No Hot Water but the Heating Works? Why a Combi Does This

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.

The Reverse Problem: Hot Water But No Heating

Our full guide to hot water but no heating covers this pattern in more depth. If your taps run hot but the radiators stay cold, the boiler can clearly fire, so the fault is in whatever tells it to heat the radiators, or in the parts that move heated water round them. Work through these in order:

• The heating isn't being asked for. Check the programmer or timer is in a heating-on period (many have separate heating and hot water channels), the room thermostat is set above the current room temperature, and any smart thermostat has power and a connection. Our guide to why a central heating programmer stops working covers the common failures.
• The diverter valve is stuck on hot water. In a combi, the same valve that causes the problem above can jam the other way, sending all the heat to the taps and none to the radiators.
• A motorised valve won't open. System and regular boilers use external motorised valves to send heat to the radiators or the cylinder. If the heating valve's motor or actuator fails, the cylinder still heats but the radiators don't.
• The pump isn't circulating. A failed or seized central heating pump can leave radiators cold, sometimes while hot water still works on older gravity-fed systems.
• Air or sludge in the system. If some radiators heat and others don't, it's more likely to be trapped air or sludge than the boiler. See why radiators heat unevenly.

No Hot Water and No Heating at All

When nothing works, start with the simple checks: is the boiler's display on, is there a fault code, does the pressure gauge read in the normal band, do other gas appliances work, and in cold weather, is the condensate pipe frozen? Our no heating and no hot water checklist walks through each one, and the boiler fault code database explains what your code means.

Boiler On But No Hot Water: Checks You Can Make

• Check the hot water temperature setting on the boiler hasn't been turned right down.
• On a system or regular boiler, check the programmer's hot water channel is on, not just the heating.
• Check the pressure gauge. Low pressure can stop a boiler firing for hot water.
• Run a hot tap fully open. A slight trickle may not register on a combi's flow sensor.
• Look for a fault code and note it down before you reset anything.

If those checks don't restore hot water, the fault is almost certainly inside the boiler. Brand-specific patterns are covered in why an Ideal boiler stops giving hot water, and our no heating and no hot water repair service covers the diagnosis.

Still no hot water? Tell us your boiler make, whether the heating works and any code on the display. Book a diagnostic visit and a Gas Safe registered engineer will find the cause before quoting a fix.