06-01-2012, 01:12 AM
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!
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!
