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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    Hello all. I'm wondering if there is an easy way to get an idea of radiator sizing needed from our SAP calculation. I'm not going to put radiators in all upstairs rooms (but I'll run water pipes), but I'd like to put in towel heaters in the bathrooms, and don't want to be hopelessly inaccurate. We have underfloor heating pipes in all of the downstairs.

    The box 37 calculation is 108 W/K. At 21ΒΊ internal -3ΒΊ external, that's a 24ΒΊ difference, so the heat required will be 2,592 watts.

    We will have a 180m^2 house, so 14.4 watts per square m. Can I just multiply that figure by the bathroom sizes?

    I'd be grateful for any comments / thoughts.
    Best to calculate radiator sizes on a per room basis. Multiply the U values of different external fabric components by their areas, sum them and multiply by the max external to internal temperature difference, then add some extra for air permeability (fan?), and a small margin.

    So for example if your bathroom has 10m2 of external wall area of U value 0.25 W/m2K, 1 m2 window of U value 1.5, 6m2 of roof/ceiling of U value 0.2 then your fabric heat loss is 5.2 W/K, so at a 24C difference, the fabric loss is about 125W. For the air permeability, for a volume of 12m3, and 1.5 air changes per hour, air heat capacity 0.33W/m3K. deltaT of 24C, you get 12 * 1.5 * 0.33 * 24 = 140W, so you would need a radiator with an output of about 125W + 140W = 265W, to which you may want to add a factor of perhaps 25% if you want to be conservative, so 330W. If you have a large internal wall area to a colder area of the house, then you may want to add a bit for this temperature difference e.g. 24C versus 18C?

    I think there are a number of online radiator sizing calculators available which will do this calculation for you.

    Also, remember the radiator output is dependent on your boiler flow temperature, which will vary depending on the boiler's technology.
    Type 22 rads to match the window size will be more than enough for most calcs. At least if you oversize you can run lower flow temps too
    @ActivePassive, that's perfect! Just what I was looking for, thank you!
    @ Victorianeco, good point about running lower temperatures, but we've external U values of 0.15 and triple glazed windows, so I'd guess a type-22 radiator would be massively oversized.
    ComeOnPilgrim, given your near passivhaus U values, then my guess is you might have MVHR, if so the air permeability calculation is likely to be much lower, possible with a deltaT of 2C, rather than 24C?
    Ah okay. I've just installed new rads at my mums house. I would highly recommend hep2o pipe and fittings if you go that route.
    Posted By: ActivePassiveComeOnPilgrim, given your near passivhaus U values, then my guess is you might have MVHR, if so the air permeability calculation is likely to be much lower, possible with a deltaT of 2C, rather than 24C?

    Good point. I was thinking about that. We have a passive heat exchanger that is about 80% efficient, but presumably the air permeability is for leaks that do not go through the heat exchanger?
    In a bathroom if you have extraction, on most normal houses (and building reg requirement?) its the extraction fan/rate which will account for most of the air loss. In the case of MVHR the extractor in the bathroom will be designed for a certain flow rate which you can use for your calculation. Do you have an extractor in your bathroom connected to your passive heat exchanger - without knowing more, I am guessing the word 'passive' means its not fan driven?

    If your heat exchanger really is 80% efficient, then your heat loss will be 20% of the deltaT between internal and external temperatures, so you might expect at 0C ambient for out going air at 20C to be replaced by incoming heat exchanged air at 16C (i.e. deltaT = (20C - 0C)*(100% - 80%)). Passivhaus requirements generally add 12% (?) onto quoted non-certified exchangers based on their testing experience, so your 80% might actually only be 68%?

    From recollection, in terms of fabric leakage, you need to multiply your designed air permeability figure or measured figure from a blower door @ 50 Pa (either in m3/m2/hr or ACH/hr) by 0.07 - which is the average loss in average wind (the 50Pa - is for a very strong wind). So by comparison with the 1.5 air changes per hour figure for bathrooms I quoted in the example above (fans etc.), your loss from poor sealing/draughts to the outside by comparison, would be 5 ACH/hr * 0.07 = 0.35 air changes per hour, so about 20% of that required by building regs/MVHR designers for a bathroom, so a much smaller affect, and would be included in the 1.5 ACH figure anyway.
    Wonderful! Thanks very much ActivePassive.

    The passive heat exchanger is from Ventive. It's working very well, even before the house is properly airtight.
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