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Discuss Boiler advice needed in the Central Heating Forum area at UKPlumbersForums.co.uk.

  1. doitmyself

    doitmyself Well-Known Member

    I moved house three weeks ago and now, for the first time, have a combi boiler - it's a WB Greenstar 37CDi.

    Among the documents given to me is a Service/Maintenance Certificate dated January 2019 whichsays that the boiler is safe to use. Attached to it is the output from a FGA which gives these figures:

    02 - 4.9%
    CO2 - 9.1%
    COppm - 25
    CO/CO2 - 0.0002

    Are these numbers what would be expected from a boiler which is nearly 11 years old?

    I am also concerned about the suitability of the boiler for heating. I did a quick whole-house type calculation which showed that the heat loss is about 9 kW. I have measured all the radiators and, using online data, worked out that they add up to about 9.5 kW. So far, so good. However the CDi 37 has a minimum output of 9.4 kW, so the boiler will be running in on/off mode all the time. I also doubt if the boiler will ever be condensing.

    Should I change the boiler now for one with a much lower minimum output or wait until the boiler dies? Ideally, I would convert to a system boiler and unvented cylinder but there is not enough space for both cylinder and boiler in the airing cupboard. One would have to go in the loft.

    I have a dilemma: what should I do??
  2. bogrodder

    bogrodder Plumber GSR

    You’ll probably only get in minimum ch output of about 5/6 kw on any combi, it’s all about the turn down ratio.

    Unvented in a loft, you’d lose pressure at your taps. Put boiler up there.

    9kw is quite low, must be a small house... so a bathroom and a half maybe, if so I’d keep your expenses down and put a decent combi in.
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  3. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    combis are sized on hot water demand

    if your unsure get it serviced by someone you know (your gas safe engy)
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  4. Millsy 82

    Millsy 82 Plumber GSR Top Contributor!!

    9.1 may be slightly low depends on how old the boiler is. But I wouldnt be too concerned as that is more fine tuning.

    I would always advise getting the boiler serviced by an engineer you trust as you never know who they got to do the service.

    As for ripping it out. Live with it for a bit and see how you get on with it. Give it the winter and then if you are not happy with it then plan to do that next summer.
  5. EvilDrPorkChop

    EvilDrPorkChop GSR

    I'd leave it till it needs replacing, at 11 years old it'll need doing soon anyway!
  6. Harvest Fields

    Harvest Fields S. Mod Trusted Plumber GSR Top Contributor!!

    Or it could be too high. Dependent on what test was done?
  7. doitmyself

    doitmyself Well-Known Member

    Which is why so many combi boilers are oversized for CH.

    That sounds like a good idea as the sevice log in the installation manual says that the last service was done in February 2012, when the boiler was just over three years old. (Service only done to comply with warranty??)
  8. Gotcha

    Gotcha GSR

    Dont worry about it. Values are good, however at 25 co ppm and R.0002 this hasnt been done on max output. But theyre allright. The boiler wont be on/off all the time, because the 9.5kw min output doesnt mean it will only ever fire up on minimum output, thus barely negating losses, no. It can and will go higher. Certainly not constantly cycling.
    Keep it running for as long as you can. Buy a spare fan if you come across one for a good price (ebay..).
    I have a 16 year old potterton system boiler and a vented cylinder on ground floor flat with tank in loft. Changed the fan on it 5 years ago. Pcb is faulty, it will constantly lock out because it thinks it overheats but resumes operation in a few minutes after, would be problem if combi but its not, didnt even notice until fan died. Its literally so old that the case looks warped and twisted, it fits fine though and doesnt leak. Dodgy gas suppliers would still charge me the same amount if I had a AAAAA++ boiler with 90% less gas usage than current one, so its literally pointless to change it. It will always be repairable, its that simple, until it melts or crumbles away.
  9. doitmyself

    doitmyself Well-Known Member

    A proper check would require the system to be checked at max and min output.

    That depends on how the boiler behaves after the internal thermostat turns it off. Many boilers, particularly older ones, always start at max output, which causes the temperature to rise rapidly. The boiler modulates down until, hopefully, equilibrium is reached and the boiler runs continuously at a low output. But if the amount of heat required is less than the boiler's minimum output, the water temperature continues to rise, the boiler shut offs, and the cycle restarts.

    Even if the boiler restarts at minimum output the same thing will eventually happen, but the on period will be longer as the rate of temperature rise will be slower.
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