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Frequently Asked Questions

Boiler Related

Q. What is a Combi boiler? Q. What is a System boiler?
Q. What is an Open Vent boiler? Q. I was told my boiler is High Efficiency. What does this mean?
Q. I have a back boiler and was told It's really inefficient? Q. How do I top up water pressure?
Q. My PRV is "letting by" and dripping? Q. What is my boiler flow switch?
Q. An engineer has told me my expansion vessel has lost pressure? Q. I have been informed my PCB is Faulty?
Q. My Diverter Valve is sticking? Q. My Boiler fan needs replacing?
Q. My Condensate pipe is frozen. How can I fix this? Q. My Taps have poor flow rate?
Q. My boiler needs a service? Q. How do I find out what my water pressure is?
Q. Which boilers are LPG compatible? Q. How do I reset my boiler?
Q. What is Frost protection?  



Controls Related


Q. What is a roomstat? Q. What is Holiday mode?
Q. Why should I have weather compensation?  
Q. What is a wireless roomstat?  



System Related

Q. What is a TRV? Q. What is meant by "flushing my boiler"?
Q. What is a central heating filter? Q. I was told my radiators need bleeding? What does this mean?
Q. I was told  I have sludge in my system. What does this mean? Q. What is Underfloor heating?



General Questions

Q. What does gas safe registration mean?








What is a Combi boiler?

A Combi boiler heats water as you draw it from the tap. It doesn't't need a water storage tank or a cylinder because the boiler's connected directly to the water mains, heating water as its needed. 







What is a System boiler?

System boilers need a cylinder to store hot water, but system components are built into the boiler itself, therefore no need for a tank in the loft. 








What is an Open Vent boiler?

An open vent boiler is so called because it is linked to an outside vent. The system provides central heating and hot water by using both a storage cylinder and water tank, which gives ultra-efficient operation. 









I was told my boiler is High Efficiency. What does this mean?

Recent changes in the law mean all new gas boilers fitted in the UK should be high-efficiency condensing models. This simply means that the boiler will extract more energy from the fuel and turn it into useable heat. Old standard-efficiency boilers can waste over a third of the energy they produce. Modern high-efficiency boilers can be 90% efficient. Upgrading to a condensing boiler can save as much as 30% on your heating and hot-water bills.









I have a back boiler and was told Its really inefficient?

You have an old boiler, located behind a wall and fire front, that is inefficient and should be replaced. Back boilers provide hot water and sometimes heat radiators. They are usually fitted behind an open fireplace or stove and are not efficient as a lot of the heat they produce is lost up the chimney:








How do I top up water pressure?

Combi and system boilers and your heating system work under pressure. If the pressure drops below a certain level the boiler will not fire (start).

Combi and system boilers need a minimum pressure within the system to work. Leaks in the system will cause your boiler to lose pressure. Leaks can be very small and you may not be able to see them. It is easy to solve this loss in pressure by topping up your system - your installer will be able to do this quickly. However, it is essential they also find and fix the leak.

It is a good idea to regularly check the pressure gauge to make sure there are no tiny leaks within your system - ask your installer
to show you how if you’re not sure. You can find the correct pressure levels for your boiler and instructions for repressurising the
system in the user instructions. If you are in any doubt, we recommend you ask a competent person, such as an installer,
to repressurise your system for you.

Please search for your boiler model in the questions bar above and click "ask" to find out more specific information about your appliance.









What is a roomstat?

You have a choice of what controls you can have fitted to help you manage your boiler. Better (more sophisticated) controls will give you greater control of your heating system and can make it more efficient.

Basic boiler controls allow you to adjust the heating temperature, the time your heating system switches on and off, and the length of time it will operate. More sophisticated controls allow you to fine-tune your system, allowing you to turn it on and off at different times in the week or at the weekend, and controlling it by thermostats so the boiler works according to room temperature.

There are many different boiler controls on the market to help you save money and give you greater control over your system. It is important to check that the controls you would like to add to your system are compatible with your boiler. Your installer should be able to advise you about this.









What is a wireless roomstat?

Radio Frequency (RF), or wireless, controls pass information between the controls and the boiler without the need for wires. Installing RF controls usually causes less disruption to your property, as you don’t need connecting wires. Often an RF control gives you more choice about where you put the thermostat than a wired version.









What is a TRV?

TRV is short for Thermostatic Radiator Valve. This is an adjustable valve connected to each radiator that allows you to alter the temperature of each room by regulating the flow of hot water to each individual radiator. The valve regulates the flow depending on
how hot or cold the room is. Your installer should assess the condition of your radiators and fittings and may recommend you use TRVs.

TRVs are a simple way of adjusting the temperature in individual rooms. Also, if you have rooms that you are not using, TRVs give you the option to not heat them at all, but they still provide low-temperature frost protection (this is the setting indicated by the snowflake symbol on the valve).









Why should I have weather compensation?

Weather compensation control is an optional control available for modern boilers. It works by monitoring the temperature outside and using this information to help the boiler work as efficiently as possible.

Weather compensation control is worth adding to a modern central-heating system as it works out when the boiler needs to operate. You simply tell the system what time you want the house to be warm and the controls take care of this in the most efficient way possible. Controls like this can also save you more on your energy bills.









What is meant by "flushing my boiler"?

Your heating system should be thoroughly cleaned by flushing it with pressurised water in line with the boiler manufacturer’s instructions. This should always be done before installing a new boiler.

This is done to clear out any sludge that has built up over time and could cause the system to run ineffectively or damage the boiler. It’s normal in older heating systems for sludge to have built up so this should be flushed away before introducing a new boiler.

The process does come with some risk - poorly made or damaged joints may leak under this extra pressure. Your installer should
warn you about any risk to your property and be ready to deal with any leaks.









What is a central heating filter?

Your boiler system will benefit from having a filter installed. A filter will help keep the water in the system as clean as possible. This will help the system run efficiently and will extend the life of your boiler.

It is sensible to add filters to your system. The water within any system can become dirty over time, creating what’s known as sludge. This sludge means your system won’t work as efficiently and can damage the boiler. There are various types of filter available and your installer will be able to advise what’s best for your system.









What is my boiler flow switch?

Flow switches are a major part of a combi boiler and would be familiar to a competent installer. The flow switch’s job is to sense when you turn on a hot-water tap and the boiler. Very occasionally, they can become blocked and need replacing.

If your combi boiler is running your central heating successfully but not producing hot water, the flow switch or diverter valve may be faulty. An installer will be able to tell you which of these items needs replacing, or if something else is causing the problem.









I have been informed my PCB is Faulty?

PCB is short for Printed Circuit Board. Modern boilers are controlled electronically and the PCB is the ‘brain’ of the boiler. If there is a problem with the PCB it may lead to the boiler working incorrectly or not at all.

Like all electronic parts, PCBs are complex electronic devices that occasionally become damaged or stop working properly. But the PCB isn’t always at fault and a good installer will check the rest of the boiler and system before automatically changing the PCB. If the PCB is faulty, the installer should replace it with a new part from the original manufacturer.









My Diverter Valve is sticking?

The diverter valve is a part inside the boiler that switches between hot water at your taps and the central heating. Valves can jam or become ‘sticky’, preventing them from working properly.

If a diverter valve sticks, you may find that you only have heating or only have hot water to your taps. In some circumstances your boiler may not work. A common cause of valve failure is sludge in the system. An installer can easily replace a faulty diverter valve but should always investigate the problem and find out what happened. Because there are so many models of boiler and exact types of valve, your installer will probably need to collect the part you need from a plumbing merchant.









My Boiler fan needs replacing?

Boilers contain a fan that pushes waste gases through a pipe (known as a flue) to the outside of your home, where they are safely released into the air. If for any reason your fan stops working, your boiler will not work, so the fan is an important safety feature.

Boilers produce waste gases which must be safely released. The fan’s job is to push these gases out of the boiler and into the flue. The flue is very similar to the exhaust pipe on a car. Replacing a fan is straightforward but your installer will need to investigate why it failed to work.









My condensate pipe is frozen. How can I fix this?

Condensation is a normal by-product of a boiler and is made when gas is burned to create heat. This condensation is discharged automatically into a drain either inside or outside the house. Unprotected or incorrectly installed condensate pipes can freeze in
extreme conditions, preventing the boiler from working.

It is best to install condensate pipes inside the house to avoid freezing. Sometimes this is not possible, in which case you can position the pipe so that it discharges to an outside drain. In this situation, the pipe must be installed in line with manufacturer’s instructions, be of an adequate size and be insulated to prevent freezing.









I was told my radiators need bleeding? What does this mean?

‘Bleeding’ is the term used for letting air out of a radiator and heating system. Bleeding a radiator involves opening a small valve at the top of the radiator to allow any trapped air to escape.

If a radiator will not heat up or if it gets hot at the bottom but not at the top, this is probably due to trapped air. Air in the system can also cause a bubbling noise when the heating is running. Bleeding all the radiators will often solve this.

You can easily bleed a radiator yourself, using a radiator key. You need to be ready to close the valve immediately once the air has
been released (at the point water starts to come out). This water could be hot and may be dirty so you should have an old towel or
something similar to hand to protect furnishings, carpets and of course yourself.

If you are the least bit unsure how to bleed a radiator you should contact your installer to do it for you.

It is often necessary to repressurise your system after bleeding radiators and your installer will be able to show you how to do this,
or do it for you.









I was told I have sludge in my system. What does this mean?

Sludge is the material that has settled to the bottom of a heating system (usually your radiators). It will make your heating system run less efficiently and can lead to boiler parts failing.

Sludge is usually caused by having air in the system, and can make the inside of radiators rusty. Sludge builds up in the bottom of the radiators and sometimes on parts within the boiler, such as the pump. You can tell if there is sludge in your system as there will usually be cold spots at the bottom of your radiators. For the system to run as efficiently as possible, a competent installer needs to remove the sludge, using chemical cleaners or a power flush (or both).









My taps have poor flow rate? 

Flow rate is the term used to describe the rate at which water comes out of taps or a shower. A poor flow rate means that the flow of water from some or all of your taps or shower (or both) will be low - your sink and bath will take a long time to fill, or the flow
of water from your shower will be weak.

Flow rate is measured in litres per minute. An installer may calculate this by timing how long it
takes to fill a measuring jug from a tap. Flow rate problems are typically found in open-vent systems. These systems rely on gravity
to push water from a storage tank to taps and showers. The greater the vertical distance between the sink or bath taps (or shower)
and the tanks in your loft, the better the flow rate will be. If the tank is too close to the outlet, the flow rate may be poor.
Your installer is likely to recommend one of the following:

• Change your open-vent heating system to one that uses either a combi or system boiler.
• Raise the height of the tanks in your loft.
• Fit a pump to independently force water to specific outlets (usually showers).

The right solution for you will depend on many things, including your budget, the space you have available and other work needed
on your system.

There are other, less common, reasons for poor flow rates. These include pipework that’s too small or too long, and blocked or
corroded pipework. A competent installer will be able to identify the cause and recommend how to fix the problem.









What does Gas Safe registration mean?

The Gas Safe Register is the official list of gas engineers who are qualified to work safely and legally on gas appliances.

The Gas Safe Register is the official gas registration body for the United Kingdom, Isle of Man and Guernsey, appointed by the relevant health and safety authority for each area. By law all gas engineers must be on the Gas Safe Register. Gas Safe Register has replaced CORGI as the gas registration body. For more information visit









My boiler needs a service?

Just like a car or other complex household appliances, regular servicing is essential to extend the life of your boiler and keep it running efficiently. It is also a requirement of the guarantee. You should have your boiler serviced each year by a Gas Safe-registered,
qualified and approved engineer.

Servicing your boiler will help to make sure it stays safe and uses fuel efficiently. There are clear regulations and standards for testing and servicing a boiler. The manufacturer’s manual will set out the procedures the engineer should follow.









How do I find out what my water pressure is?

On newer ecoTEC boilers you can check your pressure by following the guide below. 

Older boiler may have a pressure gauge. Here are some examples:








An engineer has told me my expansion vessel has lost pressure?

Expansion vessels are found in most boilers, especially combi boilers, and are needed to cope with the extra water created by expansion during the heating process. Expansion vessels work at pressure. If this pressure is lost, the expansion vessel cannot do its job
and the boiler will fail to start or will automatically shut down.

An installer can usually tell if the expansion vessel has failed as the boiler pressure gauge will show a great increase of pressure as the boiler heats up. A faulty expansion vessel will not always need replacing - often it will simply need repressurising (pumping up). Your installer will be able to tell you the most cost-effective way to fix the problem. They may recommend replacing the external expansion vessel but this can be more expensive.









My PRV is "letting by" and dripping?

PRV is short for ‘Pressure Release Valve’. This is a safety device built into a boiler that activates if the water pressure rises to a level the boiler cannot cope with. The PRV switches the flow of water to a pipe that usually discharges outside of the house.

The PRV is a key safety feature of the boiler. If you see any dripping from the PRV or discharge pipe you should contact your installer. Often they may simply have to recharge the expansion vessel. The PRV should reset and any dripping should stop once pressure returns to normal levels, but sometimes the PRV will continue to let water drip from the system. When this happens, the valve may need replacing.









Which boilers are LPG compatible?

Please see the table below of current Vaillant and Glow-worm boilers that are LPG compatible:



Boiler Model

Boiler Type

Vaillant ecoTEC plus 600 series

(NOT  612 – 615)

System boiler

Vaillant ecoTEC plus 800 series

Combi Boiler

Vaillant ecoTEC 938

Storage Combi

Vaillant ecoTEC pro

Combi Boiler



Glow-worm Energy 35 Store

Storage Combi









How do I reset my boiler?

The resetting procedure varies across our boiler ranges. Click the appropriate link below for your boiler type, identity the boiler by viewing the display icons and check under "controls"  or by downloading the attached user guide for that model. You ca also type in the boiler model, if known, into the question bar above and clicking on "ASK". For Vaillant boilers click here, for Glow-worm click here, for Heatline boilers click here, for Saunier Duval boilers click here, for all other boiler brands not listed, click here








What is Frost Protection?

Frost protection is a feature of our boilers which prevent, at low temperatures, the water in the boiler from freezing. This prevents damage to the boiler. Typically accross our boiler ranges the frost protection system operates the boiler pump as soon as the heating circuit water temperature falls between 12 degrees centigrade. The pump stops as soon as the temperature of this water reaches 15 degrees centigrade. If the heating system falls below 7 degrees centigrade, the burner ignites until it reaches 35 degrees centigrade.

This two stage frost protection feature is only active when the boiler is switched on. The boiler alone cannot ensure the installation is protected against frost. A separate forst thermostat may be neccessary to control the temperature of the system. 

The frost protection feautre can vary amongst our appliances so for more information refer to the installation and Servicing Manuals for that particular appliance. You can get more information on your boiler model by typing in the boiler model in the question bar above and clicking on "ASK". 







What is Holiday Mode?


Holiday mode can be enabled on our controls which can help you save energy. This feature will stop the boiler from unnessarily coming on when you are not there, Safety features like boiler frost protection will be still be active as long as the boiler is left on. Due to our diverse range of external controls how you do this can vary a lot. If you   







What is Underfloor Heating?

Underfloor heating is a method of heating a room by laying out a network of hot water containing pipes that are are linked to the boiler, which pumps water around those pipes.  The heat rises from the floor and combined with the fact that these pipes work at a lower temperature and distribute heat more evenly, it is an efficient way of heating a property. 










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