Glow-worm Boiler No Hot Water

Why Does My Glow-worm Boiler Have No Hot Water?

Last updated on February 20th, 2024

Having no hot water from your boiler could be happening for many reasons. Different Glow-worm boilers and setups vary a lot and so do the reasons for not getting hot water.

I have been repairing boilers full-time for over a decade and have fixed many Glow-worm boiler problems.

Mixer tap running hot water

No Hot Water From Glow-worm Boiler but Heating Working

The first thing you should know is how your hot water is being heated before you can narrow down why it’s not working.

A boiler not firing up at all could be for many other reasons, but here I’ll just focus on why the hot water isn’t working.

Hot water is typically heated by one of these methods:

  1. Comb boiler
  2. Hot water cylinder

Combi Boiler No Hot Water

Glow-worm combi boiler

A combi boiler heats water instantaneously as you open a hot tap or mixer shower to use it.

If you have no hot water from a combi boiler when the heating is working, you know you don’t have a problem with the boiler not firing up, so you can rule out certain things like no gas or electric supply.

Here are some common reasons a combi boiler not heating water:

  • Diverter valve stuck or broken: The diverter valve diverts the heated water from heating the radiators to heating the plate heat exchanger for the hot water. If you feel the flow pipe getting hot when running the hot water tap, it’s typically a sign of a diverter valve problem.
  • Plate heat exchanger blocked or covered in limescale: The plate (or secondary) heat exchanger is what heats the water before it comes out of the hot tap. These can often get sludge or debris inside from a dirty heating system or can get build-up of limescale which restricts the water from getting heated. If you run the hot tap slowly and it gets hotter, this is a sign of a heat exchanger problem. They can be removed and cleaned with acid, but replacing it is often the best solution.
  • The temperature sensor is faulty: The temperature sensor controls the temperature of the hot water to make sure it doesn’t get too hot. They can cut the boiler out too early when they’re faulty or cause your hot water to not work at all. This will need to be tested with a multimeter and replaced if faulty.
  • The flow/micro switch is faulty: The micro switch is an electric component which tells the boiler everything is good and to fire up when the hot water tap is opened. If it’s faulty, the hot water won’t work and it needs to be replaced.

Cylinder Tank No Hot Water

Hot water cylinder
Hot water cylinder

A hot water cylinder is a storage tank for hot water which is heated regularly (by a timer) so it’s always hot for when you need to use some hot water.

Hot water cylinders are heated in one of two ways:

  • Indirectly using a boiler
  • Directly using an electric immersion heater


Most hot water cylinders are heated indirectly by a boiler using 2 or 3 port valves to divert the heated water from heating the radiators to heating the hot water cylinder.

Here are some common problems:

  • 2 or 3-port valve is stuck or faulty: Motorised valves control the flow of heated water from the boiler to the radiators or hot water cylinder. They often become stuck or stop working. Sometimes you will only need to replace the motor on the outside but sometimes you will need to drain the system and replace the full valve.
  • The cylinder thermostat is turned down or faulty: A cylinder thermostat controls the temperature of the hot water in the cylinder. When it’s faulty it can cause the boiler to heat the cylinder and the thermostat will need to be replaced.
  • The programmer is not working properly: The programmer is typically next to the boiler and acts as a timer to set when to heat the cylinder to keep it hot at all the times you need hot water. When they are faulty it needs to be replaced.
Drayton boiler programmer
Boiler programmer

Immersion Heater

If your hot water cylinder is heated by an immersion heater then it has nothing to do with the boiler, the immersion heater is heated by an electric supply.

Here are some common immersion heater problems:

  • Immersion heater thermostat is faulty: The immersion heater thermostat controls the temperature of the water temperature and can be manually set to your liking. They can fail and need to be replaced.
  • No electric supply: The immersion heater might not be getting any electricity which will make it not work. You should check the power is switched on at the spur and that the fuse is good, at the consumer unit (fuse board), and that you have credit if you have a pre pay electric meter.
  • Heating element is damaged: The heating element is the metal that heats the water in the cylinder which can get damaged and need to be replaced.
Hot water cylinder heating element
Immersion heating element

Glow-worm Boiler Guides


If your central heating is working when you don’t have any hot water, you know the boiler hasn’t gone to lock out and can rule out certain things like no gas or electric supply.

You can also rule out your boiler pressure being too low as you wouldn’t have heating or hot water with low boiler pressure.

The first thing you should do is find out exactly how your hot water gets heated so you can narrow down the possible causes.

If you don’t have hot water at one tap or shower, the problem will be with the tap or shower or the pipe supplying it with water.

Any boiler part replacements and testing should be done by a Gas Safe registered engineer.

Feel free to ask me any questions in the comment section below and I’ll try my best to help.

Please share this post if you find it helpful.


  • Steven Reid

    I am a full-time plumber and Gas Safe registered engineer. I incorporated Housewarm Ltd. in 2011 to provide heating and plumbing services to homes in Newcastle upon Tyne, England. I now blog about what I've learned over the years to help DIYers and plumbers.

    View all posts
Share this post

3 thoughts on “Why Does My Glow-worm Boiler Have No Hot Water?”

  1. Hi I have a glow worm hx18 boiler, which seems to be working perfectly at heating radiators and hot water, problem is I have switched the thermostat off and I have switched off the heating on the control panel. But still both heating and water heat up when I only want hot water. I am thinking the valve is stuck open, not working properly. I am just trying to figure out if I’m on the right track and also what a valve looks like. I also have a grundfos ups15-50 fitted which switches on and gets warm…. Should this be working…is it just to increase pressure. Very confused. Please could you help shed light.

    1. Steven Reid

      Hi Kai,
      Yeah, sounds like a 2 port or the 3 port valve is stuck open.
      The Grundfos is the pump for circulating the water around the system which should always be working when heating the radiators and hot water.

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top