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

Published October 2026 | No hot water but heating works what to check

We asked our Boiler Repair engineers the questions Banstead homeowners ask most. If your radiators are firing up fine but every hot tap in the house runs cold, this guide works through the most common causes - from things you can check yourself in five minutes to faults that need a Gas Safe registered engineer.

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

Modern combi boilers handle two separate jobs: keeping your central heating circuit warm and heating domestic hot water on demand. When one works and the other doesn't, it usually means the problem sits in a component that bridges those two functions rather than in the boiler's core heat exchanger itself.

The most common culprit our engineers find in Banstead homes is the diverter valve - a motorised valve that routes hot water either to the radiators or to the hot water circuit depending on what you've asked for. When it sticks or fails, it often locks onto one circuit. If it locks onto heating, your radiators stay warm while every tap in the house runs cold.

That said, it's not always the diverter valve. Programmer settings, thermistors, motorised zone valves, and faults specific to system boilers with hot water cylinders can all produce the same symptom. The key is working through the checks in the right order rather than guessing. That's exactly how our engineers approach it when they arrive at a job in Banstead - systematic, not random.

What should I check myself before calling an engineer?

Before you book a call-out, there are a few safe checks worth doing yourself - no tools, no boiler casing, just a bit of observation.

1. Check your programmer or timer settings. A surprisingly large number of call-outs turn out to be a programmer that's been switched to "heating only" mode, or had its schedule disrupted by a power cut. Check that hot water is enabled and that the time bands are set correctly for your household routine.

2. Look at the boiler pressure gauge. Most combi boilers need to sit between 1 and 1.5 bar when cold. If the needle has dropped below 1, low pressure can restrict hot water output while central heating continues to run. You can repressurise via the filling loop yourself - your boiler manual covers the steps, and it takes about two minutes.

3. Check the hot water temperature dial. Some boilers have a separate dial for hot water temperature. If it's been accidentally turned right down, you'll get cold water from the taps even though the boiler itself is functioning normally.

4. Reset the boiler. A simple reset clears fault codes that can lock out the hot water circuit. Hold the reset button for a few seconds, allow the boiler to complete its restart cycle, then test the hot water again.

If none of those make a difference, you're most likely dealing with a component fault that needs a professional diagnosis.

What is a diverter valve and how do I know if mine has failed?

The diverter valve is a small motorised or mechanical valve inside your combi boiler that switches between sending heat to the radiators and sending it to the plate heat exchanger for your hot water. Think of it like a set of points on a railway - it routes flow to whichever system is calling for it at any given moment.

When a diverter valve fails, it typically does so in one of two ways. It either sticks fully in the heating position (so hot water never gets directed to the taps), or it jams in a mid-position where it leaks flow between both circuits. The mid-position failure is the more subtle one: you might get lukewarm water some of the time, or your heating might feel slightly less effective than usual, because the valve is bleeding flow across both circuits simultaneously.

The classic signs of a diverter valve fault are: radiators heat up normally, the boiler fires when you open a hot tap, but the water at the tap never actually gets warm. If that matches what you're seeing, the diverter valve is the most likely diagnosis.

Diverter valve replacement typically costs between 200 and 350 pounds including parts and labour. Worcester Bosch and Vaillant parts tend to be readily available and sit at the lower end of that range. Older Ideal or Potterton models can sometimes take longer to source, which can push the cost slightly higher. UK repair cost guidance from BOYLA broadly supports this range for diverter valve work.

Could it be the thermistor or a temperature sensor fault?

Yes - and this is one our Banstead engineers encounter more than homeowners might expect. A thermistor is a small sensor that tells the boiler what temperature the water currently is. If the thermistor on the domestic hot water circuit gives a faulty reading, the boiler's control board thinks the water is already at temperature and stops firing for hot water - even though it's cold at the tap.

The heating circuit has its own separate thermistor, which is why heating can continue to work perfectly while hot water fails. The two circuits are independently monitored.

A faulty thermistor will often trigger a fault code on your boiler display. On Worcester Bosch boilers, codes in the E5 or E9 series often relate to sensor faults. Vaillant uses similar error codes. Baxi and Ideal boilers have their own code sets, but sensor-related codes are usually flagged distinctly. If you've got a fault code showing on the display, note it down before you call - it helps our engineers arrive with the right parts rather than diagnosing from scratch.

Thermistor replacement is usually one of the less expensive boiler repairs, typically between 100 and 200 pounds all in. The part itself is inexpensive; the cost reflects the engineer's time and properly Gas Safe-compliant work.

What if I have a system boiler with a separate hot water cylinder?

If your home runs a system boiler rather than a combi, the picture is a bit different. A system boiler heats water and stores it in a separate cylinder - commonly in an airing cupboard. When hot water fails while heating keeps working, the fault often sits within the cylinder controls rather than in the boiler itself.

The first thing to check is the cylinder thermostat - a strap-on thermostat clipped to the side of the cylinder. If it has failed or been set too low, it signals to the boiler that the cylinder is already up to temperature, and the boiler never fires to heat it. Try turning it up to around 60 degrees Celsius and see whether the boiler then fires for hot water.

The motorised zone valve (sometimes called a mid-position valve or two-port valve) is the other common failure point in system setups. This valve directs hot water to the cylinder when heat is called for. If it jams or the actuator fails, the cylinder never gets heated regardless of what the boiler is doing. Zone valve replacement typically costs between 150 and 250 pounds.

As a temporary measure while you wait for an engineer, most cylinders have an electric immersion heater. Switching it on will heat the stored water independently of the boiler, giving you hot water in the short term without affecting anything that needs fixing.

How much does fixing no hot water typically cost in Surrey?

Repair costs vary depending on what's actually wrong, but here's a realistic breakdown based on what our engineers see across Surrey and the wider South East:

• Diverter valve replacement: typically 200 to 350 pounds
• Thermistor or temperature sensor: typically 100 to 200 pounds
• Motorised or zone valve: typically 150 to 250 pounds
• Cylinder thermostat: typically 80 to 150 pounds
• Printed circuit board (PCB): typically 300 to 500 pounds
• Call-out fee: typically 60 to 100 pounds for the initial hour

These are parts-and-labour figures. If your boiler is over 12 to 15 years old, it's worth having a frank conversation with the engineer about whether a repair makes financial sense compared to a replacement. A new boiler installation in a typical Banstead property typically costs between 1,800 and 3,500 pounds depending on boiler type, flue route, and installation complexity. 0800 HomeFix's UK heating guide notes similar cost ranges for these common fault types.

Running the Voltrade GoFIX diagnostic tool before booking a call-out can give you a more accurate estimate based on your specific boiler model and symptoms, so you're not walking in blind on costs.

Does low boiler pressure only affect hot water or heating too?

Low boiler pressure typically affects both circuits, but not always equally or at the same time. Some combi boilers are configured to prioritise the central heating circuit under low pressure conditions, which can create the impression that heating is working fine while hot water has dropped out entirely. This is why pressure is one of the first things our engineers check even when the heating appears normal.

The standard operating pressure for most combi boilers sits between 1 and 2 bar, with 1.2 to 1.5 bar being the optimal range when the system is cold. If your gauge reads below 1, repressurise via the filling loop following your boiler manual's instructions.

If the pressure drops back down within a few days of being topped up, you've got a leak somewhere in the system - possibly at a radiator valve, a pipe joint under the floor, or within the boiler casing itself. Persistent pressure loss needs investigating by a Gas Safe registered engineer. Don't keep topping it up and ignoring the root cause - it masks a developing problem that can become a significantly bigger repair if left.

What should I never attempt to fix myself?

Our engineers are direct about this: anything involving the gas supply, the internal components of the boiler, or the gas pipework is not a DIY job. Under UK law, gas work must be carried out by a Gas Safe registered engineer. This is a legal requirement, not a recommendation. Unregistered persons carrying out gas work can face prosecution, and more critically, unsafe gas repairs can be life-threatening.

What you can safely do yourself without any risk:

• Adjust programmer and timer settings
• Repressurise the boiler via the external filling loop
• Reset the boiler using the reset button
• Adjust the cylinder thermostat on a system boiler
• Switch on the immersion heater as a temporary measure

What you should never attempt without Gas Safe registration:

• Opening the boiler casing
• Working on gas pipework or valves
• Replacing any internal boiler component
• Adjusting the gas valve or burner

You can verify any engineer's Gas Safe registration on the official Gas Safe Register website before they start work. All Voltrade engineers carry Gas Safe ID and can provide their registration number on request.

Could the fault be with the boiler's printed circuit board?

A PCB failure is less common than a diverter valve or thermistor issue, but it does happen - particularly on boilers that are approaching or past the 10-year mark. The PCB is the boiler's control system, managing both circuits and all the sensors connected to them. When it develops a fault, it can produce irregular behaviour including hot water cutting out while heating continues, intermittent operation, or fault codes that appear and then clear themselves.

That intermittent pattern is the giveaway. If your hot water works perfectly for a day or two, then stops for no obvious reason, then comes back, that's a classic sign of an electrical fault developing on the board rather than a simplely failed mechanical component.

PCB replacement sits at the more expensive end of boiler repairs - typically 300 to 500 pounds - which makes the repair-versus-replace calculation more pressing on an older boiler. Our engineers in Banstead will always give you an honest assessment of whether the repair represents good value given the boiler's age and overall condition. There's no point spending 400 pounds on a PCB in a 14-year-old boiler that's likely to have another fault within the year.

How does the Voltrade GoFIX tool help before an engineer arrives?

The Voltrade GoFIX diagnostic tool is built to help homeowners across Banstead and Surrey get a clearer picture of what's wrong before an engineer comes out. You work through a series of structured questions about your boiler - what's working, what isn't, any fault codes on the display, the boiler's age and brand - and GoFIX returns a prioritised list of likely faults based on your answers.

The practical benefit is that our engineers can arrive with the right parts already in the van. A job that requires a return visit because a specific Vaillant or Worcester Bosch component wasn't stocked costs you time and potentially an additional call-out charge. GoFIX reduces that risk by giving our dispatch team the information they need to prepare properly.

It also gives you a realistic cost estimate before anyone comes through the door, which is useful if you're trying to decide between a repair and a replacement, or just want to understand what you're likely to be quoted. GoFIX doesn't replace the engineer's physical assessment - some faults can only be confirmed once the boiler is opened and tested - but it meaningfully improves the whole process for everyone involved.

If your heating is working but you've got no hot water, the most likely culprits are a stuck diverter valve, a failed thermistor, or a problem with the programmer or cylinder controls. Start with the simple checks - programmer settings, boiler pressure, a reset - and if those don't resolve it, get a Gas Safe registered engineer in to properly diagnose the fault. Most repairs in Banstead homes fall in the 100 to 350 pound range, and with the right information upfront, a good engineer will typically resolve it in a single visit.

Can I still use my heating while waiting for an engineer to fix the hot water?

Yes, in most cases you can continue using your central heating while the hot water fault is being investigated. The two circuits are largely independent in a combi boiler, and running the heating won't worsen the hot water fault or cause any safety concern. If the engineer has advised you to shut the boiler down entirely, follow that guidance - but absent specific advice, heating is usually fine to keep running.

Why does my boiler fire up when I run a hot tap but the water stays cold?

This is a classic sign of a diverter valve fault. The boiler is detecting the demand for hot water and firing correctly, but the diverter valve isn't routing the heated water to the hot water circuit - it's remaining stuck on the heating circuit. You can hear the boiler working, but the heat is going nowhere useful. A Gas Safe engineer will need to replace the diverter valve to resolve it.

How long does a typical boiler hot water repair take in Banstead?

Most common hot water repairs - diverter valve, thermistor, zone valve, cylinder thermostat - take between one and two hours once the engineer is on site with the right parts. A PCB replacement can take slightly longer. If parts need ordering rather than being carried on the van, you may need a return visit, which is another reason the GoFIX diagnostic tool is worth using before booking.

Does a no hot water fault always mean the boiler needs replacing?

Not at all. The majority of no hot water faults are caused by specific failed components rather than fundamental boiler failure, and most are repairable at reasonable cost. The exception is an older boiler where the repair cost approaches or exceeds a meaningful fraction of a new installation. Our engineers will give you a clear assessment of both options so you can make an informed decision rather than feeling pushed towards either outcome.