Category - Boilers
Guide author photo

Will Scholfield, Engineer

19 Sep : Updated 17 May ● 7 min read

Why does my hot water keep going cold?

There is nothing quite as horrible as stepping into the shower only to turn on the hot water tap and find cold water flowing out.

Your hot water could be cold for several reasons - we’ll run through all of them in this article to help you find the best solution!

10 reasons why your hot water keeps running cold

Your hot water heater - whether a combi boiler, electric heater, or gas water heater - can be acting up for several reasons.

Here are some common reasons as well as their fixes so you can enjoy a hot bath in no time!

1. Overusing the hot water

Depending on the type of boiler you have, using too much water is the most common reason why your water will be cold. Your house has a limited water supply, and if you have too many taps running at the same time, or use water quicker than it could be heated up, then you are likely going to miss out on a hot shower!


Spread your hot water use over a longer time period. Don't try to shower at the same time as someone else is attempting to wash the dishes or just after someone else has had a bath. Use hot water appliances one at a time, and give enough time between showers and baths to allow the water supply to reheat.

2. Hot water tank is too small

Does your hot water heater meet your household's needs? If it’s large enough for your own family, what about if guests come over?

If the tank is too small, it will not produce enough hot water for everyone who needs it, and someone will be surprised with icy water!


You have two options to fix this situation: either you can get a larger hot water tank, or you need to install reserve tanks. A professional plumber can then connect the tanks to ensure you have enough hot water.

3. Broken heating element (immersion)

A broken or burned-out heating element (immersion) could be the culprit for cold water in the hot water tap.

As we use our water heater, the heating element gets worn out over time, which reduces its efficiency and lowers the maximum water temperature, until it completely stops heating the water.

An electric water heater typically has two heating elements - one to heat the water coming in from the pipes and another to maintain the temperature in the water tank. Both of these can break, which will result in colder water.

Hard water or sediment in the water can also inhibit the heating element's functionality.


Booking a boiler service will ensure that you spot sediment buildup before it becomes a problem.

You can use a multimeter to test the voltage of the element - if it's not working you will have to replace it.

4. Blocked heat exchanger

A tap or shower that keeps switching between hot and cold water could be a sign that the heat exchanger inside a combi boiler is blocked.

Sludge and sediment buildup is fairly common in older boilers, especially when the filters are old. This blockage will then cause water to run hot and cold intermittently.

It’s likely that this is occurring with your combi boiler because the central heating system was not flushed properly by your engineer while installing the boiler.


If the hot water heat exchanger has accumulated sludge, you can descale it directly to get rid of any buildup.

This involves taking your combi boiler apart, so you’ll need to call in a professional Gas Safe Registered engineer with experience taking apart and servicing combi boilers. Doing it yourself is incredibly dangerous.

A severely damaged heat exchanger will require replacing and can cost around £400 - £600.

If your boiler is on the older side, it might make more financial sense to invest in a new boiler (unless your boiler is still under warranty).

Time for a new boiler?

Time for a new boiler?

Find your new A-rated boiler today with BOXT

Get a new boiler

5. Sediment build-up

We've mentioned sediment buildup on the heat exchanger, but other areas of gas and combi boilers can also be affected by sediment.

Material from corroded pipes as well as sand and debris can enter water heaters and settle at the bottom, which reduces the amount of water in the hot water tank.

It can also cause blockages in pipes.

A common sign of debris and sediment buildup is loud bangs coming from the water heater. So, if you hear your boiler knocking, it’s time for some boiler maintenance!


The easiest way to get rid of sediment is to drain and flush the water tank. Check the filters and replace any old filters (or add filters if they are missing) - this will stop the problem from happening again.

If hard water is causing blockages or build-up, install a water softener.

You may be able to carry out small scale maintenance yourself, but if you’re not sure, call on a professional.

6. Broken thermostat

Thermostats are used to set the temperature of the water heater, so if the thermostat isn't working, it's very likely you'll have cold water rather than hot water.

Thermostats not only set the temperature but also tell heaters when to turn on and off. So, if it is broken, you will be stuck with a heater not knowing what to do!


You will have to replace your broken thermostat. If you’re not a DIY expert, it might be best to call in the professionals to help you find the correct model and complete the installation.

7. Poor maintenance and old parts

Many of us take an out-of-sight, out-of-mind approach to our boilers until it is time for the annual service.

You might be surprised to hear that your water heater needs basic year-round maintenance, and if you don't keep up with a maintenance schedule, you may find yourself without hot water!


Regularly check your water heaters and fix small problems before they become large issues. If you do not feel confident in your ability to spot common boiler problems, you can contact a plumber for a bi-monthly inspection.

8. Inefficient pipes

You need an efficient pipe system to transport hot water throughout your home. If the pipe fittings are of low quality or leaking, or if the pipes are too far from the water heater, then the hot water will quickly cool down.

Hot and cold water pipes often run next to each other, and if these pipes are too close together, the cold water pipe can cool down the hot water pipe - especially if you have copper pipes.


Plumbers can help you determine how effective your piping system is. You may have to replace old pipes or tighten your fittings. Copper pipes can be replaced with less conductive materials, or should be insulated to prevent the hot water from cooling down.

9. Old heating system

Whether a tankless water heater, a combi boiler, or a gas boiler - your heating appliance has a limited lifespan.

Water heaters can last from 8 to 15 years with proper care and maintenance, but at some point, their efficiency will drop and you will be out of hot water.


It's time to replace your boiler! The newer models are incredibly efficient and do not lose a lot of heat, which means you will soon have your hot water supply back.

10. Inconsistent power supply

Since most modern homes have a water heater that runs on electricity, an unstable power source can result in a shortage of hot water.


If this problem persists, you may have to turn to alternative energy sources, such as solar power or generators, that kick in if the main source of power fluctuates.


Final thoughts

No one enjoys an unexpectedly cold shower - even if you are a fan of Wim Hof's cold water immersion therapy!

Luckily, cold water in your hot taps does not necessarily mean you have to replace the entire water heater. A few simple checks can help you narrow down the cause, and our solutions above should have you soaking in a hot bath again in no time.

And should all else fail and you need a replacement boiler, BOXT sells boilers from industry-leading brands to keep your home warm. 

Share this post