How easily can you add an unvented cylinder to a combi boiler?

Discussion in 'Plumbers' Talk' started by NickjfG, Nov 30, 2012.

  1. NickjfG

    NickjfG New Member

    We are renovating a large 5 bed semi and are being advised that the 7-year old Worcester Hi-Flow 24kW boiler (with an 18L delivery rate) won't be able to cope with our plans to add a bathroom and an ensuite; we are also being quoted some high rates (well over 10 grand) for installing a new system boiler with unvented cylinder and new radiators.
    I recently stayed in a house which appeared to have a combi boiler feeding hot water to the kitchen AND a megaflow serving 3 bathrooms...this got me thinking (having previously thought, this wasn't possible!). Obviously our 24 kW boiler is on the small side for the size of the house, but what is to stop us doing this?  We obviously want to have decent shower pressure in the showers and be able to run up to 3 showers at the same time. Also, can you get electrically-assisted unvented cylinders, in case we have guests staying and need more hot water than usual?
    Any advice gratefully received!
  2. hibeealex

    hibeealex New Member

    This is sometimes done in cafe's & restaurants, you could put a motorised valve into the heating side with a tee piece to the cylinder that way heat can  be diverted to the cylinder, I think all cylinders come with a thermostat socket so you can have electric"boost" if you wish, you could leave the kitchen or nearest hot tap  fed from the boiler just to function the flow divertor
  3. NickjfG

    NickjfG New Member

    Thanks for your answer. Do you think, though, that we are being foolish keeping this boiler (given that it is not condensing and that it is only 24kW)?
    It is located at the back of the kitchen which is in an extension out the back; so there will be quite a distance between it and any tank we put into the loft.
    One last Q: there has been quite a lively discussion in these forums in the past over the merits of unvented cylinders vs. Thermal Stores.  If we kept this old boiler, do you have any ops as to what would be most sensible (we have radiator heating throughout, and since we will have to retile the roof in the foreseeable future, we won't be wanting to put up solar panels in the short term)?
    Thanks again, Nick
  4. petertheplumber

    petertheplumber New Member

    As long as your gas supply pipe is increased in size, at the moment I bet its 15mm but it needs to be 22mm or if its long enough some of may need too be 28mm if you go to 28kw or greater. 28kw will give 11.5ltrs /min of hot. 32kw will give 13ltrs a min make sure you have enough cold water flow too, 13ltrs of hot needs 15ltrs/min minimum of cold coming into the house for it.
  5. hibeealex

    hibeealex New Member

    Nick 1st things 1st a condensing boiler is always quoted as saving 30% this is ******** the usual saving is 15%- 20% but they quote the higher figure to sell them, nearly every boiler on the market over 24KW is downrated to 60% of output for heating so a 24KW boiler actually gives you more heating than a 28KW or a 30Kw, its only when giving hot water that the higher demand is used, you can increase the output quite easily if you have the right literature, a 7 year old boiler isnt old at all there are many worcesters and others going strong after 20 years and plenty of 15 year old combi around, if you did replace the boiler consider putting it in the attic, its not something you go to every week and keeps kitchen space clear, unvented cylinders are better than heatstore IMHO they dont sludge up, your best bet is to get a decent plumber round and see what he says
  6. NickjfG

    NickjfG New Member

    Many thanks, Hibeealex, this is helpful and reassuring!  We will definitely  keep the old boiler for the time being and I will quizz the plumber we  use on the heat store question.
  7. NickjfG

    NickjfG New Member

    Thanks, Peter, for this...didn't understand it, though!  If we add a tank in the roof, does the gas supply pipe matter? (we've decided to keep the extg boiler in the short term and add a cylinder in the roof).  Isn't the water flow and pressure more of an issue?  We haven't had this checked yet, but will do so.

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