Back Boiler Service

Servicing a Back Boiler

Now is certainly not the time that you would normally worry about the boiler, after all it is 35 degrees outside! However now is a good time to give your boiler its annual service as the heating specialists are generally no where near as busy. Also if you have any major problems with your heating system and require some serious repair work then you can take the time to get a few quotes in, save up a bit of money etc. Wait until the cold nights of November and you'll probably be willing to pay out any amount of money to who ever can fix it the quicker!

Update: This article historically refers to Corgi but you should now check that the operative is on the Gas Safe Register and not Corgi .

My heating system relies on the old back-burner style gas boiler, the ones that are considered the most dangerous from a Carbon Monoxide point of view, and it is almost impossible to get hold of spare parts for them. On the plus side they are pretty simple devices and so there isn't a whole lot that can go wrong with them (other than gassing people to death of course:)

My boiler has a carbon monoxide detector right next to it. If you don't have one then stop reading this newsletter and go and buy one, right now. Carbon Monoxide is a colourless and odourless gas, and is completely lethal.

My back boiler didn't look entirely healthy, it has a lot of visible muck and debris in its innards (including one particularly unfortunate moth) and the pilot light was burning a yellow smoky flame instead of the bright blue one that you usually see. Also the pilot light sparker system was clearly broken. Instead of giving a nice blue spark to light the pilot light it tended to give you a small electric shock. The pilot light just got lit using a match after the first time that happened!

Mysterious Hot Water

My hot water also steadfastly refused to go below 50C, even with the hot water control switched off. At first I assumed that this was due to the immersion heating, but I double checked that that was switched off (if you are unsure whether the immersion is on or off go check your electricity meter, throwing the switch one way will cause the thing to go mad and quickly start totting up the electricity units - you know the immersion heater is on).

Next I checked the gas boiler, switching it down to 'zero' which prevented the main gas flame from firing. Still the hot water stuck at 50C.

Even after I took a long hot bath and completely drained the hot water tank (don't worry all the water ended up on my parched garden) the hot water tank steadily cranked the heat back up over night until it was 50C again.

Time to get a Gas Man in!

With this strange seemingly free source of hot water I decided that it was about time to get the gas man in. Quick, take me to

After searching around for a while and deciding that it was pretty impossible to distinguish between the hundreds of heating and plumbing one-man-bands around I just called one at random. After explaining the situation to him, and getting the usual sucking of teeth, the tutting, and the explanation that you cant get parts for these old boilers, I decided to ask if he was Corgi registered.

"but of course"

"excellent, what is your Corgi number?"

"err, uhm"

:Phone goes dead:

If you want to check that someone is Corgi registered go to The Corgi Site and enter their Corgi number. According to Corgi

"Unregistered installers are dangerous, breaking the law and should be stopped. ".

So don't mess about and always make sure your gas heating is serviced by a Corgi registered installer.

Second Time Lucky

The second phone call went a lot better, the guy had a Corgi number that checked out and explained that these old back-burners were pretty easy to fix up and anyway there were lots of second hand parts available as most people were replacing them with the fancier combi-boilers. He promises to come over the next morning.

The Gas Man arrives

The gas man arrives at 8am on the dot and quickly gets to work. First of all he checks my ventilation. Apparently a room with a back boiler in it must have adequate ventilation to make sure there is enough oxygen in the room; not enough oxygen and you can get Carbon Monoxide being produced. More modern boilers pull their air in from outside, which is one of the reasons why they are favoured nowadays.

Next he started work on the boiler itself, taking the front off the system and having a good look around. Apparently the boiler is getting pretty old but should last for a couple of more years yet. He cleaned the whole thing out with a hoover and then took at look at the pilot light which was causing me some concern.

The pilot light took a bit of work, but essentially it just needed a good cleaning!! The gas man also replaced the thermocouple (temperature sensor) which is a safety feature to check that the pilot light is on. If the pilot light goes out then the temperature sensor gets cold and the system shuts off the gas flow preventing a build up of unburned gases in your house. Very handy.

Lighting the pilot light

The sparker unit was more problematic. It was pretty broken with all the cables corroded. It wasn't dangerous but was completely useless for relighting the pilot light. The gas man suggested that he could call around for a replacement but that I'm probably better off just leaving it and using matches to relight the pilot light (it hardly ever goes out anyway). I decided to leave it.

But why is my hot water always hot?

The last area of concern was my hot water tank. Why was it still getting hot? Once the gas man had taken the front off the boiler it was pretty obvious. The pilot light burns directly onto the heat exchanger for the hot water system. Even with a properly burning pilot light it transfers some heat into the hot water system, and with my clogged up pilot light it was getting the heat exchanger hotter than normal. With the ambient temperature at around 35C at the moment it doesn't take much gas to heat the water up to 50C.

Recommendations for a new boiler system

Even though my back burner boiler is doing reasonably well I had a chat to the gas man about options for a new boiler. He talked putting a condensing boiler either in the loft or in the kitchen. Apparently putting the system in the loft would probably be more expensive as a vent would have to be put in the roof (which would require getting a roofer in - i.e. expensive). Putting a boiler in the kitchen would be slightly cheaper but would use up some valuable cupboard space (my kitchen is very small) and would require running hot water a long way to the hot water tank. Neither option sounded particularly appealing and both would cost in excess of £4,000. Ouch, that can wait until another day.

Getting the heating service bill

After about 2 hours work and 4 cups of tea the gas man was finished and wrote out a bill for £70. This was £50 for the labour and £20 for the thermocouple. Now I know that these things really cost about 50p each, but I was happy to pay his mark up. I felt he did a good honest job and even recommended that I don't get him to do the work to fix the sparker system, saving me a fair amount of money.

Getting my boiler Serviced

I would recommend getting your boiler serviced in the summer months, even though I had no major problems it was good that the gas man could come out the very next day, and hopefully come winter time I will have a hassle free heating system

"Last winter I had an problem with my Potterton boiler and of course, like a lot of people I was scared of the price and how big the job will be. I used the company Boiler Repair and I was very happy with the job and price. They were a great find! I hope this helps anyone who is experiencing problems."

Claire Shreen

"We are about to buy a house with a gas central heating system run by a back boiler. I have no experience of this type of system before. Can anyone explain in simple terms how they work, especially in the summer months???"


"As a Gas safe registered engineer myself,that sounds a fair price.I usually charge a similar price for that myself. Anything under for that I don't think you would get a proper job done (Admin: Depending on area though?)"


"I have a 30 year old Ideal back boiler. My corgi man fixed it by cobbling together the igniter and adding a couple of (minor) parts. He charged me 170 pounds but spent most of this time on the igniter. It now lights the back boiler but not the front fire. He charged 85 pounds for the minor parts. I had to use him as I couldn't access the parts myself. Outrageous price for a registered gas safety engineer, especially as I am a pensioner"


"Helpful info in this article, never had a back boiler but moving into house with one so it helped explain how they worked."

M Macalister

"What area are you in? If local I could do with this gas man helping me with similar problems!"


"my boiler is working well but my heaters distributing the heat is not working well due to excess water in them. How do I release the excess water in the heaters?"


"I have had a back boiler for 15 years without anything going wrong and when serviced I\'ve been told it\'s in fantastic condition for its age. Then we got a carbon monoxide alarm a year ago (which I put right next to the boiler - perhaps a mistake?). The alarm, which was an expensive one recommended by B Gas went off one night two months ago. B Gas came out and disconnected me saying there was a flue or chimney fault. They tried to sell me a new system for over £5000. No way can I afford that! So I got an expert chimney man out and he tested it and said it was absolutely fine. Called British Gas back and they said the chimney again wasn\'t drawing properly and there was spillage into the room. However as the chimney worked perfectly when the chimney expert came they suggest it\'s atmospheric pressure that\'s stopping the chimney working and that it\'s probably intermittently happened ever since I got the boiler (15 years ago). They also imply I can\'t even have a gas fire because if sometimes the chimney doesn\'t draw well there will always be spillage. This is ludicrous - for 15 years we\'ve lived with it and it\'s been fine and nobody has had any headaches or been sleepy so the level of carbon monoxide must be very very small. Other houses in my terrace with chimneys the same all have boilers or gas fires but British Gas refuse to reconnect me now and I\'m stuck with no heating. The cost and practicalities of installing a boiler elsewhere make it not viable. I can\'t believe this has happened and that probably if I hadn\'t called B Gas we\'d still be connected with a boiler that would work fine for years to come. Perhaps the solution should have been to leave me connected but with instructions to turn the system off whenever the alarm went off (since it\'s a weather related intermittent problem)."


"Very interesting comment about the elements. We have exactly the same problem - Gas man has switched off the fire because elements are broken. We are now searching high and low to try to source some spares - but proving difficult so far - anyone able to help supply parts for a Gloworm \'Co-ordinat\' Fire type FRB ??"


"I sympathise with these comments - we had similar problems last october. British gas condemned ours due to a 1mb gas leak and quoted £5k to replace the entire system. We got an independant (corgi reg) maintenance company in who changed a leaky valve (that britgas couldn\'t find), recommended we get it regularly serviced and then turned our gas back on! We don\'t let british gas through the door anymore. However my concern now is how to afford to run these reliable but somewhat inefficient old systems with the price increases?? am considering getting some electric radiators..."


"very interesting especially the bit about parts, i have a problem in that the boiler has just been serviced and is working fine after 35 years (they don\'t make em like that anymore! do they??, however the gas fire on the front has a broken element and was condemned and disconnected by the brit gas man, i am now wondering where can these parts be obtained, otherwise for the sake of a few pounds for this part i am faced with the prospect of a new boiler and all that entails, at what cost 4k more!!!! "


"very useful information especially as I have a back boiler gas fire system myself and I agree that the usual comments by gas repair men are that no spares can be had for them. I feel at the moment that if everything is working alright don't knock it."

S. Black

"Your right. I think the authors mistake over thermistor and thermocouple caused the confusion. Note he also said he was happy with the overall price."


"Thermocouples for your boiler cost from £8 to £10 not 50p. Most Corgi Engineers have personal integrity. £70 was the right price for the job. "

Stuart (Corgi Engineer.)

"Thanks Dave (corgi registered), I have updated the page."


"For thermistor I think you mean thermocouple"

Dave the gas man (Corgi Reg)