Cold water from a tap that should be hot is one of those problems that sends homeowners straight to the phone to call their boiler engineer. But the boiler is often blameless. Here are five causes of missing hot water that most people never consider.
1. A Faulty Cartridge or Mixing Valve
In single-handle faucets, a cartridge controls both the temperature and the flow rate. When the cartridge fails, it can get stuck in a “cold only” position even when the handle is moved to hot. This is a particularly common failure mode after 5–8 years of use.
Test by removing the handle and temporarily bypassing the cartridge. If hot water flows freely when the cartridge is out, you have found your culprit. Replacement cartridges for most major brands (Grohe, Hansgrohe, Bristan, Ideal Standard) are widely available and straightforward to swap.
2. Crossed Connections in the Supply Lines
After a recent plumbing job, it is possible the hot and cold supply lines were accidentally crossed — meaning the hot supply was connected to the cold inlet and vice versa. This explains why the tap produces cold water even when turned to hot.
To check: run the cold tap. If it gets warm after a minute, the connections are swapped. A competent DIYer can fix this under the sink in minutes.
3. Sediment in Your Hot Water Tank
If you have a traditional hot water storage cylinder, sediment from hard water accumulates at the bottom over time. This acts as insulation between the heating element and the water, drastically reducing heating efficiency. You may get lukewarm water from all hot taps, or hot water that runs out unusually fast.
The solution is to flush the tank annually. Connect a hose to the drain valve at the base, open it, and let the tank flush until the outflow runs clear. It takes about 30 minutes and makes a dramatic difference.
4. Undersized Hot Water Cylinder or Combi Demand
Combi boilers heat water on demand — they do not store it. If multiple hot taps are used simultaneously, the boiler may not be able to keep up with demand, and you will get cool or lukewarm water from some outlets. Similarly, if your household has grown since the cylinder was installed, you may simply be outrunning your hot water capacity.
The solution is upgrading your boiler to a higher flow-rate model, or adding an unvented cylinder for stored hot water capacity.
5. Anti-Scald Device Set Too Low
Many modern faucets and shower valves include a built-in anti-scald device — a thermostatic limiter that prevents the water from reaching dangerously high temperatures. On some models, this device can be set too conservatively, capping hot water at what feels like a lukewarm temperature even when the boiler is heating to 60°C or higher.
On most cartridge faucets, there is a small rotary limit stop behind the handle that can be adjusted clockwise to allow higher temperatures. Consult your faucet’s installation manual or look up the model online for specific adjustment instructions.
When It Really Is the Boiler
If hot water is missing at all taps simultaneously, the boiler or hot water cylinder is the likely cause. Check for error codes on the boiler display, ensure the pilot light is on (older models), check that the programmer and thermostat are set correctly, and verify that the cylinder thermostat is set to at least 60°C. If in doubt, call a Gas Safe registered engineer.
