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Measuring KW of rads before ordering gas boiler

  • 11-09-2019 2:22pm
    #1
    Geuze
    Registered Users Posts: 13,051 ✭✭✭✭


    Hello,

    I plan to get a new gas boiler, to replace existing Ferroli Tempra 24 gas boiler.

    In order to choose the output of the new boiler, I am trying to measure the radiator KW. I would appreciate advice.

    There are 14 radiators, all 450mm high, they appear to be Purmo Compact, this model:

    https://www.purmo.com/int/products/panels/purmo-compact.htm

    They seem to be Type 11 and Type 22, with lengths range from 600mm to 1600mm. Using this data:

    https://www.purmo.com/docs/Purmo_tech_catalogue_panel_radiators_full_PR_05_2017_EN_PL.pdf

    I added up all the watts to get 13.3 KW.

    I used the 75/60/20 degree figures.

    Does that sound correct?


    An employee for a boiler installation firm estimated (14)(1.5 KW) = 21 KW


Comments

  • #2
    John.G
    Registered Users Posts: 3,466 ✭✭✭John.G


    Rad outputs are based on "50 Deg" which is the (flow+return temps)/2 minus the required room temp, so if 20C is the typical required room temp 75C/65C/20c will give you this... ((75+65)/2 - 20) = 50C. You choose "47.5 deg) rads ((75+60)/2 - 20) = 47.5C. so you will have to x the rated outputs by a factor of 1.08 to get the same output so you will now have to go to the next size up rad(s) or be happy with 93% of the rated output. Also to get most or some of the condensing effect of your boiler you should aim for a return temp of 45 to 50C, if you use 50c then you would require a flow temp of 90C to achieve this, (90+50)/2 - 20. If you use your 47.5C number then a flow temp of 85C will be sufficient, (85+50)/2 -20, so oversizing the rads by 10% or so is no harm IMO as you can achieve very good condensing effects without a very high flow temperature.

    As you will be replacing the boiler with a modulating one then even if you install a 24Kw new one, it will still modulate down to ~ 5 or 6 kw which should be fine.
    but 20kw may do be sufficient with further downturn to ~ 4 or 5 kw.


  • #3
    Geuze
    Registered Users Posts: 13,051 ✭✭✭✭Geuze


    Sorry, sorry, that was a typo, the Purmo Compact rads factsheet displays the wattage according to 75/65/20.

    My typo, sorry.


  • #4
    Geuze
    Registered Users Posts: 13,051 ✭✭✭✭Geuze


    I asked about Viessmann Vitodens gas boilers, and the rep suggested the 100W model, output either 26kw or 30kw.

    They did their sums as follows: (14 rads)*(1.5kw) = 21kw +3kw for the hot water cylinder = 24kw.

    They suggested going for the 30kw, to be sure.

    But if my sums are correct, based on the Purmo factsheet, I have 13.3kw of rads?


  • #5
    John.G
    Registered Users Posts: 3,466 ✭✭✭John.G


    If you are happy with the existing rad(s) output of 13 kw then IMO a 24/26 kw boiler would be more than adequate and leaves you with room for any further expansion.
    I have a 20 kw oil fired boiler with total rad output of 12.5kw (12 rads), some of my neighbours have, over the years installed gas fired boilers in the range 15/18 kw and are quite happy with them but personally I would like to have a bit to spare.


  • #6
    Dtp1979
    Registered Users Posts: 12,725 ✭✭✭✭Dtp1979


    30kw is ridiculous overkill and not needed. 18kw will do fine. I’m not nearly the mathematician that John is so I won’t attempt the sums.
    If you install a 30kw boiler on a relatively small system then the boiler will blast 30kw into the system on initial fire up. This is wasteful. Yes, the system will prob heat up faster but the boiler wont be in condensing mode for long until it modulates.
    Oversizing a condensing boiler reduces efficiency.


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  • #7
    Geuze
    Registered Users Posts: 13,051 ✭✭✭✭Geuze


    Thanks everybody.

    A final query - should I be looking for system or combi boiler?

    https://www.viessmann.co.uk/products/gas-boilers/gas-condensing-boilers/vitodens/vitodens-100-w


    Mine will be a replacement for existing Ferroli Tempra 24, in regular house with tank in attic, and cylinder in hot press.


  • #8
    Dtp1979
    Registered Users Posts: 12,725 ✭✭✭✭Dtp1979


    Geuze wrote: »
    Thanks everybody.

    A final query - should I be looking for system or combi boiler?

    https://www.viessmann.co.uk/products/gas-boilers/gas-condensing-boilers/vitodens/vitodens-100-w


    Mine will be a replacement for existing Ferroli Tempra 24, in regular house with tank in attic, and cylinder in hot press.

    If you have a cylinder then you need a system boiler. Personally, I wouldn’t rate viesmann boilers. Overpriced and overhyped


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