gas boiler thermocouple

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This my third go at getting somewhere with my ailing boiler. The problem now appears to be that the thermocouple is failing at high temperatures. So it will stay on all night but it will stop working after the boiler runs for 10 minuites or so. Does anyone have any experience of this? I have put in a new thermocouple but the same thing is happening.
Any thoughts
Stony
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If your boiler is old enough to take a thermocouple, you might want to consider replacing it. A modern boiler would have 90%+ efficiency, instead of the 50-65% you are likely to be getting now. It may be worth making a judgement on whether or not a replacement would make financial sense, particularly if the old boiler is starting to keel over. There's a lot of gas to be saved.
Christian.
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Can you explain how you calculate efficiency - is it for example : Given the amount of heat ( in say joules ) that is released by the burning a specific quantity of gas, find the amount of heat transfered to the water and take the quotient ?
Steve
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a
It is far more complicated than that. In fact, under the calculation method used, it is even possible to get more than 100% under certain conditions. However, it does approximate the potential absolute efficiency in translating fuel energy into useful heat energy that may be distributed around the system. See www.sedbuk.com for details.
Christian.
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burning
method
So this calculation may give an efficiency of greater than 100% - if so, I believe this is breaking the well tested empirical laws of thermodynamics. ( Can any physicists help, my memory is bad ). The boiler manufacturers should immediately publish this result :-) I suspect that that, rather than this being the case, the boiler is nowhere near 100% efficient - the obvious example is the fact that they all chuck heat out the flue - even the condensing ones.
What I would be intersted in is : - A sensible definition of efficiency - the reason why modern boilers claim such a big efficiency increase over the older designs - are you comparing older non-condensing boilers with newer condensing designs.
I couldnt find the calculation details you mention - where is it on the sedbuk site ?
Steve
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There isn't an actual methodology on the site that I've found. However, it does state that the calculation makes various assumptions and calculations based on the likely heating profile required in the UK. It isn't a simple energy out/energy in calculation at all.
Christian.
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I see - I did find another site that had some more info - still not enough. However it is does appear ( at first ) a little misleading to see efficiences of 100% or more percent quoted !
I talked to a guy at work and he says that they basically fiddle the "energy into the system" value to be a low value and this is how they achieve such a high efficiency. He also said that modern boilers used insulation to stop wasting heat by transfer into the surrounding space and that they do achieve lower flue temperatures which means they are extracting more heat from the gas.
Thanks for the info.
Steve
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it
calculations
simple
enough.
"energy
The higher efficiencies are due to better combustion, alloy condensing heat exchangers, etc. The over 100% is because when they made the scale latent heat was not taken into account. Condensing boilers make a mockery of this. Very few boilers have insulation around the heat exchangers.
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Keston does. However, that might be more to prevent frying the electrics inside the boiler casing, as most of the heat which escapes from the heat exchanger into the boiler case would simply serve to preheat the combustion air, since the case is vented only out through the combustion chamber. Thus the heat would not be lost -- it cleverly goes back into the combustion chanber. The front of the casing does get a bit warm just round the heat exchanger, so a little is lost that way.
One figure Keston don't give is the heat input to the room from the boiler casing, which would have been useful to know when I originally did all the heating calculations (some other manufacturers do). I just ignored it, and that doesn't seem to have caused any great error in that room, so it's probably not a lot.
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Andrew Gabriel

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writes:

One of the few and they tend to be the upmarket boilers.

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Indeed, although it is the much larger, more efficient heat exchanger which does this, rather than insulation.
A well designed condensing boiler installation might have a flue temperature of 50C, compared to an old boiler with 300C. The flue temperature being considerably lower than 100C is the cause of all the pluming. Although people worry about it, thinking it is a health hazard, the emissions in that plume are much better than those in the invisible 300C one. But you can't see the 300C version, so no-one cares.
Christian.
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which
temperature
that
Thanks again - and now the real question, are condensing boilers unreliable ? Here is an example of what I want from a boiler :
- Run for 18 years with minimal ( 3 or 4 thermocouples ) maintenance ( my old fashioned boiler ) - I find it amazing that modern combis seem to need new bits after a couple of years
Am I being unrealistic - are all modern devices complicated and unreliable ? :-)
Steve
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by
flue
can't
No. They are same as a system boiler except they have a plastic drain pipe attached.

That is possible, but you have to buy a top of the range boiler. As in all aspects of product prices, more expensive "generally" means better quality. Not always though.

couple
Depends on what make and model again. Cheap units break down more. Poorly installed units break down more too. A combi is just a system boiler with a water section. No reason for it to be unreliable at all. Some combi's have all the functions integrated to ensure no operating problems. The Microgenus is one boiler with all integrated.

yes.
No. Depends on what you buy.
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being
in
pipe
all
quality.
a
have
A fallacy is that combi's are unreliable. Some are, some are not, there are the Ladas of the combi world as well as the Mercs. Some conventional boilers are unreliable. A friends once had a Fuelsaver, much simpler than a combi, and gave continuous trouble. He eventually replaced it after only 6 years with a "combi".
"All" the system except the rads is in the box. In a cylinder/tank setup, if the ball-cock has trouble in the cold tank in the loft, no one blames that on the boiler downstairs.
Compare a boiler/cylinder/cold tank system with a combi, you have to take all of the servicing of the "system" (and that includes rads) and compare it with servicing of a combi system (and that includes rads). Assuming the rads and thermostat valves on each system are of equal quality.
You have to compare like-with-like. Comparing a combi "white box" with a simple cast-iron boiler "white box" is ridiculous. You have to compare total system vs total system.
All the old wives tales and fallacies about combis no longer exist. They can do two bathrooms, they are high quality and reliable, they do have instant hot water at the taps, they are cheap to install offsetting the high initial "box" price, they are economical to run.
I had a Potterton Neatheat regular boiler for over 20 years. The boiler was very reliable in itself because there was not much in it. In that time I went through:
Outside the boiler: 3 pumps, a zone valve, a zone valve motor, a cylinder, cylinder stat, room stat and a programmer.
On the boiler: a relay, flue fan and a pressure differential switch.
Overall the "system" was not that reliable. I am certain a good Vaillant combi would have given far less trouble over 20 years.
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Wonder why you installed a quality boiler with shite peripherals?
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Dave Plowman snipped-for-privacy@argonet.co.uk London SW 12
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Also worth mentioning that they have acid continuously running over the heat exchanger. Some have stainless steel heat exchangers, whilst others have aluminium alloy heat exchangers. I would not be surprised to find a significant difference in the life of these two types. I suspect that a heat exchanger failing in a condensing boiler more than perhaps 6 years old is not likely to be regarded as an economic repair.
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pipe
Installed properly they should both last quite a while. There are plenty of Ideal aluminium heat exchanger condensers about, and have been around for many years.

Depends on price or repair and a new item, as always. Some Ideal heat exchangers are not that big, so can't be that expensive to a new boiler. Also new condensing boiler prices are dropping, and soon will be the norm, so prices will drop further.
So you are in the situation of 15 years ago when 99.9% of boilers were regular boilers. Some where expensive to change the heat exchangers, other, mainly the finned copper tube one piece exchangers were cheap and super simple to do. see: http://tinyurl.com/r1xg The heat exchanger just slides out.
A spiral tube heat condensing heat exchanger is a matter of screwing off the pre-mix burner and the flue cxn.
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Yes. By turning such products into consumables rather than capital purchases, the manufacturers keep themselves in business. OTOH, such products are vastly cheaper in real terms than their older long-lasting predecessors.
I seriously doubt you'll get 18 years out of any current boiler without some major parts breaking. I also seriously doubt any manufacturer will supply spares for anything like 18 years.
Thinking back over many threads in this newsgroup, the combi part of a combi boiler does seem to be cause for many failures. If you were looking for something trouble-free, you might want to avoid a combi.
I have seen fewer problems than I might have expected specific to condensing boilers reported here, so that might not be something to worry about so much. However, that could just be because there are relatively few condensing boilers used by participents in this newsgroup -- that's hard to tell. The few problems which have come up here have been initial teething problems.
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writes:

unreliable ?

As most have pumps inside , pumps don't last that long.
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If the pump is a generic part, that's not a problem. If the pump is a special part manufactured for that boiler, then the boiler is probably dead within some 5 years or so of manufacturer ceasing making it.
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Andrew Gabriel

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