No hot water but heating works? Causes and quick checks

If your radiators are warming up nicely but the hot tap is stone cold, it can be confusing and worrying. The good news is that this fault usually comes down to a few common issues you can quickly identify with some simple checks. Often, it’s a problem with your boiler settings, valves, or thermostat rather than a major breakdown. By following a straightforward checklist, you can pinpoint the cause and decide whether it’s something you can fix yourself or if you need to call a professional.

This guide will help you understand the key causes and what to do next, saving you time and stress.

Step 1: Work out what type of boiler you have

The first step in any no hot water fault is to know whether you have a combi boiler or a system/regular boiler with a hot water cylinder. The checks you need to do are slightly different for each.

A combi boiler heats water directly from the mains whenever you open a hot tap or shower. There is no separate hot water cylinder. A system or regular boiler heats water in a stored cylinder, usually found in an airing cupboard.

• Combi boiler signs: No cylinder, boiler usually in the kitchen or cupboard, fires up when a hot tap is opened.
• System/regular boiler signs: Hot water cylinder in a cupboard, possibly a small header tank in the loft, boiler heats both cylinder and radiators at set times.
• Mixed symptoms: Heating working but no hot water, or vice versa, can point to valves or sensors rather than the boiler itself.

Once you are clear on the boiler type, you can follow the most relevant checks below without guessing.

Step 2: Check the programmer and hot water demand

Before assuming a fault, make sure the controls are actually calling for hot water. For system and regular boilers, look at the programmer or time clock. You should see separate settings for heating and hot water.

Confirm that hot water is set to "On" or "Continuous" as a test, rather than relying on timed schedules. On combi boilers, check that any hot water temperature dial is not turned right down and that any "heating only" or "eco" mode is not limiting hot water output.

Basic thermostat and cylinder stat checks

If you have a hot water cylinder, look for the cylinder thermostat strapped to the side. It is usually set between 55 °C and 65 °C. If it has been knocked or turned down, the water may feel lukewarm or cold.

Turn the cylinder stat up slightly and listen for a click. That click means it is calling for heat. If nothing happens after a reasonable time and your heating still works, the issue may lie with a motorised valve or wiring rather than the boiler itself.

Combi boilers: diverter valves, heat exchangers and flow

With a combi boiler, a few key parts decide whether the boiler sends heat to your radiators or your taps. When the heating works but the taps are not getting properly hot, these components are common suspects.

Diverter valve symptoms

The diverter valve directs hot water to either the radiators or the domestic hot water side. If it sticks towards heating, your radiators will work, but your taps will stay cold or only get warm when the heating is on.

Typical signs include hot radiators whenever you run a hot tap, taps that only get vaguely warm, or hot water that comes and goes. The valve itself is inside the boiler, so replacement is a job for a gas engineer.

Plate heat exchanger and limescale issues

The plate heat exchanger is the part that transfers heat from the boiler water to your tap water. In hard water areas, limescale can build up inside it and restrict flow and heat transfer.

This can lead to lukewarm hot water, good heat at low flow, but cold when you open the tap fully, or hot water that only lasts briefly before going cooler. Descaling or replacing the plate heat exchanger should be done by a professional, but you can reduce future problems with regular boiler servicing and suitable scale protection.

Low pressure and flow sensor faults

If your system pressure is too low, a combi boiler may refuse to fire for hot water. Check the pressure gauge on the front of the boiler; it is usually best around 1 to 1.5 bar when cold.

Many boilers also use a flow sensor to detect when you open a tap. If that sensor sticks or fails, the boiler will not recognise hot water demand even though the heating still works. This usually needs a trained gas engineer to test and replace safely.

System and regular boilers: motorised valve problems

In a system or regular boiler setup, motorised or zone valves control where the heated water goes. One valve will serve the heating, and another the hot water cylinder. If the hot water valve fails, you can get excellent heating with no hot water at all.

Signs include the hot water light showing as "On" at the programmer, the boiler firing for heating, but the pipework to the cylinder staying cold. You may hear the valve trying to move or feel that its lever is loose or stuck. Internal repairs are for an engineer, but you can note what you see to help their diagnosis.

Everyday scenarios and what they suggest

Hot water runs lukewarm

Lukewarm water is often linked to a temperature setting turned down, a scaled plate heat exchanger, or a poorly set cylinder thermostat. In some cases, mixed taps or showers can draw in too much cold, giving the impression of a boiler fault.

Try a different hot tap fully open and check the temperature controls on the boiler and any mixer shower. If only one outlet is lukewarm, it may be a local mixer or shower issue, not the boiler.

Hot water cuts out after 30 seconds

Short bursts of hot followed by cold can point to a restricted plate heat exchanger, a problem with sensors, or poor flow rates confusing the boiler. This pattern is particularly common where hard water limescale has been building up for years.

In these cases, a professional clean, replacement parts and ongoing servicing usually give the most reliable fix.

Only one shower is cold

If your kitchen tap is piping hot but one shower is stubbornly cold, that often indicates a problem with the shower mixer, not the boiler. Thermostatic shower valves can clog with limescale or debris and fail to blend correctly.

Try other hot taps and another shower if you have one. If the boiler works fine elsewhere, a plumber can focus on that single shower valve instead of the heating system.

What you can safely check now

There are a few quick, safe checks you can do without taking any covers off the boiler or touching gas components. These can rule out simple control issues and help you describe the problem clearly to an engineer.

• Confirm your boiler type and note which outlets are affected.
• Check programmer settings, room stat and cylinder stat positions.
• Look at boiler pressure and warning lights or error codes.
• Try more than one hot tap or shower at different flow rates.
• Note any patterns, such as hot water only when heating is on.

Most modern boilers display fault or error codes when something is wrong. These codes vary by brand, so check your user manual or the manufacturer's website to see what the code suggests, then share that with your heating engineer.

When to call a professional engineer

If your checks point towards diverter valves, plate heat exchangers, motorised valves, sensors or wiring, it is time to call in a professional. These parts are inside the boiler or near live electrics and must only be handled by a qualified engineer.

A thorough fault-finding visit will test controls, valves, sensors and safety devices, then repair or replace what is needed. Regular boiler servicing also helps pick up sticking valves, scaling and control issues before they leave you without hot water.

Get your hot water back on

You do not need to put up with cold taps while the radiators are roasting. Work through the safe checks above, note any error codes and symptoms, then let an expert take it from there.

For friendly, professional help with diagnostics and repair, contact Summit Plumbing & Heating Solutions Ltd on 02033557452. An experienced gas engineer can quickly track down the fault, get your hot water running properly again and help keep your boiler reliable in the future.