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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorjms452
    • CommentTimeSep 13th 2020 edited
     
    Hi All,

    I got asked what our kWh/m2 for heating was recently. It's a standard number but seems very hard to work out without lots of assumptions.

    There are three of us in a 90m2 detached victorian house using 3700kWh/year of gas for heating, hot water and cooking on the hob and I need to work out heating usage...

    BRE say hot water usage average in the uk is 4kWh/day for an average 2.4 person house
    https://www.bre.co.uk/filelibrary/SAP/2012/STP09-DHW01_Analysis_of_EST_DHW_data.pdf

    That gives us about 25kWh/m2 for heating and hob cooking, assuming a child as 0.4 persons.

    However we get some hot water from solar PV in the summer and I often have a shower at work - (also makes an experiment between summer and winter hard).

    What would be the standard approach to giving a kWh/m2 for heating (that doesn't require a 5 minute explanation).

    Thx in advance

    I should say gas usage was 12700kwH/year for two people before the renovation :bigsmile:
    •  
      CommentAuthorDamonHD
    • CommentTimeSep 13th 2020
     
    A common approach is to compute a regression line (which you can in Excel).

    Another is simply to assume that 20%-ish of all annual gas consumption is DHW.

    Neither of those addresses your shower and solar leakage issues, but you could separately estimate their kWh/year and add/substract as appropriate from the metered gas kWh.

    Rgds

    Damon
    •  
      CommentAuthordjh
    • CommentTimeSep 13th 2020
     
    I think I would start by looking at the numbers for days/weeks/months when the heating is not on to try to estimate the background, all-year-round, non-heating energy use. Then subtract that from the times when the heating is on and see if you can puzzle out any variations.

    It's all a lot easier when your heating is all-electric :) Much easier to monitor and separate out the consumption.
    • CommentAuthorjms452
    • CommentTimeSep 13th 2020
     
    Posted By: DamonHDA common approach is to compute a regression line (which you can in Excel).


    of what? kWh/day versus degree days?

    Posted By: djhI think I would start by looking at the numbers for days/weeks/months when the heating is not on to try to estimate the background, all-year-round, non-heating energy use


    Now it's in black and white it's made me think I can add an energy meter to our Optiplug on the immersion heater.

    It's also a fringe benefit of moving to induction hob which is planned with a kitchen refresh...
    •  
      CommentAuthordjh
    • CommentTimeSep 13th 2020
     
    Posted By: jms452Now it's in black and white it's made me think I can add an energy meter to our Optiplug on the immersion heater.

    Or just turn off the solar diverter for a week or so? Or even the entire solar system.
  1.  
    >>>> compute a regression line (which you can in Excel).

    That's the best way, plot the energy used each day/month Vs the local average temperature in that interval. Do a curve fit with a polynomial which should have a kink in it at the point you turn the heating on, every thing warmer than that point is the non-solar portion of the dhw/cooking, which you subtract from the energy bills to leave the heating, and the slope in the low temperature region can give you the overall U value for the house.

    A simplified version of that is to check your energy bill for June-August (= non-solar DHW, cooking), multiply by 4 for the other months, and subtract that from the whole year total bills to give the heating.

    It's not essential to know the solar contribution but you could estimate it by looking at your summer usage before and after the solar was fitted, if those numbers are available and nothing else changed that year.

    25kWh/m2/yr for a Victorian house is excellent, well done! At that level, incidental heating from cooking etc are an integral part of the heating of the house, so it's difficult to disentangle them.
    • CommentAuthorjms452
    • CommentTimeSep 13th 2020
     
    Posted By: djhOr just turn off the solar diverter for a week or so? Or even the entire solar system


    Couldn't bring myself to do that!

    We just have a 1kW immersion running of an optipug (switches on only when net exports >1kW), it's all on three pin plugs for the optiplug so easy to add a plug in energy meter too. There are lots of weeks in summer where that supplies all the hot water so easy to note down.


    Posted By: WillInAberdeen>>>>
    25kWh/m2/yr for a Victorian house is excellent, well done! At that level, incidental heating from cooking etc are an integral part of the heating of the house, so it's difficult to disentangle them.


    'Thanks - The BRE 4kWh/day for hot water seems like an over estimate for us so all I can say hand on heart until I work out the domestic hot water is that it's definitely <40kWh/m2.
    •  
      CommentAuthorDamonHD
    • CommentTimeSep 13th 2020 edited
     
    Posted By: jms452
    Posted By: DamonHDA common approach is to compute a regression line (which you can in Excel).


    of what? kWh/day versus degree days?

    Posted By: djhI think I would start by looking at the numbers for days/weeks/months when the heating is not on to try to estimate the background, all-year-round, non-heating energy use


    Now it's in black and white it's made me think I can add an energy meter to our Optiplug on the immersion heater.

    It's also a fringe benefit of moving to induction hob which is planned with a kitchen refresh...


    Yes. An approach we've been using across our recent Radbot trial (>100 homes) is to take the *median* daily gas consumption in July/August as an estimate of baseload. Yes, that will be knobbled by solar hot water (thermal or PV diversion). 10% of total winter gas use was also reasonably good, and would likely work OK with the above solar DHW or where summer figures weren't available.

    Rgds

    Damon
    • CommentAuthordaserra
    • CommentTimeSep 15th 2020
     
    As a family of four, we always use about 1 bottle of butane (15kg) every 6-8 months for our hob, the oven is electric, if that helps any.
    • CommentAuthorjms452
    • CommentTime7 days ago
     
    Posted By: DamonHDYes. An approach we've been using across our recent Radbot trial (>100 homes) is to take the *median* daily gas consumption in July/August as an estimate of baseload.


    Presumably I can go as far into the shoulder months as possible (providing the heating isn't on) to get a more accurate reading (so we have less sun, colder incoming water and as we tend to eat less stew in summer!).


    Posted By: daserraAs a family of four, we always use about 1 bottle of butane (15kg) every 6-8 months for our hob, the oven is electric, if that helps any.


    thanks - nice to have a number - 13.7 kWh per kg and circa 20Kg per year is ballpark 280kWh/yr.
    •  
      CommentAuthorDamonHD
    • CommentTime7 days ago
     
    Yes, I'd imagine that the shoulder months might help. If my own diverter project had not been ... diverted ... I'd be in a position to give you some more direct insight on that!

    Rgds

    Damon

    PS. Some numbers and graphs that may be of interest...

    http://www.earth.org.uk/statscast-202006.html

    http://www.earth.org.uk/saving-electricity-2020.html
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