Last updated on January 1st, 2025
Having no hot water from your boiler could be happening for many reasons. Different 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 encountered and fixed many boiler problems. Here I’ll show you the most likely reasons for no hot water based on the many situations I’ve come across.
How is your water heated?
The first thing you must 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 I’ll just focus on why the hot water isn’t working.
Hot water is typically heated by one of these:
- Comb boiler
- Hot water cylinder
Combi boiler
A combi boiler heats water instantaneously as you open a hot tap or shower. If you have no hot water from a combi boiler when the heating is working, the boiler is firing up so you can rule out certain things like no gas or low boiler pressure.
Common reasons a combi doesn’t heat water:
- Diverter valve stuck
- Plate heat exchanger blocked
- Faulty temperature sensor
Diverter valve stuck
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. These can be fixed by stripping and cleaning or replacing the diaphragm, but might need to be replaced.
Plate heat exchanger blocked
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 a 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.
Temperature sensor 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.
Cylinder tank
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 some hot water.
Hot water cylinders are heated:
- Indirectly using a boiler
- Directly using an electric immersion heater
Boiler
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 stuck or faulty
- Cylinder thermostat is turned down or faulty
- Programmer is not working properly
2 or 3-port valve
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.
Cylinder thermostat
A cylinder thermostat controls the temperature of the hot water in the cylinder. When it’s faulty it can cause the boiler to not heat the cylinder and the thermostat will need to be replaced. Turn the temperature up to maximum to see if it will work first.
Programmer not working properly
The programmer is typically next to the boiler and is used to set times for heating the cylinder. This keeps it hot at all the times you need hot water. When they are faulty it needs to be replaced.
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.
Common immersion heater problems:
- Immersion heater thermostat faulty
- No electric supply
- Heating element faulty
Immersion heater thermostat
The immersion heater thermostat controls the temperature of the water 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 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
The heating element is the metal that heats the water in the cylinder which can get damaged and need to be replaced.
Summary
If your central heating is working when you don’t have hot water, you know the boiler is working and can rule out certain things like no gas or low boiler pressure. Find out 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.
Have any questions? Drop them in the comments below, I’ll do my best to help! If you found this post helpful, don’t forget to share it with others who might benefit.
Hi.
I have a Glow worm 18HXi boiler with a hot water cylinder. I have recently had a new hive control system fitted that seems to be working fine. I have had a new pump and new 3 way diverter valve. The heating side is working fine but when I put on the hot water, either with or without the heating, the boiler kettles after about 20 seconds and cuts out. My heating engineer has kindly washed his hands of it because he doesn’t know what it issue is. Any ideas what the problem could be?
Many thanks
Could be air in the system. There should be a bleed valve next to the hot water cylinder. Keep bleeding the air out and turning the hot water on. Sometimes takes hours to get all the air out.
Water pressure drops over a period of 18 hours hence no hot water it’s a 2 year old combi boiler. I have checked for leaks throughout the house and everywhere is dry. Any suggestions please thankyou
Sounds like expansion vessel has no air in it or pressure relief valve is letting by. You’ll need to recharge the expansion vessel (or sometimes replace) or replace the pressure relief valve, potentially both.