Boiler Not Heating Water – Causes & Fixes
Few things are more frustrating than turning on the hot tap and getting nothing but cold water. Whether you have no hot water at all or the water is only lukewarm, your boiler is struggling and the cause needs to be identified quickly. At Next Gen Boilers Ltd, our Gas Safe registered engineers in Manchester and Salford diagnose hot water faults every day. Here are the most common causes and what to do about each one.
Quick Answer
Why is my boiler not heating water?
If your boiler is running but not producing hot water, possible causes include a diverter valve fault on a combi boiler, a thermostat or control fault, low pressure, a pump problem, or an issue with a hot-water cylinder system. A diverter valve controls whether a combi sends heat to radiators or a hot tap, but the symptom alone cannot identify the failed part. A Gas Safe engineer in Manchester can diagnose the problem; call 07305 687 331 or use the boiler repair service.
Diverter Valve Fault (Combi Boilers)
If you have a combi boiler and your radiators get hot but you have no hot water from the taps — or vice versa — a diverter valve fault is one possibility. This valve controls whether the boiler is in heating mode or hot water mode. A thermostat, sensor, control or flow problem can produce similar symptoms, so diagnosis is needed before replacing a part.
A diverter valve is not a DIY repair — it requires a Gas Safe registered engineer to diagnose and replace. Once the cause is confirmed, the engineer can explain whether the repair can be completed during the visit and provide the price before work starts.
Low Boiler Pressure
Low pressure affects both heating and hot water performance. Check your boiler's pressure gauge — it should read between 1 and 1.5 bar when cold. If it is below 1 bar, repressurise the system using the filling loop beneath the boiler.
If you are not sure how to repressurise your boiler, call our team on 07305 687 331 and we can walk you through it over the phone. If pressure keeps dropping after you repressurise, there is likely a leak in the system that needs professional attention.
Broken or Misset Thermostat
If your hot water thermostat (often a dial on the boiler itself or a separate cylinder thermostat for system boilers) is set too low, the boiler may think it has already reached the target temperature and switch off before the water is hot enough.
Check that your hot water temperature is set between 55°C and 60°C on the boiler's hot water dial. If you have a separate hot water cylinder with its own thermostat, make sure that is also set correctly. A faulty thermostat will need to be replaced by a Gas Safe engineer.
Boiler Ignition Fault
If your boiler is not firing at all — not just failing to heat water — there may be an ignition fault. You may notice the boiler attempts to start, makes clicking sounds, and then locks out. Common causes include a dirty or failed ignition lead, a faulty gas valve, or low gas pressure.
Try resetting the boiler once and waiting two minutes. If it locks out again, call a boiler repair engineer near you. Repeated attempts to reset a boiler with an ignition fault can cause additional damage.
Warning: If you smell gas at any point, do not attempt to use the boiler. Leave the property, do not use any electrical switches, and call the National Gas Emergency Service on 0800 111 999.
Faulty Pump
Your boiler's pump circulates hot water around your system — to your radiators and your hot water cylinder (on a system or regular boiler). If the pump fails or seizes, heat cannot be distributed effectively. Signs of a pump failure include the boiler firing but radiators or hot water staying cold, unusual grinding or humming noises from the boiler, and overheating of the boiler unit itself.
Pump replacement is a straightforward job for a Gas Safe engineer in Manchester and can usually be done in a single visit. Next Gen Boilers Ltd offer fixed prices for pump replacements with no hidden charges.
Airlock in the System
An airlock occurs when trapped air in your heating system prevents water from circulating properly. This can cause certain radiators or the hot water supply to be affected. On a sealed system (most modern boilers), airlocks can sometimes be resolved by bleeding your radiators. Start from the ground floor and work upward.
If bleeding radiators does not resolve the problem, or if the hot water issue persists, a Gas Safe engineer can carry out a full system diagnosis. Our team cover boiler repair across Manchester, Salford, Irlam, Eccles, and the whole of Greater Manchester. Boiler Care The Next Gen Way.
Frequently Asked Questions
Who should I call if my boiler is not producing hot water?
Call a Gas Safe registered engineer. Next Gen Boilers Ltd cover Manchester, Salford, and Greater Manchester — call 07305 687 331 for same-day diagnosis.
Is it safe to use a boiler that is heating but not producing hot water?
Do not assume it is safe simply because the heating still works. If there is a fault code, repeated lockout, unusual smell or noise, switch the boiler off and seek professional advice. A Gas Safe engineer can confirm whether limited use is appropriate.
How quickly can a hot water fault be repaired?
Most hot water faults — including diverter valve replacements, thermostat faults, and pump failures — can be diagnosed and repaired in a single visit. We carry common parts for all major boiler brands.
When should I replace my boiler rather than repair the hot water fault?
If your boiler is over 10 years old and facing a major repair such as a heat exchanger replacement, a new boiler installation from £1,699 is often the better investment. Our engineers will give you an honest recommendation.
Need a Gas Safe engineer? Call: 07305 687 331 — Manchester & Salford.