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LPG Condensing Boiler Efficiency

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  • 13-10-2019 10:07am
    #1
    barry181091
    Registered Users Posts: 1,593 ✭✭✭


    After my last thread on the (ludicrous) price of Calor gas, now my attention has turned to the efficiency of our boiler that is installed.

    We have a Promax System HE Plus LPG 24 boiler which, according to the documents, is a condensing boiler with an efficiency of 93%.

    Currently the boiler is set to heat the water to 61 celcius but from my research I have found out that the actual efficiency falls off a cliff if it is over a certain temperature (Almost log scale).

    I cannot for the life of me find any sort of recommended temperature in the documentation so I am stumped here.

    I know it all depends on the temperature outside too but if we presume an average of 8-10c, surely an educated guess can be made.

    I did put a thermometer on the boilet outlet pipe and that was at, at least, 45c. This seems like a decent amount of heat loss, unless my standards are too high? :o

    Boiler: https://www.potterton.co.uk/-/media/websites/pottertonuk/files/discontinued-boiler/potterton_promax_combi_he_plus_lpg_install.pdf


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    #2
    Markcheese
    Registered Users Posts: 8,429 ✭✭✭Markcheese


    The lower you can set the boiler thermostat the better... But obviously the lower it is the longer it runs for to get your rads and rooms to your desired temperature, too low and the rooms may not get to your desired temp at all... (that could depend on the size/style of rads, the levels of insulation, amount of air change/ventilation in a room...

    Slava ukraini 🇺🇦



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    #3
    John.G
    Registered Users Posts: 3,467 ✭✭✭John.G


    To get the full condensing benefit (~ +7%) the boiler flue gases would need to be as low as 40C which means a boiler water return temp of even lower, say 30C to 35C. however to avoid heat exchanger corrosion some boiler manufacturers specify a minimum boiler water return temp of ~ 45C, 50C seems to be a fairly practible return temperature you will still get a partially condensing boiler at this temperature and compared with standard (non condensing) efficiency boiler will result in a flue gas temperature ~ 120/150 C lower which will result in a efficiency gain 0f ~ 5 to 7%.

    This 50C return can be achieved in a number of ways and this in turn will have a big effect on the radiator(s) output which are now based on a European standard of 50C and are called "50 deg" rads, it is calculated on the mean radiator temperature minus the desired room temperature which is often based on 20C so a 50deg rad based on a return temp of 50C would require a flow temperature (boiler thermostat setting) of 90C....(90+50)/2 - 20 = 50. Again, it would be fairly normal to have a boiler setpoint of 70C (required to heat the hot water cylinder to 60/65C) so in this case the radiator would be a "40 deg" rad, (70+50)/2 - 20, but its output would the be (40/50)^1.3 or 75% of its design so when sizing the radiator you would oversize it by ~ 34% or go to the next size up.

    This is why using existing radiators with a Heat Pump flow and return temp of 40/35C will only give a radiator output of ~ 25% which just may give enough heat to keep your house warm if run 24/7. but unlikely?.


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    #4
    [Deleted User]
    Posts: 0 ✭ [Deleted User]


    65c is the usual recommenced temperture setting to allow a slight temperature drop between the boiler and hot water cylinder which must be heated to a minimum of 60c.


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    #5
    John.G
    Registered Users Posts: 3,467 ✭✭✭John.G


    I suppose, theoretically at least, the return temperature really governs the combustion efficiency, obviously the higher the flow (boiler) temperature the greater the pipework radiation losses etc so it (flow temp) has a practical effect on the overall efficiency.

    The accompanying chart shows that condensing doesn't commence until 55C with a efficiency of 87%, 50C= 90% and 40C = 95% so if one could maintain a consistent say 45C return temp then there is a additional efficiency gain of ~ 6% greater than running in the non condensing mode. This is probably far easier to achieve with a modulating gas boiler and a circulating pump with proportional pressure control than it is with a "fixed" output oil boiler with the same type of circ pump. It would be very interesting if one had a recording thermocouple on the return pipe and monitor it, maybe some modern gas boilers do this anyway or at least displays it..


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