10 Most Common Vaillant Boiler Problems & Solutions

Let’s get straight to it—most boiler troubles are not as bad as they seem. A few faults may be solved with a simple reset. Some may require a gas fitter. But knowing what is happening allows you to take swift action and prevent complications further down the line.

1. Vaillant Boiler Problems No Hot Water

No hot water from your Vaillant boiler? This is one of the most frequent complaints, and nine times out of ten, it is due to something simple.

Here’s what to check:

• Power supply: Ensure that the boiler is switched on and receiving electricity.
• Thermostat levels: Is the temperature sufficiently high? If the thermostat is at a low setting, the boiler probably won’t come on.
• Boiler pressure: The majority of Vaillant boilers require a pressure of 1.0 to 1.5 bar. If it is low, hot water won’t be delivered.
• Pilot light or ignition: For older models, the pilot light may have extinguished. More recent models will indicate a fault on the display.

If you have tried all of the above and it still doesn’t work, it may be a faulty part or a cracked internal component within the boiler. It is always best to get a Gas Safe registered engineer to come and take a look.

2. Boiler Leaks

Leaks from boilers are a serious issue. A drip that is small may expand into costly repair work if left for so long.

Some of the usual culprits of leaks are:

• Excessive boiler pressure: If the pressure is excessively high, water might leak out via the pressure relief valve.
• Rusted joints or pipes: Metals become weak and corroded with time, often happening with older plumbing systems.
• Worn-out parts: A defective compressor, seal, or valve within the boiler may cause internal leaks.
• Weary seals: Rubber seals may crack after a long time, particularly after undergoing frequent thermal and freezing cycles.

Never attempt to open the boiler casing yourself — it is unsafe and will invalidate your guarantee.

What to do instead:

• Switch off the boiler at the main switch.
• Place the towel or container underneath the leak to collect the water.
• Call a qualified heating engineer as soon as possible.

Leaks may even cause electrical issues or mould growth in your house. So act quickly, even if it seems like a small leak.

3. Vaillant Boiler Problem Low Pressure

Low pressure is a really common Vaillant boiler issue. If the pressure is too low, your boiler could fail to function effectively, leaving you without hot water or heat.

You may be able to check the pressure on the front of your boiler. It is displayed on a small digital or circular dial on most Vaillant boilers. The ideal pressure is typically about 1.5 bar.

Why does the boiler pressure drop?

• Leaks: A slight leak within the system is enough to cause the pressure to dip.
• Bleeding radiators: Also, this lowers pressure since the water is escaping.

How to Increase Vaillant Boiler Pressure?

• Find the filling loop (usually a silver spiral hose beneath the boiler).
• Open both valves slowly until the pressure gauge reaches around 1.5 bar.
• Close the valves tightly.
• Reset the boiler if needed.

4. No Heat or Hot Water

Waking up with no hot water or no heating is frustrating, particularly on a cold winter morning. A few typical reasons may be to blame.

First, check these simple things:

• Power supply: Check if the boiler is not switched off or unplugged accidentally. A power outage is also a possible cause for it.
• Boiler pressure: The boiler may shut down if it is too low or too high. The correct reading is normally 1.5 bar.
• Frozen condensate pipe: During very low temperatures, this pipe may freeze, rendering your boiler useless. Pour over the pipe with warm (not boiling) water to defrost it.

Other potential causes:

• The supply of gas or water was shut off.
• Air locks or clogs in the pipes.
• A failure within the system initiates a shut-off for safety reasons.

5. Hot Water But No Heating

You’ve got hot water, but your radiators remain cold. You are not on your own. This is a frequent problem. It may occur due to several reasons. Check the following:

Combi boiler settings

• In case you own a combi boiler, it could be configured to only heat the water and not the central heating.
• It usually occurs during summer and is forgotten when it becomes chilly once again.
• Look at your boiler control panel or display.
• Make sure it is set to both heating and hot water.

Radiators need bleeding

• At times, the boiler is fine, but the radiators don’t function.
• It may be due to air being trapped within them.
• Bleed the radiators to expel the air from them.
• You’ll require a radiator key for that.

There’s a system fault

Have you tried adjusting the settings and bleeding the radiators, but nothing gets hot at all?

• Perhaps the issue is with the heater or the heater system.
• This may be an issue with a valve or a defective part.
• You’ll require a Gas Safe engineer to have a look at it.

This is a common problem for most homeowners. If the Vaillant boiler is not working but hot water is, do not take it for granted — small issues may become bigger over time.

6. Vaillant Boiler Problems No Heating

And if your Vaillant boiler is not delivering any heat at all, there is probably something amiss within the system. Here are some possible reasons:

Low boiler pressure

• Your boiler could cease to heat if the pressure plummets too low.
• You should check the pressure gauge, which should read about 1.5 bar.
• If it’s under 1, try re-pressurizing the system using the filling loop.

Broken thermostat

• If the thermostat is faulty, the boiler won’t be able to tell when to switch on the heating.
• Change the temperature setting or replace the batteries.

Faulty parts inside the boiler

• Things such as pumps, valves, or sensors may fail.
• In that event, only a registered Gas Safe engineer is able to repair it safely.

7. Vaillant Boiler Showing Error Codes

Noticing an error on your boiler screen? Don’t panic. Vaillant boiler error codes can really inform you what is wrong with it. Your boiler is directing where to look.

Below are some of the common error codes along with their meanings:

Error Code	What It Means	What to Do
F13	The cylinder temperature sensor is short-circuited.	This needs a professional. Call a Gas Safe engineer.
F22	Water pressure is too low.	You can try topping up the pressure. If that doesn’t work, call an expert.
F23	Big temperature difference between the flow and return pipes.	Could be trapped air, faulty sensors, or a pump issue. Best to call an engineer.
F25	Flue gas temperature is too high.	This needs expert help. Do not attempt to fix it yourself.
F27	Faulty flame sensor, gas valve, or PCB.	A Gas Safe engineer needs to check this.
F28 / F29	The boiler won’t ignite. Lockout.	Could be gas, ignition, or air flow issues. You’ll need an expert.
F62	Delay in flame switch-off.	Possible gas valve or PCB problem. Call a pro.
F75	Faulty pump or pressure sensor.	Needs a replacement part. A heating engineer can fix it.

8. Vaillant Boiler Not Heating Radiators

If radiators are cold when the boiler is on, something is wrong. 

Begin by verifying the boiler’s pump. If the pump is faulty, hot water won’t flow to the radiators.

Proceed to bleed radiators. Air that gets stuck may prevent heat from penetrating.

Not working yet? Perhaps a blockage within the system or a defective component is the problem. Then call a heating engineer.

9. Boiler Failing to Respond to Thermostat

Your boiler is not turning on despite turning the thermostat up? Let’s take a look at a couple of things. First, ensure that the thermostat is set higher than the current room temperature. If it is not higher, the boiler won’t come on.

And if the room is cold but the boiler is not reacting, replace the thermostat batteries. That usually solves the problem. Also, ensure the thermostat is close to its receiver if it is wireless.

Should that not work, the thermostat could possibly be faulty and require a new one.

10. Boiler Keeps Switching Off

Is your boiler shutting itself off on its own? There may be several reasons why:

• Low or high pressure
• Trapped air
• A faulty thermostat
• An accumulation of debris or sludge within the system

In some cases, the solution is easy. At other times, it may require a repair, possibly a new boiler. A professional engineer would be able to advise you definitively.