My Boiler Has No Hot Water but the Heating Works: It is a frustrating scenario: your radiators are scorching hot, but as soon as you turn on the tap, the water stays stone cold.

When your central heating works but your hot water doesn’t, it’s a sign that your boiler is “half-functioning.” While it might seem like a total mystery, this is actually a very common issue that the team at MJS Appliance & Boiler Repairs Ltd encounters across Stockport and Manchester.

Here is a breakdown of why this happens and what you can do about it.

1. The Diverter Valve is Stuck

This is the “usual suspect.” In a combi boiler, the diverter valve is a small mechanism that opens and closes to direct hot water either to your radiators or to your taps and showers.

  • The Problem: Over time, these valves can become stuck or clogged with “central heating sludge.” If it’s stuck in the “heating” position, the hot water stays in the radiators and never makes it to your taps.
  • The Fix: This isn’t a DIY job. A Gas Safe engineer needs to inspect the valve to see if it can be cleaned or if it requires a full replacement.

2. Low System Pressure

Your boiler needs a specific amount of water pressure to operate correctly. If the pressure drops too low, the boiler might prioritise the heating circuit over the hot water.

  • The Check: Look at the pressure gauge on the front of your boiler. It should typically be sitting between 1 and 1.5 bar.
  • The Fix: If it’s below 1 bar, you may need to repressurise the system using the filling loop. If the pressure drops again shortly after, you likely have a leak that needs professional attention.

3. Thermostat or Timer Issues

Sometimes the hardware is fine, but the “brain” of the system is confused.

  • The Check: Check your wall thermostat and your boiler’s digital display. Is there a “Hot Water” icon flashing or crossed out? Has a power cut reset your timer?
  • The Fix: Ensure your hot water is set to “On” or “Auto” and that the temperature dial for the domestic hot water (usually marked with a tap icon) hasn’t been turned down accidentally.

4. An Air Lock in the Pipes

If you have a traditional system (with a cylinder in the airing cupboard) rather than a combi, you might have an air lock.

  • The Problem: A bubble of air can get trapped in the hot water pipes, physically blocking the water from reaching the tap.
  • The Fix: While there are DIY “hosepipe” tricks to clear air locks, they can be messy. It’s often safer to have a plumber flush the system to ensure there’s no underlying blockage.

5. Broken Diaphragm or Sensor

In some older boilers, a rubber diaphragm or a “flow sensor” tells the boiler when you’ve turned a tap on. If this part perishes or fails, the boiler simply doesn’t know you want hot water, so it doesn’t fire up.