No hot water but heating works: causes and what to do

If your radiators are lovely and warm but the hot tap only gives cold water, it can be both frustrating and confusing. Luckily, this common problem usually has a straightforward cause. By running through a few simple checks, you can quickly identify the issue and decide the best next steps. Understanding what’s going on will save you time and help you avoid unnecessary costs. Plus, knowing when to call a professional can keep your home comfortable and your boiler running smoothly.

How your boiler gives heating and hot water

Most homes in Blyth, Cramlington and Ashington use either a combi boiler or a system with a hot water cylinder. In both setups, the boiler has to decide whether to heat your radiators or your hot water when you turn on a tap.

Inside the boiler or near the cylinder, there is usually a valve that sends heat to the right place. Sensors, pressure and controls all play a part. If any of these go wrong, you can be left with heating that works but no hot water.

Quick checks you can safely do yourself

Before assuming the boiler has a major fault, work through a few basic checks. These are safe for most homeowners and can save time when you speak to an engineer.

• Check your controls: Make sure your hot water is actually set to be on, not off or on a different timer.
• Try more than one tap: Run the kitchen and bathroom hot taps to see if the problem is everywhere.
• Watch the boiler when a tap runs: See if the boiler fires up or shows a flame symbol when you open a hot tap.
• Look at the pressure gauge: On a combi, check the pressure is usually around 1.0 to 1.5 bar when cold.

If you are unsure where any of these things are, check your boiler manual or the manufacturer’s website. Never remove any covers or panels unless the instructions clearly say you can.

Common causes when heating works but hot water does not

There are a few faults that often lead to this exact problem. Some are relatively minor, while others need proper investigation.

The diverter valve is stuck or has failed

In many combi boilers, a diverter valve decides whether to send heat to radiators or hot water. If it sticks in the heating position, your radiators get hot, but your taps stay cold.

You might notice that the hot water only works when the heating is already on, or that it goes hot for a moment, then runs cold again. A diverter valve fault is not usually something you can fix yourself, as it involves draining and working on the boiler internals.

Limescale and plate heat exchanger issues

Inside a combi boiler, there is a component called a plate heat exchanger that transfers heat from the boiler side to your tap water. In hard water areas, this can become restricted with limescale or sludge.

The result can be lukewarm water, hot water that runs hot then rapidly cools, or the boiler overheating and switching off. Descaling or replacing the plate heat exchanger is a job for a qualified gas engineer.

Flow sensor or turbine problems

The boiler needs to know that you have opened a hot tap. It usually does this with a flow sensor or a small turbine that spins as the water moves.

If this sticks, fails or gets blocked, the boiler may not “see” that the tap is open, so it does nothing. You may notice that sometimes it works and sometimes it does not, depending on how far you open the tap.

Low system pressure

If your boiler pressure is too low, the boiler might still manage to run the heating but refuse to fire properly for hot water. Many modern boilers will show a fault code when this happens.

Check the pressure gauge on the front of the boiler. If it is below the level your manual recommends, you may be able to top it up using the filling loop, but only if you feel confident and follow the instructions carefully. If the pressure keeps dropping, you need an engineer to find the leak or fault.

Controls, timers and thermostat settings

On systems with a hot water cylinder, separate timers and thermostats control when the cylinder is heated. It is easy for a setting to be changed without you realising.

Check that the hot water programme is set to “on” or “timed”, and that the cylinder thermostat is not turned down too low. A thermostat set below about 55°C can feel like you have no hot water at all.

Warning signs to stop and call a professional

There is a big difference between checking your controls and trying to repair a gas appliance. If you notice any of the following, stop investigating and contact a Gas Safe engineer straight away:

• Smell of gas anywhere near the boiler or meter
• Water leaking from the boiler or pipework
• Burning, scorching or sooting around the boiler
• Repeated boiler lockouts or flashing error codes
• Boiler making loud banging or knocking noises

Do not attempt to reset the boiler over and over again. A single reset following the manufacturer’s instructions is usually fine, but repeated lockouts mean something is wrong and needs proper attention.

Information that helps your engineer fix it faster

A bit of preparation before you call can make your repair visit quicker and more cost-effective. Try to have the following details to hand:

Boiler make and model: This is usually on the front panel or inside the drop-down flap. If you have a Baxi boiler, note any fault code on the screen so the engineer can refer to specialist Baxi fault code guidance.

What exactly happens: For example, “radiators are hot, boiler lights for heating, but when I open a hot tap, the boiler does nothing, and the water stays cold”.

When it occurs: Is it all taps or just one? All the time or only at certain times of day? Does it improve if the heating is already on?

Recent pressure or leak issues: Let the engineer know if you have topped up the pressure recently or seen any damp patches around pipes or radiators.

Preventing repeat hot water problems

Many issues with diverter valves, sensors and limescale show up gradually before they fail completely. Regular servicing means an engineer can spot worn parts, clean filters and advise on limescale protection before you lose hot water altogether.

If you live in a hard water area, ask about scale reducers or water treatment that can protect the hot water side of your boiler. Keeping the system pressure stable and dealing with small leaks quickly will also help your boiler run smoothly.

Need help in Blyth, Cramlington or Ashington?

If you are stuck with no hot water but the heating still works, it is usually a boiler or control fault that needs a professional eye. Gas Wizard Heating Services Limited can diagnose issues with diverter valves, sensors, plate heat exchangers and more, and get your taps running hot again.

For prompt boiler repairs in Blyth, Cramlington and Ashington, or to book a boiler service to reduce the risk of future breakdowns, contact Gas Wizard Heating Services Limited on 07890242485.