No Hot Water But Heating Works in Ashford - What to Check

Published September 2026 | Boiler Repair

Why Your Heating Works But There's No Hot Water

This is one of the most common boiler faults our engineers deal with across Ashford and surrounding parts of Kent. The reason it's so specific - heating on, hot water off - comes down to how a modern combi boiler divides its workload between two separate jobs.

A combi boiler heats water in two distinct ways. For the radiators, it circulates water around a sealed central heating circuit. For the taps, it heats cold mains water on demand whenever you open a tap. These two functions use different internal components, which is why one can fail entirely while the other keeps working without any sign of trouble.

Here are the most common reasons this happens.

Faulty Diverter Valve

The diverter valve is the most likely culprit. It's a motorised valve inside the boiler that physically switches between sending heated water to your radiators or routing it through the domestic hot water circuit. Over time, these valves can stick - often in the heating position - or the small motor that drives them can burn out. When that happens, your radiators stay warm while your taps run cold. Boilers from Worcester Bosch, Vaillant, Ideal, and Glow-worm all use diverter valves, and all of them can and do fail, particularly in boilers more than eight years old.

Failed Thermistor (Temperature Sensor)

Your boiler uses thermistors - small electronic temperature sensors - to monitor water temperature on different circuits. The domestic hot water thermistor tells the boiler whether water at the taps is reaching the target temperature. If it fails or starts sending inaccurate readings, the boiler may not fire up for hot water at all, even while heating runs without any issue. This is a less dramatic fault than a diverter valve failure but produces similar symptoms.

Scaled-Up or Blocked Plate Heat Exchanger

Combi boilers heat domestic hot water using a plate heat exchanger - a compact unit where heat from the primary heating circuit passes across to fresh mains water. In areas with harder water, limescale builds up on the plates over time and restricts the flow. Parts of Kent have moderately hard water, which makes this worth considering for Ashford homeowners. A partially blocked heat exchanger typically produces lukewarm rather than fully cold water; severe scaling can cut hot water output off almost entirely.

Faulty Flow Sensor

Many modern boilers rely on a flow sensor to detect when a tap has been opened. That signal is what triggers the boiler to switch into domestic hot water mode. If the sensor fails, the boiler never receives the instruction to produce hot water - so the central heating carries on normally while the taps stay cold. This fault can be intermittent at first, which sometimes makes it harder to diagnose.

Motorised Valve or Cylinder Issues (System Boilers)

If your home has a system boiler with a separate hot water cylinder rather than a combi, the picture is different. In this setup, a motorised valve controls whether hot water is directed to heat the cylinder. If the valve or its actuator fails, the cylinder stops being heated even though the radiators are fine. A failed cylinder thermostat or a broken immersion heater element can produce the same result. This type of system is less common in newer Ashford properties but is still found in many older homes.

Step-by-Step: What to Check Before Calling an Engineer

Before booking anyone out, run through these checks. Several of the causes above have symptoms you can identify - or even resolve - without opening the boiler casing.

1. Check your programmer and hot water settings. It's easy to overlook, but confirm that hot water isn't simply turned off on the timer. Many boilers in Ashford homes have separate heating and hot water schedules, and a power cut or accidental button press can switch hot water off while leaving heating active. Check the control panel, confirm hot water is enabled, and make sure the temperature is set to at least 50 to 60 degrees Celsius.

2. Check the boiler pressure. Look at the pressure gauge on the front of the boiler - it should read between 1 and 1.5 bar when cold. Very low pressure can cause erratic behaviour, and some boilers deprioritise domestic hot water when pressure drops. If it's below 1 bar, repressurise via the filling loop. Your boiler's manual will explain how to do this safely.

3. Run the hot tap fully open for two minutes. If the water comes out cold and stays cold, the problem is likely a diverter valve or flow sensor. If it comes out lukewarm or takes a long time to warm up, a scaled plate heat exchanger or a partially failing thermistor is more probable. This simple test helps narrow down what your engineer will be looking for.

4. Check for error codes. Most modern boilers - including Worcester Bosch, Viessmann, and Vaillant models - display fault codes when something's wrong. Note down any code showing on the display and look it up in the manual. You can also run the code through the Voltrade GoFIX diagnostic tool for a plain-English explanation of what the fault means and how serious it is.

5. Check the hot water cylinder if you have one. If your system includes a separate cylinder, feel the pipes around it during the hot water schedule. If they're cool, either the motorised valve isn't opening or the cylinder thermostat isn't calling for heat. You should also be able to hear the motorised valve click when it activates - silence during a hot water period can indicate a failed actuator.

6. Try a boiler reset. Some intermittent faults clear after a reset. Hold the reset button for five seconds as directed in your manual and see whether hot water returns. If it does, monitor over the next 24 to 48 hours. A fault that clears on reset but keeps returning is a reliable sign of a component that's deteriorating and needs replacing properly.

DIY vs Professional - Knowing When to Stop

The line between what you can do yourself and what legally requires a professional is clear and worth understanding before you go any further.

You can safely handle the following without any specialist knowledge: adjusting programmer settings, checking and repressurising the boiler, resetting the unit, and noting fault codes. None of these involve opening the boiler casing or touching gas or refrigerant components.

If you're in Ashford and you've run through the checks above without fixing the problem, the right move is to book a registered engineer. Don't be tempted to remove the boiler casing or adjust internal components yourself, regardless of what you find on YouTube.

What a Qualified Engineer Will Do

When one of our Gas Safe registered engineers attends a property in Ashford, the first job is systematic diagnosis rather than immediately ordering parts. Jumping straight to replacements without proper diagnosis wastes time and money.

The engineer will start by reading the boiler's live fault data and error history. For boilers without a display, they'll use test equipment to check voltage signals at the diverter valve motor, read resistance values from the thermistors, and verify whether the flow sensor is triggering when a tap is opened. This takes 15 to 30 minutes and typically makes the fault clear.

If the diverter valve is stuck mechanically rather than electrically failed, an experienced engineer will often attempt to free it before recommending a replacement. A stuck valve that can be worked free saves the customer the cost of a new part. If the motor has burned out or the valve is physically damaged, a replacement is needed - this involves draining part of the system and fitting the correct part for that boiler model.

For a scaled plate heat exchanger, the engineer may recommend a targeted chemical descale rather than a full power flush, depending on how badly the plates are restricted. In cases where the heat exchanger is damaged by limescale or corrosion beyond what descaling can fix, replacement is the only option - it's a more involved job but still comfortably done within a single visit in most cases.

Thermistor and flow sensor replacements are typically quick. These are small, accessible components in most boiler designs, and the work usually takes under an hour from diagnosis to completion. After any repair, the engineer will test hot water across multiple taps, verify boiler pressure, and confirm the central heating is still functioning correctly before finishing.

Costs and What Affects the Price

Boiler repair costs in Ashford vary based on what's failed, how long the work takes, and parts availability for your specific boiler model. Here's what you can typically expect to pay.

Diverter valve replacement is usually the most expensive of the common fixes. Expect to pay between 200 and 400 pounds for parts and labour combined. Vaillant and Worcester Bosch parts tend to carry a higher price than those for budget brands, but they're more commonly stocked by local engineers, which can reduce waiting time.

Thermistor replacement is one of the more affordable repairs - typically 80 to 150 pounds all in. The component itself is inexpensive and the labour time is short.

Flow sensor replacement generally falls between 100 and 200 pounds, depending on the boiler model and whether the part needs ordering.

Plate heat exchanger descaling costs around 100 to 200 pounds for a chemical treatment. If the exchanger has to be replaced rather than descaled, costs rise to between 300 and 500 pounds - it's a more substantial repair with a costlier component.

Motorised valve replacement on a system boiler typically runs between 120 and 250 pounds.

Most engineers in Kent charge a call-out fee in the region of 60 to 100 pounds, which may or may not be absorbed into the final repair quote. Always ask upfront. It's also worth checking any boiler cover policy you hold before booking - some plans include call-outs and repair labour as standard.

If your boiler is more than 12 to 15 years old and a major component has failed, your engineer may raise the repair-versus-replace question . A new combi boiler fitted in Ashford typically costs between 1,800 and 3,000 pounds installed. That's a bigger upfront figure, but it includes a manufacturer's warranty of five to ten years and eliminates the risk of repeated repairs on aging components.

How to Prevent This in Future

Most of the faults above are wear-and-tear issues that become more likely as a boiler ages. You can't prevent them entirely, but consistent maintenance substantially reduces the risk and catches failing components before they stop working completely.

Get an annual boiler service. A Gas Safe registered engineer will inspect internal components during a service, including the diverter valve mechanism and heat exchanger. Catching a part that's starting to degrade costs far less than an emergency repair and avoids the inconvenience of losing hot water unexpectedly. In Ashford, an annual service typically costs between 70 and 120 pounds. Annual servicing is also a requirement of most boiler manufacturer warranties - skipping it can void your cover.

Fit a magnetic system filter if you don't already have one. These devices capture iron oxide particles - the dark sludge that accumulates in older heating systems - before they circulate through the pump and valves. Sludge accelerates wear on the diverter valve and can contribute to blockages in the heat exchanger. Ask your engineer to fit one at the next service.

Add a corrosion inhibitor to your heating system. This chemical treatment protects internal components from scaling and oxidation. It's particularly worth doing in harder water areas of Kent. Inhibitor levels should be tested and topped up during each annual service - it breaks down over time and loses effectiveness if left unchecked.

Monitor the boiler pressure regularly. Pressure that consistently drops below 1 bar puts additional strain on seals and components. Check it every couple of months and repressurise when needed.

Don't ignore fault codes, even intermittent ones that clear themselves. A code that appears and disappears is often a sign that a component is starting to fail. Log any codes you see and mention them at your next service visit so the engineer can investigate before a complete failure occurs.

Frequently Asked Questions

Why does my combi boiler heat the radiators but not the hot water taps?

In a combi boiler, the diverter valve switches the system between heating mode and domestic hot water mode. When this valve sticks or its motor fails, the boiler gets locked into heating mode and can't switch over to serve the taps. It's the most common cause of this exact symptom and can be diagnosed and repaired by a Gas Safe registered engineer, typically in a single visit.

Can I fix a diverter valve fault myself?

How long does a boiler repair typically take in Ashford?

Most common repairs - diverter valves, thermistors, flow sensors - are completed within two to three hours from arrival. If a part isn't on the engineer's van and needs ordering, you may be waiting a day or two for a return visit. Engineers covering Ashford typically stock common parts for Worcester Bosch, Vaillant, and Ideal boilers, which often means same-day completion for these brands.

Will a boiler service prevent me losing hot water?

Not guaranteed, but it reduces the risk significantly. During a service, the engineer checks internal components for early signs of wear - including the diverter valve and temperature sensors. Catching a part that's starting to degrade before it fails completely is considerably cheaper than an emergency repair. Annual servicing is the most cost-effective preventive measure available to homeowners with combi boilers.

My boiler is over 12 years old and the heat exchanger has failed - should I repair or replace?

At that age, a failed plate heat exchanger is often the point where replacement makes more financial sense than repair. The repair cost is significant, and other components on an older boiler are increasingly likely to follow. A new combi boiler installed in Ashford typically costs between 1,800 and 3,000 pounds and comes with a manufacturer's warranty of five to ten years. An honest engineer will give you a clear view of both options without pushing either way.