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Condensing Boilers

By Gordon Shaw

Condensing Boilers are more efficient than the traditional boilers they are designed to replace.

Condensing Boilers:
In principle, domestic condensing boilers fit into the scheme of energy saving in a similar way to Micro CHP boilers. Both achieve a high efficiency by using heat that would otherwise be wasted. The technical difference is in the way that the additional energy is applied. Condensing boilers simply increase the heat transferred to the water while CHP boilers provide electricity via an engine. In practice there is a more commanding difference in that condensing boilers are available and have a track record in the UK and the Netherlands. In 2006, Micro CHP boilers had yet to penetrate the market, and so far as we can judge there is no recognised standard for defining the efficiency such as SEDBUK* (we are informed that work is in progress to deliver an alternative to SEDBUK for Micro CHP boilers whereupon they should be acceptable under the new regulations [see below]).
Condensing boilers are more complex than their predecessors which makes them more expensive to buy and, almost certainly, less reliable over their lifetime.
In Britain (England and Wales*) regulations issued by the Office of the Deputy Prime Minister (at that time John Prescott, no less, was the DPM) require that all new gas boilers fitted from April 2005 should be 'A' or 'B' SEDBUK* efficiency rated and that effectively means condensing boilers (regulations for oil were due to come in two years later). Within the regulations exceptions are allowed and it is possible that a certain amount of abuse could occur because there is not sufficient policing to prevent bending of the rules for monetary gain. It is therefore expected that many of the older boilers will still be fitted by individual installers under pressure from customers to cut costs, but house owners who 'economise' in this way should be aware of a nasty sting in the tail.
How does the Condensing Boiler work?
A condensing boiler is a development of traditional domestic water-heating boiler. Conventional boilers lose a substantial amount of heat with the products of combustion. This heat is virtually wasted and only serves the purpose of encouraging vigorous convection to discharge the exhaust through a flue.
The Condensing Boiler incorporates an extra heat exchanger so that the hot exhaust gases give up much of their energy to pre-heat the water in the boiler system. When working correctly, much of the water vapour produced in the combustion process condenses back into liquid form releasing its latent heat.
The most common fuel in domestic applications is natural gas which is largely methane but boilers can also operate with LPG or oil. Since, normally, fossil fuels are being used, these boilers contribute to global warming.
  What are the gains in practice?
In principle more heat is being extracted and so the Condensing Boiler is more efficient. That is to say it will give the same heating output for lower cost and correspondingly less pollution. In practical terms you should expect a condensing boiler to be 'A' rated (which means a stated efficiency greater than 90%) and the gains depend on what you compare it with. Stated efficiencies rely upon a system operating under carefully prescribed conditions and they are not always realised in practical operation. Nevertheless, comparing like with like, a modern non-condensing boiler could have an efficiency in the region of 80% indicating a theoretical gain of the order of 10% to 12%. Old boilers may be down at 65%, or even very old, heavy-weight boilers down to 55% but since boilers have limited life spans, we guess that there are likely to be few of the very inefficient ones in service. So we think that advertising which claims 40% benefit, or thereabouts, is clearly sales exaggeration.
However, even with a theoretical improvement of 10-12%, all is not plain sailing and the extra processing associated with the condensing technology has its downsides.
What are the disadvantages compared to the Non-condensing types?
Some drawbacks are very obvious and measurable, others, based on anecdotal evidence, are arguable and not easily measured nor anticipated.
The clear disadvantages are associated with the extra complexity:
  • the exhaust gases are relatively cool and a fan is needed to compensate for the lower convection flow.
  • the condensation process produces a liquid which is acidic (due to sulphur and nitrogen impurities), which requires the use of materials not needed in conventional boilers and necessitates the addition of a fluid draining system
  • this draining system can be awkward to route to a suitable drain point and must be adequately insulated against severe frost (something easily overlooked as we can vouch)
  • the exhaust will appear as a continuous plume of steam which can be off-putting in some positionings and gives less flexibility in placement options
  • fitting a condensing boiler to an existing system may require the system to be power-flushed and that can add several hundred pounds of extra expense
The arguable disadvantages come from anecdotal evidence but that does not mean that they should be overlooked:
  • because the technology is relatively new and plumbing expertise is well based in existing technology you may find some difficulty in locating installers capable of doing the work to your satisfaction at a reasonable price and (in the UK) being thorough enough to provide a valid log
  • for the same reason you may find problems with locating plumbers who are capable and willing to repair faults when they occur
  • not all boilers have a good reputation so it is important to choose one with a good record and long guarantee, and this can be a difficult exercise (The consumer magazine 'Which?' did an in-depth test on gas-fired condensing boilers in September 2006). We have been told that several boilers, apparently from different manufacturers, have the same innards.
  • condensing boilers can easily go wrong and can be costly to maintain and it is rumoured that their lifetime can be only half that of traditional boilers
  • the new systems are more sensitive to the method of operation, tuning and state of repair, for example if not working at full load the efficiency may be impaired
  • if excessive servicing is required the cost benefit to the consumer can be reduced or cancelled and the extra fuel burnt by repair vans reduces the environmental benefits
  • We can add our own bit here since in October 2006 we had a new gas-fired system installed (with a Glow-Worm, heat-only boiler). Everything appeared to go well but our plumber (who did a first class job) refused to give a final verdict on how good they are saying there needs to be a 10 year track record before a reliable judgment can be made
What is the current law in England and Wales?
From 1 April 2005 all central heating boiler installations fell under the control of building regulations (the change did not apply to oil-fired central-heating boilers until 1 April 2007). The new legislation states that all gas boilers fitted in both new and existing homes must be condensing boilers with either an ‘A’ or ‘B’ efficiency rating (A= greater than 90%, B= 86%-90%). There are several important points which need to be noted:
  • homeowners are responsible for ensuring their heating complies with Building Regulations
  • if fitting a new gas boiler check that the installer is Gas Safe* (or in the case of oil, OFTEC*) registered
  • your local authority must be told when you have a new or replacement boiler fitted
  • make sure that your installer does this for you and provides you with a certificate to confirm that the new boilers conform to building regulations
  • keep the certificate safe as you will need the document when you come to sell your home
  • there are exceptional circumstances (for example where it is not practical or economic) when your installer will be able to legally fit a non-condensing boiler but make no mistake, if you sell your house, you will then have to produce an ‘exception certificate’ provided by the competent installer
* Notes:
In Britain:
England and Wales were required to conform with the new regulations from 1 April 2005. As far as Northern Ireland is concerned our information is undermined because the government keeps moving the goal posts (this is the norm with government departments, for example see the WEEE legislation for a gross illustration). However, we understand from communications that oil-fired boilers are common in NI and at the last account (Q4 2006) we understand that condensing boilers must be fitted in 'new-builds' but for replacements the legislation should have come into force in April 2007. A useful telephone number for EST energy advice in NI is 0800 512012 and you may find it best to check with them. We understand that in Scotland there were no immediate plans to introduce this requirement, which surprises us.
Gas Safe Register replaces the CORGI register and all gas engineers must, by law, be so registered in Great Britain and the Isle of Man.
OFTEC is the Oil Firing Technical Association (sic)
SEDBUK is a British standard designed to be a realistic measure in practice, it stands for: Seasonal Efficiency of Domestic Boilers in the UK.
Energy rating has often been expressed in BTU/h (British thermal units per hour) but the increasing trend has been to express it in kW (thousands of Watts). Note that these definitions describing the rate of energy production or consumption. To find the total energy produced or consumed they should be multiplied by time so that the units of energy become BTU (or kBTU) or kWh.
To convert from kW to kBTU/h multiply by 3.413
To convert from kBTU/h to kW multiply by 0.293
Condensing boilers, potentially, offer a significant but not overwhelming advantage in terms of cost and environmental pollution, unfortunately the benefits are not always fully realisable. Despite John Prescott's belief in 2005 that the technology was then proven and accepted, the evidence suggests otherwise.
The plumbing trade, in our opinion, has often been slow to adapt to new technology and combined with the customers urge to try to get a good deal it is likely that installations could be dogged by problems for years to come. At the time of drafting (2005), feedback from homeowners who had already been involved with condensing-boiler installations painted a picture that was quite off-putting. Our own later experience was not like that. Not that this makes much difference in future because legislation is in place (in most of the UK) which makes adoption of the new technology mandatory.
The new legislation should be welcomed by the trade because the market is potentially very large and manufacturing and installation standards should rise to embrace the windfall which has been offered.
Likewise, customers should take the opportunity to gain benefits from the potential savings in fuel costs. The outlay will be more initially and they will have to deal with suppliers and installers who have proven themselves competent, and that is likely to increase costs further. Added to that is the question of reliability and higher maintenance costs. However, the most expensive way forward would be to penny pinch and negotiate special deals with inexperienced tradesmen who may not achieve either competence or compliance. Those, in England and Wales (and later for NI) who have an installation from April 2005 and have not complied with the legislation will find that the normally traumatic experience of moving house just got worse.
OK, so we've painted a depressing picture but let's realistic. The pressure is there, the opportunities are there, some of the existing customer feedback has been congratulatory and we are assured from what we read that the situation has improved. Our personal experience is positive, as of Q1 2013, (except for the fitters insulation error mentioned above) and so we feel that modest long term benefits for the individual and for the environment are likely to be accomplished.
Comparisons we made late 2006 between condensing and micro-CHP boilers left us in no doubt that, in the UK, condensing boiler technology is far preferable to the (notional?) CHP product for individual dwellings. When we had to carry out a domestic installation from scratch,about that time, we opted for a gas condensing boiler system and the choice was clear for a variety of reasons. Further, the two highly regarded heating installation engineers (aka CORGI (Gas Safe) plumbers at that time) we consulted (not to mention British Gas) had no experience and virtually no knowledge of micro-CHP as an alternative.
If you want to know more about CHP including micro-CHP we recommend you see our reference article on that subject (see the drop-down from Renewable Energy in the menu above).
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Originated: 13 June, 2005,  Last amended: 29 October, 2013