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Boiler flue and bathroom extractor exiting at same point?

Discussion in 'Plumbing and Central Heating' started by iwilson16, 24 Nov 2019.

  1. iwilson16

    iwilson16

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    Can I have the output of a bathroom extractor fan adjacent to the boiler flue at the point where both exit the property? Ideally I’d like one to be 100mm below the other.

    It’s a first floor bathroom, so this would be approx 5m off the ground. Nearest window opening would be 800mm away. It would be 200mm from nextdoor property.

    Thanks,
    Ian
     
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  3. Madrab

    Madrab

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    Nope ..... In the absence of a specific boiler's MI then, if I remember correctly, building regs state that a flue must be at least 300mm above or below an opening up to 32Kw. I believe it is the same 300mm from a boundary.

    To be absolutely sure though, you would need to check the boiler MI's.
     
  4. iwilson16

    iwilson16

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    Ok, thanks. it’s the Viessmann 100-W 35kw boiler I’m considering. (210 Hw cylinder, 3 bed, 2 bath flat.)

    I can run the extractor to the outside wall, and instead take the boiler flue straight up through false wall in bathroom above, into roof space and out roof. It would be approx 10m long with two 45 degree bends (no 90 degree out top of boiler). I think this is borderline for the smaller flue diameter.
     
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  5. muggles

    muggles

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    Why the 35kW boiler? That's likely to be massively oversized for a flat. The 19kW should be more than enough. They do an 11kW now and I'd have thought that even that would be enough unless your flat is enormous.

    You can have the flue run through a false wall, but if you do so you need to have inspection hatches installed so that every flue joint can be inspected. Madrab is correct in saying you need 300mm between your extractor and the flue
     
  6. iwilson16

    iwilson16

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    It’s a 160 sq.m. Victorian town house conversion With 3.4m high ceilings. 3 double bedrooms, large lounge with big bay window, 7x5m kitchen-dinner, and 10m full height hall. Maybe 35 is too much, but I’d think at least 30kw would be needed ...?
     
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  8. muggles

    muggles

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    I think you should do a proper heat loss calculation rather than guessing. An oversized boiler will be inefficient, and therefore expensive to run.
     
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  9. iwilson16

    iwilson16

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    I do suspect the flat has more radiator power than really needed. When I moved in it was extremely draughty (stripped floorboards, single glazing, badly fitting windows, no draughtproof strips etc) and with no insulation to speak off.

    Living room was the worst - huge draught from bare floorboards, under skirting, loose windows ... plus an open fireplace. Any heat would be literally be sucked I’m the chimney. Now it has carpet, dbl glazing, good fitting windows, skirting sealed up, no escape route up chimney.

    So do I spec the boiler based on current radiator demand, or on heat loss per room? There’s currently 11 radiators with approx 18kw of radiator potential (delta 40 degrees, I think ... or was it 50?). Add onto that the demand for the hw cylinder?

    one advantage of all the radiators is that rooms heat up fast when required. I have a smart control system, so only those rooms needed are heated.
     
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  10. muggles

    muggles

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    If you've only got 18kW of radiators, it's really very pointless having a 30 or 35kW boiler. It'll cost more to run and will suffer from increased wear and tear due to cycling. If the house gets warm enough with the radiators you have now and you're not planning to extend, the 19kW boiler will be ample.
     
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  11. iwilson16

    iwilson16

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    But what allowance needs to be made for the hot water cylinder ...?

    Muggles - many thanks for the prompt advice. I’ll definitely do a proper heat loss calculation taking into account mods I’ve made Before I decide on boiler size.

    interestingly, just looked at the home report from when I bought the flat - it says space heating demand is 19kw and hot water demand is 3kw. The current boiler is a Bosch Greenstar 30 Ri, installed by previous owner.

    I’m now thinking of placing both boiler and hw cylinder to an upstairs store room (1.5x1.1m). It’s right beside the upstairs bathroom, and downstairs bathroom is below upstairs bathroom. So means much smaller pipe runs to the main bathroom.

    The upstairs location also means it’s much easier to run pressure relief and condensate drain off to outside. Also, the flue pipe can run straight up into the loft then out the roof. Maybe 3 or 4m max, with 2-4 45 degree bends.

    my only concern then would be noise from the boiler being heard in the bedrooms. But if I put it in a small cupboard within the larger walk-in cupboard (without compromising air supply) then that should be ok.
     
    Last edited: 25 Nov 2019
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