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baldkev

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Hi all, our boiler is acting up. I turn on the hot tap, it fires, gets up to mabe 70° and stops burning, water temp on boiler readout drops down to 50 and then it kicks in again and gets back to about 70, then stops heating the water etc etc. So running a bath with just hot tap gets you warm water with intermittent hot water. It also has ( a couple of times ) heated the radiators even though the central heating is 'off' at the room thermostat. We only have one thermostat ( small house )
Im thinking diverter valve, but that doesnt account for the intermittent hot water.

The boiler is 13ish years old and in march im hoping to fo an extension, so i would like to get this thing limping through until march and then I'll buy a new one. My usual plumber dislikes this boiler and last time it went wrong we got worster bosch to fix it ( fixed price of 280 quid, but the engineer did absolutly loads of jobs.... but he said next time it goes wrong, just bin it 😒 )
Its a buderus boiler
 

Spectric

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but he said next time it goes wrong, just bin it
I would take his advice, that is an old combi boiler and you will be throwing good money at it for little return. Talk with your plumber about fitting a system boiler and un-vented hot water cylinder rather than a combi, much better and efficient system, less complicated and uses off the shelf generic components rather than boiler OEM specific. A combi boiler is a rather complex animal, a self contained system that was originaly intended for flats, works under higher thermal stress and rather than proven zone valves has a divertor valve which do cause a lot of issues. If your divertor is sticking then the fact your radiators get warm with heating off is a sure sign, try putting the heating on and then try hot water. The divertor should prioritise the hot water over the heating.
 

baldkev

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@Spectric Thanks for the reply 👍 yes it does prioritise the water but it would be interesting to see if the boiler heats up and drops off whilst doing the heating, so thanks, that'll be a good test.

The boiler is on the wall in our kitchen ( we have a modern rabbit hutch )

Part of the reason for the march timeline is that we will be remortgaging to pay for it, in the mean time, we are trying to buy a piece of land for the extension so need to save as much as possible 😐 a lottery win would help 😀
 

baldkev

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With just the heating on, it slowly built up to 84° on the boiler then cut out. It slowly dropped down to 36 and didnt kick back in ( it hadnt reached thermostat goal temp and the heating indicator stayed on, whereas it usually goes off once the boiler stops firing )
Turning on the water forced it to fire up again, then after the water was turned off the heating actually activated again.


Terrible timing. We will be moving the kitchen into the extension, at which point our current kitchen becomes a utility room, so we could have a cylinder in there at that point....
 

John Brown

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In my limited experience, most problems are caused by diverter valve failure. It seems to be the weak link in combination boilers.
Whaddya know? I find myself agreeing with Spectric.🙂
 

Fergie 307

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If your radiators, usually the one closest to the boiler in the circuit, get warm when you are running hot water with the heating off, this does indicate a stuck diverter valve. Your hot water symptoms sound like you might have a partially blocked heat exchanger. Try holding the hot water out pipe just below the boiler when someone runs a hot tap. If it gets very hot just before the boiler cuts out this suggests the heat exchanger is partially blocked. Basically if it is blocked them water can no longer flow through it as quickly as it should, and so becomes very hot. This causes the thermistor to register excess heat and shut down the burner. If you leave the tap running then once the temperature at the outflow drops below the trip temperature the burner will fire up again, and so on. Depending on the boiler this could be an easy repair if you know what you are doing, and shouldn't involve any messing with the gas. Most important is to ensure you have a good filter in the system, preferably one that does particulates as well as magnetic stuff, otherwise it will just happen again. If you haven't serviced the system for a few years then take the opportunity to drain and flush it, then refill with a good inhibitor. Really annoys me when people immediately go for the "you need a new boiler" option. If you Google replace diverter valve on, insert your boiler make and model, you will almost certainly find a You tube video showing how to do it. Then you can make up your mind if you feel it is something you are competent to take on. But if I'm any doubt then get a plumber in.
 

baldkev

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Your hot water symptoms sound like you might have a partially blocked heat exchanger
Ahh, yep! We did in fact replace it about 5 years ago, my plumber said it was a pain in the ass to get to. As it is on the water side, is it legal to do it myself? I seem to recall someone telling me only gas registered plumbers are allowed to work on gas boilers.
Also, being as i am replacing the boiler when we build the extension, should i probably be able to get away with flushing the existing heat exchanger?

Thanks, kev
 

Fergie 307

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Standard trick is to remove it and soak it in spirits of salts for an hour or so. This will dissolve any crud. Be careful as its highly corrosive.
 

baldkev

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I looked on google after asking the question and apparently vinegar is also used... spirits of salts apoears to be hydrochloric acid! Ive got citric acid and oxalic ( and vinegar )
 

baldkev

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Just looked this up too, so yes, you can replace items which do not have any gas fittings / supply etc connected to the item. If you need to do anything involving any part of the gas system, you have to have a registered gas safe plumber
 

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John Brown

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I changed the heat exchanger on our boiler a few years back. I was confident that it was "water side", as opposed to "gas side". I was chatting with the service man the next time he came, and he reckoned that strictly speaking, I shouldn't even have removed the cover to the airtight chamber.
Not saying he was right, but it seems to be a bit of a grey area.

May be covered in that doc, but too small to read on my phone.
 

baldkev

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It basically says you can change parts that do not interfere with gas in any way, but you cannot touch any gas fitting.


"What gas work can be undertaken by other tradespersons?
If the work required only involves the replacement of a non gas component such as a water circulating pump
or central heating control valve, housed within the boiler’s decorative casing, the work could be undertaken
by another competent tradesperson e.g. plumber/electrician. Providing the work could be undertaken without
having to break a combustion chamber seal "

It says some covers have part of the system built in:

"Integral casing: Many appliances have cases, which in addition to being decorative, function as a part of the
combustion circuit and form an important seal around the gas carrying components e.g. burner, combustion
chamber, gas valve etc. If removing the case involves undoing a number of screws, this normally means it is
a functional case and it should not be removed by the consumer or a person who is not Gas Safe registered"
 

Cozzer

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I looked on google after asking the question and apparently vinegar is also used... spirits of salts apoears to be hydrochloric acid! Ive got citric acid and oxalic ( and vinegar )
Cheers, baldkev.
Not the thing to use if you're a tad clumsy, then!
 

Gerry

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I just use kettle descaler with boiling water on my old Myson and let it stand for a good hour or two before rinsing with fresh water. It come up like brand new.

Gerry
 

Fergie 307

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Sorry, yes it is hydrochloric acid. Just much faster and more thorough than descaler, but also much more dangerous. Best thing I did was to fit a dual action filter, I think its called a twin flow, about £150. Has the normal magnet to catch ferrous debris, but also a brillo pad type filter which catches anything else like limescale. First few times cleaning it it was full of rubbish, last time clean as a whistle. Dont really know where it all comes from as it had been powerflushed. Can only assume there was a lump of stuff somewhere that had defied even the powerflush, and would gradually release a bit to bung things up. Ultimately it doesnt matter how good your boiler is you will get issues if you have bits of rubbish floating around the system.
 

Fergie 307

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It basically says you can change parts that do not interfere with gas in any way, but you cannot touch any gas fitting.


"What gas work can be undertaken by other tradespersons?
If the work required only involves the replacement of a non gas component such as a water circulating pump
or central heating control valve, housed within the boiler’s decorative casing, the work could be undertaken
by another competent tradesperson e.g. plumber/electrician. Providing the work could be undertaken without
having to break a combustion chamber seal "

It says some covers have part of the system built in:

"Integral casing: Many appliances have cases, which in addition to being decorative, function as a part of the
combustion circuit and form an important seal around the gas carrying components e.g. burner, combustion
chamber, gas valve etc. If removing the case involves undoing a number of screws, this normally means it is
a functional case and it should not be removed by the consumer or a person who is not Gas Safe registered"
Very true, however the bit you need to get at, the hot water heat exchanger, is usually quite seperate. Generally you have a sealed compartment at the top containing the burner, fan and central heating exchanger, and everything else is exposed underneath, so the hotwater exchanger, valve block etc. Good idea to see if you can download the maufacturers installation and maintainence insructions. You should replace any disturbed seals if possible, and the manufacturers documentation will hopefully give you part numbers and or sizes of any O rings or fibre seals you might need. You might get away with reusing them, but always better to use new ones.
 

Steve_Scott

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Diverter valve may be at fault but I'd also suggest you whip out the plate heat exchanger (primary loop/domestic) and give that a clean. Despite living in a hard water area, my domestic side stays clean but the primary side filters out crud from the central heating loop and reduces flow. This seemingly causes the hot water temperature to cycle in the way you describe. I just jet mine out with a garden hose every time I start to notice the cycling when running a bath.

Another factor to consider, although it might not be a feature on your combi, is if the boiler is preheating the plate heat exchanger. My Viessmann did this and it basically fires and circulates around the plate heat exchanger even when there isn't hot water demand. Flow temp skyrockets and when its done, it opens up the diverter to the rads and dumps a slug of really hot water around the primary loop (when there is no demand). Silly feature if you ask me, solved by removing any timing program for hot water (which is crazy on a combi boiler).
 

baldkev

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Thank you all for your replies, very helpful....

@Fergie 307 , yep we got a digital copy of the manual 5 or 6 years ago when it started going wrong, theres a full instructional for the heat exchanger, which is indeed a seperate unit, down low under the combustion chamber etc.... i have a large selection of o rings and fibre washers in various plastic organisers 🤣 im a sucker for pucking up those selection packs 'just in case', but its suprising how many R clips and split pins, washers etc i get through!! When something breaks over a weekend or at night, ive often got enough bits kicking about to repair 😆😂
 
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