boiler insulation

[Ed_Dinning]

Hi Gents, I just thought I'd get some opinions from forum members.
I have a 30 year old Glo-worm gas boiler that works very well and is fitted to a fully pumped and controlled system with zone valves.
People are telling me to get a modern condensing boiler, but the need to totally change the system and remove the copper cylinder would give a long pay back period.
I also intend to fit a solar hot water collector of my design and feed it into the tank on a new coil as a feedwater heater. This I could not do if I remove the cylinder.

The idea is to insulate the free standing boiler to reduce heat losses from its water jacket by means of a modern insulating blanket that is a similar material to shuttle tiles. This will fit inside the boiler sheet metal casing and be clipped against the outside of the boiler, obviously leaving gaps where there are air intakes, flues and other combustion access. This should considerably reduce losses from the boiler shell.

Your thoughts Gents?


Ed






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[pwdrive]

Hello Ed, no expert on gas, but do I take it that the water in the gas boiler circulates through a coil in a seperate hot water cylinder as opposed to an "on demand" via small heat exchanger.

Lawrence.

[Yorkie]

Hi Gents, I just thought I'd get some opinions from forum members.
I have a 30 year old Glo-worm gas boiler that works very well and is fitted to a fully pumped and controlled system with zone valves.
People are telling me to get a modern condensing boiler, but the need to totally change the system and remove the copper cylinder would give a long pay back period.
I also intend to fit a solar hot water collector of my design and feed it into the tank on a new coil as a feedwater heater. This I could not do if I remove the cylinder.

The idea is to insulate the free standing boiler to reduce heat losses from its water jacket by means of a modern insulating blanket that is a similar material to shuttle tiles. This will fit inside the boiler sheet metal casing and be clipped against the outside of the boiler, obviously leaving gaps where there are air intakes, flues and other combustion access. This should considerably reduce losses from the boiler shell.

Your thoughts Gents?


Ed


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You are confusing a condensing boiler with a combi boiler Ed.

To replace your 30 yr old Glow Worm boiler with a condensing boiler is about a day's job, assuming that your Glowworm is a balanced flue boiler on an outside wall and the new boiler is going in the same position. A condensing boiler retains your hot water cylinder and cold feed tank, but you will possibly find that to comply with current codes of practice, the installer will want to update your programmer and room thermostat.

A condensing boiler is about 95% efficient - your Glow Worm will at best be about 60% effcient, and also has a permanent pilot usding gas 24/7/365, which is not permitted on modern boilers. It will certainly be such a 'gas guzzler' that it would have been on the list of boilers elgible for the £400 'Gordon Brown grant' for replacing inefficient boilers with condensing boilers. My 20 year old Glow Worm was on the list, so I took advantage of the grant a couple of years ago.

A combi boiler is quite a different animal and the explanation that follows has no relevance to your situation, but may help to clarify what a combi boiler is: It doesn't use a hot water cylinder or cold feed tank so no hot water is stored - it is supplied on demand by the boiler when a hot tap is turned on on a sink or bath or shower. The heating system is sealed, and pressurised to 1 - 2 Bar (about 14 - 28 PSI). This is simply done by turning a bypass valve usually located near to or within the boiler while observing the pressure gauge on the boiler until it reaches the correct pressure, then turning off the valve. There is small pressure gauge on the boiler and the householder needs to keep an eye on that because very small leaks on radiator valves can cause the pressure to fall, and of course, if a radiator is removed to decorate behind, as soon as the radiator valves are tuned back on, the pressure is lost in the system. If the pressure drops too low, the boiler closes down.

The main drawback with combi boilers is the poor delivey rate of hot water. Not too noticeable if you only use showers, but very noticeable if you want to fill a bath. This is even worse in winter when the cold water from the mains is so much cooler, because to provided a constant output temerature with a fixed heat input to the boiler, the only way to maintain the temperature is for the boiler to slow down the water flow rate. Combis have their place, especially in single floored apartments where there is no 'head' for a cold water feed tank for a conventional boiler, and for a new installation, there is a saving on plumbing costs - not cold feed tank, overflow pipe, hot water cylinder etc.

None of that has any relevance at all to you Ed, so file it under 'burn before reading!.

More relevant to your situation are condensing boilers:

All current production gas boilers must by law be condensing boilers, and confusingly perhaps, that includes combis. (So while all current combis are condensing boilers, not all condensing boilers are combis!). Prior to condensing boilers, gas central heating boilers were about 65 - 70% efficent - the rest of the heat went out through the flue outlet in the form of water vapour and carbon dioxide at quite a high temperature. The heat exchanger of a condensing boiler is so efficient in extracting heat from the flue gases that while to carbon dioxide is emitted from the flue, the gases fall to a temperature at which the water vapour condenses into water, which is connected to a drain, which ideally should be indoors and connected to say the waste pipe of a sink, as for example would be a dishwasher, washing machine etc, but are often installed externally. That is very bad practice as the 22mm diameter pipe will freeze even if their is onlty a light frost. The reason for this is that when the boier is operating, the water goes down the drain in a trickle rather than a great gush, so will strat to freeze on the inside wall of the 22mm drain, mkaing the internal diameter progressively smaller, until it freezes solid, then the boier shus down.

Condensing boilers are room sealed - they take their combustion air from a concentric flue pipe with the inner pipe being the 'exhaust' and the outer concentric one being the freash air inlet. Because the flue is fan-assited, it is small in diameter (about 75mm) and can run for quite long distances of several feet horizontally or vertically, so the boiler siting can be more flexible that older balanced flue boilers, which had to be sited in an outside wall.

Rather a ramble, but I hope it helps rather than confuses!

[Skywave]

Quote 'Yorkie':
The main drawback with combi boilers is the poor delivery rate of hot water. Not too noticeable if you only use showers, but very noticeable if you want to fill a bath. This is even worse in winter when the cold water from the mains is so much cooler, because to provide a constant output temperature with a fixed heat input to the boiler, the only way to maintain the temperature is for the boiler to slow down the water flow rate.

Unquote.

Very recently, I replaced my ageing 'Glow-Worm' gravity-fed boiler with a Vaillant combi. The house here is a typical 1930's 3-bed semi. None of the problems reported by Yorkie above are manifest, although I must add that I have experienced those problems with other makes in other places where I have lived.
I can adjust the temperature of the hot water from the taps to the sinks in the lavatories and kitchen with the control on the boiler. I can adjust the temperature of the radiators by another separate control on the boiler. All the radiators have their own thermostatic control valves.
As a consequence of that replacement, I now no longer have a cold water tank nor a header tank for the boiler in the attic: more room to store vintage radio stuff! And the hot water cylinder in the airing cupboard has gone too, so bigger cupboard (but I am forbidden to put 'radio junk' in there! ).

Overall, I am happy with my new system, especially as I know it will be cheaper to run. I have no pecuniary interest in Vaillant nor their products.

Al.

[Mark Hennessy]

Hi,

A good explanation David. People often confuse condensing with combination - an easy enough mistake.

A few minor clarifications if I may; your explanation of combi boilers implied that only combi boilers have a pressurised circuit. It's true that combi boilers require this, but an awful lot of non-combi systems are also sealed/pressurised. In fact, that's far and away the preferred mode of operation.

Also, unless this has changed very recently, the law doesn't state that condensing boilers are required; rather, it states a minimum value of efficiency. In general, to achieve this efficiency, a condensing boiler is selected, but there are (or were, perhaps) a small number of non-condensing boilers that could get there...

A third clarification (sorry!): better boilers use a syphon in the condensate drain, so that a decent amount whizzes out each time - much less likely to freeze than the gentle trickle. But freezing condensate is definitely a big issue. My new boiler installation isn't quite finished - as I need to start rebuilding the kitchen next, but the condensate pipe will be entirely internal when done. Meanwhile, the outside part of the run is in 50mm pipe. I'd like to see that freeze!

Important points I'd add to the conversation:

First, please don't assume that a new boiler will last anything like 30 years - today, 10 years is considered to be good. Sad but true. Will the fuel savings over the next 10 years pay for a new boiler, taking into account the repairs needed? Impossible to judge really - but don't replace your boiler on those grounds. Replace your boiler to get improved controls and comfort, and certainly don't mess about trying to improve the efficiency of your existing boiler by trapping heat inside - it's designed to emit heat when operating - far better to use that heat for space heating IMHO. Any such modification could be incredibly dangerous :omg:

Secondly, to elaborate on my point about sealed systems. The "traditional" system, with a small header tank in the loft, is a problem because the water in the system is always being exposed to oxygen, which means the system is corroding from the inside. You should of course be maintaining a level of inhibitor in the system, but any system leaks slightly, and your system will be continually being topped up with fresh tapwater containing more oxygen, and diluting the inhibitor. Hence, the system will be sludging up. A sealed system in much, much better in this regard, and if you change nothing else, I'd recommend making the change to sealed, providing your old boiler is happy with this. You only need install an expansion vessel, and these are cheap. There's a kit of parts that includes a pressure gauge, pressure relief valve and filling loop - these normally are installed with the pressure vessel in some convenient spot near mains water and the system pipework.

(All me a brief rant: Mum has a British Gas policy, several hundred pounds a year, including an annual "service". What "service" means in this context is a brief safety check; i.e. the minimum they can get away with. Last time I was down, I discovered a BG van parked in my usual parking space, and inside was an engineer telling Mum the reason the system failed was because it was all sludged up. The only solution was a "Powerflush", at the cost of £700. If Mum declined, BG would refuse to fix the system ever again. My question is a simple one: do you check the strength of the inhibitor at each service? Answer was "no". "In which case, if you're not taking such basic preventive measures, the system is guaranteed to sludge up, requiring this Powerflush. How on earth can you justify charging (double the going rate) for this problem that you have failed to prevent?". There was no answer to this question. Unfortunately, Mum had already signed the form. The problem with the system, BTW, was a faulty pump. Nothing at all to do with sludge.)

Back to potential replacement boilers:

I appreciate that you might not be planning to change yours, but for anyone who is, here is the most important fact: the majority of modern boilers use aluminium heat exchangers. This is bad! The flue gases contact the heat exchanger and condense, and the condensate is mildly acidic. This reacts with the aluminium to form aluminium hydroxide, which is a nasty, rock-like substance. In other words, they are self-destructing. It doesn't matter how careful you are about maintaining the correct level of inhibitor (which you must - steel, brass, copper and aluminium in the same system!), the damage is happening on the flame side of the heat exchanger.

I have just replaced an early condensing boiler made by Ideal (there's an ironic name if every there was one!). As part of the annual service you are supposed to thoroughly clean the deposits from the heat exchanger - not that British Gas would bother! But, it's absolutely impossible to reach everywhere, and when doing the "post mortem" on it, I found these deposits blocking channels and routes that are impossible to reach without a major rebuild. It was really shocking - every bit of aluminium in the entire boiler was being rotted away by the condensate. In a few places, there were holes that allowed combustion gases to be recirculated by the intake fan, which would have affected the efficiency adversely. I'm glad the thing was room-sealed, and that I kept a carbon monoxide detector nearby.

So, look for a boiler with a stainless steel heat exchangers - they are slightly more expensive, but it's money well spent in my opinion.

David has already explained that pretty much all new boilers are room-sealed (it is possible to find room-ventilated models for special installations), and that a fan is part of this. Indeed, the fan pre-mixes the gas and air to improve efficiency. Be aware that some models are quieter than others, and that depending on where in your house your boiler is installed, this could be a problem. My boiler is in a cupboard in a bedroom, and the old Ideal was very noisy indeed.

Another feature with modern boilers is that they modulate. My old one couldn't; it could only produce 14kW or nothing. Which of course means it cycled on and off, which is really inefficient. When you think about it, any system will do this when the outside temperature is above the design temperature of -3C or whatever. A modulating boiler avoid this by turning down the "wick", meaning that it can burn longer. My new boiler can produce anything between 4.8kW and 19kW - which is more than needed (I don't have 19kW of radiators), but was the smallest available in this range...

Which leads me onto (yet!) another problem with combi boilers; to get a decent flow rate, you need at least 24 to 28kW. But when the boiler is heating the house rather than water, it's massively over-rated for the sort of house that can use a combi (smaller houses and flats). Which means that combi boilers will be less efficient than an appropriately sized normal or system boiler.

I must admit that when I first moved into this house, I automatically assumed that when it was time for a boiler change, I'd go for a combi and reclaim the space taken from the bedroom that has the airing cupboard. But now I'd never consider a combi. Quite apart from the efficiency issue, being able to take a shower while the rest of the family are washing and the washing machine is filling with hot water is well worth finding space for a hot water cylinder.

When you change to a condensing boiler, in order for it to actually condense, the return temperature must be lower than 57C. With an old system, you would most definitely avoid this because condensation in a cast-iron heat exchanger is bad news indeed! So, to get all of the efficiency improvements from the new boiler, you might need to increase the size of some radiators. Of course, in moderate weather, you won't need the full output from your system, and with the right controls, the flow temperature should be reduced so that maximum condensing can take place...

Which leads me on to the subject of controls. We as a nation are amazingly backwards about all of this! Very few people fit Weather Compensation to their systems, yet in terms of efficiency and comfort, this is so much better than a simple room thermostat. Indeed, my system no longer has a room thermostat. A simple sensor mounted outside on a north-facing wall samples the outside temperature and decides the flow temperature based on the "set point" you have asked for. In the time I've had it working on my new system, I've been very impressed indeed. The house always feels warm, but not too hot, and there is never any time when you feel uncomfortable. But, when you step outside on different days, you really can tell that the system is absolutely magic!

The flow temperature is typically somewhere in the mid-40s. The return is correspondingly lower, meaning maximum latent heat is being extracted from the flue. The radiators feel warm, not hot. But the house is always comfortable. There are digital thermometers in each room, and they all read 20C, amazingly. The TRVs are all wide-open (they rarely work well, despite what the government seem to think about them - they are not an alternative for good design and good controls elsewhere).

I have to explain that I really couldn't see how this would work. With the old system, the rads would be too hot to touch, but the house would be really cold - especially downstairs where we have draughts (Victorian terrace) - so the easy assumption was that my radiators were all too small. But of course, I was doing the typical "twice a day" cycle, where most of the energy going in to the system was warming up water, air and building fabric needlessly. As an engineer, I knew that it would be more efficient to leave the system running continuously at a reduced temperature, but didn't trust the old boiler enough to do that. But the new system does just that, and the gentle background heat is much better for the building as well as comfort in general. I'm awaiting the next gas bill, and perhaps I'll rethink all of this, but given that the boiler is mostly only generating 5kW or so, I ought to be OK... It goes to a "setback" temperature overnight, which you can adjust - currently set to 10C. It only runs when it's very cold outside; indeed, if it's below 3C (IIRC) it runs the pump (and the burner if absolutely necessary) to avoid freezing.

People don't like Weather Controls because installers don't really understand them, so don't explain and promote them. People expect red-hot rads that they can dry clothes on (causing no-end of other problems!). Then their installers have to remove the sensor because they failed to put them on a north-facing wall, and then wonder why the house cools down when the sun comes out (true story!). But so far, I'm converted. Incidentally, if the system doesn't seem to work correctly, there is infinite scope for adjusting the "curve" the boiler uses, so in other words you tell the system your house's heat loss. I haven't fiddled yet - it seems to work as supplied. Time will tell...

Another thing; have you got a "Y-plan" system? If so, you'll need to rip that out. To get the most benefit from a condensing boiler, you need low flow and return temperatures. But, to heat the water, the flow should be somewhat above the target DHW temperature. And certainly the water needs to be hot enough to avoid Legionnaires disease. If you have the common Y-plan system, the boiler doesn't know if it's heating water or the house, so the installer has to compromise by setting a high flow temperature. Bad news all round...

The boiler I bought is a "system boiler". This means that the pump and diverter valve is internal, and there are 4 pipes emerging; 2 go to the radiators, the other 2 go to the DHW cylinder. The boiler does "hot water priority", and when heating water, the flow temperature goes up to >70C. When the water is satisfied, the valve trips over to the heating position, and the flow temp falls back to 40-something. It would be possible to operate this boiler in a 2-pipe scheme, and I could have retained the Y-plan valve, but I would have been absolutely mad to do so.

The boiler I got is here: http://www.viessmann.co.uk/en/products/g...200-w.html

It's amazingly intelligent, and is compatible with solar heating; indeed Viessmann make a lot of these products and I'm quite tempted to install a system. The controls for this plug straight into the boiler, and you can view the "solar yield" on the display. So far, I've been very impressed by it. It wasn't especially expensive either - the gas man who connected up the gas pipe was very impressed as well. He didn't want to get involved with the installation - fear of the unknown - but when I told him how much I got it for, it was less than he would have charged me for a Worcester Bosch (who use aluminium heat exchangers). Incidentally, the boilers British Gas supply (for around £3k!) are rebadged Worcester Bosch models...

I'm sorry to have rambled on, and I know that a lot of this might not be directly applicable to the original question. But, sooner or later, you will need a new boiler, and hopefully some of this might be useful to you and maybe others in a similar position.

All the best,

Mark

PS: another energy saving technique is to use a variable speed brushless DC motor on the pump. I didn't choose this option as it was an extra £300 (at list price), but I like the idea. But apparently it will be mandatory before too much longer. Oh, and Weather Compensation is mandatory in many European countries now, so it won't be too long before we all have to understand it!

[Yorkie]

An excellent post Mark - there's nothing I'd quarrel with there. I tried not to over-complicate my earlier post, and said for example that combi boiler systems don't store water in a cylinder, unlike open vent systems which have a cold feed tank and cylinder. That isn't the whole story - large systems can and do have a hot water cylinder if desired, but not a conventional open vent one. My son's house in Putney has a large combi boiler and a cylinder about 5Ft tall and 2 ft diam, which is a swelf contained fully insulated jobbie.

As to British Gas, I spent my 40 yr career with them, joining as an apprentice in 1954, progressing through sales and service management, a spell in training service engineers on central heating fault dignosis, then in the labs during the natural gas conversion phase desigining burner kits for obsolete appliances which BG would otherise have had to replace at their cost, then I became service manager at Grimsby, Mansfield and Sheffield, before moving to Hull as General Mananger for North Humberside, where I sank gently to earth in a 'golden parachute' in 1994!

Much has happened since, and time has passed me by, so I can't claim to be up to speed on technical developements. However, on the question of an 'Annual Service ontract' from my experience, any unnnecesary dismantling of boilers for a 'service' to try to demonstate value for money to customers risks making the boilers less reliable. The mere act of distrubing thermocouples, burners, removing the cover of the boiler rirks introducing faults. The cal back rate after an annual 'service' hoverved between 10% - 15% mostly within 24 hours. Open flued boilers did need regulary serving especially where there are pets or when irnoning of clothes takes place in the same room as a boiler (kitchen etc) as the venturi of the burner will draw lint into the burner and cause it to 'lint up'. This elongates the flame, causes incomplete combustion, flame impingement on the heat exchanger, causing 'sooting up' and risks carbon monoxide being generated. There are fewer and fewer open flued boilers, so over time, that problem disaappears.

The benefit of such contracts is the annual breakdown cover, but there's so much small print and exclusions, that personally, I think it's best to stand the risk, and call out a GasSafe installer as needed.

Room sealed boilers rarely need to be dismantled, and a combustion check on the flue gases will reveal if the boier is working at peak efficeincy or needs adjustment. Over the 20 years we had our balanced flue boiler (which I've just remembered was an Ideal Standard - not a Glow Worm!) it never needed touching, but I did however have to replace the pump three times.

As to 'power flushing'' - a nice little earner!

It's true that if a system is drained down, or householders are in the habit of removing and refittng radiators to decorate, and in neither instance flussh th syste ad put inhibitor in whenthey fill up, sludge (corrosion of pressed steel radiators etc) can arise, and itslso true that whn a boiler is changed, it's a requirement to power-flush the system. But other than that, it's a needless expense. Any outfit that wants to charge £700 for it should be laughed off the face of the planet.

BG training was, and I guess still is, second to none, and if their servie engineers din't pass the City and Guilds intermediate a Final exams, they didn't qualify. I don't want to bite the hand the firm that pays my pension, but personaly, if I needed any installtion orservice work done, I'd use a GasSafe (Corgi as was) instller who is Ex BG. Such installers live and work in the neighbourhood, their kids go to school with the kids of their customers, they often operate from home with low overheads, will be trained in diagnostics by the companies whose boilers thay install Vaillant, Worcester Bosch, Baxi etc), and they have a personal reputation to protect. Generally, they'll do go job at a fair price.

[pwdrive]

Gosh, all this puts my backwoods log fed system in the dark. In response to Ed's question about in effect modifying his gas boiler (insulation upgrade) then I personally would not touch anything in a gas appliance except user defined operations as defined in the book of words. I can certainly see where Ed is coming from as regards energy costs, it's a real pain, If the whole nation went solar then a solar tax would follow at some point to be sure. As regarding diy solar it would be an idea that would appeal to many including myself, when I collected our wood stove from the suppliers warehouse/workshop I got chatting to their solar installation engineer, he pointed out to me that they don't use conventional plumbing pipes/fittings etc due to the extreamly high temperatures the modern solar panels can attain, all their pipe work was s/steel with a special high temp insulation used to insulate them, no copper in sight.
For the future then solar combined with other sources is the probable way to go using mass hot water storage, and passive systems, these systems are already on the market but are not always compatible with the average housing stock/property, this is I feel where the problem really lies, our houses in general are very poorly designed when it comes to heat retention and solar gain wether the solar system is dynamic, passive or whatever.
To many vested interests I fear. Energy conservation is the way forward for the consumer.

Lawrence.

[Michaelr]

Very interesting thread.

I must say David , it is pleasing to hear of an honest opinion on maintenace contracts from the gas board especially from a senior manager.

I stopped our full contract which I was paying over £30 a month for ( it was the full monty) and had been paying for over 10 years with no claims. A silly situation whereby we had to call back the engineer at every yearly check , these checks used to take about and hour , 15 minutes of actual enginnering interspersed with frequent use of their mobile phones followed by a 40 minutes period to fill in the paperwork while they try to sell you system flushes and whatever else they were promoting at the time.Constant reminders that the combi boiler was getting old and spares maybe a problem and how crap "Halstead " were as a quality of boiler.

The final straw was when one of the bathroom towel radiators was corroding and I was worried about a failure. BG refused cover as it was a towel rail not a radiator.

Since I stopped the contract and got over the phone calls every 2 days by BG to encourage me back I have been putting the £30 a month into a maintenance account. I wished I had done it 10 years ago I could have bought the wife a new car.

We have quite a large bungalow ( 1700square feet) and have always had a combi boiler in the 23 years we have lived here, heating and hot water have never been a problem . Our first unit was a huge Worcester floor standing unit ( about the size of a chest freezer) that we replaced after 10 years purely to move to a wall mounted more compact unit. We have had this unit which is a dreaded "Halstead" that most plumbers referr to as the "devil's spawn" yet has given us trouble free operation and is still doing so , though having finished this posting I do not know how long it will continue to do so ( knowing my luck). I must say this year I intend to replace the boiler with a more modern unit so I am finding the info in this thread most useful.

I am afraid I am a big believer of "each to his own" when it comes down to DIY re engineering. Modifying a boiler even with just lagging would make me feel uneasy, I would work with a knowledgeable decent heating engineer if you intend to reengineer.

Mike

[AlanBeckett]

We had a Halstead Combi boiler at our last house. I chose it because it was the only Company that had a Technical Department that appeared to know what they were talking about - I was worried about whether the pressure and flow available were adequate. I installed it and then got a Gas Fitter to connect it up and test it. It worked faultlessly for ten years and then began to trip out intermittently - overheat. I couldn't find anything wrong so I summoned up the experts. Two of them poked about for a bit and then decided that it must either be the PCB or Heat Exchanger, either of which would be more than the boiler was worth. I persuaded them to ring Halstead Technical Dept. who immediately diagnosed the problem as a dirty fan. They cleaned the fan, tested - it worked fine - stuck a sticker on the boiler proclaiming that they were agents for another (Italian) Manufacturer, charged us about £80 for the privilege and went away. They also told stories about how dreadful Halstead boilers were. Perhaps they didn't break down often enough, or their complexity exceeded the intellect of the average Fitter. After we moved, as far as I know, the boiler continued to work for several more years before it was replaced during a general house refurbishment and extension building programme. A colleague where I used to work has an even older model which he continues to refurbish. Spares don't appear to be a problem.
Alan

[Mark Hennessy]

Hi David,

I'd forgotten that you worked for BG - apologies! I fully admit that I have no training in these matters, but have spent a lot of time researching and planning my installation, and have built up a fair amount of plumbing experience of the last few years. That said, I have a handful of minor plumbing queries that you might be able to help with - I'll start a new thread later on to save taking this too OT...

Sadly, however, I fear that British Gas are rather trading on their reputation. Many people still think of them as "the gas board", despite the fact they have been a commercial company (Centrica) for many years now. I'm no businessman, but I like to think that if I had the massive brand recognition and trust that came with British Gas, I wouldn't abuse it in quite the way BG seem to. A lot seems to depend on the contractor that turns up - many of them aren't BG employees, of course - apparently, if you get a good one, it's worth persuading them to work for you outside of BG. There is a fair amount of discontent out there - just search for "british gas powerflush" to uncover countless complaints about their Powerflush policy. I've never used their products or services, so have no particular axe to grind personally, but still feel cross about Mum's system. If I lived nearer to her, I'd hire the machine and buy the chemicals for around £100, then convert her system to sealed. She could have my old expansion vessel as it's only 3 years old, and the new boiler has its own. Of course, a magnetic filter is a good way to control the sludge: http://www.spirotrapmb2.co.uk/

You make a good point about the servicing, and I fully admit that I'm of a similar school of thought: if it's not broke, don't try to fix it. However, with condensing boilers, they absolutely must be cleaned every year - the aluminium hydroxide deposits clog up the heat exchanger and the channels for the condensate - not to mention the condensate syphon (if fitted). With my old Ideal, these deposits would build up in the bottom and eventually interfere with the ignition and flame-sensing electrodes - after about 13 months the boiler would reliably break down and require a service to get rid of these. New designs put the burner and electrodes in the top so the deposits are building up at the bottom away from the electrodes, but they still need hoovering up. Also, the large rubber O-ring that seals the combustion chamber on most modern boilers deteriorates and should be replaced at each service - when these fail, the consequences are really scary. And the electrodes need cleaning and realigning as they distort in the heat of the flame. I've read that this is important to avoid explosive ignition, and also to insure the ionisation flame-sensing electrode works correctly. In fact, my new boiler measures the ionisation current to determine and correct the fuel/air mixture, like an oxygen sensor in a modern car. I presume that the spacing between burner and electrode might be an important factor here...

I suppose some believe that responding promptly to a breakdown "seems" better to paying customers than preventative maintenance and no breakdowns (I once had a boss who felt the same), but I could never come to terms with this mentality. Apart from anything else, a failure to perform the manufacturers maintenance will invalidate the warranty.

I also agree that system choices are complicated, and while there used to be a time when you'd only ever see combi or open-vent systems in most domestic installations, the situation is much more complex today. With worries about water storage these days, there is a trend towards using smaller cylinders (and loft tanks) in "conventional" systems, allied with high-recovery cylinders and boilers that do DHW-priority. The smaller the cylinder, the more like a combi system you have! Indeed, Viessmann make a kit that I could add to my system boiler that converts my system boiler to a combi - it's just a plate heat exchanger and some switches/sensors - in other words the bits that would have been added to mine in the factory if I'd bought the combi version, but in an external box. While I doubt I'd want to do it, it's nice to know I can switch to combi operation without having to throw away the existing boiler...

Larger combi boilers have water storage built in, constantly kept up to temperature so that there's no waiting for hot water when the tap is first turned on. I think of these as using a very small water cylinder with a high-recovery coil rather than a plate heat exchanger (in simple terms - I've never investigated further). Of course, unvented cylinders and thermal store options are very interesting, and when I need to replace my cylinder, I might consider the latter (mains pressure hot water without the compromises of combi operation). A thermal store can be heated via solar or heat pumps, with the gas boiler providing the final input.

Mike, paying the best part of £4k to British Gas for so little in return is shocking! The final insult is when they convince you the parts are no longer available for your boiler (which actually means they don't stock them in their central warehouse), then quote you over £3k for a really simple boiler swap. This might be OK if the replacement was something like a Viessmann with new radiators and a replacement stainless steel DHW cylinder, but for a mid-range model (rebadged Worcester-Bosch) that's simply criminal. I would strongly urge people to listen to David's advice about finding a local installer - not always easy, but via word of mouth it should be possible. The chap that helped me was recommended by friends, and his wife works in our local butcher, so my wife knows her well...

All the best,

Mark