Hot water not working is most often caused by low boiler pressure, an active fault code, a stuck diverter valve, or a frozen condensate pipe. Central heating working normally while taps run cold usually points to the diverter valve specifically, while a total loss of both heating and hot water suggests the boiler has shut down entirely.

You turn on the tap or step into the shower, expecting hot water, but instead the water runs cold. This is one of the most common and disruptive plumbing problems in London homes, often bringing daily routines to a sudden halt.

This article explains the most likely reasons behind hot water not working, including boiler and system faults, and walks through what you can safely check yourself before calling a professional. It also outlines when it is necessary to bring in a qualified engineer.

From simple pressure or thermostat issues to more complex boiler faults, this guide helps you quickly narrow down the cause and understand your next steps.

Why Is My Hot Water Not Working?

When your hot water is not working, the issue usually comes down to a few common faults. Identifying the pattern helps narrow down the cause quickly and decide whether a simple fix is possible or if an engineer is needed. Below are some of the most common faults:

Fault 1: No Hot Water with a Combi Boiler

This is one of the most common causes in modern homes. A combi boiler heats water on demand, so any internal disruption can instantly stop hot water. Typical causes include low boiler pressure, an active fault code, a frozen condensate pipe in cold weather, or a failing diverter valve.

Fault 2: Boiler Not Working, and No Hot Water

If there is no response from the boiler at all, and both heating and hot water are affected, the system has likely shut down. This may appear as a blank screen or an error code. In these cases, the boiler is not firing, and no heat is being produced.

Fault 3: Central Heating Working But No Hot Water

If radiators are heating normally but taps run cold, this is a strong indicator of a diverter valve fault in a combi boiler. This component controls whether heated water is sent to the radiators or the taps. When it sticks, heat is diverted away from domestic hot water.

Fault 4: No Hot Water Pressure From Your Hot Taps

Weak flow or a trickle from the taps often indicates a pressure-related issue. This can be caused by low mains water pressure, a partial blockage in the pipework, or a failing pump. Even when the boiler is functioning, insufficient pressure prevents proper hot water delivery.

Fault 5: Not Enough Hot Water or Water Not Getting Hot Enough

If water is lukewarm or runs out too quickly, the issue may be temperature- or capacity-related. Common causes include a thermostat set too low, a failing heat exchanger, or, in older systems, an undersized or poorly maintained hot water cylinder.

What to Check Before Calling a Plumber

Before booking a repair, there are a few safe and practical checks that can help identify the cause of boiler hot water not working issues.

1. Check The Boiler Pressure Gauge

Most modern boilers should sit between 1 and 1.5 bar. If the pressure drops below 1 bar, the system may shut down or struggle to heat water properly.

• Make sure the boiler has cooled down before touching the filling loop; system water can be hot
• Locate the filling loop, usually a flexible silver hose connected under the boiler
• Slowly open the valve at each end of the loop
• Watch the gauge and stop as soon as it reaches the correct level, between 1 and 1.5 bar
• Do not overfill past the green zone on the gauge
• Close both valves firmly once done
• If the pressure keeps dropping after topping up, this points to a leak elsewhere in the system; call a Gas Safe registered engineer rather than repeat the process

2. Check For Fault Codes on The Boiler Display

Modern boilers communicate faults through error codes shown on the digital display.

• Note the exact code shown on the display
• Check the boiler manual or the manufacturer’s website for what the code means
• Codes vary between brands, so confirm you are looking at the right manual for your model
• Some codes, such as a low-pressure warning, can be resolved yourself
• Codes relating to ignition or the PCB should not be repeatedly reset, as this can mask a fault that needs professional attention

3. Check The Thermostat and Timer Settings

Incorrect settings are a common reason for heating and hot water not working properly.

• Confirm the system is set to “on” or “auto” rather than “off”
• Check the hot water schedule covers the current time
• Confirm the temperature dial has not been turned down
• Check for recent power cuts or daylight saving changes, which can reset settings without you realising
• Recheck even if the settings look familiar, since accidental adjustments are easy to miss

Check for a Frozen Condensate Pipe

In winter, the external condensate pipe can freeze and trigger a safety shutdown.

• Locate the condensate pipe, usually a white plastic pipe running from the boiler to an outside wall
• Only attempt this if the pipe is easily and safely reachable at ground level; do not attempt this on a ladder or if it is above head height
• Wrap the pipe in a warm towel or hold a hot water bottle against it rather than pouring water directly
• Never use boiling or very hot water; sudden heat can crack the pipe
• Take care of icy or slippery ground when working outside
• Once thawed, reset the boiler following the manufacturer’s instructions
• If the pipe freezes repeatedly, this usually means it needs better insulation, which is a job for a Gas Safe registered engineer

Check The Mains Water Supply

A partially closed stop valve can restrict water flow and mimic a boiler fault.

• Locate the stop valve, usually under the kitchen sink or at the mains entry point
• Check the valve is turned fully anti-clockwise to the open position
• If it feels stuck or won’t turn, don’t force it; this may need a plumber
• If it was partially closed, opening it fully may resolve the issue immediately
• Rule this out before assuming the problem is with the boiler itself

Diverter Valve Faults Explained: When Heating Works But Hot Water Does Not 

When your heating works normally, but hot water does not, the issue is often linked to a faulty diverter valve inside the boiler. A diverter valve is a key component in a combi boiler that controls where heated water is directed, switching between the central heating system and the hot water supply depending on demand.

When this valve sticks or fails, the boiler continues to produce heat, but it is sent only to the radiators. This leads to the central heating working but no hot water, which can be confusing because part of the system still appears to be functioning normally.

A diverter valve replacement or repair is not a DIY job. It involves opening and working inside the boiler, which must only be carried out by a qualified professional. You should always use a Gas Safe registered engineer when dealing with internal boiler components.

Absolute Plumbing Ltd are Gas Safe registered (registration 955302) and is also verified on Checkatrade, carrying out expert boiler diagnostics and repairs across South West London.

Hot Water Not Working in a System or Conventional Boiler

Not all homes use combi boilers. In system or conventional setups, hot water is stored in a cylinder, which introduces a different set of potential issues. If the cylinder thermostat is set too low or has failed, water will not reach the correct temperature, leading to poor or inconsistent heating performance.

An airlock in the hot water circuit can also prevent water from reaching the taps properly, resulting in no flow or intermittent cold water. This can sometimes be mistaken for a more serious boiler fault, when in reality it is caused by trapped air in the system.

Most homes with this type of setup also include an immersion heater inside the cylinder, which acts as a backup heat source. This is useful for diagnosing where the problem lies: if the immersion heater works but the boiler does not, it usually indicates the issue lies within the boiler system rather than the cylinder itself.

Over time, sludge and debris can build up inside older heating systems, reducing efficiency and restricting heat transfer. This can contribute to hot water not getting hot enough, and in more severe cases, a power flush may be required to clear the system and restore proper circulation and performance.

Total Boiler Shutdown: Heating and Hot Water Not Working Together

When both heating and hot water stop working at the same time, it usually indicates that the boiler itself has stopped functioning rather than a problem within the wider system. In these cases, the issue is typically internal and requires professional diagnosis.

Common causes include a loss of pressure, a tripped safety switch, an electrical fault, or internal component failure, such as a broken pump or a faulty printed circuit board. Any of these can prevent the boiler from operating at all, leaving the home without heat or hot water.

If you are experiencing heating and hot water not working together, it is important not to attempt repairs yourself. Gas appliances are complex and potentially dangerous if handled incorrectly, and any internal work must only be carried out by a qualified Gas Safe registered engineer.

Absolute Plumbing Ltd are listed on TrustATrader and provides expert boiler repair and installation services across South West London, ensuring faults are diagnosed safely and resolved efficiently. 

FAQ: Hot Water Problems Explained

Why is my hot water not working, but my heating is fine?

This is most commonly caused by a faulty diverter valve in a combi boiler. The heating circuit continues to function, but hot water is not directed to your taps. 

How do I fix no hot water pressure?

Start by checking your mains water supply and boiler pressure. Also, check the pressure relief valve and ensure the pump is operating. If the issue continues, a plumber should inspect the system for blockages or failing components.

Can I fix hot water problems myself?

You can safely check boiler pressure, thermostat settings, and basic supply issues. However, any gas-related work or internal boiler repairs must be carried out by a Gas Safe registered engineer.

How much does it cost to fix a boiler with no hot water?

Costs vary depending on the fault. Minor issues, such as pressure correction, are usually inexpensive, while part replacements, like pumps or diverter valves, can cost significantly more in London. 

For an accurate diagnosis and pricing, get in touch for a free quote:

When to Call a Heating Engineer

Most cases of hot water not working come down to a clear, identifiable fault, whether that’s pressure-related, a thermostat issue, or a component inside the boiler itself. While some problems can be narrowed down with basic checks, many require a trained engineer to diagnose accurately and prevent further damage.

Absolute Plumbing Ltd provides expert general plumbing services across South West London, with fully qualified Gas Safe-registered engineers (955302) who can quickly and safely identify and resolve boiler and hot water issues.

If you are dealing with ongoing problems or a complete loss of hot water, call us on 07341 547739 for professional assistance.