There are a number of reasons why your hot water does not get hot enough or there is no hot water at all. And it depends on the type of heating system you have. Vaillant combi boilers are known to be reliable but sometimes things can go wrong. Here, we look at some of the main causes of a Vaillant boiler not giving hot water.
Combi boiler no hot water: Causes
If your Vaillant boiler has no hot water, the most common cause is boiler pressure being too low or high. Other causes include seasonal temperature fluctuations, water flow rates in the system, and faults with electrical boiler parts. Some issues you will be able to fix yourself while others will need a gas engineer to resolve.
Most houses have a combination boiler that combines into one unit instantaneous hot water for the taps and heating for the radiators. Vaillant combis are among the most popular.
Vaillant Boiler Pressure Too High: How To Bring It Down
Causes of no hot water from a Vaillant boiler
We look at the main causes and also give you tips to fix these issues before you call an engineer.
Hot water demand sensor not working
One of the first things to check is if the boiler knows that the hot tap has turned on. In most boilers, a light displays on the front showing it has recognised that somebody has turned a hot tap on. Depending on the model, you may even see the temperature on the display rising as the water is heated up. If this light doesn’t come on, it could mean that sensor is not working.
Blocked heater exchanger
For a Vaillant combi boiler, the plate heat exchanger which heats the water for the taps can become blocked with limescale and will need cleaning or be replaced altogether. If in that heat exchanger, the heating side becomes blocked, it will likely be due to sludge build up. This can affect the hot water part too.
Why Does My Vaillant Boiler Keep Losing Pressure?
Faulty diverter valve
The diverter valve directs where the hot water goes – either to the taps or the radiators. In a Vaillant combi boiler, a faulty diverter valve can mean hot water only works when the heating is on.
This is obviously not good in summer when you only need hot water and not heating. You normally notice this fault in the summer.
Alternatively, it could be passing heat down the central heating flow pipe, causing lukewarm water at the taps.
Thermistor out of calibration
Another cause of no hot water is one of the thermistors being out of calibration. This will send the wrong information to the PCB (printed circuit board) so the water temperature never rises to become hot.
Blocked hoses
A particularly common problem with older Vaillant boilers is rubber hoses in them that become easily blocked with dirt and debris that tend to stick better to rubber. They can quickly leak or in certain extreme situations, burst. It may be a simple fix to take then out and clean.
No water coming out of hot tap
If your central heating is working fine but having issues with hot water, the likely cause could be one of two things: a faulty diverter valve – run a hot tap for 10 minutes and check if the central heating system is warm but not the hot tap, this is the reason. Secondly, the plate heat exchanger is blocked up with sludge or debris, especially without a boiler filter.
If no hot water comes out of one tap or the shower, the reason could be a clogged up pipe to that tap – from limescale or other minerals; or there may be an airlock in that branch of pipework. This can happen when work has been done on any part of the system.
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Wrong sized boiler for the property
The Vaillant combi boiler may be too small for the size of the property or the number of people in the household who use water at the same time. The boiler could be too small for the demand for hot water it receives and it cannot heat the water up quick enough. You should look to upgrade to a higher kW boiler if this is the case.
Thermostat set too low
If the water isn’t hot enough, it could be easily fixed by checking the hot water control on the boiler. Check the water temperature setting on the boiler itself, this could be set too low. If you are getting only lukewarm water, try turning the setting to maximum.
Faucets and taps causing hot water to become cool
Mixer taps and shower mixers especially those that have a single cartridge have in them rubber O rings that can wear out, so the cold water can cross over into the hot side. The fix for this is to replace the cartridge with a new one which should solve the problem.
Vaillant Combi Boiler Hot Water Goes Cold: Problems And Solutions
Seasonal temperatures
Sometimes there is nothing wrong with the boiler but it’s the season that affects the water temperature. The combi boiler is designed to increase the water temperature by 40⁰C from the cold mains.
In the winter months, the groundwater temperature is colder so the boiler raises it by 40⁰ as it should, but it feels colder than in summer simply because the summer water temperature in the mains can be as high as 19⁰ whereas in winter it can be as low as 2⁰.
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Check water flow through the boiler
The flow of water through the boiler may be going too fast. Combi boilers are designed to heat water optimally at certain flow rates stated as litres per minute l/m. Different-sized boilers can handle differing flow rates. Vaillant combis range from 12 to 20 litres depending on the boiler size.
Maybe the water flow rate is too fast for the Vaillant boiler you have installed. You should call an engineer who will check the water flow rate and if it’s going too fast, may be able to adjust it down when it goes into the boiler so the boiler has more time to heat the water up.
If the boiler shows it’s heating the water to a high enough temperature, but you still feel the temperature isn’t hot enough, you can try to adjust the flow of water down to a particular hot tap, say, in the kitchen sink. You don’t need to adjust anything on the boiler.
With the slower flow of water to a particular hot tap, it should raise the temperature of the water. You can usually find a valve on the inlet pipe to the hot tap and turn this to slow the flow. This will mean the heat exchanger has more time to heat the water and so give you hotter water.
It is usually only the kitchen sink hot tap where you want hotter water, the bath and basin temperature can become too hot to bathe and wash in.
Vaillant Boiler Not Firing Up: Problems And Cures
Another way to get hot water quickly is after turning the tap on to let the boiler fire up and then slow the tap rate down so it’s running slowly, i.e. partially close the tap so it’s not at full flow, and this will give the boiler more time to heat the water.
You use less water this way and get hot water quicker too.
Summary
These are the main causes of a Vaillant boiler that’s not giving hot water:
- Blocked heat exchanger.
- Faulty diverter valve.
- Internal temperature sensor not working.
- Seasonal temperatures.
- No water flow to the boiler.
- Thermostat set too low.
- Boiler that isn’t powerful enough for the hot water demand.
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