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

Published September 2026 | No Hot Water But Heating Works What to Check

We asked our Boiler Repair engineers the questions Alton homeowners ask most. If your radiators are firing up fine but you're standing at the sink waiting for hot water that never arrives, you're dealing with one of the most common boiler complaints we see across Hampshire - and the good news is that it's usually fixable without replacing the whole unit.

Why Does My Boiler Heat the Radiators But Not the Hot Water?

This is the question that kicks off most of our call-outs in Alton, and the answer depends on what type of boiler you have. In a combi boiler - which is the most common type fitted in Hampshire homes - the boiler handles both central heating and domestic hot water from a single unit. There's no separate cylinder. When heating works but hot water doesn't, the most likely culprit is the diverter valve. This valve switches the flow of hot water between your radiators and your taps. When it sticks or fails, it often gets stuck in the heating position, so your radiators get everything and your taps get nothing.

In older system boilers or conventional boilers with a separate hot water cylinder, the problem is usually somewhere in the cylinder circuit rather than in the boiler itself. That might be a failed immersion heater element, a faulty cylinder thermostat, or a motorised valve that's stopped operating. Understanding which type of boiler you have is the first step, because the diagnosis path is quite different for each.

How Do I Know If My Diverter Valve Has Failed?

The diverter valve is a small but critical component inside your combi boiler, and it's one of the parts most likely to cause exactly this symptom. When it fails, it commonly gets stuck in one position. If it's stuck in the central heating position, your radiators work perfectly but hot water from taps runs cold or barely lukewarm. If it's stuck in the domestic hot water position, you get hot water from taps but your heating won't come on - that's a separate but related fault.

A good way to test this at home is to turn your heating off completely and then run a hot tap. If hot water suddenly appears, that's a strong indicator the diverter valve is stuck and isn't switching properly when it should. You might also notice that hot water only comes through when the heating is running, or that it takes an unusually long time to arrive at the tap. Our engineers use the Voltrade GoFIX diagnostic tool to quickly pinpoint whether the valve is the issue before opening the boiler, which saves time and means we can often arrive with the right part already. Diverter valve replacement on a combi typically costs between 150 and 350 pounds depending on the boiler brand and the valve type required.

Could the Plate Heat Exchanger Be Blocked?

In a combi boiler, the plate heat exchanger is responsible for rapidly heating cold mains water as it passes through. It does this by sitting adjacent to the primary circuit water, transferring heat across without mixing the two water supplies. Over time, particularly in areas with hard water, limescale builds up inside the plate heat exchanger and reduces its ability to transfer heat efficiently. In severe cases it can block almost completely.

Hampshire sits in a region with moderately hard water, and Alton is no exception. If your boiler is more than five or six years old and hasn't had any descaling work done, limescale is absolutely worth investigating. The symptoms often include hot water that starts warm and goes cold within a minute or two, or water that only reaches a lukewarm temperature rather than getting fully hot. An engineer can sometimes descale the heat exchanger in situ using a descaling agent, which costs roughly 80 to 150 pounds. In worse cases, replacement is needed, which typically runs between 200 and 500 pounds depending on the boiler model. Brands like Worcester Bosch, Vaillant, and Ideal all have proprietary heat exchangers, so parts costs vary considerably.

What Should I Check on My Programmer and Controls First?

Before calling an engineer, it's worth spending ten minutes checking your boiler's programmer and controls. It sounds basic, but this catches a surprising number of call-outs. Here's a logical sequence to work through:

1. Check that your programmer or smart thermostat has a hot water schedule set and that it's currently within an active time slot.
2. Look at the boiler display for any error or fault codes - consult your manual or search the code with your boiler brand to get a starting point.
3. Confirm the hot water temperature dial on the boiler (usually marked with a tap symbol) is turned up to at least 55 to 60 degrees Celsius.
4. If you have a hot water cylinder with a separate programmer, check that both the programmer and the cylinder thermostat are set correctly.
5. Try switching the boiler off at the isolator, waiting 60 seconds, and switching it back on. Some fault states clear on a reset.

Many Alton homeowners on smart thermostats like Hive or Nest find that a software update or connectivity dropout has reset their hot water schedule without them realising. Worth checking the app directly rather than relying on a schedule you set months ago.

What Is a Thermistor and Could Mine Have Failed?

A thermistor is a temperature sensor inside your boiler that monitors how hot the water is getting. Combi boilers typically have two or more: one for the primary circuit and one specifically for the domestic hot water flow. The hot water thermistor tells the boiler's circuit board when the water passing through has reached the target temperature, so it can regulate the burner accordingly.

When the domestic hot water thermistor fails, it can send incorrect readings to the control board. In some failure modes, it tells the boiler the water is already hot enough when it isn't, so the burner never fires hard enough to produce hot water at the tap. In other cases it causes an error code and the boiler shuts the hot water circuit down as a safety measure. Thermistors are relatively cheap components - typically 20 to 50 pounds for the part - and the labour to replace one is usually an hour or less. Total cost is commonly in the range of 80 to 150 pounds. It's a repair that a Gas Safe registered engineer can carry out quickly, and it's one of the faults our Voltrade GoFIX diagnostics flags reliably when we run a pre-visit check.

I Have a Hot Water Cylinder - What Should I Check?

If your home has a separate hot water cylinder (common in older Hampshire properties and houses that still have a conventional or system boiler), the diagnosis is quite different. Your heating circuit and your hot water circuit are largely independent, which is why the heating can work perfectly while hot water fails.

The most common causes in cylinder-based systems are:

1. A failed immersion heater element. If your cylinder uses an electric immersion heater as a backup or primary source, the element itself may have blown. Replacement typically costs between 100 and 250 pounds including labour.
2. A faulty cylinder thermostat. This sits on the outside of the cylinder and cuts power to the immersion heater once the water reaches a set temperature. If it fails in the open position, the cylinder never heats. Replacement is usually 80 to 150 pounds.
3. A stuck or failed motorised valve. This is the valve that opens to allow boiler heat into the cylinder coil. If it's jammed shut, the cylinder won't heat from the boiler. Motorised valve replacement typically runs 150 to 300 pounds.
4. A failed pump. If the pump that circulates water through your heating circuit has failed, neither heating nor hot water may work properly - though in partial pump failures, one circuit can be affected more than the other.

Can I Check or Fix Anything Myself Before Calling an Engineer?

There are a handful of checks any homeowner can safely do without touching gas components or opening the boiler casing. Anything involving the gas valve, heat exchanger, or internal components must be carried out by a Gas Safe registered engineer - that's a legal requirement, not just a recommendation.

What you can safely check yourself:

1. Check the boiler pressure gauge. Most combi boilers need to sit between 1 and 1.5 bar when cold. If pressure has dropped below 0.5 bar, the boiler may not fire correctly. You can repressurise using the filling loop - your boiler manual will show you how.
2. Look for any visible error codes on the display and note them down before calling an engineer. This gives them a head start.
3. Check that the hot water temperature setting on the boiler hasn't been accidentally turned down.
4. If you have a hot water cylinder, check the thermostat dial is set between 60 and 65 degrees Celsius.
5. Bleed your radiators. It won't directly fix hot water, but if trapped air is causing circulation issues it can have knock-on effects.

Beyond that, please don't attempt to open the boiler casing or interfere with any gas fittings. Gas Safe registration is legally required for any work on gas appliances, and for good reason.

How Much Does It Cost to Fix No Hot Water in Alton?

Costs vary depending on what's actually wrong, but here's a realistic breakdown of what repairs typically cost for Alton homeowners in 2026:

1. Diverter valve replacement: 150 to 350 pounds. More at the upper end for older or less common boiler models where parts are harder to source.
2. Plate heat exchanger descale: 80 to 150 pounds. Full replacement, 200 to 500 pounds.
3. Thermistor replacement: 80 to 150 pounds all in.
4. Motorised valve replacement: 150 to 300 pounds.
5. Immersion heater element: 100 to 250 pounds including labour.
6. Cylinder thermostat: 80 to 150 pounds.

Most diagnostic call-outs in Hampshire include an initial inspection fee, commonly 60 to 100 pounds, which is often absorbed into the repair cost if you proceed. The total for most hot water faults lands between 150 and 400 pounds for a single component repair. Where engineers find multiple issues - for instance, a diverter valve that failed partly because of limescale buildup in the heat exchanger - costs can be higher. Getting a clear quote before authorising any work is always sensible.

Which Boiler Brands Are Most Prone to This Fault?

No boiler brand is immune to diverter valve or heat exchanger issues, but our engineers do see certain patterns across Alton properties. Worcester Bosch boilers - one of the most widely fitted brands in the area - occasionally develop diverter valve problems in units that are seven or more years old, particularly the older Greenstar range. Vaillant ecoTEC models can develop thermistor faults and sometimes plate heat exchanger scaling issues. Baxi and Ideal boilers occasionally present with motorised valve failures in older installations.

For cylinder-based systems, the brand of the cylinder matters less than its age and whether it's been serviced. Immersion heater elements in cylinders from manufacturers like Megaflo or Telford typically last eight to twelve years before they start failing, though water quality plays a significant role. If you've got hard water and no water softener, elements can calcify and fail earlier. That's something worth considering for Alton properties given local water hardness levels.

How Can I Prevent This Happening Again?

Keeping your boiler and hot water system in good shape reduces the chance of this fault recurring. The single most effective thing you can do is book an annual boiler service. During a service, a Gas Safe engineer will check the heat exchanger for early signs of scaling, test the diverter valve operation, check thermistor readings, and inspect the condition of internal components. Catching a part that's starting to degrade before it fails completely is far cheaper than an emergency call-out.

For homes in Alton with hard water, fitting an inline scale reducer or magnetic filter to the boiler's cold water inlet can significantly reduce limescale buildup in the plate heat exchanger. Filters like the Fernox TF1 or MagnaClean are commonly fitted during services and cost around 80 to 150 pounds to install. Power flushing the central heating system every five to eight years also helps keep the circulation clean and protects components including the pump and motorised valves. Annual service costs in Hampshire typically run between 80 and 120 pounds - a fraction of most repair bills.

If your heating is working fine but hot water has stopped, the most common cause across Alton properties is a diverter valve or plate heat exchanger issue in combi boilers, or a motorised valve or immersion heater fault in cylinder-based systems. Most of these repairs are well within the scope of a single visit from a Gas Safe registered engineer, and catching them early keeps costs manageable.

Is it safe to keep using my boiler if the heating works but there's no hot water?

In most cases, yes - a boiler with a stuck diverter valve or failed thermistor isn't a gas safety hazard in itself. However, you should still get it checked promptly. Leaving a faulty component in place can cause additional wear on other parts, and some fault states do carry a small risk of further damage if ignored.

How quickly can a Gas Safe engineer diagnose the fault?

Most experienced engineers can narrow down the cause of a no hot water fault within 30 to 45 minutes of arriving, particularly if you've noted any error codes in advance. Using diagnostic tools like the Voltrade GoFIX system can speed this up further by providing pre-visit fault data to the engineer before they've even knocked on the door.

Will my boiler warranty cover a diverter valve or thermistor repair?

It depends on the age of your boiler and the terms of your warranty or boiler care plan. Most manufacturer warranties cover parts and labour for five to ten years from installation. If your boiler is within warranty, contact the manufacturer directly first. Unauthorised repairs by non-approved engineers can sometimes void a warranty, so check your documentation before booking any third-party repair.

Can limescale really stop hot water working even if my boiler seems to be running fine?

Absolutely. A partially blocked plate heat exchanger can allow the boiler to fire normally and heat the primary circuit - which feeds your radiators - while failing to transfer enough heat to the domestic hot water passing through. The boiler appears to work, your radiators are warm, but the heat simply isn't getting to your taps. Descaling resolves this in most cases where the blockage isn't too severe.