When you turn on the tap expecting a warm flow and instead get nothing, a trickle, or cold water, it can be frustrating. The issue is often something simple that you can safely check yourself before calling in a Gas Safe engineer. If you live in areas such as Bordon, Whitehill, Liphook, Alton or Farnham, similar symptoms tend to stem from the same handful of faults, especially in combi boilers common across GU35, GU34 and GU9 homes.
Understanding why your hot water stops flowing
Most hot water issues come from blockages, pressure problems, diverter valve faults or thermostatic mixing valve (TMV) issues. Before assuming the worst, follow the steps below to narrow down what is actually happening.
Quick checks you can do now
- Is the boiler powered on with no warning lights?
- Are you getting normal cold water pressure from taps?
- Does the boiler fire up when a hot tap is opened?
- Have you recently had plumbing work done?
- Is your shower mixer producing any warm water at all?
If the answer to any of these is unusual, the sections below will help you pinpoint the likely cause.
Check 1: Low system pressure and how it affects hot water not coming through
Combi boilers rely on stable water pressure to produce hot water. If the pressure is too low, the boiler may refuse to ignite or may cycle on and off rapidly, resulting in weak flow or no hot water. This is especially common after bleeding radiators or during colder months when system demand increases.
To check your pressure safely, look at the gauge on the boiler’s front panel. Most manufacturers recommend a pressure between 1.0 and 1.5 bar when the system is cold.
- If the pressure is below 1.0 bar, the boiler may not fire.
- If it is over 2.0 bar, this may indicate overfilling or an expansion vessel issue.
Repressurising is usually straightforward: locate the filling loop (a braided metal hose) and open the valves until the gauge returns to the correct level. Close them immediately afterwards. If pressure keeps dropping, you may have a slow leak in the system, which needs professional attention.
Check 2: Diverter valve faults and how they stop hot water flow
The diverter valve is a key component in a combi boiler. It decides whether the boiler’s heat goes to your radiators or your hot taps. When it fails or sticks, you may notice symptoms such as the following:
- Radiators heating up when you run a hot tap
- Hot water coming through for a moment before turning cold
- No boiler response at all from hot taps
Diverter valves can fail due to age, limescale or worn seals. In hard-water areas around Bordon, Liphook and Farnham, this is particularly common. You can carry out a simple homeowner-friendly check: run a hot tap fully open and observe the boiler.
Checklist for spotting a diverter valve issue:
- Does the boiler display a hot water symbol but fail to fire?
- Do radiators begin to warm even though heating is off?
- Does hot water start but refuse to stay consistent?
If the symptoms match, the diverter valve likely needs repair or replacement. This is not a DIY fix, but good diagnosis helps your engineer resolve the issue quickly.
Check 3: Thermostatic mixing valve (TMV) problems
TMVs are found on many modern systems, particularly on baths and showers. They blend hot and cold water to ensure a safe output temperature. If a TMV becomes clogged or faulty, you might experience weak flow, temperature fluctuations or only lukewarm water.
Common signs of a TMV issue include:
- Cold water overriding the hot water entirely
- Very slow flow from the hot side of a bath or shower
- Temperature stuck at one level no matter how you adjust controls
You can do a light check yourself: isolate the hot tap feeding the mixer (usually a small valve under the bath or near the pipework), then open the tap to see whether the flow improves. If the issue disappears, the TMV is likely clogged with limescale.
Homes in Whitehill, GU35 and nearby often experience this due to hard water content. Cleaning a TMV requires disassembly and should be done by someone experienced, but identifying the cause is extremely helpful.
Check 4: Airlocks and flow restrictions
Airlocks can prevent water from moving through the system efficiently. They commonly affect older pipework or systems that have recently been drained. You may notice gurgling sounds, spurting taps or uneven flow.
A simple first step is to open all hot taps fully for several minutes, one by one, allowing trapped air to move through the system. If the flow briefly improves and then drops again, there may be a partial obstruction or scale buildup.
Homes in Liphook and Alton often have older copper pipework where limescale can accumulate, but these issues are manageable once identified.
Check 5: Boiler temperature settings and sensor faults
This step is often overlooked. If the boiler’s hot water temperature is set too low, or if a sensor is misreading the temperature, you may feel as though hot water is not coming through even though the system is technically working.
What to check:
- Is the hot water temperature dial set to at least medium-high?
- Is there a flashing light or fault code on the boiler display?
- Does the boiler fire briefly and then shut off?
Sensors and thermistors do fail with age, especially in combi boilers that cycle frequently. If settings look correct but you still have no reliable hot water, the sensor may require replacement.
Check 6: When simple faults become safety issues
Most hot water faults are annoying, not dangerous. However, repeated boiler lockouts, unexplained pressure drops or burning smells should be treated seriously. In these instances, always turn the boiler off and avoid attempting repairs.
As a rule, if your boiler behaves unpredictably or you are unsure about anything involving pressurised systems or gas components, a qualified engineer should inspect it.
Bringing it all together
Working through checks step by step helps narrow things down quickly. Low pressure is the most common culprit, followed by diverter valve faults and TMV blockages. In towns such as Bordon, Whitehill, Liphook and surrounding GU34 and GU9 postcodes, we regularly see these same issues crop up due to similar boiler models, local water quality and house layouts.
By following the guidance above you can often identify the problem before calling in a professional, saving time and avoiding unnecessary disruption.
Next steps: if you need a Gas Safe engineer to diagnose or repair your hot water system, you can book a visit at https://www.embassygas.com/book