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Common Viessmann Boiler Problems & Repair Advice

Rob Wells
By: Rob Wells
Updated: 27th March 2024

If you have a Viessmann boiler, this handy guide will take you through some of the most common problems their boilers experience. Taking you from identifying a fault right through to fixing it, or knowing that you'll need an engineer. Should the need for engineer arise, and if in doubt always contact one, use our free service to get quotes from reliable engineers near you.

Common Viessmann boiler problems

Viessmann boiler problems Error code
Frozen condensate pipe F4 EE
Low boiler pressure Pressure gauge reading lower than 1 bar
Boiler leaking Pressure gauge reading lower than 1 bar
Burner blockage F1
Radiators not heating up properly
No hot water but heating works 50, 51, 58, 59

Frozen condensate pipe (F4 EE)

With new condensing boilers, the small amount of waste gas that boilers naturally produce and turn to water as it cools is drained outside by the condensate pipe. In the UK we're usually experiencing cold conditions, which means that it's common for the water to freeze and cause a blockage in the condensate pipe. When this happens, the boiler will lockout until the frozen blockage has been thawed out.

Thawing a frozen condensate pipe

Thawing out a condensate pipe that has frozen over is something you can do yourself. Simply, pour hot water (make sure it’s not boiling) over the blocked area of the pipe or hold a hot water bottle over the area. For full instructions, follow our guide on how to thaw a frozen condensate pipe which includes a video walkthrough.

Low boiler pressure

When your boiler has low boiler pressure, it can lead to your central heating system failing to function properly. Checking the pressure of a boiler couldn't be simpler, all you need to do is look at the built-in pressure gauge, if you see anything below 1 then the pressure is too low.

Increasing low boiler pressure

Low boiler pressure is something you can increase yourself without the need for a Gas Safe registered engineer. Find out how to repressurise your Viessmann boiler with our guide to low boiler pressure.

Boiler leaking

Even the smallest of leaks can be a serious boiler issue. A leak could happen for one of many reasons but it tends to indicate that an internal component has broken, normally a seal or valve. This can lead to corrosion, rust or cause electrical components within the boiler to short circuit.

How to fix a leaking Viessmann boiler

Don't try to repair the fault yourself, instead contact a Gas Safe registered engineer as soon as possible.

Burner blockage (F1 or A4)

The burner is an important part of your Viessmann boiler as it’s needed to supply the correct amount of fuel and air for generating heat. In the case that it gets blocked, the boiler won’t be able to operate.

How to unblock a burner

You should never attempt to do any repairs on the burner yourself, always hire a Gas Safe registered engineer. They will safely be able to diagnose and repair the issue. Depending how old the burner is, it could be worn down beyond repair and need replacing altogether.

Radiators not heating up

Your radiators not heating up can be caused by a number of things, but most commonly it's as a result of sludge or air build-up in the system. If the bottom of your radiator is getting hot but not the top, you may need to bleed it. Unless you'd like an engineer to carry this out, you can bleed the radiators yourself which our guide to bleeding radiators can take you through.

No hot water but heating works

If you find that the heating works but you’re not getting hot water, or the heating comes on when you turn the hot water on then there’s most likely an issue with the diverter valve.

The diverter valve directs the hot water to the radiators or taps, depending whether you make a call for heating or hot water. Occasionally, they can get stuck in one position so, in this case, the hot water is always flowing to the radiators, rather than hot water taps.

How to fix a diverter valve

A Gas Safe registered engineer will be able to repair the diverter valve but if your boiler is particularly old (over 10 years) then you might be better off replacing your boiler.

Need a boiler repair?



Which Viessmann boiler problems need an engineer?

If in doubt about any boiler issues, always contact a Gas Safe registered engineer, but there are some common faults you'll be able to resolve yourself. Use our table below to recognise any faults you might be experiencing with your boiler.

Problem Possible cause Do you need an engineer?
No heat or hot water Can be caused by several things including broken diaphragms and airlocks, failure of motorised valves, issues with the thermostat or low water levels Yes
Leaking and dripping Most likely to be a broken internal component but it will depend where the water is leaking from Yes
Banging, whistling or gurgling noises Air in the system could be causing this, otherwise it could be that the water pressure is too low Yes
Pilot light goes out The thermocouple could be broken which is in turn stopping the gas supply Yes
Losing pressure Could be a water leak in the system, the pressure relief valve needs replacing or due to recently bleeding radiators Only if there's a leak
Frozen condensate pipe This fault should be displayed on your boiler as an error code or warning notification Yes
Thermostat issues Thermostat could be turned off or not set to the correct settings (time etc.) Consider replacing
Kettling Limescale or sludge building up on the heat exchanger Yes
Radiators not getting hot Sludge or air building up in the system or an unequal distribution of heat Your radiators will need to be bled which doesn't require an engineer but if you don't feel like carrying out the task yourself
Boiler keeps switching itself off Could be low water pressure, a problem with the thermostat or a lack of water flow due to a closed valve, air or the pump not circulating the water in the system properly Yes

Viessmann boiler warranty

The majority of Viessmann boilers come with a 5 warranty as standard that covers parts and labour. Use our guide to Viessmann warranties for more information.

Once the warranty has begun, an annual service of your Viessmann boiler by a Gas Safe engineer will need to be arranged to keep the warranty valid for the entire duration. In addition to keeping the warranty valid, a service will help to keep the boiler running safely and efficiently for as long as possible.

Boiler insurance and ongoing cover

If your Viessmann boiler warranty has expired, then boiler insurance is something you might want to consider. Keep an eye out for any insurance that offers an annual service and safety inspections. You can take out insurance with your energy supplier but you're absolutely no obligation to do so.

Is it time to replace Your Viessmann boiler?

When your boiler has been providing heating and hot water to your home for a fairly long period of time, you might be thinking about replacing it. Those of you that live in a detached home could be making potential savings of as much as £300* a year. Take a look at the table below to see how much you could be saving depending on your current boiler rating and property.

House Type G (less than 70%) F (70-74%) E (74-78%) D (78-82%)
Semi-detached £195 £140 £125 £110
Detached £300 £210 £165 £135
Detached bungalow £155 £110 £100 £90
Mid-terrace £170 £120 £105 £95
Mid-floor flat £80 £55 £50 £45

How to find the best installers for a Viesmmann boiler replacement

If you're convinced that a new boiler is for you, then you'll need to to think about who will do the installation.

Any installation should be carried out by a Gas Safe registered engineer, that's the simple part. The big decision you'll have to make, comes down to a local engineer or a national company.

With national companies, you'll not only be getting your boiler replaced but be offered extra services like ongoing boiler cover and big brand protection, always a plus.

Local engineers, can offer extremely competitive rates and you're likely to get a personal service throughout the installation from the first phone call to the last.

When it comes to getting quotes, we recommend getting up to 3, rather than just 1, so that you know you're getting the best deal for you.

To find trusted engineers near you, use our service to receive free quotes for a new boiler installation.

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*According to the latest figures from the Energy Saving Trust

Rob Wells

About the author

Rob Wells

Rob is another of our resident home heating experts. He manages our external partnerships, making sure you get the most up to date information.

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