No Hot Water But Heating Works in Barnsley - What to Actually Check

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

Most people assume that if their heating is running fine, their boiler must be working properly - and that assumption sends them in completely the wrong direction. When your radiators are warm but your taps run cold, there's nearly always one specific component at fault. And in most cases, you don't need a new boiler to fix it.

Myth: If Your Heating Works, Your Boiler Is Fine

The reality

This is the single most common misconception our engineers encounter across Barnsley and South Yorkshire, and it's understandable. If the boiler's firing and heat is reaching the radiators, surely the main unit is working? Not quite.

A combi boiler manages heating and hot water through separate internal pathways. The component that switches between them is called a diverter valve. When your thermostat calls for heating, the diverter valve routes hot water around the central heating circuit. When you open a hot tap, it's supposed to redirect that flow to supply domestic hot water instead.

If the diverter valve sticks - which is one of the most frequently reported faults in combi boilers across the UK - it can lock itself in the heating position. Your radiators keep working perfectly. Your hot water taps run cold. The boiler is technically firing, but one internal component has failed and is holding the system in heating mode.

This is not a boiler replacement job. A diverter valve repair or replacement typically costs between 150 and 350 pounds depending on your boiler's make and model. BOYLA's hot water diagnostic guide confirms that diverter valve faults sit among the most frequent causes of this exact symptom pattern - heating on, hot water gone.

Myth: No Hot Water Means You Need a New Boiler

The reality

When a Barnsley homeowner rings us to say their hot water has stopped while the heating runs normally, there's often a fear that this is the end of the road for their boiler. In most cases, that fear is unfounded.

When heating works but hot water doesn't, the fault is commonly traced to one of a handful of specific components. Here's what our engineers check first:

1. A faulty diverter valve - the most frequent cause in combi systems
2. A failed thermistor - the temperature sensor that tells the boiler when water has reached the right heat
3. A blocked or failing plate heat exchanger - the part that heats domestic hot water in a combi boiler
4. A stuck motorised zone valve - more relevant in conventional or system boiler setups with hot water cylinders
5. A faulty programmer or timer that isn't correctly calling for hot water

A thermistor replacement typically costs between 100 and 200 pounds for parts and labour. A plate heat exchanger is more involved - expect between 250 and 550 pounds in most cases - but that's still well below the cost of a new boiler. Plumbing Correct's breakdown of nine likely causes for this symptom is a useful reference, and it highlights how many different faults can produce the same result. Getting the right diagnosis first is the only way to avoid paying for the wrong repair.

Myth: Checking Boiler Pressure Will Reveal the Problem

The reality

Checking boiler pressure is a sensible reflex when something goes wrong, and our engineers always encourage homeowners to know where their pressure gauge is. But there's a common leap from "the pressure looks fine" to "pressure definitely isn't the cause" - and then confusion about what to look at next.

Here's the nuance: low boiler pressure typically causes both heating and hot water to fail together, or triggers a complete boiler lockout. If your heating is running normally and only the hot water has stopped, pressure is very unlikely to be the direct cause. The heating and domestic hot water circuits in a combi boiler are managed separately enough that a pressure fault doesn't usually knock out one while leaving the other unaffected.

What's actually worth checking is your boiler's display panel. Most modern boilers from Worcester Bosch, Baxi, Ideal, Vaillant, and Glow-worm display fault codes when a component fails. That code is your starting point - it tells an experienced engineer which direction to look before they open the casing. If the display is blank or flashing without a code, note that too; it still carries diagnostic information worth passing on when you call.

The Voltrade GoFIX diagnostic tool gives our engineers a structured way to cross-reference reported fault codes and symptoms before they arrive on site, which cuts diagnostic time considerably and means fewer open-ended exploratory visits.

Myth: Turning It Off and Back On Again Will Sort It

The reality

Resetting your boiler is a legitimate first step and our engineers do recommend trying it once. But the distinction that matters is this: a reset is a diagnostic step, not a fix. Treating it as a solution will leave you in the same position a week later - or sooner.

If a reset brings your hot water back and it stays working, great - monitor it closely. If the problem returns within hours or days, the reset cleared a fault code temporarily while the underlying cause remained untouched.

Some faults do temporarily respond to resets, for specific reasons:

• Thermistors that are degrading but not yet completely dead can cause intermittent faults that clear on reset, then return as the sensor continues to fail
• Diverter valves that stick but free up when the system cools and pressure normalises
• Genuine control board glitches that a reset actually resolves - though this is the least common explanation by some margin

If you're resetting more than once or twice for the same symptom, you need an engineer. Repeated resets without addressing the root cause can also mask a fault that's actively worsening, which makes proper diagnosis harder and potentially more expensive when the engineer does arrive.

Myth: Cylinder-Based Systems Are Far More Complicated to Fix

The reality

There's a persistent belief that if you've got a conventional or system boiler feeding a hot water cylinder in the airing cupboard, any fault is automatically going to be complicated and expensive. That's not how it works in practice.

Because the hot water system is physically separate from the heating circuit in these setups, faults are often more targeted to identify - not less. When heating is working but the cylinder isn't producing hot water, here's what to check:

1. The cylinder thermostat setting - it should typically sit between 60 and 65 degrees Celsius, which covers both efficiency and legionella prevention
2. The immersion heater element - this can fail completely independently of the boiler, and replacement typically costs between 80 and 180 pounds
3. The motorised zone valve serving the cylinder - if this has seized in the closed position, no hot water will flow in to be heated regardless of what the boiler is doing
4. The programmer or timer settings - verify that hot water is actually being called for at the right times of day

In Barnsley properties with older conventional boiler systems, the motorised zone valve is a frequent culprit. These valves have a limited lifespan and can seize up over time, particularly in systems that haven't been serviced regularly. Replacement typically costs between 120 and 250 pounds - a far cry from a system overhaul.

Gas Safe Solutions notes that this fault pattern - heating working, cylinder cold - is commonly encountered in South Yorkshire, particularly in homes with older system configurations where zone valves have been in service for many years without attention.

What Actually Matters - Expert Advice

When your heating runs fine but your hot water has stopped, here's how an experienced engineer works through the problem - and what you can do before picking up the phone.

Step 1 - Read the fault code. Before touching anything, note what your boiler display is showing. Take a photo with your phone if you can; it speeds up the call considerably when you ring for help.

Step 2 - Identify your boiler type. Is it a combi (compact unit only, no cylinder), a system boiler (with a sealed cylinder but no loft tanks), or a conventional boiler (cylinder plus a feed-and-expansion tank in the loft)? Each type has different likely fault causes for this specific symptom.

Step 3 - Check the obvious settings first. Is your programmer or smart thermostat actually set to provide hot water? Has someone inadvertently turned off the immersion heater wall switch, or adjusted the cylinder thermostat down? These checks take two minutes and occasionally reveal that no repair is needed at all.

Step 4 - Try a reset once. Power the boiler off, wait two minutes, power it back on. If hot water returns and remains stable, monitor it closely. If the problem returns, move to step 5 rather than resetting again.

Step 5 - Call a Gas Safe registered engineer. Any internal boiler repair legally requires a Gas Safe registered engineer under the Gas Safety (Installation and Use) Regulations 1998. This isn't optional and it isn't a technicality - gas work carried out by an unregistered person is illegal. You can verify any engineer's current registration on the Gas Safe Register website before they start work, and you should.

A diagnostic callout in Barnsley typically costs between 60 and 120 pounds, with repair costs added on top depending on findings. Getting a proper diagnosis before ordering parts is almost always cheaper than guessing.

Myth-Busting Questions

Can I repair a diverter valve myself to cut costs?

Is losing hot water a sign that my boiler is dangerous to use?

Not automatically. A failed diverter valve, thermistor, or motorised valve is a functional fault, not a combustion problem, and doesn't typically create an immediate safety risk on its own. That said, if you can smell gas, hear unusual noises from the boiler, or notice the boiler flame appears yellow or orange rather than blue, those are separate and serious warning signs. In that case, call the National Gas Emergency Service on 0800 111 999 and do not use the boiler until it has been assessed by a Gas Safe registered engineer.

How long does a hot water repair typically take?

Most component replacements - diverter valves, thermistors, motorised zone valves - take between one and three hours for an experienced engineer. Plate heat exchanger replacements can take longer depending on the boiler model and how accessible the casing is. Engineers working across Barnsley and the wider region commonly carry parts for popular makes including Worcester Bosch, Baxi, and Ideal, which reduces the need for return visits and gets your hot water back faster.

Frequently Asked Questions

Why does my boiler show no fault code even though my hot water has stopped?

Not all faults trigger a fault code, particularly on older boiler models or when a component is failing gradually rather than failing completely all at once. A diverter valve that has stuck but hasn't fully seized may not generate a code - the boiler continues operating in heating mode without flagging a problem. This is one reason why a physical inspection by a Gas Safe engineer is important even when your boiler's display looks completely normal.

How often should a boiler be serviced to help prevent faults like this?

An annual service is the standard recommendation, and most boiler warranties require it to remain valid. A proper service includes checking internal components like the diverter valve and thermistor, cleaning the heat exchanger, and verifying that all safety controls are functioning correctly. Regular servicing doesn't prevent every fault, but it does catch components that are beginning to degrade before they fail entirely - which typically means a cheaper repair when the time comes.

At what point does it make more financial sense to replace a boiler rather than repair it?

As a general rule, if your boiler is over 12 years old and facing a repair that costs more than a third of what a replacement would cost, it's worth getting an engineer to assess both options. Older boilers also tend to be significantly less efficient than modern condensing models, which means a replacement can reduce energy bills enough to offset some of the upfront cost. That said, many boilers run reliably well beyond 15 years with consistent servicing - so age alone isn't the deciding factor.