James Elston

James Elston

Published April 9, 2024

If your combi boiler hot water goes cold, it could indicate a faulty thermostat or heat exchanger, built-up limescale, or power supply issues. 

While combi boilers usually supply an endless stream of hot water, thanks to their ability to heat water directly from the mains, sometimes, there may be an issue that prevents this from happening. This could leave you with lukewarm water when you turn on your taps or water that starts off hot but slowly turns cold as your faucets run. 

In this guide, Eco Happy will discuss why your combi boiler hot water goes cold and how you can easily troubleshoot these issues to restore your hot water supply. 

Why Your Combi Boiler Hot Water Goes Cold

Understanding the relationship between the boiler’s internal mechanisms, their operational roles, and the resulting hot water allows for better combi boiler troubleshooting.

Let’s look at a few common reasons why your combi boiler’s hot water goes cold.

High hot water demand

Before you call in the professionals, ask yourself a couple of these questions regarding the loss of hot water.

  • Have you had guests over recently who all took a shower or bath at the same time?
  • How many people normally live in your home?
  • What size is your current heating system and boiler – is it big enough to cope with your lifestyle and water demands?

Depending on the size of your boiler, using too much hot water at once is the most common reason for an insufficient supply. When a boiler’s size doesn’t match our lifestyle and heating demands, the hot water supply may be inefficient to meet our needs. Should this be the case, it might be time to consider a bigger combi system.

Heat exchanger issues

A heat exchanger is the element inside a combi boiler that warms cold water.

There are a few potential causes of temperature fluctuations, such as switching between hot and cold water:

  1. Blocked heat exchanger: Central heating units clog up with sludge and limescale over time. This causes the heat exchanger to become partially or fully blocked. The blockage affects the flow rate of water entering the system. When water pressure builds up due to sediment, it tells the thermostat to switch off. This ‘switchover’ is why taps are running cold.
  2. Dip tube issues: Dip tubes keep hot and cold water separate in the boiler tank. Hot water is stored at the top for quick access and supply. Cold water is kept at the bottom – where the dip tube is found. Hot water turns lukewarm or cold when this device is faulty.
  3. Malfunctioning heat exchanger: A modern boiler lasts for around 15 years. At this time, you can expect certain issues to arise, including those related to heat exchangers, pressure valves, and thermostats.

Thermostat malfunctions

Thermostats are responsible for regulating and maintaining boiler temperature. They’re designed to keep boilers within optimal range and at your desired setting preferences. When they are working properly, you will have consistent water temperature. 

When a thermostat is faulty, it may indicate that your boiler has reached its desired temperature, even if the hot water runs cold. 

Signs could show up as a complete boiler shutdown or erratic temperature fluctuations. These issues signal deeper problems with your heating system.

Sludge build-up

As mentioned, our central heating system can clog up with debris over time. This is often due to corrosion (rust), limescale, and sludge. When combi boilers aren’t heating up, or not providing enough hot water, it could be because your boiler has a debris blockage.

Sludge affects all areas of the boiler and heating system, including the heat exchanger, dip tube, heating element, and pipes. It acts as an insulator, preventing efficient warmth transfer through the system.

The sludge can damage boiler components like pumps and valves, shortening the unit’s lifespan.

Common signs of the problem include:

  • Limited-to-no hot water – including radiators taking long to heat up
  • Cold spots on radiators
  • Strange noises coming from the boiler unit
  • Increased energy and heating bills – with no logical explanation

Low water pressure

Boilers need stable water pressure to function properly. The ideal pressure for combi boilers is generally between 1 and 2 bars, as indicated on the pressure gauge on the display panel. 

Low boiler pressure restricts water flow, limiting its reach to the boiler’s heat exchanger. This problem often shows up as:

  • Lukewarm water – in severe cases, cold water is only supplied
  • Spluttering taps
  • Cold radiators

Some boilers require a minimum water pressure to ignite the burner. So, if the pressure is too low, your heating system may not start.

Common causes of low pressure include:

  • System leaks
  • Faulty valves
  • Main water supply issues
  • Airlocks and/or sludge build-up

Top tip: As a pressure guideline, water flow rates below 10 litres per minute are considered low, between 10 and 15 is average, and 15+ is good.

Power supply problems

An electric combi boiler relies on a steady flow of electricity to power the:

  • Ignition – the spark that lights the flame
  • Pumps – that circulate the water flow rate
  • Controls – to regulate the thermostat and burner operation

Symptoms of boiler power issues include:

  • Sudden cold water coming from taps
  • Intermittent heating
  • Error codes
  • Strange noises, like clicking or humming, from the boiler unit

Let’s move on to how you can fix these issues.

DIY Troubleshooting For Quick Solutions

These simple troubleshooting steps can often solve your hot water crisis and save you a call-out fee.

But before we continue, understanding how boilers work is crucial. This is good to know before you try any DIY fixes, as repairing a gas boiler yourself or fiddling with boiler parts can be a major safety concern (and it’s illegal).

But don’t stress – these tips are designed for homeowners! For ease of reference, we’ll arrange these solutions to match their issues from the above section.

High hot water demand

If your boiler can’t cope with your current hot water demand, there’s not much you can do in terms of DIY repairs.

You can either:

  1. Reduce your hot water consumption by limiting showers, turning down radiant heating systems, or adjusting your routine to match the boiler’s capacity.
  2. Opt for a bigger boiler system when lifestyle routines won’t allow for heat reduction. If this is a serious consideration, consult a Gas Safe registered engineer for a full assessment and installation of a new unit, if necessary. You can also consult our guide on how to calculate the boiler size you need to meet your heating demands. 

Heat exchanger issues

As mentioned, there are a couple of reasons for faulty heat exchangers. Here are their solutions:

  1. Blockages: Use a central heating inhibitor (cleaner) to rid your system of built-up sludge and scale. Refer to your boiler manual to ensure you choose a compatible product. Check out our guide on the best central heating inhibitors on the market.
  2. Damaged dip tube: Replacing this component can be tricky, but not impossible. Refer to your boiler manual for its precise location and follow the steps they suggest. The cost of replacements ranges between £20 and £80.
  3. Malfunctions: If only cold water comes out of your taps, it may be time to replace your boiler. Heat exchanger replacements are expensive, ranging between £800 and £1,000. When you compare this to the price of new combi boiler installations, it may be cheaper to get a new one – and call professionals to install it!

Thermostat malfunctions

Turn up the thermostat to see if your boiler kicks into heating mode – usually a dial on older boilers and an electronic display on newer models. If it functions and heats up normally, the thermostat is NOT the problem. There’s probably another area that’s malfunctioning, which you’ll need assistance from heating experts to determine.

Sludge build-up

  • When combating sediment build-up, use an inhibitor.
  • Install magnetic filters to catch suspended debris before it blocks up the system.
  • Do regular system flushes (every 1 to 2 years) to restore heating and water flow efficiency. It is very handy to know how to clean a central heating system yourself. If there’s a lot of build-up, you might have to call an expert to perform a power flush.
  • Bleed radiators to release built-up air and remove scale residues. Know how to turn off a radiator, follow the necessary steps, and ensure all radiators are cool to the touch before you begin.
    Locate the bleed valve on the side of your radiator and turn the bleed key (or a flathead screwdriver) anti-clockwise to open the valve. You should hear a hissing sound. Use a bucket to catch excess water and sludge. Once the excreted system water runs clear, close the valves and turn the radiators back on. Repeat these steps for all radiators in your home.

Low water pressure

Do a boiler pressure check (check the water pressure gauge usually on the front display panel).

If the pressure is below 1 bar, your system needs to be repressurised.

  • First, turn off your boiler and allow it to cool down.
  • Top up the boiler, via the filling loop – found underneath the boiler or near a sink – until it reaches the optimal range (between 1 and 2 bars).
  • Once the ideal range is reached, turn the boiler back on and allow it to restart and heat up.

Top tip: Be careful not to overfill the boiler as this can result in high boiler pressure, which is equally damaging to your heating system.

Power supply problems

  • Check that electricity supply cables are insulated and in good condition. Ensure all trip switches are up on the main electrical board. Loose cables or open wires pose a short-circuiting risk which can cause fires.
  • Ensure there is no electrical maintenance work happening in your area.
  • Similarly, for gas boilers, ensure the gas connection is stable.
  • Next, check if the boiler pilot light has gone out or if it ignites – a healthy power supply should ignite the flame without any issues.

When to call a professional

Knowing your DIY limits is important. When the task at hand becomes too complex or risky, it’s best to call in the professionals. Hire certified boiler technicians or Gas Safe registered engineers for complex issues, such as:

  • Sounds coming from the boiler
  • The smell of gas from the boiler or in the event of electrical sparks
  • Various error codes are flashing across the boiler display panel
  • Pilot light won’t ignite or goes out soon after ignition
  • Boiler pressure drops significantly, even after you have repressurised the system
  • Erratic thermostat readings
  • Persistent cold water, especially after trying all DIY solutions

Maintenance Tip To Keep Your Hot Water Flowing

Preventive boiler maintenance is a cost-effective solution if you want to avoid expensive repairs in the long run. Most of these maintenance tips can be easily factored into your daily, weekly or annual routines.

Pressure checks

Checking pressure regularly is a quick way to gauge if your system is working properly or not. Ensure readings are within the ‘green zone’, between 1 and 2 bars.

Be thermostat savvy

Keep an eye on your boiler’s thermostat to ensure it’s within optimal range. The ideal temperature range for combi boilers is between 50 °C and 60 °C.

Check power supply

Periodically check electrical (and gas) connections from the boiler unit itself to the mains power supply. Call a professional promptly to replace or repair any faulty-looking cables and pipes to prevent nasty accidents.

Routine maintenance

  • System flushes should be done at least every 1 to 2 years to keep systems in tip-top condition.
  • Use inhibitors and magnetic filters to reduce sludge build-up.
  • Schedule annual boiler services with heating experts for a full assessment of your heating system. These professionals will address any safety concerns and replace and repair any faulty hardware if needed.

By following these practical tips, you will prevent common boiler and heating issues.


Should I replace my boiler if it’s constantly running cold?

If your combi boiler constantly experiences hot water problems and is over 15 years old, replacing it would be advised. Modern boilers are more energy efficient and will save you money on your heating bills in the long run. 

However,  always consult a Gas Safe registered engineer to assess your boiler and advise you on the best course of action.

Why do I get hot water from some of my taps but not others? 

If you only get hot water from some of the taps in your house but not others, it may be due to the proximity of your combi boiler in relation to the faucets. If the tap is located too far away from your combi boiler, it could cause the water to cool down in the pipes as it travels to the faucet. 

Alternatively, there could be a blockage in one specific pipe that is preventing the hot water from reaching its end destination. 


Our guide has revealed why your combi boiler hot water goes cold and identified the culprits behind these common problems. 

From faulty thermostats and a blocked heat exchanger to power supply and pressure issues, these are quick to identify and repair. However, if your cold water problems persist after you’ve followed these DIY steps, it’s best to call in a Gas Safe registered or heating engineer. 

With a few simple steps, regular boiler maintenance, and some advice from professionals like Eco Happy, you’ll be reunited with a hot water supply in no time! Call Eco Happy today for tips, guidance and all your boiler needs.

James Elston

James Elston

Boiler Expert

James Elston is the top boiler replacement and heating expert at Eco Happy. He has over 20 years of experience in the industry, focusing on Gas Safe boiler installations and offering home-heating and energy-saving solutions to homeowners across the UK. From sourcing the most energy-efficient combi boiler to providing specialist heating advice, James ensures that Eco Happy maintains the highest standards and best customer service.

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