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Discuss New central heating and HW system, ? Unvented Cylinder - what size, what boiler? Confused! HELP!! in the Plumbing Forum area at

  1. Matt Davies

    Matt Davies New Member

    So I'm new to Internet forums and new to commissioning a new CH system... any help would be greatly appreciated, as I'm pretty confused by all the differing things I've read and been told regarding my current heating dilemma.

    Background - I've just purchased a large 1900s 4 bed semi with 1 bathroom. Currently there is a traditional vented system taking up most of the single bathroom, microbore rads and an old (20-30 yrs) BAXI boiler in the hall way.

    I'm renervating the house to be my family home for my wife and 2 young kids. Full re wire, back to brick replaster etc.

    We have plans for a large single storey kitchen/open plan extension to encorperate a new wet room and possibly underfloor heating. We will also be looking to undertake a loft conversion to add a 5th bedroom and ensuit. Thus I want to future proof my CH plans to accommodate a 5bed 3bathroom house.

    General plan is for an unvented sysytem to be installed at the back of the garage which is attached to the side of the house, and bring the pipes in through the floor and up to the 1st and 2nd (loft) floor. Guidance thus far is that a combi isn't really an option for this size of house? And I'm keen to avoid a gravity fed system as I'd like to develope the loft as discussed.

    The pressure to the property is reasonable, and the flow has been measured at 14lpm. However, the house is currently supplied by a 1/2" lead supply from the off-cock on the drive approximately 40m to where it enters the house - right at the rear of the property. I am planning to have this replaced with plastic following discussions with Severn Trent. I'm hoping this will improve the flow but appreciate the pressure is whatever I have from the supply.

    Apologies for the long winded introduction!!.. some general questions, if you would be so kind as to give me some guidance...

    1. What sort of flow should I be looking for to enable a unvented system to work "well". Is upgrading my water main a sensible thing to do for the long term? Is this likely to improve the flow in my tap/to the CH system.

    2. If unvented is appropriate what size of tank would be suitable? I know this is a broad question, but everywhere you look there is a different answer. Any recommendations on type of tank and manufactures?

    3. Should I look to install a "heat only boiler" or a system boiler? As I've had different people come along and recommend/offer to install different types altogether. And also, what size boiler would do the job?

    4. What are people thoughts on secondary returns? It seems to be a toss up between insulating all the "extra" pipes or waiting a bit longer for the water to be drawn down through the pipes or am I oversimplifying?! I'm not on a water meter.

    That's all for starters!! Any help will be most appreciated.

    Cheers, Matt (new owner of a semi-derelict family home!!)
  2. king of pipes

    king of pipes Trusted Plumber GSR

    21 litres a minute on a new 25mm mdpe main at 3 bar pressure hopefully but this may drop off a bit on 40 + meter run , 200 - 250 litres for hotwater storage , heat only boiler with seperate expansion vessel and kit , secondary return could be a option but if your could get the cylinder installed central in the house i would do that. Regards kop
  3. Matt Davies

    Matt Davies New Member

    Thanks for your reply kop - much appreciated. The 40m run will be cut down to about 10m as main will run straight up the drive and into the garage/boiler - 21l/m enough to run this type of system?

    Is a system boiler a big no no or is it a matter of preference? Any thoughts on size of boiler. Assuming 15ish rads and potential under floor?
  4. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    best to either go with a 25 or 32mm alk main and 20-30lpm + depends whats at the mains seven will be able to give you an idea what your likely to get more the better

    300l and gledhill are very good

    heat only boiler best as there less to go wrong and all off the self part eg controls etc

    and me personally i would run a secondary return to the furthest outlet

    boiler size your looking at a total shot in the dark anywhere from 30kw upwards
    • Like Like x 1
  5. king of pipes

    king of pipes Trusted Plumber GSR

    Yes 21 litres a minute will work for you 25 mm pipe minimum , problem with a system boiler is the pump is not adequate for the size of the system ? and the expansion vessel inside the system boiler will not accommodate 15 rads + underfloor and cylinder primaries, secondary pump is a option but do you really need it how will it be controlled ie timed ? you dont need it running when theres no demand for hotwater. Boiler size 30 kw + cheers kop
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