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

Published September 2026 | Boiler Repair

Your radiators are toasty, the boiler is firing up, and there are no error codes flashing on the display. So the problem must be somewhere in your plumbing - right? That is the most common assumption our engineers hear from homeowners across Amersham, and it is almost always wrong. The fault is nearly always inside the boiler itself, sitting in the components that handle domestic hot water on demand rather than anything hidden in your pipework.

Myth: If the Heating Works, Your Boiler Must Be Fine

The reality

A combi boiler - which is the type installed in the vast majority of UK homes, including most properties across Buckinghamshire - does two completely separate jobs. It heats your radiators through one circuit, and it heats water for your taps and shower through another. These two circuits can fail independently of each other.

When heating works but hot water does not, the boiler is doing one job correctly and failing at the other. The error is not system-wide. It is contained to the domestic hot water side of the unit. This is why homeowners often call a plumber first and end up disappointed - there is nothing wrong with the plumbing. The fault is mechanical, electrical, or hydraulic, and it lives inside the boiler casing.

Popular combi boilers you will find in Amersham homes include the Worcester Bosch Greenstar range, Vaillant ecoTEC Pro and Plus models, Ideal Logic, and Baxi 800. All of them can present with exactly this symptom, and in all of them, the root cause follows a familiar short list of components that wear out over time.

Myth: It Is Probably a Plumbing Problem, Not a Boiler Problem

The reality

When hot water suddenly stops working, the first instinct is to check the taps, inspect the showerhead, or call a plumber to look at the pipework. In most cases, this is wasted time and money. If the heating is working properly, the water supply to your property is confirmed as live, and the boiler is firing without fault codes - your pipework is almost certainly not the issue.

The real culprits are nearly always inside the boiler itself. Our engineers use the Voltrade GoFIX diagnostic tool to run through likely fault codes and system readings before opening up a boiler, and the results consistently point to four main components: the diverter valve, the plate heat exchanger, the flow sensor (also called the flow thermistor), and in colder months, the condensate pipe.

A plumber who is not also a Gas Safe registered engineer cannot legally open or work on the gas-related components of your boiler. If you call one out first, they will often just refer you straight to a heating engineer anyway. Starting with the right trade from the beginning saves you time, a second call-out fee, and the frustration of waiting an extra day without hot water.

Myth: Low Boiler Pressure Is Causing the Problem

The reality

Boiler pressure is one of the most misunderstood aspects of home heating. Homeowners see the pressure gauge drop or notice it sitting below the green zone, and assume it explains everything - including a complete loss of hot water. The logic sounds reasonable. It does not hold up in practice.

Here is why: low pressure affects both the heating and the hot water circuit simultaneously. These two circuits share the same pressurised water loop inside a combi boiler. If pressure were genuinely causing your hot water to fail, your radiators would also be performing poorly - they would be cold at the top, inconsistent across the house, or the boiler would be locking out entirely with a low-pressure fault code on the display.

When heating works fine and only hot water fails, pressure is rarely the cause. That said, checking your pressure is still worth doing first because it takes about ten seconds and costs nothing. Look at the pressure gauge on the front panel - it should sit between 1 and 1.5 bar when the system is cold. If it reads below 0.5 bar, use the filling loop to repressurise the system and then test again. Just do not assume that fixing the pressure will resolve your hot water fault. In most cases, it will not, and you will need to work through the other likely causes.

Myth: Repressurising the Boiler Will Sort It Out

The reality

This myth follows directly from the previous one. Because repressurising a boiler is one of the few maintenance tasks homeowners can safely carry out themselves, it tends to get applied to almost every boiler problem - including ones where it is completely irrelevant. Our engineers in Buckinghamshire hear this frequently: "I topped it up but nothing changed."

Repressurising will not fix a stuck diverter valve. It will not unblock a scaled-up plate heat exchanger. It will not replace a failed flow sensor. And it will not thaw a frozen condensate pipe. These are all component-level faults that require a Gas Safe registered engineer to diagnose and repair properly.

There is also a secondary issue worth knowing: if your boiler loses pressure repeatedly - needing a top-up every few weeks - that is a separate fault in itself, usually pointing to a leak somewhere in the system or a failing pressure relief valve. Repressurising repeatedly without finding the source of the pressure loss will eventually damage the boiler further and lead to higher repair bills down the line.

If you have checked the pressure, it reads fine, and you still have no hot water, the problem is definitely mechanical or electrical. At that point, the next call should be to a Gas Safe registered engineer - not another attempt to top up the system.

Myth: No Hot Water Means You Need a New Boiler

The reality

This is perhaps the most expensive myth on the list. Homeowners who lose hot water frequently jump to the conclusion that the boiler has completely failed and needs replacing. Some less scrupulous engineers are not quick to correct this assumption. The reality is that the most common causes of this specific fault are all repairable without touching the rest of the unit.

Here is what our engineers typically find and what it costs to fix in 2026:

1. Diverter valve failure - The diverter valve switches the boiler between heating mode and hot water mode on demand. When it sticks or fails, the boiler defaults to heating the radiators and cannot divert energy to the hot water circuit. This is by far the most common cause our engineers encounter. Replacing a diverter valve typically costs between 200 and 400 pounds including parts and labour, depending on the boiler make and model. Worcester Bosch and Vaillant parts tend to sit at the higher end of that range due to manufacturer pricing on OEM components.
2. Plate heat exchanger blockage - The plate heat exchanger transfers heat from the boiler's primary circuit to your domestic hot water without the two water sources mixing. In hard water areas - which includes much of Buckinghamshire - limescale builds up inside it over time and gradually reduces flow until hot water either runs lukewarm or stops entirely. Cleaning or replacing this component typically costs between 250 and 500 pounds. A magnetic filter fitted during the repair and annual servicing can prevent the problem recurring.
3. Flow sensor or thermistor failure - The flow sensor detects when you open a hot tap and signals the boiler to fire specifically for hot water. If the sensor fails, the boiler does not register that hot water is being requested and simply does not respond. Replacing this component is generally a quicker job and typically costs between 100 and 200 pounds for parts and labour combined.
4. Frozen condensate pipe - During cold spells, the condensate pipe - the plastic waste pipe that carries acidic condensate from the boiler to an outside drain - can freeze and cause the boiler to lock out. This is the one fault that homeowners in Amersham can sometimes resolve themselves. Carefully pour warm water (not boiling, which can crack the pipe) over the exposed external section. Once thawed, reset the boiler and test the hot water. If this keeps happening each winter, an engineer can re-route or insulate the pipe to prevent it.

On a boiler that is less than ten years old, any of these repairs is worth carrying out without hesitation. A full replacement, by comparison, typically costs between 2,000 and 3,500 pounds installed - which makes a 250-pound diverter valve job look like exactly what it is: a sensible repair, not a crisis.

What Actually Matters - Expert Advice

When our engineers attend this fault in Amersham, the diagnostic process follows a logical sequence. Working through these steps before calling anyone out will speed up the job considerably and, in some cases, save you the call-out fee entirely.

1. Check the boiler pressure gauge. It should read between 1 and 1.5 bar when the system is cold. If it sits below 0.5 bar, repressurise through the filling loop and test hot water again.
2. Check for error codes on the boiler display. Worcester Bosch, Vaillant, Baxi, and Ideal all display fault codes that point directly to which component has failed. Run the code through the Voltrade GoFIX diagnostic tool for an immediate interpretation alongside likely repair costs and options.
3. If it is cold outside and your boiler is mounted on an external wall or in an unheated utility room, locate the condensate pipe and check whether it is frozen. Warm water poured carefully over the external section can resolve this in minutes.
4. Press the boiler's reset button and hold it for three to five seconds. This clears temporary lockouts and occasionally restores hot water if the original cause was a momentary sensor fault rather than a failed component.
5. If none of the above resolves the problem, the fault is internal. At this point you need a Gas Safe registered engineer. Gas Safe registration is a legal requirement for anyone working on gas appliances in the UK - it is not optional, it is not a guideline, and it is not worth bypassing to save money. You can verify any engineer's registration at the Gas Safe Register website using their ID number before they start work.

Expect a call-out and diagnostic fee in the Amersham area of typically 60 to 100 pounds. Most reputable engineers will deduct this from the final repair bill if you proceed with the work on the same visit.

Myth-Busting Questions

Can I keep using my heating while I wait for an engineer?

In most cases, yes. If your heating is working and the boiler is not showing an error code that causes it to lock out completely, it is typically fine to continue running the central heating while you wait for an engineer. The fault is isolated to the hot water circuit. If your thermostat or programmer allows it, switch the boiler to heating-only mode and turn the hot water schedule off to avoid putting unnecessary demand on the faulty component until it is repaired.

How long does a diverter valve replacement actually take?

Most diverter valve replacements are completed within one to two hours from the point the engineer arrives. An experienced Gas Safe engineer will typically carry the correct valve for common boiler brands like Worcester Bosch, Vaillant, and Ideal Logic, or have one available on next-day delivery from a local merchant. The job involves isolating the boiler, removing the old valve, fitting the new one, and testing both heating and hot water circuits before leaving. Same-day resolution is realistic when you book through a service that prioritises local engineers.

Will my boiler warranty cover a failed diverter valve or heat exchanger?

It depends on the boiler's age and whether the warranty is still active. Most manufacturers offer warranties of five to ten years, but these typically require annual servicing by a Gas Safe registered engineer to remain valid. If your boiler has been regularly serviced and is within the warranty period, contact the manufacturer's customer line before authorising any paid repair - the component and labour may be covered at no cost to you. Always check this first, because a valid claim can save you several hundred pounds.

FAQ

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

The most common reason is a failed or stuck diverter valve. This component directs energy from the boiler either to the central heating circuit or to the domestic hot water heat exchanger. When it fails, the boiler defaults to heating mode only and cannot serve the hot water side at all. A Gas Safe registered engineer can diagnose and replace this part, typically at a cost of between 200 and 400 pounds including parts and labour.

Is it safe to run cold taps and use cold water when the boiler is not producing hot water?

Yes, completely. Your cold water supply runs independently of the boiler and is entirely unaffected by any boiler fault. Cold taps, toilets, and cold feeds to appliances will all work normally. If you need a temporary warm water source, an electric shower heats water independently and will work without a functioning boiler. Do not attempt to open or adjust any internal boiler components yourself - gas-related work must be carried out by a Gas Safe registered engineer.

How much does it cost to fix a boiler that has heating but no hot water in Amersham?

Repair costs vary by fault. A flow sensor or thermistor replacement typically costs between 100 and 200 pounds. A diverter valve replacement usually comes in at 200 to 400 pounds. A plate heat exchanger repair or replacement - particularly relevant given the hard water in parts of Buckinghamshire - commonly costs between 250 and 500 pounds. Call-out and diagnostic fees in the Amersham area typically range from 60 to 100 pounds, and reputable engineers usually deduct this from the total if you proceed with the repair on the same visit.