder
  • Hot water coming from certain cold taps
  • Steam/moisture in the roof
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