Boiler sizing - smaller or larger?

Discussion in 'Plumbers' Talk' started by diymostthings, Dec 13, 2012.

  1. diymostthings

    diymostthings Well-Known Member

    No 2 son needs a new boiler. Done the heat loss/DHW calcs. and they've come out at about 16kW. We've decided on the Baxi Solo heat only "HE" series.

    Question: As we are near the upper limit of one boiler (the 18HE) and the lower limit of the next boiler (the 24HE) which one should we choose? One would work hard, the other would coast along!

    Thanks

    diymostthings
     
  2. Like most (all?) modern boilers, these fellows 'modulate' which means they'll turn down the burner to suit the demands of the system - a bit like turning down your gas hob to a simmer when your potatoes boil.

    So I'd be inclined to go larger - the 24 - as it'll definitely do the job, with summat in reserve if needed. Whereas the smaller boiler can only go higher than its prescribed rating by bursting into flames - it could well be found wanting at times, and will have to be going flat out.

    If you son is replacing any rads - espec in the living area - then I'd also recommend he goes 'large' there too. Eg: fit doubles where single-panels were there, or ones with convector fins, etc. This way he'll be able to run his CH at a lower temp which will increase the efficiency of the boiler.
     
  3. diymostthings

    diymostthings Well-Known Member

    Thanks for the words of wisdom DA - as always! Well I agree to some extent but my concern would be whether the larger one would short cycle? (more wear and tear?)

    diymostthings
     
  4. It doesn't 'cycle' as much as turn the bruner down to a lower rate. My own 30kW jobby (it's that 'big' 'cos it's a combi and needs it for the DHW supply) will modulate it's burner down to around 7kW if that's all that's needed. You can see the condensate vapour from the flue go down to a whisp at these times, whereas at full flow it looks like the shuttle taking off...

    I have to confess, tho', that I'd overlooked your calculated demand when I made my first reply; if your 16kW demand is correctly worked out, then the 18HE will in theory handle that comfortably too, so will likely be more than adequate.

    T'Pros will be on to advise shortly, I'm sure :)
     
  5. hibeealex

    hibeealex Member

    We boiler size for a reason, so if you think its 16Kw go for the 18 in case you have miscalced or add a rad later, there is absolutley no reason to go for a 24Kw, re sizing the rads as stated in a waste of money and complete nonsense, condensing boilers only condence until they reach 50 Deg approx, an 18kw modulates exactly the same as a24Kw so its not working any harder than any other boiler, when temps are being reached the boiler modulates down
     
  6. diymostthings

    diymostthings Well-Known Member

    "We boiler size for a reason..."   "....18kw modulates exactly the same as a24Kw so its not working any harder than any other boiler,..."

    Thanks for that hibeealex - We''ll probably go for the 18 then. Incidentaly your two statements above seem to be contradictory?

    best

    diymostthings
     
  7. diymostthings

    diymostthings Well-Known Member

    Thanks again DA - yes we will go for the 18HE. Now looking for a supplier of Main boilers (they are the same as Baxi but I heard a bit cheaper for the same "innards" - none of the national chains seem to sell them though).

    Best wishes diymostthings
     
  8. hibeealex

    hibeealex Member

    I merely meant that regardless of the size of a boiler it modulates the same ie varies heat output, so a 24Kw will have a range and will work within it and a 18Kw will have its range and work within that
     
  9. diymostthings

    diymostthings Well-Known Member

    OK hibeealex - That's understood. I wasn't trying to be difficult - just needed to be sure of what was said for my own benefit!

    Thanks again

    diymostthings
     
  10. "...re sizing the rads as stated in a waste of money and complete nonsense..."

    What I stated was "If you(r) son is replacing any rads - espec in the living area - then I'd also recommend he goes 'large' there too. Eg: fit doubles where single-panels were there, or ones with convector fins, etc. This way he'll be able to run his CH at a lower temp which will increase the efficiency of the boiler."

    I certainly wouldn't advocate replacing rads if not required - as you say, this would be a waste of money and the small gain in efficiency would take an age to recoup.

    However, I believe having larger rads on a condensing system is more efficient.

    A cond boiler will condense based on the water return temp. With larger rads, the flow temp can be turned down lower and the return will therefore also be lower. Ergo the boiler will condense more effectively and remove more heat from the combustion gases.
     
  11. tom.plum

    tom.plum Screwfix Select

    long ago when I was a boy ( and moses was alive) we had range rated boilers, where you could set the output of a boiler just by setting the gas pressure, it was always a good place to park your car too, outside the terminal as it would stop your car frosting up, alas since then things got complicated with pcb's and condencers and i don't understand them anymore, apparently its no longer a good place to park your car near the terminal, because there's no heat coming from it and also the condencing water that comes out of the terminal will eat through your car quicker then aliens spit.
     
  12. hibeealex

    hibeealex Member

    DA take it from me having large rads is not more efficient and neither is turning down the flow temp. In reality condensing boilers are a gimmick, when setting up a boiler the usual requirement is 82Deg flow and 71 deg return or 11 deg between the 2, As stated most condensing boilers stop condensing at around 50Deg so in effect they are most efficient at the start of the heat cycle and get less efficient as the temp increases, the best use of a condensing boiler is in underfloor heating systems where the flow temp is up to 50 Deg so the boiler is continually condensing.
     
  13. That's the theory behind larger rads - they'll give out the same amount of heat as smaller rads whilst running at a lower temp. So the flow temp can be correspondingly lower.

    No doubt what you say about underfloor heating is correct - if you can have your boiler running that low, then it'll be at max efficiency. But I don't think it completely stops condesning at temps above 50oC, tho'. I sked the very same thing a couple of years back on this forum as I'd assumed the same as you - how on earth can a boiler still condense when the return temp is as high as, say, 60oC? The answer I was given (can't remember who by) was that if you have enough water vapour being produced, it'll condense out on any surface it can even if that surface ain't as cool as you'd imagine - even warm surfaces will have condesnation forming on them if there's enough moisture being produced. And combusting gas produces a lot.

    Even when my GlowWorm has the rads up to max temp, and is cycling down to suit, there's still a plume being produced and a steady 'trickle' sound coming from the boiler.
     

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