All About Boilers

Welcome to the boiler section of In this page we aim to provide as much information as possible related to boilers, So in the end you may have a better understanding of them to ultimately make an informed choice as to which boiler will be most suited for your hot water and central heating demands.

We aim to cover all of the important topics related to gas and electric boilers, aswell as breif overview of LPG and Oil boilers.

Due to the extensive industry experience we pocess, which is the result of over 17 years of experience working in the industry, we aim to cover every single Boiler Manufacturer we have worked on !

Some of the topics we shall cover include: 

  • What is a Boiler
  • How Boilers Work
  • Boiler Repair
  • Boiler Replacement
  • Getting Boiler Quotes
  • Government Boiler Grants
  • Combi Boilers
  • System Boilers
  • Different forms of boilers
  • Boiler Service
  • Boiler Installation
  • Boiler Insurance
  • Cheapest Boiler Option
  • Who is Qualified to Install Boilers
  • Heat Only Boilers
  • Instantaneous Water Heaters

Boilers What Are They & How They Work

In the uk, when people mention boilers they are generally refering to gas boilers, as that is the most common form of a boiler to be found in almost evey home in the uk.

And within this gas boiler category there are various other categories of boilers, Such as:

  • Combi Boilers
  • Regular Boilers
  • Domestic Boilers
  • Water Heaters
  • Electric Boilers
  • Heat Only Boilers 
  • Floor Standing Boilers
  • Commercial Boilers
  • Electric Boilers
  • Oil Boilers
  • System Boilers
  • Wall Mounted Boilers
  • High Output Boilers
  • LPG Boilers
  • Storage Boilers

The most common form of boilers we get asked about is the combi boiler. Almost every day we are quoting customers for a combi boiler installation. And most of the time it's usually to replace the boiler. 

As the combi boiler is such a popular boiler, we would be discussing this in thorough detail as well .

So what is a boiler & what does it  do ?

Simply put, A boiler is generally used to heat water ! And that water is generally used for the purpose of washing, bathing or heating and is non-potable (not suitable for drinking). 

Hot Water Boilers

Although we are talking about gas boilers here, there is another very poplar form of water heating, known as: storage hot water cylinders. (more on this topic later) 

The gas water heaters are usually instaneous, ie they heat the water instantly or simultaneously as the water enters into the boiler via the cold water inlet pipe. 

What causes the boiler to start ?

Simple ! As you turn on the hot water via either a hot water tap, bath mixer or shower, the movement of the cold water entering the boiler causes a clever little devise inside the boiler called a flow switch or flow sensor to signal the boiler to come on. 

What happens inside the boiler after it starts ?

After the boiler has been triggered to come on via the opening or turning of a hot water outlet, the boiler through 3 stages;

  1. Ignition Process
  2. Combution Process
  3. Cool Down Process

The Ignition Process

The boiler ignition process or ignition sequence is a series of steps that take place after receiving the signal from the flow switch or flow sensor. 

The first thing that happens during the boiler ignition process is, the the boiler fan comes on. This is to ensure that there is sufficent air or oxygen available for the combustion process.

This stage is a very critical stage, as insufficent air or oxygen will result in a very dangerous and potentially lethal condition known as incomplete combustion,  And as you might have guessed incomplete combustion produces the dreaded Carbon Monoxide.

The Components used in this process are mainly the Air-Pressure Switch and Fan.

The fans job is to remove the products of combution and simultaneously draw in fresh air from outside.

And the Air-Pressure switch's job is to signal to the boiler PCB or control board that:

" The Air Intake is fine You May Initiate The Combution Process"

The Boiler Combustion Process

After the boiler control board receives the signal from the air pressure sensor/switch the boiler now starts to burn the fuel source, which in this case is gas. The flame is sitting directly below what is called a heat exchanger. 

The Heat Exchanger is the part of the boiler that assists in the transfer of heat. The heat produced by the burning of gas inside the boiler is aimed at the heat exchanger.

The heat exchanger obsorbs that heat which is instantly transfered to the cold water which is passing through it. 

The Cool Down Process

The cool down phase of the boiler is nothing more than, the boiler trying to reduce it's own temperature after it has been used. Why ? So it can prolong the life of it's internal components. 

Just imagine yourself having a shower for 10 mins, and during this 10mins,  the boiler has been generating immense heat by the burning of gas. 

Now if the boiler suddenly stops, result of you turning off the shower, preventing fresh cold water to come in and assist the heat to be transfered via the heat exchanger (as discussed earlier). Where is that heat going to go ?  effectively nowhere !

All that heat is trapped within the boiler and has the potential to cause some serious damage to the components within.

So how does the boiler cool itself down? 

It does it buy a very simple and effective process known as overun.

The overun process varies from system to system. In our case it's pretty simple. After the consumer turns off the tap. The boiler will keep the fan and pump running for a while, Simple. 

The heat inside the boiler is simply dissipated via the continuous operation of the fan and pump. Althogh we say continuous, what we mean here is that,  the fan and pump continue to run until a signal has been received from another boiler component known as the temperature sensor. 

The temperature sensor monitors the temperature of various components and areas  inside the boiler. So in our case the temperature sensor we are concerned with is the one on the heat exchanger. 

So all the temperature sensor is doing os continuously sending a signal to the boiler circuit board. And the boiler Circuit board is the one that acually decides if the fan and pump should remain on or off. 

What is a Combi Boiler ?

Above we have discussed a very simple boiler, which had the only function of  generating hot water. A Combi Boiler on the other hand is a boiler designed not only to generate hot water but also heating.

A Combi boiler is actually a combination of 2 boilers hence the name " combi ". 

The 2 boilers are; 1) The instantaneous water heater and 2) The heat only regular boiler.

So basically it posesses the components inside to function as an instantaneous water heater and also has the components inside to just heat up the central heating.

How does it know when to switch over from heating water to heating the radiators ? 

The primary component inside the combi boiler which causes the boiler to either give hot water or central heating is known as the diverter valve.

The diverter valves function is to divert the heated water to either the hot water circuit or the central heating circuit.

Diferent Brands Of Combi Boilers:

Why Do You Need To Replace Boilers ?

As you may have gathered by now, a gas boiler does generally go through a good beating every single day !  let alone a month or a year.

Now, If a boiler gets used extensively, day in day out without even a service for several years, more often than not a boiler replacement is due immediately ! due to the irreversible damages caused to the boiler seals, burner, flue and combution area.

And we are talking from experience here! 

We often get called by landlords and estate agents to issue landords gas safety certificates. And whilst carrying out gas certificates what we see sometimes are absolutely shocking !

Here's a few examples:

Shocking Boiler Conditions We Come Accross

Vaillant Combi Boiler 

At first glance to the unsespecting eye, it might appear that there is nothing majorly wrong with this boiler.

But to a qualified gas heating engineer this is nothing less than "ID".  ID stands for immediately dangerous, in gas safe register terms. 

ID or immediately dangerous is the highest category a gas appliance can be after the category of "At Risk". 

When a gas boiler appliance falls under immediately dangerous, it's compulsory for the gas engineer to isolate the appliance to prevent it from being turned on until the fault is rectified. 

In the case with this vaillant boiler, most likey a new boiler installation might be the most economical option, because of the damage cased by the lack of servicing.

Different Brands Of Boilers We Repair & Install






Worcester Bosch


Glow Worm