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Last updated: 5th April, 2024


Shower Going Hot and Cold - Combi Boiler

Shower Going Hot and Cold - Combi Boiler

Key takeaways

  • A combi boiler can cause hot and cold fluctuations in the shower when malfunctioning.
  • Temperature inconsistency often stems from a damaged or blocked heat exchanger.
  • Regular maintenance and professional inspection can mitigate shower temperature issues.

Within homes across the UK, the combination boiler, commonly known as a combi boiler, serves as a high-efficiency water heater and a central heating unit.

Experiencing a shower that fluctuates between hot and cold can be a source of frustration and discomfort. This particular issue is often observed in homes equipped with combination boilers, commonly known as combi boilers.

A combi boiler is an ingenious space-saving idea, as it combines central heating with hot water in one unit. However, they are complex systems and can lead to temperature instability in your shower when not functioning correctly.

Temperature variations during a shower can emerge from different malfunctioning components within the combi boiler system.

The heat exchanger, a crucial part that transfers heat from the boiler's burner to the water that will come out of your taps, can become damaged or blocked. This can result in the boiler being unable to maintain a stable hot water temperature.

Additionally, issues such as faulty thermostats, limescale build-up, or even temporary disruptions in gas supply can result in a shower that unpredictably alternates between hot and cold.

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Understanding Combi Boilers

Within homes across the UK, the combination boiler, commonly known as a combi boiler, serves as a high-efficiency water heater and a central heating unit. This dual-purpose function eliminates the need for a separate water tank, providing hot water on demand from a compact and singular source.

Components of a Combi Boiler

Combi boilers possess several key components that enable them to provide heating and hot water efficiently. At the core sits the heat exchanger, which transfers heat from the burner to the water without intermixing the two substances. Controlling the system, the thermostat allows users to set their desired temperature, while various sensors monitor boiler performance.

  • Heat Exchanger: Transfers heat to water.

  • Burner: Ignites to produce heat.

  • Thermostat: User interface for temperature control.

  • Sensors: Monitor pressure, flow, and temperature.

How a Combi Boiler Works

Once the thermostat signals the boiler, the gas burner ignites, heating the water within the heat exchanger. As water passes through the network of pipes, the heat exchanger warms it to the set temperature. For central heating, the heated water circulates through radiators throughout the property. When hot water is required, it is delivered directly to taps and showers without the delay often experienced with systems reliant on a separate water tank.

  • Step 1: Thermostat signals for heat.

  • Step 2: Gas burner heats water in the heat exchanger.

  • Step 3: Hot water circulates to radiators or taps.

Benefits of Combi Boilers

These boilers are celebrated for their efficiency and space-saving design. Their ability to provide hot water on demand means no energy is wasted in heating a separate tank, reducing both fuel consumption and bills. Additionally, their compact size makes them ideal for smaller properties where space is at a premium. With fewer components than traditional systems, combi boilers often result in lower installation and maintenance costs as well.

  • Efficiency: No need to heat a separate water tank, reducing energy consumption.

  • Space-Saving: Compact design frees up space within the home.

  • Cost-Effective: Often lower installation and maintenance costs.

Identifying Common Issues

When troubleshooting combi boiler issues that lead to water temperature instability, it's crucial to understand the underlying causes. This section will guide homeowners through the typical problems and help them identify possible breakdowns.

Temperature Fluctuations

Temperature inconsistency, often experienced as water suddenly going hot and cold, can be symptomatic of a heat exchanger malfunction. This critical component is responsible for transferring heat to the water, and any damage within can cause noticeable temperature swings—a common issue in combi boilers. Sludge buildup or scale might also hinder its performance.

Low Water Pressure

If a shower delivers only a trickle of water or fluctuates wildly, low water pressure in the system could be at fault. Boiler pressure should be within the range of 1 to 1.5 bars; anything lower might lead to inadequate water flow. A possible cause for low pressure can be leaks or recently bled radiators.

Boiler Not Firing Up

A boiler that doesn’t ignite, thus providing no hot water, may have an issue with its thermostat or electrical supply. It's essential to check that the boiler's pilot light is on or whether the electrical panel has tripped any circuits. Sometimes, these are straightforward fixes.

Frozen Condensate Pipe

During cold spells, the condensate pipe may become frozen, especially if it is exposed externally. This pipe safely disposes of the condensate by-product generated from the boiler's operation. If it freezes, the boiler can shut down as a safety measure, resulting in no hot water or heating. Thawing the pipe carefully can resolve this issue.

For each of these common issues, thorough fault-finding is advised. Regular maintenance can preempt these problems, ensuring an uninterrupted supply of hot water.

Diagnosing Shower Temperature Problems

When assessing a shower's unstable temperatures, considering the condition of the mixer valve, pipes, and boiler components is crucial to pinpoint the fault correctly.

Shower Mixer Valve Faults

Shower mixer valves regulate the mix of hot and cold water to deliver a consistent shower temperature. A malfunction here could result in abrupt temperature swings. Signs of a faulty valve include erratic temperature control or a complete failure to mix the water temperatures properly. A professional assessment might reveal whether a cleaning or a replacement of the valve is necessary.

Debris and Scale Buildup

A common culprit for shower temperature instability is blockage caused by debris or scale—a chalky deposit that accumulates over time, especially in hard water areas. This impediment in the waterways can severely impact the boiler's ability to deliver consistent hot water. Descaling the boiler and associated pipes often rectifies this issue, but persistent problems might warrant more extensive interventions such as replacing affected parts.

Diverter Valve Issues

The diverter valve in a combi boiler directs hot water either to the radiators or to the taps as needed. If a fault occurs, the boiler may struggle to maintain a steady supply of hot water, leading to a combi boiler's hot water going cold. Reasons range from mechanical failure to lime scale encrustation. Addressing diverter valve issues typically involves cleaning or, in more severe cases, full replacement to ensure proper function.

Maintenance and Prevention

Maintaining a combi boiler ensures a consistent supply of hot water, avoiding the discomfort of showers fluctuating between hot and cold. Regular attention to the hot water heat exchanger, filters, and boiler pressure can prevent common issues.

Routine Descale and Cleaning

In any household, the accumulation of scale and sludge can impair a boiler's performance. Descale the hot water heat exchanger annually with a commercial descaler, ideal for removing mineral deposits. Likewise, cleaning the filters will fend off blockages, enhancing the longevity and efficiency of the boiler.

Checking for Airlocks and Blockages

Airlocks in the system may disrupt water flow, leading to temperature inconsistencies. Expelling air from radiators and checking for blockages can restore stability in water temperature. Filters should be inspected for sludge buildup; if present, a power flush of the system might be necessary.

Setting Correct Boiler Pressure

Boiler pressure requires close observation, as it affects the boiler's operation. Confirm that the boiler pressure gauge reads between 1 and 1.5 bars, the ideal range for proper functionality. If the pressure falls below 1 bar, repressurising the boiler will be essential to re-establish hot water supply continuity.

Professional Repairs and DIY Fixes

Addressing fluctuations in your shower's temperature can range from straightforward DIY fixes to complex repairs best left to professionals. It's important to discern when to undertake a task yourself and when to enlist the support of a Gas Safe registered engineer.

When to Call a Gas Engineer

If you're confronted with a persistent problem with your shower's heat, it may be indicative of a deeper boiler-related issue. Professional help is paramount when it comes to complex boiler repairs or the need to replace the heat exchanger. DIY intervention is not just discouraged but could be unsafe. If your combi boiler requires intricate repair work, or you suspect it might be time to invest in a new boiler, contacting a Gas Safe engineer ensures the job is conducted with expertise and compliance to safety standards.

  • Signs that necessitate a professional's touch:

    • Erratic temperature changes too complex to diagnose

    • Boiler's heat exchanger shows signs of damage or corrosion

    • The repair work involves gas or boiler disassembly

    • Faulty parts that require precise calibration or adjustment

(All of iHeat's engineers are Gas Safe and fully trained!)

Common DIY Solutions

Some combi boiler issues can be resolved with simple DIY measures. Regular maintenance, such as checking pressure levels, can prevent certain faults. When the issue is minor, like air trapped in the system or a simple blockage, here are steps you can take:

  • Routine maintenance tasks you can do:

    • Bleed radiators to eradicate air pockets that affect heating consistency

    • Conduct a boiler pressure check and repressurise if it falls below 1 bar

    • Clear out debris from the shower head or faucet to improve water flow

*Note: Any DIY intervention should be carried out with the power off and following the boiler manual's instructions. If at any point you're unsure, it's safer to consult a professional.

Improving Water Pressure and Flow

Maintaining consistent water pressure and flow rate is essential for a comfortable and functional shower experience. In the UK, combi boilers are often central to both issues, directly affecting the performance of showers in homes. Two effective methods stand out in addressing pressure and flow concerns: installing booster pumps and thoroughly inspecting water sources.

Installing Booster Pumps

To effectively increase water pressure, especially where the mains pressure is insufficient, homeowners may consider installing a booster pump. These devices amplify the water pressure, thereby improving the flow rate to water outlets, including showers. The target flow rate for a satisfactory performance is around 12-15 litres per minute, achievable with a quality booster pump. It's critical, however, to ensure compatibility with your specific combi boiler model to avoid contravening any regulations.

  • Important Consideration: Booster pumps should be installed onto the cold water feed pipe to enhance overall system efficiency.

Inspection of Water Sources

A well-functioning combi boiler hinges on a stable feed from multiple water sources. Routine inspection of these sources includes checking the mains and any connected storage tanks.

  • Initial Check: Verify the mains pressure is within the suitable range for your boiler system.

  • Secondary Action: Examine other water sources for potential obstructions or defects that could impact water flow to the boiler, consequently causing pressure fluctuations.

By keeping a close eye on these components, one can often preempt issues with water flow and pressure.

  • Professional Recommendation: Regular servicing by a qualified technician can identify and mitigate intricate problems early on.

Do I Need A New Boiler?

Deciding if you need a new boiler hinges on several specific factors, including efficiency and the ability to meet your heating and hot water needs. Evaluating the age and condition of your current system is critical.

When to Invest in a New Boiler

A new boiler is a significant investment and should not be taken lightly. Consider a replacement when:

  • Your current boiler is over 15 years old.

  • Repair costs are becoming frequently expensive and extensive.

  • Efficiency has dropped, leading to higher energy bills.

An inefficient boiler doesn't just lead to erratic temperatures in your shower; it affects the entire heating system. The hot water storage cylinder often found in regular boiler systems may lose efficiency over time, indicating the potential need for a new system.

Upgrading Your Heating System

Upgrading your heating system could entail transitioning from a regular boiler system, which includes separate hot water storage, to a combi boiler, which provides heating and hot water on demand without the need for a cylinder.

Key considerations for upgrading should focus on:

  • Efficiency: Modern combi boilers offer significantly better efficiency ratings.

  • Space: A combi boiler frees up space by eliminating the need for a hot water storage cylinder.

  • Water Pressure: Ensure the selected combi boiler can deliver adequate water pressure for your shower and other needs.

Furthermore, the specific demands of your home’s heating requirements will dictate whether a powerful, higher capacity boiler is necessary. Assessing your home's heating needs with a Gas Safe registered engineer can provide clarity on whether an upgrade is the most cost-effective and efficient solution.

Cost Implications

When managing a combi boiler's temperature control issues, homeowners must be vigilant about both immediate and long-term costs which include energy consumption and the feasibility of repairs versus wholesale replacements.

Energy Bill Impact

Identifying when a combi boiler causes the water temperature to fluctuate can be key in understanding its effect on energy bills. When a boiler works erratically, it frequently switches on and off, using gas each time to raise the water temperature. This could lead to an increase in gas usage, reflected as higher energy bills. Consistent monitoring of the boiler's performance may help in detecting inefficiencies early on, potentially avoiding a spike in costs.

Cost of Repairs Versus Replacement

The decision between undertaking boiler repair and investing in a new unit is complex. Factors such as the severity of the damage and the age of the boiler play critical roles. Repairs by a Gas Safe registered engineer may resolve the issue at a lower upfront cost; if they are minor, such as cleaning a filter. However, continuous repairs can add up, tipping the scale in favour of replacement for a more sustainable, long-term solution. It's essential to compare these costs:

  • Repair: The baseline cost for an engineer’s visit, plus parts and additional labour.

  • Replacement: The price of a new boiler including installation fees.

Pursuing repairs for minor issues may be cost-efficient, but for recurrent or major faults, investing in a new, energy-efficient boiler could be more economically judicious in the long run.

iHeat New Boiler Costs

Boilers can be a daunting purchase for many people as they’re an appliance with a lot of responsibility, providing heat for you and your family is something you want to get right. Boilers aren’t exactly a quickly disposable item either, potentially lasting you a decade.

Effectively, new boiler cost can be split into two segments: the first is the actual boiler itself (unit price), and the second is the cost of the boiler being installed (set up) in your property by an expert engineer. 

Here at iHeat, we want to remove all of this undue stress and make the decision making process of upgrading to a new central heating system, as easy as possible.

Boiler costs can vary depending on a number of factors including their brand, model, fuel, output, warranty, labour and installation type. Typically a new boiler will cost between £1,845 and £3,500, below is a list of average boiler installations offered by iHeat (guide only).

Installation Type

Price (inc VAT)

Combi to combi swap


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System to combi conversion


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New boiler install


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Back boiler to a combi


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System to system


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Frequently Asked Questions

Why does my shower keep running hot and cold?

The heat exchanger in the combi boiler often acts as a culprit when the shower water cannot maintain a constant temperature. This inconsistency can arise from a damaged heat exchanger or blockages caused by sludge in the system.

How do you fix a fluctuating shower temperature?

Addressing a fluctuating shower temperature requires inspecting the heat exchanger, checking for any blockages, and ensuring the boiler pressure is at the correct level. It may also involve cleaning or replacing the filters if they are faulty.

Why does my shower go cold after a few minutes?

A decrease in boiler pressure or an issue with the diverter valve could cause the shower to turn cold after several minutes of use. It's important to monitor the pressure gauge and assess the functionality of the diverter valve.

Why is my hot water not staying hot on my combi boiler?

If the hot water from a combi boiler fails to remain hot, it could suggest that the appliance is experiencing fluctuating pressure levels or there might be a problem with its internal components, such as the heat exchanger or sensors.

Why does my hot water stop after a few minutes on my combi boiler?

The hot water may cease after a brief period if the combi boiler has a failing diverter valve or if there is an issue with the boiler's ability to sustain an adequate flow rate due to limescale or similar obstructions.

How much does it cost to fix a diverter valve?

The cost to replace or repair a diverter valve fluctuates based on location, the complexity of the job, and the model of the boiler. Typically, homeowners may encounter expenses ranging from £150 to £250.

How do I know if my diverter valve is broken?

Symptoms of a malfunctioning diverter valve include inconsistent water temperatures, radiators heating up when the hot water tap is turned on, or the hot water failing to heat up when the central heating is off. An inspection by a certified technician can confirm such faults.

Stephen Day profile photo
Article by
Stephen Day | Co-founder
Gas Safe registered and FGAS certified engineer with over 20 years experience in the heating and cooling industry.