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How can I avoid Megaflow and get the best from my Combi ?

Discussion in 'Plumbing and Central Heating' started by platforminc, 29 Dec 2018.

  1. platforminc

    platforminc

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    Hi All.

    We currently live in a 4 bed house which is going to be extended to a 5 bed, 1 bath & 1 shower. Now the current boiler is a 34cdi. Cold water pressure is good, I dont know what people count as good, but I dont suffer from low pressure etc, the pressure is adequate. At the moment, in the bathroom if the basin tap is opened, and the shower tap is on, the pressure drops drastically.

    However if the kitchen hot water is on, the bathroom one is only slightly affected but still gets good hot water. So I dont know what to make of it.

    As part of the extension, below is my requirement/some facts.


    • The old lead pipe will be upgraded to a plastic pipe, say 32mm or the next size down.
    • I prefer Combi as I dont have where to put a mega flow tank/pipe work etc.
    • The only time the showers will get used is in the morning, small chance of 2 getting used at thesame time. and even if that was the case, it would not be for long, say 5 - 10 mins max.
    • I plan to have underfloor heating on an area of about 60sqm, if I only do the centres i.e in a ktichem lets say 50sqm.
    • During the day after the shower period, omst hot water demand is from the kitchen/toilet basins.
    • We previously had a system boiler before combi 3yrs ago, so I have a feeling some of the pipes were not correctly sized, also the heating engieer cut corners, for example the boiler is located in the kitchen, and the downstairs bath/toilet is next to it, only a wall seperates them, however the pipe work is such that when the hot water is opened, it uses the old system boiler pipe route, so the water goes up to the first floor and then comes back down, even though the demand is from the adjacent room where the boiler is.
    • I have read on here that one can have a combi + a cylinder. I like the idea although I want to know if the cylinder ought to be upstairs or downstairs or it doesnt matter.
    • If the cylinder + combi is the way forward, can someone please advise on who should be placed on cylinder and who should be direct from combi.
    • The proposed extension is such that the boiler is intended for the new utility room which is downstairs, where it says bathroom downstairs in the picture floor plan below.

    Thanks in advance.

    I attach the proposed floor plan.

    site2.PNG
     
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  3. dilalio

    dilalio

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    You need an appointed heating engineer not a DIY forum.
     
  4. MeldrewsMate

    MeldrewsMate

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    If your d/s bathroom is to become a utility room then that is the best place for a hot water cylinder. The distances to the two bathrooms above would be at a minimum. A cylinder also allows 'free' hot water from solar PV panels in the future.

    Suggest using the Megaflow to feed all outlets except one in the utility room that will come from the combi directly.
    Clearly you will need to renew the hot water distribution pipework to keep the hot water transit times to a minimum.

    Noted your desire to avoid a cylinder, but there are good reasons to have one (see above), as there are to keep a d/s toilet if you can.
    If you must insist on a combi only solution then consider flow reducers on the individual hot feeds, to reduce the inevitable interaction when more than one outlet is open concurrently.
     
  5. platforminc

    platforminc

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    Hi.

    Thanks for the reply. The existing bathroom which is downstairs will now become a utility room/toilet. It's only a 2m x 2m room. It's also likely to be the location of the combi boiler, the boiler may also need to go into the garage as if you notice unless the garage door is moved backwards, there is no where for the flue pipe to come out to. Even if the garage door is moved, it means one can't have a traditional garage door because it must not close up to the flue . So the boiler may well need to be in the garage.


    When you say mega flow in your post above are you referring to a Stand alone cylinder or an unvented system setup?

    Flow reducers, do you mean an Inline valve and have it half open for example?


    Thanks
     
  6. platforminc

    platforminc

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    Anyone please.
     
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  8. flameport

    flameport

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    A megaflow is a particular brand of unvented hot water cylinder. Other makes are available, most of which are significantly better and in many cases cheaper as well.
    If there really is no space in the house for one, put it in the loft.

    Flow reducers are flow reducers - devices designed to restrict the flow. Also called flow regulators. Here is one example: https://www.bes.co.uk/flow-regulator-ball-valve-cp961-15mm-17527
    A valve could be used for a similar effect but is likely to be noisy as the water passes through it, and won't really work all that well if the water pressure varies at all, as higher pressure will result in more flow.
     
  9. platforminc

    platforminc

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    I meant an unvented cylinder, combined with a combination boiler. Not a complete unvented system setup with a system boiler.
     
  10. hard-work

    hard-work

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    General points for main pressure systems:
    • Fit a dedicated 22mm cold feed to the combi from the mains stopcock, irrespective of bore size of mains pipe. This assists in giving flow priority for the shower;
    • Fit a full bore stopcock;
    • At the stopcock fit a tee to supply all the cold outlets, except the shower which tee off the cold supply to the combi, 'near' the combi;
    • Dedicated hot pipe to showers from combi/unvented cylinder;
    • Fit combined isolators/flow regulators (using a cartridge for litre/min) on all H&C outlets - to balance the system. Not on the hot and cold to the shower
      https://www.bes.co.uk/flow-regulator-ball-valve-cp961-15mm-17527 ;
    The above ensures the supply prioritises the combi with minimum influence on the shower. You could use hot and cold manifolds with pipes to each outlet from each manifold. Then only 12mm pipe need be fitted to most outlets - 15mm for the showers. In softer water areas, even smaller bores. This gives quicker DHW delivery at the taps as the volume of cold water in dead-leg pipe is less.
     
    Last edited: 3 Jan 2019
  11. hard-work

    hard-work

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    The garage looks good for a combi. Short lengths of hot water pipe to the bathroom, en-suite and kitchen.

    You are wise in avoiding space consuming unvented cylinders. A decent high flowrate combi is all you will need in that house. Many here are on about the new Intergas Xtreme 36 combi that has an efficient flue heat recovery system and decent hot water delivery.
     
    Last edited: 3 Jan 2019
  12. hard-work

    hard-work

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    It is best to fit in pipes in to accommodate a solar pre-heat cylinder for when one is being fitted in the future. Probably best to a have a safer, low pressure, vented, DHW only thermal store. The hot water outlet may have a diverter valve that sends the solar heated hot water directly to the taps by-passing the cylinder. When the temperature is below hot water setpoint the water is directed to the combi to be topped up.
     
    Last edited: 4 Jan 2019
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