Hot water isn’t hot enough: 9 possible reasons

If your hot water isn’t hot enough, it may be due to your boiler, thermostat, or even the time of year. Whether you have a combi boiler or a hot water cylinder, we’ll go through several possible causes.

Hot water isn’t hot enough – Contents

My hot water isn’t hot enough and I have a combi boiler

If you have a combi boiler, hot water isn’t heated and stored in advance in a cylinder. Instead, it’s heated on demand from the mains as soon as a hot tap is opened. Any interference in this process is highly likely to affect the water temperature. You may find that closing the tap slightly to reduce the flow produces hotter water, or sink hot taps produce hotter water than the bathroom hot tap. These are classic signs of a combi boiler that’s struggling. The bottom line is that the boiler is unable to transfer heat energy to the water effectively. Slowing the flow rate of water through the boiler allows more heat to be applied.

The basics

The first thing to check is the temperature setting on the front of the boiler – make sure that this is set properly.

The winter

There are several possible reasons for this. One of the most pervasive reasons is winter. At this time of year, mains water is much colder. Consequently, the boiler must transfer much more energy to it in order to bring it up to temperature. With the tap open at full bore, the average combi boiler can only raise the water temperature by a maximum of around 35 degrees Celsius.

This is no problem for the boiler in the summer months, when the mains water temperature may be as high as 15-20 degrees. The boiler can easily raise the temperature to 50 degrees and above – a temperature which will quickly scald you. However, during an icy winter spell, the mains water may be as low as 2-4 degrees. The boiler may, therefore, struggle to heat it above 40 degrees. In terms of a bath, you may find this temperature cooler than normal, and that your bath gets colder quicker than usual.

Limescale & sludge

Another possible reason is the presence of limescale or sludge inside the boiler. These can prevent the transfer of heat effectively, much as in the same way a kettle whose element is covered in limescale takes longer to boil. Adding descaling chemicals to the system and powerflushing can tackle this problem. Boilers affected by the build-up of limescale often make abnormal sounds – see the article on boiler kettling for more information. A particularly strong indicator of limescale buildup being a problem is if the boiler cuts out while the hot tap is open – this is a safety feature to protect the boiler from being damaged by the excess heat which cannot be transferred to the water.

Faulty diverter valve

In a combi boiler, the diverter valve controls whether heat from the boiler is used to heat water for the radiators or water for the hot taps. A classic sign of a faulty or jammed diverter valve is the radiators getting hot when a hot tap is opened, even though the central heating is off or it’s not cold enough in the property for the thermostat to activate it. The hot water itself will be lukewarm or not hot at all. A Gas Safe-registered engineer should repair or replace the faulty valve.

Faulty thermistor

The thermistor is a sensor fitted inside your boiler. It determines how much heat is needed in order to bring the water up to the desired temperature. A faulty thermistor can easily affect the temperature of the water. A faulty thermistor can only be replaced by a qualified heating engineer.

My hot water isn’t hot enough and I have a hot water cylinder

Hot water demand

If your hot water isn’t hot enough and you have a hot water cylinder, the first thing to consider is the demand for hot water in your home. As hot water is drawn off from the cylinder, it’s replaced with cold water from the mains or from the storage tank in the loft. If there has been a rush of people using the bathroom to get ready for work or school, you may have no choice but to wait a few minutes for the boiler or the immersion heater to bring the cylinder back up to temperature. You may be able to use the immersion heater’s boost function to speed this up, but this is likely to have an impact on your energy bills. A long-term solution may be to upgrade to a larger hot water cylinder, or to one with a faster recovery time.

The thermostat

Check the thermostat on the front of the cylinder – it should be set to 55-60 degrees Celsius. Anything lower than this risks the growth of Legionella bacteria, which are responsible for Legionnaire’s disease.

The coil

If you have an indirect cylinder, your hot water cylinder will have a coil inside it. Hot water from the boiler flows through this, heating the rest of the water in the cylinder. Any hindrance in this process will inevitably affect your hot water. The most common issues are:

  • A faulty motorised valve. This valve controls whether hot water from the boiler flows through the coil or through the radiators. If your hot water isn’t hot enough but your radiators come on even though the central heating is off, this is highly indicative of a faulty motorised valve.
  • Sludge/air locks. These can prevent water from flowing through the coil properly.

Hot water isn’t hot enough – Economy 7 cylinder

If you’re on an Economy 7 tariff, your cylinder will have two electric immersion heaters. The lower heater heats the water at night on the cheaper night rate, and the top heater keeps the water up to temperature as it is drawn off during the day.

The immersion heaters should be connected to two separate switches in your airing cupboard. The bottom one should be left on all the time. The top one should be switched on when necessary to boost the water back up to temperature. Leaving the top one switched on all the time will allow it to boost the temperature whenever it drops below the set figure, but this will have an impact on your energy bills.

The immersion heater thermostats are likely suspects for this problem – you may also notice that there isn’t enough hot water despite the size of the cylinder. Fitting a thermostat which is too short for the immersion heater can also cause water temperature issues. Caution: Immersion heaters run on mains voltage electricity. Never attempt to work on them if you are not competent. If you are in any doubt, contact a tradesman.