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Thread: Boiler Efficiencies

  1. #1
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    Default Boiler Efficiencies

    My 16 year old Ideal Classic boiler had its annual service and MOT yesterday. The engineer left a copy of the flue gas analysis taped to the boiler front which showed a Net Efficiency of 91.1% which I think equates to a Gross Efficiency of 82.8%. Looking back to the original boiler specs, it is listed as having a Gross Efficiency of 76.1%.

    I naively thought that a boiler's efficiency would reduce with age. I appreciate that the flue gas analysis was carried out after the burner etc had been cleaned and the boiler checked, but to me the efficiency value looks odd. I have used the same engineer for the past 10 years so I have no reason to believe that his test kit is out of calibration.

    Am I missing something or do boilers become more efficient with age?

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    Not that I can see..

    How exactly does this flue gas analyser determine the efficiency anyway ?

    Surely the only way to measure efficiency properly would be to measure gas flow rate, (energy in) output and return flow temperatures and flow volume. (energy out)

    If you know the temperature differential between flow and return, and the flow rate you can calculate the total amount of energy being put into the circulating water by the boiler heat exchanger, and the total amount of energy going into the boiler is indicated by the gas meter flow rate. This would give a very accurate determination.

    I can't see how you could get an accurate determination simply by measuring the flue gas contents. An approximation based on a number of assumptions perhaps.
    Last edited by DBMandrake; 7th January 2017 at 10:56 PM.

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    The level of detail on the analysis print out is quite detailed and analysers are used when commissioning new boilers. The full readout was as follows:

    O2% 9.6

    CO (ppm) 27

    CO2% 6.4

    Ratio 0.0004

    Pressure (mBar) -0.07

    Temp Net (C) 135

    Temp Flue (C) 157

    Eff Net (%) 91.7

    XS air (%) 85.1

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    wondered the same, and found this :

    https://www.kane.co.uk/knowledge-cen...ler-efficiency

    Chris

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    Boiler heat exchangers scale and foul reducing thermal heat transfer efficiency

    Combustion efficiency is only one part
    I work for Resideo, posts are personal and my own views.

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    Quote Originally Posted by chris_j_hunter View Post
    wondered the same, and found this :

    https://www.kane.co.uk/knowledge-cen...ler-efficiency

    Chris
    Hmm...

    The efficiency of modern condensing gas boiler can be over 100% as heat is extracted from the incoming air.
    No it isn't - a boiler is not a heat pump. The efficiency can exceed 100% because most boiler manufacturers 'cheat' when quoting efficiency by using a rather artificial definition of efficiency, 'net efficiency' designed to make boiler efficiencies look better than they really are. The calculation uses the net calorific value of the gas rather than the gross - ie. the energy content of the gas as measured without the water vapour in the combustion products being condensed.

    The author must know this so it seems odd that he got this wrong as it isn't something you could attribute to a typo. But he then goes on to say:

    Net combustion efficiency calculations assume that the energy contained in the water vapour which is formed as a product of combustion is recovered and is not exhausted from the flue or stack.

    Gross combustion efficiency calculations assume that the energy contained in the water vapour is not recovered.
    Which are the wrong way round. Oops! Ok, nitpicking perhaps and I'm sure most people here already know all this but it would be confusing to those who aren't familiar with the subject.

    A good explanation of gross and net efficiencies, and why boiler manufacturers tend to use the latter, can be found here:

    http://www.lovekin.net/boiler-effici...s-and-net.html

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    Quote Originally Posted by top brake View Post
    Boiler heat exchangers scale and foul reducing thermal heat transfer efficiency

    Combustion efficiency is only one part

    Any reduction in efficiency due to heat exchanger fouling will cause the flue gas temperature to be higher than when clean. The combustion efficiency analyser described in that link isn't just determining combustion efficiency (how efficiently the energy in the gas is converted to heat) but it also calculates how much of that energy is extracted by the heat exchanger by measuring the temperature of the flue gases.

    The main inaccuracy arises from combustion heat which doesn't make it to either the water or the flue, Eg. heat through the case - but that will be useful heat in many cases anyway. The only other source of inaccuracy that I can think of could be due to the temperature and humidity level of the incoming air (humid air will require more energy to heat than dry air and presumably will affect the rate of condensation); I don't know if the analyzers measure these - and I don't believe it's easy to measure humidity to better than a few percent anyway.

    How accurate the analyzers actually are in measuring temperatures, oxygen and CO levels etc. and thus their calculated efficiencies, is another matter but I guess they should be pretty good, especially as they are supposed to be calibrated annually.

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    On various Dutch websites, Intergas is claiming that their new Xtreme boiler has a Net Efficiency of 122%.

    https://www.warmgarant.nl/intergas-xtreme.aspx
    Last edited by HenGus; 10th February 2017 at 05:43 PM.

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    Quote Originally Posted by HenGus View Post
    On various Dutch websites, Intergas is claiming that their new Xtreme boiler has a Net Efficiency of 122%.

    https://www.warmgarant.nl/intergas-xtreme.aspx
    Well that helps deal with global warning then - we need lots more of this technology that puts out more energy than you put in! Wonder if it feeds gas back into the mains if you turn the output low enough?

    Of course there are a few more plausible possibilities given that it shouldn’t be possible to achieve more than 111% (net) efficiency with natural gas:

    1) An accredited independent Dutch test company is in danger of losing its accreditation
    2) An instrument calibration service company somewhere is due a good b**licking
    3) Intergas have incorporated new technology to continuously extract energy from subspace/parallel universes (they probably nicked the idea from Stargate Atlantis).
    4) Zaphod Beeblebrox was visiting nearby at the time they were testing and the results were influenced by the infinite improbability drive
    5) The heat exchanger, combustion chamber or some other internal parts are made of combustible materials or have a combustible coating
    6) The test house happened to get an especially potent supply of natural gas with extra oomph at the time of their testing
    7) The test house got some exceptionally good herbal product with extra oomph at the time of their testing
    8) The announcements were a publicity stunt for marketing purposes - e.g. the incoming combustion air was at 40C and the return water temperature was 20C

    After much searching, this is what I found; it turns out that the real answer is probably rather more mundane than most of the above and it’s down to the definition of efficiency as required by EU energy labelling directive 2010/30/EU. If you look at the (attached) label for the Intergas Extreme 36 you see that the heating efficiency (Opwekkingsrendement) is 95%. The (Hs) means that it is based on the gross calorific value of gas. I assume this a seasonal rating.

    The 122% claim is for the water heating efficiency, ‘Jaartaprendement (Hi) 106.4% - 122%’

    None of the online dictionaries or translators can translate jaartaprendement but I believe it means annual (yaar) tap water heating efficiency. Hopefully a Dutch speaker can help out. This is a net efficiency (Hi).

    The relevant directive I believe is this one:

    ‘Commission Delegated Regulation (EU) No 811/2013 of 18 February 2013 supplementing Directive 2010/30/EU of the European Parliament and of the Council with regard to the energy labelling of space heaters, combination heaters, packages of space heater, temperature control and solar device and packages of combination heater, temperature control and solar device’
    http://publications.europa.eu/resour....0006.01/DOC_1

    The above, 811/2013, details the relevant calculations and factors for space and water heaters.
    There is also 812/2013 for water heaters, hot water storage tanks and packages of water heater and solar devices which seems to contain pretty much the same information as in 811, but only that relating to the water heating rating of combination boilers. As such 812 is easier to navigate.

    http://eur-lex.europa.eu/legal-conte...3R0812&from=EN

    812, Table 1, Annex II (attached) details the required efficiencies for the energy label ratings G to A+++ for various sizes of water heaters. The Intergas combi is size XL and its gross efficiency of 110% means it gets an A rating as it’s in the range 80% <= eff < 123%. Anybody else think that is a huge band rendering the band ratings useless as practically all current appliances will be in the same band?

    To get an A+++ rating the efficiency of an XXL sized heater would have to exceed 213% - but presumably those ratings are only going to apply to heat pumps or solar heaters which are also covered by this labelling standard.

    For measuring efficiency, 812 annex VII, table 3 specifies 24 hour load profiles for hot water usage ie. drawing off certain specified quantities of water (actually quantities containing specified, useful, heat energy rather than a particular volume) at specified flow rates and temperatures to model typical household usage. The efficiency is calculated from the total amount of useful heat (that is water > 25C) obtained in the 24h period, divided by the amount of fuel and electricity used. The calculation is specified in 812 annex VIII 3 (a). Annex I defines the relevant terms.

    The efficiency that must be listed in the product’s documentation is specified in Annex IV, 1.1:

    (e) the water heating energy efficiency in %, rounded to the nearest integer and calculated in accordance with point 3 of Annex VIII, whereby: for solar water heaters and heat pump water heaters, under average climate conditions;
    For non-solar/heat pump heaters there doesn’t seem to be any reference to climate conditions (ie. outside, combustion air temperature and humidity) or the incoming water temperature which are bound to impact efficiency. I wonder if this is an oversight or perhaps they simplified (not a common EU trait) by assuming that incoming water temp is pretty stable at around 5 to 10C all year round? Does this give manufacturers a loop hole to test with, for example incoming water at 1C and saturated outside air at 30C to exaggerate the efficiency their appliances - ala European car official fuel consumption figures? Perhaps they are specified elsewhere in the regulations.

    So how do they achieve efficiencies above 100%? I guess it’s because with incoming water at (say) 5C and outside air at (say) 20C, then it’s possible to have exhaust flue gases leaving at a lower temperature than the incoming air. That allows additional energy to be extracted from:

    1. the sensible energy from cooling the nitrogen and inert gas content of the intake air from outside ambient to exhaust temperature (ignoring NOX)

    2. latent energy from the condensation of any humidity contained in the intake air

    3. sensible energy from the cooling of the water condensed from the incoming air’s humidity from ambient to exhaust temperature

    4. sensible energy from the cooling of the condensate and other combustion products (mostly CO2) from 25C to the exhaust temperature. That is because the gross calorific heating value of the gas is defined as the amount of heat released by a specified quantity (initially at 25°C) once it is combusted and the products have returned to a temperature of 25°C, which takes into account the latent heat of vaporization of water in the combustion products. From this will have to be deducted the sensible heat required to heat the mains gas to 25C (again incoming gas temperature doesn’t seem to be specified) and the oxygen content of the incoming air to 25C.


    I can't think of any other way that efficiency could exceed 100% (gross), 111% (net) - particularly as the electricity for gas valves, pumps etc. have to accounted for (including 60% for average power station generation efficiency). Start-up losses also have to be accounted for as water delivered below a minimum (specified in the load profile tables) doesn’t count as useful energy.

    Interestingly (to some maybe!) larger heaters should have an advantage over smaller ones because of the load profiles; an XL has to deliver 19.07kWh in 24h of which 87% is in a few (6 out of 30), relatively larger deliveries. An M sized heater has to deliver 5.845kWh but only 66% in larger deliveries (4 out of 23) and will likely suffer somewhat higher start-up losses, though possibly partially compensated by being able to use a lower volume heat exchanger.

    Published efficiencies, being measured for very specific load profiles, may be very different from those achieved under different usage scenarios – such as someone using 25 cu meters/day I'm not sure as to how useful having one efficiency figure is; I wonder why they didn't demand ratings for at least a low load and a high load profile for each size boiler to allow the system design to better choose a boiler for different end uses - eg. a house with no baths and one with long pipe runs where the boiler start-up losses become less significant? Too much information for most end users perhaps but important for smaller commercials users for example.

    Finally, from 811/2013 annex IV:

    2. The heater model shall be considered to comply with the applicable requirements set out in Annex II to this Regulation if:
    (c) the water heating energy efficiency ηwh is not more than 8 % lower than the declared value at the rated heat output of the unit
    So that means your appliance could be up to 8% less efficient than declared. I wonder how much, if at all, this gets abused? It gives a manufacturer an opportunity to select the unit used for certification, or even modify/tweak it knowing that the almost all real production models will be below that – so long as they can guarantee the worst is no more than 8% below the tested model. There must be some temptation to game the rules to try to get at the top of the efficiency rating lists.

    In conclusion I expect that the 122% net efficiency claim is plausible (but it could not be achieved for central heating other than for starting up a very cold system). If they really were trying to exploit loopholes I’d expect other manufacturers to be screaming foul in short order – though it’s early days yet for this particular boiler.

    I think I’ve had my fill of reading EU regulations for the time being; I don’t suppose anyone’s still reading this but if anyone can shed any light on the air, water and gas temperatures and humidity test conditions I’d be interested to hear.

    [EDIT] Added missing attachments
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    Last edited by splin; 12th February 2017 at 05:14 AM. Reason: Missing attachments

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