Boiler not heating hot water? Common causes and fixes
Few things are more frustrating than turning on the tap and getting nothing but cold water, especially when the central heating seems to be working perfectly fine. If your boiler is producing heating but not hot water, or if the hot water is lukewarm and inconsistent, there are several common causes — and some of them you can check yourself before calling an engineer.
We deal with this issue regularly in homes across Sussex, from Victorian terraces in Brighton to modern builds in Crawley. Here is a practical guide to what might be going wrong and what to do about it.
1. Faulty diverter valve
This is the single most common cause of a combi boiler producing heating but not hot water. The diverter valve is the component inside the boiler that directs hot water either to your radiators or to your taps and shower. When you turn on a hot tap, the diverter valve is supposed to switch the flow away from the heating circuit and towards the domestic hot water outlet.
If the diverter valve is stuck, worn, or faulty, it may fail to redirect the water properly. The result is that the boiler continues heating the radiators but does not send hot water to your taps. In some cases, you might get lukewarm water because the valve is partially stuck, allowing some hot water through but not enough.
Can you fix this yourself?
Unfortunately, no. The diverter valve is an internal boiler component and must be repaired or replaced by a Gas Safe registered engineer. It is one of the more common combi boiler repairs and typically costs between £150 and £350 depending on the boiler model and whether the valve itself needs replacing or just its cartridge.
2. Thermostat issues
Your boiler relies on thermostats to know when to fire up and when to stop. If your room thermostat or the boiler's own internal thermostat is set incorrectly, malfunctioning, or has dead batteries, the boiler may not be receiving the right signals.
On some boilers, the hot water temperature is controlled separately from the heating. Check whether the hot water temperature dial or setting on the boiler itself has been accidentally turned down. This sounds obvious, but it is a surprisingly common cause of the problem.
Can you fix this yourself?
Yes, in many cases. Start by checking the following:
- Make sure the thermostat batteries are not dead (wireless thermostats especially)
- Check that the hot water temperature setting on the boiler is turned up to an appropriate level
- If you have a programmer or timer, verify that the hot water schedule is set correctly and that the clock has not reset
If adjusting these settings resolves the problem, no engineer visit is needed. If the thermostat appears to be working but the boiler is still not responding correctly, the issue is likely internal and requires professional diagnosis.
3. Airlocks in the system
An airlock occurs when air becomes trapped in the pipes or the boiler itself, blocking the flow of water. This can prevent hot water from reaching your taps even though the boiler is firing. Airlocks are more common after the system has been drained for maintenance or if a radiator has been removed and refitted.
Can you fix this yourself?
Sometimes. If you suspect an airlock in a specific tap, you can try this method: attach a length of hose between the hot and cold taps on the same sink, turn on the cold tap fully, then open the hot tap. The mains pressure from the cold supply can sometimes push the airlock through the system. Run it for a few minutes and then check.
For airlocks within the boiler itself or deeper in the pipework, a Gas Safe engineer will need to bleed and repressurise the system properly.
4. Low boiler pressure
If your boiler pressure has dropped below 1 bar, many modern boilers will stop producing hot water as a safety measure. Check the pressure gauge on the front of your boiler. If the needle is in the red zone or below 1 bar, the system needs repressurising.
Can you fix this yourself?
Yes. Locate the filling loop — usually a braided silver hose beneath the boiler — and slowly open both valves until the gauge reads between 1 and 1.5 bar. Close both valves firmly afterwards. If the pressure keeps dropping repeatedly, there may be a leak in the system that needs professional attention. Our guide on boiler losing pressure covers this in more detail.
5. Frozen condensate pipe
During cold weather, the condensate pipe — which carries acidic water from the boiler to an external drain — can freeze. When this happens, the boiler detects the blockage and shuts down as a safety precaution, meaning no heating and no hot water.
Can you fix this yourself?
Yes. Locate the condensate pipe, which is usually a white or grey plastic pipe running from the boiler to an outside drain. Pour warm (not boiling) water over the frozen section to thaw it. Once the ice has melted, reset the boiler and it should fire up again. If the pipe keeps freezing, an engineer can reroute it or add insulation to prevent it happening repeatedly.
6. Faulty heat exchanger
The heat exchanger is the core component of your boiler. It transfers heat from the gas burner to the water. If the heat exchanger is cracked, blocked with limescale, or failing, the boiler may produce heating at a reduced level but struggle to heat domestic hot water effectively.
In hard water areas like Sussex, limescale build-up inside the heat exchanger is a common issue, particularly in boilers that have not been fitted with a scale inhibitor or magnetic filter.
Can you fix this yourself?
No. A faulty heat exchanger is a serious repair that must be carried out by a Gas Safe registered engineer. Depending on the boiler's age and the cost of the part, this repair can run from £300 to over £500. If the boiler is more than 10 years old and the heat exchanger has failed, it is often more cost-effective to consider a boiler replacement rather than a repair.
7. Motorised valve failure (system boilers)
If you have a system boiler with a hot water cylinder, your setup uses motorised zone valves to control the flow of water between the heating circuit and the hot water cylinder. If the motorised valve responsible for the hot water circuit fails or gets stuck, hot water will not reach the cylinder.
Can you fix this yourself?
On some older systems, a stuck valve can sometimes be freed by gently moving the manual lever on the valve head. However, if the motor has failed, the valve will need to be replaced by a qualified engineer. This is a relatively common and straightforward repair, typically costing between £100 and £250.
When to call an engineer
As a general rule, if you have checked the thermostat, repressurised the system, and ruled out a frozen condensate pipe without resolving the problem, it is time to call a Gas Safe registered engineer. Internal boiler faults such as diverter valve failures, heat exchanger issues, and motorised valve problems require professional diagnosis and repair.
If your boiler is over 10 years old and experiencing these kinds of issues, it is also worth having an honest conversation about whether a repair or a replacement makes better financial sense in the long term.
FAQ
Why does my boiler heat radiators but not hot water?
The most common cause in a combi boiler is a faulty diverter valve. This valve switches the flow between heating and hot water, and when it fails, the boiler continues to heat the radiators but does not redirect water to your taps. In a system boiler, the cause is often a stuck or failed motorised zone valve. Both require a Gas Safe engineer to diagnose and repair.
Can I fix my boiler's hot water problem myself?
Some issues you can check and resolve yourself: thermostat settings, repressurising the system, thawing a frozen condensate pipe, and clearing simple airlocks. However, internal faults like diverter valve failure or heat exchanger problems must be handled by a Gas Safe registered engineer. Never attempt to open the boiler casing or work on gas components yourself.
How much does it cost to fix a boiler that is not heating hot water?
Costs vary depending on the fault. A diverter valve replacement typically costs £150 to £350. A motorised valve replacement is usually £100 to £250. A heat exchanger replacement can be £300 to £500 or more. If the repair cost is approaching half the price of a new boiler and the unit is over 10 years old, replacement is usually the more economical option.
Is it dangerous if my boiler stops producing hot water?
A boiler that stops producing hot water is not usually dangerous in itself, but it can indicate an underlying fault that should be investigated. If you smell gas, notice a yellow or orange flame instead of blue, or your carbon monoxide alarm sounds, turn off the boiler, open windows, leave the property, and call the Gas Emergency Service on 0800 111 999 immediately.
Should I replace my boiler if it keeps losing hot water?
If the problem is recurring and your boiler is over 10 years old, replacement is often the better long-term decision. Repeated repairs on an ageing boiler add up, and a new A-rated boiler will be more efficient, more reliable, and come with a manufacturer warranty. Get in touch with Platinum Boilers for a free, no-obligation assessment of your current setup.
If your boiler has stopped heating hot water and you are in Sussex, Platinum Boilers can help. We provide fast, professional diagnostics, repairs, and new boiler installations across the whole of East and West Sussex, from Brighton and Hove to Eastbourne, Worthing, and Crawley. Get in touch today for honest advice and a fixed-price quote.
