Why is there hot water coming from the cold taps?

Hot water coming from the cold taps – it may seem bizarre, but it can and does happen. On some occasions it is due to a hopeless plumber – this author once encountered a toilet which had been plumbed into the domestic hot water supply. Residents of the house could flush the toilet and watch the gas meter go up as the boiler heated the water to fill up the cistern again!

However, hot water coming from the cold taps is no laughing matter. Not only is it a waste of money, the consequences can be deadly. The cause is usually a failed immersion heater thermostat.

Overview

In a typical open vented hot water cylinder – that is to say, a hot water cylinder fed by a cold water storage cistern – the hot water is heated via one or two large elements known as immersion heaters. The immersion heaters may be the only means of heat, or they work in compliment to a heat exchanger inside the cylinder: a coil of pipe which carries hot water from the boiler.

Each immersion heater is equipped with a thermostat, which turns off the electricity to the immersion heater when water inside the cylinder reaches a preset temperature (usually 55 – 60 degrees Celsius). This thermostat is located underneath the metal cap which covers the top of the immersion heater, and should not be confused with the thermostat that is strapped around the outside of the cylinder.

The problem of hot water coming from cold taps or cold outlets occurs when the immersion heater fails in the ‘on’ position. This will heat the water up in the hot water cylinder indefinitely. This extremely hot water expands up the expansion pipe and vents back into the cold water storage cistern. But since the hot water cylinder is supplied with water from the cold water storage cistern, this effectively creates a loop whereby the water in the cold cistern is being heated by the immersion heater.

Since the cold water cistern usually supplies the cold taps in the bathroom, this is why a failed immersion heater thermostat will result in them delivering hot water. The kitchen tap will be unaffected as it is supplied from the mains. However, even in households where the bathroom cold taps are supplied from the mains, you may still notice this problem in the form of exceptionally hot water from the hot taps.

Danger

If left long enough, a failed immersion heater will bring the water to a boil, including the water in the cold water cistern in the loft. The danger of having up to 50 gallons of water at a rolling boil in the loft speaks for itself. While most modern plastic cisterns are able to accommodate water at such temperatures for extended periods of time, there have been incidents where improperly supported plastic cisterns have split, dumping a quarter of a ton of boiling hot water onto residents below, seriously injuring them or even killing them. In once instance, the cistern was installed on a wooden door laid across the joists.

Signs

As well as hot water coming from the cold taps, there are other warning signs which may be indicative of this problem. As mentioned by the Health and Safety Executive, keep an eye out for:

  • Excessively hot water coming out of the hot water taps;
  • Excessive noise or ‘bubbling’ from the hot water cylinder;
  • Steam/moisture in the roof space.

Solution

If the cause of this problem is indeed a faulty immersion heater thermostat, then replacing it with a new thermostat with a safety cut-out feature should solve this problem. The cold water storage cistern should be inspected for any damage, and should be checked to see that it is situated on a flat, stable, continuous base which supports the entire base of the cistern, leaving no part of it overhanging.

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