Why Is There No Hot Water But the Heating Still Works?

The Clue Is in the Pattern

When your heating system works perfectly but you turn on a hot tap and get only cold water, it's a very specific and telling symptom. For homes with a combi boiler, this pattern points directly to an internal fault that is preventing the boiler from switching into its hot water production mode. It's a well-understood diagnostic sign, not a random mystery.

The Most Common Culprit: The Diverter Valve

Inside a combi boiler, a component called the diverter valve is responsible for directing hot water where it's needed. When your heating is on, it sends hot water to your radiators. When you open a hot tap, it should instantly switch (or 'divert') the hot water to the hot water heat exchanger instead. If this valve gets stuck, jammed with debris, or its motor fails, it can remain in the 'heating' position. This is why your radiators get hot, but your taps run cold—the boiler is physically incapable of sending heat to the right place for hot water.

A Gas Safe registered engineer will check the valve's operation, often by listening for a motorised movement when a tap is run, and may test it manually. If stuck, it can sometimes be freed and cleaned, but often a replacement is the reliable fix.

Other Possible Faults Inside the Boiler

While the diverter valve is the prime suspect, other internal components can cause the same symptom.

• Plate Heat Exchanger: This is a separate, compact heat exchanger dedicated to hot water. It can become severely scaled or blocked with sludge, especially in hard water areas. When this happens, the heating circuit water can't transfer its heat effectively to the cold mains water flowing through the other side of the plates, resulting in lukewarm or cold water at the taps.
• Flow Sensor or Diaphragm: To initiate hot water mode, the boiler must detect that you've opened a tap. A flow sensor (often a small paddle wheel) or a pressure-activated diaphragm tells the boiler's circuit board that water is now flowing. If this sensor fails or the diaphragm perishes, the boiler simply doesn't 'know' a hot tap has been turned on, so it never activates the diverter valve or the burner for hot water.

Important Distinction: If You Have a Hot Water Cylinder

The diagnosis above applies specifically to combi boilers, which heat water on demand. If your home has a system or regular boiler with a separate hot water cylinder, the cause of this fault is completely different. In that setup, the boiler heats the cylinder (like a kettle), and a separate thermostat on the cylinder controls the temperature. If your heating works but water from the cylinder is cold, the issue is likely with the cylinder thermostat, a failed immersion heater (if you use one), or a motorised valve controlling flow to the cylinder. Check which system you have before considering the causes.

What an Engineer Will Do

A diagnosis for this fault is systematic. The engineer will first confirm the boiler type. For a combi, they'll run the heating and then a hot tap to observe the boiler's response, checking for error codes. They'll listen for the diverter valve motor and may test its operation. They'll check the hot water flow sensor and examine the condition of the plate heat exchanger. The fix depends on the confirmed faulty part, but the symptom pattern makes finding the root cause a straightforward process for a qualified professional.