Why is there hot water coming from the cold taps?

Hot water coming from the cold taps is usually due to an immersion heater thermostat failing in the ‘on’ position. Hot water from the hot water cylinder eventually fills the cold water cistern in the loft, which supplies the bathroom cold taps.

This is a potentially dangerous fault which can increase the risk of scalds and burns in the house. Plus, if the cold water storage tank splits, it will release up to 50 gallons of scalding hot water onto the residents below.

So why is there hot water coming from the cold taps?

This fault 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 then watch the gas meter go up!

However, this fault is no laughing matter. Not only is it a waste of money, but the consequences can also be deadly.

However, the cause is usually a failed immersion heater thermostat. 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. In a direct cylinder, the immersion heaters are the only means of heat. As for indirect cylinders, the water is heated indirectly by a heat exchanger. This carries hot water from the boiler and it’s usually in the shape of a coil.

Each immersion heater is equipped with a thermostat. This shuts off the immersion heater when water inside the cylinder reaches a preset temperature (usually 55 – 60 degrees Celsius). This thermostat is located underneath a metal cap which covers the top of the immersion heater. It shouldn’t be confused with the thermostat that is strapped around the outside of the cylinder.

So what exactly causes this issue?

The problem of hot water coming from cold taps occurs when the immersion heater fails in the ‘on’ position. This will heat the water in the hot water cylinder indefinitely. The extremely hot water expands up the expansion pipe and vents back into the cold water storage cistern. But since the storage cistern also supplies the hot water cylinder, this effectively creates a loop whereby the cylinder’s immersion heater is heating the water in the cistern too.

In this situation, you notice hot water coming from the cold taps because they are usually supplied by the storage cistern, especially the bathroom taps. The toilet may also flush with hot water, if it is supplied by the storage cistern. The kitchen cold tap will be unaffected as it is supplied directly by the mains. However, even if all of the cold taps in your house are supplied by the mains, you may still notice this problem in the form of exceptionally hot water from the hot taps.


If left long enough, a failed immersion heater will bring the water to a boil. This includes the water in the cold water cistern. 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.


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.

Preventing hot water coming from the hot taps

If the cause of this problem is indeed a faulty immersion heater thermostat, replacing it with a new thermostat with a safety cut-out feature should solve the problem. The cold water storage cistern should also be inspected for any damage.

Note that immersion heaters run on mains voltage electricity. Never attempt to work on them yourself if you are not competent. If you are in any doubt, contact a qualified tradesman.

Cold water storage cisterns should always be situated on a flat, stable, continuous base. The base should extend the underneath of the cistern by at least 50 mm. It should retain its structural integrity even if it gets wet. For this reason, chipboard should never be used.