What does a boiler service involve?

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I have put together a little page regarding what happens on a boiler service. There doesn’t seem to be much on the internet regarding the detail of an annual boiler service. There are often vague descriptions but I haven’t found anything in great detail so I thought I would add this.

Having been on various forums it is clear that some customers are advocates of the annual boiler service, whilst others are dubious and see it as a waste of time and money. I think its fair to say that not all boilers are the same, some have different requirements when they are serviced, hence the length of time one spends servicing an appliance does vary. I think its also fair to say that you get what you pay for, and the amount of detail an engineer will go into on a boiler service also varies.

A boiler service is first and foremost to check that the appliance is working safely, and will continue to do so for the foreseeable future(the next 12 months). It also involves checking the appliances components which are responsible for keeping the boiler running. So I want to make sure it is working safely and then do whatever is necessary to keep the boiler running sweet until its next service is due. Components which are failing can be pointed out to a customer who ultimately decides if they want to replace them – assuming they don’t affect safety.

An annual service for your gas appliance(s) is recommended by Gas Safe. Although you are not legally obliged to have your gas appliances serviced annually, you are ultimately responsible for your gas appliances. You cant service your own gas appliances legally unless you are competent, qualified and Gas Safe registered etc. Hence you need to commission someone who is to carry out any work relating to your gas appliances.

Ideal Isar service

This is an example of a service on an Ideal Isar and what I generally do to make sure the boiler is working safely and continues to work until its next service is due. Obviously what needs to be done on a service varies depending on the make and model of the appliance. An Ideal Isar is an example of a boiler which will stop working if maintenance is neglected. This usually happens as the spark electrode that lights the burner get fouled up over time and stops working. The appliance stops working and exhibits the ‘FL’ fault code at some point, which is usually preceded by a loud bang every time the boiler fires up.

So this is what the Isar looks like with the main combustion cover removed.

ideal isar

Drain the appliance

So first of all with boilers I generally check that the appliance is working before I carry out any work. The Isar has a permanent preheat function which means it is always warm, so this is a good indication.

With all combi boilers I generally drain down the appliance first so I can check the expansion vessel. The expansion vessel is a cylinder with a tube inside of it which is inflated to a set amount, the vessel is what regulates the pressure in the system when the central heating is on. If the tube is flat they don’t work. With Isar’s I generally drain them down using the filling loop, although this is not always possible. There is a little drain off point on the flow pipe which is on the right which can be used.

draining ideal isar

Remove fan and burner

So the fan is removed and then the burner…

ideal isar fan removedideal isar burner removed

Check expansion vessel

Right, so what I check first is the pressure of the expansion vessel to determine if it needs to be reinflated. That
red thing in the middle of the image below is the expansion vessel and there is a schrader valve where you can check the pressure and recharge the vessel if required. On the right which is out of sight unless you look for it is an automatic air vent, which bleeds air out of the system as the name suggests. I generally check these as its not uncommon for these to leak so it makes sense just to make sure it isn’t.

ideal isar expansion vessel

Once this has been checked the appliance can be repressurised with water. The pump is lubricated by the central heating water, so when you fill the boiler back up you have to bleed this as well. The image below shows the diverter valve on the left and the pump on the right. The big black things in between them is the printed circuit board which controls everything the appliance does.


Check burner, spark electrode, ionisation probe and burner pads

Next its onto the burner, spark electrode, ionisation probe and burner pads. The image on the left is what the burner looks like. On some boilers the burners can fail, its not so much of an issue on an Isar so a quick visual inspection is all that is needed. These don’t generally require cleaning, but they are made from some sort of ceramic material so you have to be careful as not to break it. Like most Isar spares, the burner is not a cheap item so you definitely do not want to damage it!

The next bit is quite important. You can see what I’m talking about in the second image.

On the Isar there are two retainer clips which keeps the burner pads in place, one at the front and one at the back(the burner pads are just pieces of insulation essentially). Sometimes the retainers break and or bend and the burner pads can fall down. In extreme cases this can cause the heat from the burner to be directed to the side of the heat exchanger and burn a hole straight through it! Which is why it is important to check these!

In the second image you can see the ionisation probe on the left, which is responsible for detecting the flame from the burner. The thing in the middle is the retainer which keeps the burner pad in place – there is one in the front and one at the back. On the right is the spark electrode which works just like a spark plug on a car and lights the burner when the boiler fires up. On this appliance over time the ionisation probe and spark electrode get fouled up and stop working eventually. The last image is what they look like after they have been cleaned

ideal_isar_burnerideal isar heat exchangerideal isar heat exchanger

Check condensate trap

After the above checks have been carried its time to replace the fan and burner etc. The next check is on the bottom half of the appliance, the condensate trap in particular.

On condensing appliances there has to be a means of collecting the condensation that builds up at the bottom of the burner. This is where the condensate trap comes in. Obviously you are looking for leaks in general but the condensate produced by condensing natural gas boilers is slightly corrosive. If a condensate trap leaks onto some metallic it will corrode it. On Isar’s its not uncommon for them to leak onto the plate to plate heat exchanger which sits directly beneath it. This heat exchanger is responsible for producing the hot water that comes out of the tap, it has mains pressure water running through it and obviously if it starts to leak you have an unlimited supply of water running through it which can create quite a mess.

I often come across boilers and installations where a condensate trap or its pipework is leaking. This is particularly dangerous when it is leaking onto a gas pipe for example. I have come across this before. This is why you should get your boiler serviced annually.

Apart from checking for leaks the condensate trap can be removed and cleaned where necessary. The image below on the right is a boiler I have come across where the condensate trap has been leaking for quite some time. You can see where it has corroded the stainless steel heat exchanger. If left long enough it would have started leaking. What was worse in this example is that the appliance had just been recently serviced, but even worse still is that the boiler was up in the loft. If the heat exchanger had started leaking there would have been an unlimited supply of water leaking from it – you can imagine how much damage that could cause to a property.

ideal_isar_condensate_trapisar condensate trap leaking

Diverter valve

Whilst checking the bottom half of the boiler it makes sense to check the diverter valve as well. This is just to the left of the condensate trap. The electrical actuator can be removed to check for leaks etc.

FAGS checks

‘FAGS’ stands for Flue, Air, Gas and safety(device).
Once the boiler has been put back together and filled up with water it is time to test it.
Each appliance has a specification and one of the checks involves checking to see how much gas the appliance is using.

The flue gasses are also analysed with a flue gas analyser. This tells you how efficient the appliance is, how hot the flue gasses are and how much CO etc the appliance is producing. This has to be within a certain tolerance.

The safety device on the appliance is tested by turning off the gas. The boiler should try to reignite itself and go to a lockout state within 3 attempts of ignition.

The flue integrity and termination point are also checked as well as the air supply. Modern domestic boilers are usually ‘room sealed’. This means their air supply from outside and expel the flue gasses outside as well. They are only room sealed when the case seals that seal the main combustion cover are intact, so this is checked as well.