No hot water but heating works? Likely causes

If your radiators are heating up nicely but there’s no hot water coming from the taps, it can be both frustrating and confusing. The good news is that this issue usually comes down to just a few common problems. These are often straightforward for a qualified engineer to diagnose and fix quickly, helping you get your hot water flowing again without delay. Understanding the likely causes can also help you feel more confident when discussing the problem with your heating engineer.

Quick safe checks before you call anyone

Before assuming the boiler is badly broken, there are a few simple, safe checks you can do yourself. These can sometimes solve the problem, and even when they do not, they give your engineer a head start.

First, check your controls. Make sure the hot water is actually switched on at the programmer or timer, and that any room thermostat or hot water setting is not turned down too low. If you have a cylinder, check the cylinder thermostat is set around 60 degrees.

Next, look at the boiler itself. Check:

• The boiler pressure gauge on a combi is usually around 1 to 1.5 bar when cold

• The hot water temperature dial is not turned down too low

• There are no fault or error codes on the boiler display

• The boiler is not showing a “heating only” or “summer” mode that turns hot water off

If you have the manual, have a quick look at any error code displayed. You can mention this code when you call D.E.S. Gas Safe Solutions so they can advise whether it is safe to keep using the boiler while you wait for a visit.

Heating works but hot water does not

This is one of the most common complaints with combi boilers. A combi should prioritise hot water when you open a tap, temporarily pausing the heating. If that is not happening, there are a few likely culprits.

Diverter valve problems on combi boilers

The diverter valve is the part that sends hot water either to your radiators or to your taps. If it sticks in the “heating” position, your radiators will get hot but your taps will stay cold or lukewarm.

Typical signs include radiators getting hot whenever you try to run a bath, or hot water that only appears if the heating is already on. A Gas Safe engineer may be able to free the valve, but on older boilers it often needs replacing.

Hot water temperature sensor faults

Modern boilers use sensors to measure the temperature of the water leaving the boiler. If a hot water sensor fails, the boiler may think the water is already hot and refuse to fire properly, or it may overheat and shut itself down.

You might notice that water starts warm then goes cold, or that the boiler briefly fires when you open a tap and then cuts out. A sensor is a relatively small part, but it must be tested and replaced by a competent engineer.

Flow switch or flow turbine issues

On a combi, a flow switch or small turbine detects that you have opened a tap and signals the boiler to fire for hot water. If this device sticks or fails, the boiler does not realise you want hot water, so it sits there doing nothing while your heating still works.

Sometimes the boiler may fire for hot water only at very high flow (tap fully open) or cut in and out. This is not something to fix yourself, but it is a clear line of investigation for your engineer.

For cylinder systems: heating on, hot water off

If you have a hot water cylinder, your boiler heats the cylinder through a coil and controls on the pipework. In this case the boiler and radiators can work, but the cylinder never warms up properly.

Motorised valve and cylinder thermostat issues

Motorised valves control whether hot water from the boiler goes to the radiators, the cylinder, or both. If the valve that feeds the cylinder sticks or fails, your radiators will still be fine but the cylinder stays cold.

The cylinder thermostat also plays a key role. If it fails, becomes loose from the cylinder, or is set too low, the boiler may never get the signal to heat your hot water. An engineer will test whether the thermostat and valves respond as they should when hot water is called for.

Hot water works but heating does not

Sometimes you have decent hot water at the taps but the radiators refuse to heat up. Many of the same components are involved, but the symptoms point in the opposite direction.

On combis, a diverter valve can stick in the hot water position, so you get good showers but cold radiators. On system and regular boilers, a failed motorised valve on the heating circuit or a fault in the room thermostat or programmer can leave the boiler thinking no heat is required.

Check that the heating is turned on at the controls, the room thermostat is turned up, and the programmer is not in “hot water only” mode. If those look fine, it is time for professional testing.

Hot water is only lukewarm

Lukewarm water is often harder to pin down, but there are some common patterns. On combis, low boiler output, incorrect temperature setting, a partially blocked plate heat exchanger or a faulty sensor can all cause tepid taps.

If you have a cylinder, a low cylinder thermostat setting, shortened heating times on the programmer, or a sluggish motorised valve can all mean the cylinder never reaches proper temperature. Lukewarm water can also be a sign of flow problems, especially on long runs to bathrooms.

Safety and comfort always come first

If you notice any water leaks, unusual smells, scorching or sooting around the boiler, or if your carbon monoxide alarm sounds, turn the boiler off and ventilate the area. Do not attempt to reset or restart it until a Gas Safe engineer has checked it.

Lack of hot water is inconvenient, but your safety is more important. In Sheffield, Rotherham and Barnsley, D.E.S. Gas Safe Solutions can advise over the phone whether it is safe to leave the boiler on for heating while you wait for a visit.

What not to do with a faulty boiler

Modern boilers are sealed gas appliances. You must not remove the boiler casing, adjust internal components, or attempt to bypass any safety devices. Doing so is unsafe and can invalidate warranties and home insurance.

Avoid constantly resetting the boiler in the hope that it will “clear” the fault. Repeated resets can mask a developing problem and sometimes cause more damage. If your boiler keeps locking out or showing the same error, note the code and call an engineer instead.

What a Gas Safe engineer will test

A qualified engineer will start with the basics: boiler pressure, flue condition, and any fault codes. They will then test the controls, such as the programmer, thermostats, motorised valves and, on a combi, the diverter valve and flow switch.

They may run the boiler in hot water and heating modes separately, checking temperatures and how quickly pipes heat up. Electrical tests help confirm whether sensors and valves are sending the correct signals, and any suspect parts can then be repaired or replaced safely.

Gas Safe registration matters because it confirms that the engineer is properly trained and legally allowed to work on gas appliances. It is your assurance that combustion and flue checks will be carried out correctly, protecting your home and family.

Simple checklist to share on the phone

Before you call for help, it can be useful to jot down a few key details. This helps your engineer arrive with the right parts and saves time on site.

• Boiler make and model, and whether it is a combi or has a cylinder

• Exact problem: heating ok / hot water off, or vice versa, or lukewarm

• Any error codes showing on the boiler display

• Current boiler pressure and hot water temperature setting

• Anything else unusual: noises, smells, leaks, or CO alarm activation

You can also mention any changes you have made recently, such as new programmers, smart thermostats, or radiator work. Control changes can sometimes trigger hot water issues without the boiler itself being at fault.