Condensing boilers


Have just been looking around for suitable replacements for my present gas central heating boiler fitted by myself a lot of years ago, as I have increased the demand on the boiler by house extension.
I read that the new building regs from April 2005 require I fit a condensing boiler as a replacement or else have a justification prepared by an installer (suitably qualified), saying that it is not possible or economic to fit the said condensing boiler. My wishes in all this are to be ignored in the form to be filled in.
I reecently read that condensing boilers are pretty useless in standard CH systems, as the full efficiency is not attained unless the water temperatures used in the system are kept around 50 C, to allow the exhaust to be condensed and for higher temp water, efficiency is about the same as standard boilers exept at startup, whilst the water in the system is cool. As my present radiators require water temps around 70 C to meet the heat load, it seems to imply the use of new low temp rads. Along with the boiler costs being around twice that of a standard boiler, the new rads needed would be huge or fan assisted, and would make the costs punitive, as well as the lifetime of condensing boilers being lower than standard types.
Its taken a long time to get to the point:- But have I got it all wrong and modern condensing boilers are indeed approx 90% efficient for all circulating water temps or as usual has some government department with no more knowledge of heating techniques than my dog dreamed up some arbitrary rules in the name of "Doing our bit for global warming", (NOT)? which has put another nail in the coffin for DIY.
Is there anybody in the group who knows the efficiency variation of condensing boilers with water temp or who actually owns a condensing boiler with high temp rads and has seen the running costs reduce significantly. Various British Gas blurb suggests one third reduction in running costs, is this attainable?
Has anyone filled in the required form to meet the building regs.
What is the end result of ignoring the rules, dark threats in the document seem to indicate inability to sell ones house in the future if rules are ignored.
Looking forward to informed comment
Dave
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I changed from a standard boiler with a HW cylinder, to a condensing direct heat system. After 7 months My gas bill is all ready down over 25% (there was a price rise during this time) and my "leccy" bill down 5%. As I am on a monthly payment system it takes 12 months for the full price savings to cut in. I estimate! that the final gas saving will be about 40%. During the summer the bedroom next to where the HW cylinder was a lot cooler, it used to get uncomfortably hot. The rooms warm up a lot quicker. The only down side is the bath takes a bout 3 minutes longer to fill up.

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There's a points scheme for justifying a non-condensing boiler, and no, your wishes don't count for any points.

The information you read was inaccurate.

You only need the max temperature on the very coldest days of the year, and there are actually rather few of them. Throughout most of the winter, your radiators will still heat the house even with lower temperature circulating water, and the boiler operating in condensing mode. On the few coldest days of the year, the boiler might need to operate in non-condensing mode.
Secondly, even ignoring the condensing side, the heat exchangers in condensing boilers are more efficient than the ones in non- condensing boilers. The waste flue gas temperature is typically only 5C hotter than the water flow temperature in a condensing boiler, verses something between 50C-150C hotter than the water flow temperature in a non-condensing boiler. (This is why some condensing boilers can use plastic drainpipe for the flue.)

The information you read was misleading.

Yes, but only if your existing boiler is a very old and inefficient one (e.g. > 30 years old, or a back boiler). Look up the efficiency of your old boiler, verses a new one, at the SEDBUK Boiler Efficiency Database, http://www.sedbuk.com /

I did mine in 2002, before that was required (and very deliberately so). However, I still fitted a condensing boiler, even back then when it wasn't required.

Possibly. Many of these rules have been in effect since April 2002, and I don't think they have caused many problems for people who have ignored them so far (double glazing have more so). Might be different when seller's packs come along.
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Andrew Gabriel

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<snip>

So whats the best temprerature to set the bolier to?
I have a 1 yr old Glowworm condesning boiler and have no idea what to set the thermostat to.
Steve
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The best, as in most efficient, is the lowest temperature which will maintain the house at the correct temperature. If you get this just right, the boiler will run continuously just short of tripping off at the room stat. Boilers with more advanced controls will handle this temperature adjustment automatically. My condensing boiler doesn't do this automatically, but I have found that a flow temperature of 45C works well on my system (with radiators sized for condensing operation) for most of the winter. Only on very cold days does it need turning up higher, and initially to speed up heating the house from cold.

Condensing operation requires the return temperature to be below 55C, which is the dewpoint of the flue gasses. However, it's not a sudden step function, and the lower you can get it, the more efficiently the boiler runs. Condensing systems are usually designed to operate at 70C/50C flow/return (i.e. average radiator temperature of 60C) when outside temperature is at -3C. At lower outdoor temperatures, they will need to provide hotter water and will move out of condensing mode.
Note that for a non-condensing boiler, the rules are reversed. The return temperature must be high enough to ensure there is no condensing in the heat exchanger, or it will very quickly corrode. The safest minimum return temperature is typically 60C in this case.
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Andrew Gabriel

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writes:

I think Andrew you have just made my point, to benefit from condensing boilers require low temperature heat emitters. Though you say that this is not often a problem in this country as outdoor temperatures don't often stay low for long spells, I would say that in the North of England and Scotland you might be wrong. My system was designed with a flow temperature of 80 C and I need that temperature to maintain my house comfortable during quite a lot of the Winter, I'm afraid I undersized in the first place due to physical contraints on radiator sizes. I boost my cold spells with gas fires. The other point is that my system runs at high water temps but maintains room temperatures by upstairs and downstairs motorised valves and occasional rad stats. I don't really want to be constantly varying boiler temp as the boiler is sited away from living space. If this can be automatic thats fine. But how far do I have to go to get a condensing boiler working at its maximum efficiency, saving me enough money to pay for it over its life and reducing my monthly bills, with new emitters and controls. I'm sorry but I don't think man induced Global warming enters the equation. I don't believe in it. I think Steve Rainbird also brings home the point that people are being forced to install a technology that doesn't give them the savings promised by people like British Gas, (Boiler change only will save you 30% on your heating bill.)
So once again I ask who will tell me the efficiency of condensing boilers at various flow temps, and not just the best tweaked value.
Dave Ball
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You get maximum benefit that way, but even when operating in non-condensing mode, they are more efficient than a non- condensing boiler -- see my first post.

Well, sounds like you are starting off with a badly undersized system. Builders usually oversize radiators in new installations, as they aren't compitent to work it out correctly, and simply oversizing makes very little difference to the cost. As a result, many people find their existing convensional systems easily convert to condensing boilers.

Even if you need new radiators, you probably don't need to replace them all. Some maybe OK, but even if not, you can probably just replace the biggest, and then move the old ones to replace the next biggest, etc.

The government has decided the cost of meeting CO2 reductions is to be borne largely by the public, rather than by industry, which is comparatively being let off. So whilst you are required to operate boilers with >90% efficiency, power stations running at only 30% efficiency are still commonplace. We probably wasted 50% of our natural gas in inefficient usage, but mostly in industry. If we hadn't, we would still have years more self-sufficiency left. Global warming isn't the only issue -- the fact we've run out of gas is another one.

It usually says in the instructions, which you can download from manufacturers' websites.
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     snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) writes:

The SEDBUK figures are not best tweaked values, but are in conditions of typical usage throughout a whole year. For example systems with permanent pilot lights get hit quite badly because they waste around 200W of heat continuously all summer.

Just checking my Keston Celcius 25 manual, the boiler is 10% more efficient at 50/30C than it is at 80/60C. (The return temperature is probably the only one that's important in working out the efficiency, as that's what governs the waste flue gas temperature.)
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Andrew Gabriel

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writes:

Andrew,
Thanks for replying but now I'm even more confused. :)
The display on the front of my boiler says 69. Should I adjust that to 45 as in the 1st figure you quote or leave it so it matches (nearly) the second figure of 70C
Steve
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If the room stat keeps cutting in and out, then you can get more efficient boiler operation by turning down the water temperature. The optimum water temperature is the point where the room stat just stops needing to trip off the boiler. What this optimum temperature is will depend on your system and the outside temperature -- it is unlikely to be the same as mine so I can't tell you what yours is.
Note that if the boiler also heats a hot water cylinder, this is probably also the temperature of the water used to heat that, and if you turn it down low, your hot water won't be so hot either. If it's a combi boiler, it probably has a separate temperature setting for the hot water, although I don't know your model.
The 70/50 figure is the normal design temperature at a temperature of -3C outdoors. Even systems quite accurately designed to this criteria are unlikely to find this is the optimum in practice, as there are a number of potential inaccuraces in the calculations. Most systems aren't accurately calculated in the first place anyway.
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increase in oil prices my total heating bill averaged about 300 per year for a three bedroom bungalow in Scotland (Perthshire) I don't know the efficiency of the boiler but I run the boiler temperature at 70Deg in the winter and 60 deg in the summer. Blair
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tank with non-condensing boiler + gas cooker, but I assume being a modern house is better insulated.
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As far as my understanding of what a condensing boiler does is it circulates the water around the flue to absorb as much heat as possible from the gases before the water goes into the main heat exchanger
i am assuming that its the cold supply for the DHW
The temperature the water is then heated to i dont think has much to do with the efficiency but then i could be wrong
George

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The lower the water temperature the better the efficiency of the boiler. It's impossible for the flue gases to be lower than the temperature of the water so the cooler the water the cooler the gases will be. At their most efficient, condensing boilers run at around half the temperature of a standard boiler. This is what makes them more efficient. Trouble is if you fit a condensing boiler to an existing system and run it at its most efficient then the heat output from the radiators will be about half of what they should be, but this is only a problem when the weather is particularly cold and the radiator temperature has to be raised to overcome the heat losses.

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