If your radiators are heating up as normal but you’re standing at the sink with cold water running, you’re not alone. This is a common issue with combi boilers that many homeowners face. Several factors could be causing this problem, from simple settings and flow issues to parts like the diverter valve or heat exchanger not working properly. Understanding the possible causes can help you spot what’s wrong and decide when to call a professional for a quick fix.
First checks when heating works but hot taps are cold
Before worrying about expensive repairs, there are a few quick and safe checks you can do yourself. These can help you rule out simple setting or pressure issues, and give an engineer useful information if you do need to call one.
Always read your boiler manual alongside these steps and never remove the boiler case or touch internal parts. Anything involving gas, the burner or wiring must be left to a Gas Safe engineer.
Check the boiler is in the right mode: Ensure it is set for heating and hot water, not heating only.
Confirm the pressure: Look at the pressure gauge when the boiler is cold and see if it is within the normal range shown in your manual.
Run one hot tap fully open: Check if the boiler fires up when a single hot tap is run at full flow.
Note any error lights or codes: Write down any flashing lights, fault codes or patterns on the display.
Boiler controls, thermostats and programmers
If your heating works, the boiler is clearly capable of firing, which is good news. The issue may be with how it is being asked to provide hot water rather than with the main burner itself.
On a combi boiler, hot water is usually provided on demand when a hot tap is turned on. Check that:
The boiler is not set to a summer or heating-only mode, which can sometimes disable stored or priority hot water functions. Also, make sure any wall thermostats or programmers are not set in a way that accidentally restricts hot water times.
If your boiler has separate temperature controls for heating and hot water, ensure the hot water dial has not been turned right down. Adjust it gradually rather than spinning it from minimum to maximum in one go.
System pressure, flow and tap aerators
Combi boilers need a suitable system pressure and adequate water flow to operate properly. Low pressure or very low flow can stop the boiler recognising that a hot tap is open.
With the boiler cold, look at the pressure gauge and compare it to the range in your manual. If the needle is outside this range or swinging around a lot, do not keep resetting the boiler. Make a note of the reading and speak to a Gas Safe engineer.
Flow can also be restricted at the tap itself. Modern kitchen and bathroom taps often have aerators or flow restrictors fitted. If they are badly blocked with limescale or debris, the flow rate might be too low for the boiler to detect.
Try removing the aerator on one hot tap, if it is user-removable, and run the tap fully open. If the boiler now fires and you get hot water, the tap end may simply need cleaning or replacing. If you are unsure how to remove it safely, leave it for a plumber.
Common combi boiler hot water faults in more detail
Diverter valve issues when heating works
The diverter valve is a mechanical part inside the boiler that directs heat either to the radiators or to the hot water side. If your heating works but your taps are cold, the valve could be stuck prioritising the radiators.
Signs can include radiators getting hot when you run a hot tap, or lukewarm tap water that never reaches a proper hot temperature. The diverter valve is an internal gas appliance component, so testing or replacing it must always be done by a Gas Safe engineer.
Domestic hot water flow sensor faults
When you open a hot tap, a flow sensor tells the boiler to fire up for hot water. If that sensor is faulty or dirty, the boiler may not realise that you are demanding hot water.
You may notice that the boiler does nothing when you open a hot tap, even though pressure and flow seem fine. Again, internal sensors should not be tampered with by homeowners, but it is helpful to describe these symptoms to your engineer.
Blocked plate heat exchanger and limescale
In hard water areas, limescale can build up inside the boiler. The plate heat exchanger, which transfers heat from the boiler water to your tap water, is particularly vulnerable.
Typical signs are fluctuating hot water temperature, hot water that starts warm then cools quickly, or good heating performance but poor hot water flow. A Gas Safe engineer can clean or replace the plate heat exchanger and advise on protecting against future limescale.
Safe reset steps and what to note
If your manual states that a basic reset is safe, you can usually press the reset button once and wait to see if the boiler restarts normally. Never repeatedly reset a boiler that keeps locking out.
Before resetting, write down any error codes or note any flashing light patterns. If the fault returns, tell the engineer what you saw and how often it happens. This can speed up diagnosis and potentially reduce labour time.
Only reset once, following the manufacturer instructions.
Stop using the boiler if it keeps locking out or shows the same fault repeatedly.
Turn the boiler off and call an engineer if you smell burning or see scorch marks.
When to call a Gas Safe engineer
Some symptoms mean you should stop DIY checks and get professional help. Safety comes first with any gas appliance, even if the fault seems minor.
Call a Gas Safe engineer if you notice any of the following:
A burning or electrical smell from the boiler or nearby wiring. Repeated lockouts or the same error code returning after a single reset. Visible water leaks from the boiler, pipework or safety pipe outside, or pressure that rises or falls sharply in a short time.
You should also call an engineer if the boiler does not ignite at all when you run a hot tap, despite normal pressure and good flow, or if you are uncomfortable performing any of the basic checks mentioned above.
How regular servicing helps avoid hot water problems
An annual boiler service is one of the best ways to prevent hot water faults. During a service, a Gas Safe engineer will clean key components, check gas safety, and look for early signs of wear.
On a combi boiler, this can include checking the diverter valve operation, inspecting and cleaning the plate heat exchanger, confirming the correct operation of flow sensors and temperature sensors, and checking inhibitor levels and system water quality to reduce sludge and scale.
Regular servicing can often catch small issues before they become a full loss of hot water, saving you from sudden cold showers or emergency call-outs.
Need hot water restored in Bradford, Leeds or Huddersfield?
If you have tried the basic checks and still have no hot water but your heating works, it is time to get professional help. A trained heating engineer can safely test the diverter valve, sensors, heat exchanger and controls to find the fault quickly.
KSM PLUMBING LTD provides combi boiler repairs and servicing across Bradford, Leeds and Huddersfield. If you need your hot water restored, we can diagnose the problem quickly and get you back to normal as soon as possible. To book a visit or discuss your symptoms, call KSM PLUMBING LTD on 07704373410.
