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    • CommentAuthorkates
    • CommentTimeAug 22nd 2017
     
    Hi.

    We are part way through a 4 bed detached house refurbishment of a 150 m2 house which involves insulation, triple glazing, MVHR, a new gas boiler and 12 aluminium radiators, and a 4,120 kWh/yr PV system with solar iBoost. Our SAP predicts the house will need a total of 17660 kWh/yr input (11576 kWh/yr for space heating and 3540 for HW).

    There will normally be 3 occupants but 6 fairly regularly, and we will have one bathroom (bath/shower) and one separate shower. One of us has a medical condition that requires plenty of mains pressure HW for decent showers.

    Does a 18kW regular boiler with a 250l vented thermal store sound sensible for our needs? Any other advice also welcome.
    • CommentAuthortony
    • CommentTimeAug 22nd 2017
     
    I would not use a SAP calculation as the basis for boiler size,

    I would also add more insulation during the reverb while it is easy.
    •  
      CommentAuthordjh
    • CommentTimeAug 22nd 2017
     
    Does SAP predict a peak power load as well as an annual demand? Or even give monthly demand figures? That would be most useful for sizing the boiler.

    We have a 250 L thermal store that is heated only by a solar diverter from a 4 kWp system during most of the year, but there are generally only two of us in the house. Our system copes with two or three poor days without having to resort to a mains boost. With 3 people you will probably occasionally need to use the boiler and with six I'd be amazed if you don't need the boiler as well except in gloriously sunny weather.
    • CommentAuthorkates
    • CommentTimeAug 22nd 2017
     
    Thanks both.

    We're putting in the maximum thickness of Pavadentro for IWI and 200mm of graphite EPS EWI where possible. We're not expecting all of our hot water to be solar, especially with visitors, but maybe a 300l store would be enough for the 3 of us most of the time?

    The peak monthly requirements have been calculated as 2065 kWh/month for space heating and 270 for hot water (although I wonder if 270 for HW is only for 3 people and might therefore need to be increased).
  1.  
    Over here the graphite EPS is not worth the extra expense if you can accommodate the extra thickness for the normal EPS. I.E. its cheaper to use normal EPS with increased thickness to get your desired u value.
    It might be worth the calculations for where you are.
  2.  
    You can use SAP if you get access to the full 'SAP Worksheet' - I use Stroma's Free FSAP 2012 which generates a number of reports including the 'SAP Worksheet' which contains the detailed energy calculations. You may be able to get a copy of the worksheet from whoever did you SAP calculations - I am not sure whether all rdSAP calculators provide this detail though.

    Calculation result 39 from the full 'SAP Worksheet' (standard SAP 2013 nomenclature) gives you the heat loss parameter in Watts per Kelvin (both monthly and annually). So for example if you want to calculate the maximum heat requirement, and your heat loss parameter is 300W/K , then multiply this by the maximum difference you might expect between inside and outside temperatures for your region, for example (20C - -5C) = 25C temperature difference. So 300W/K x 25C = 7500W or 7.5kW maximum heating requirement - sometimes people add a margin on for this, but you generally ignore all other heat gains in a home (solar, appliances, people) - for the purposes of this calculation. If you have a very energy efficient home, then your hot water requirements might override and determine this maximum (not an issue in your case?).

    My gut feel, given the information you provided is that you boiler and hot water specifications are about right. If you are concerned I would just pay for a more powerful boiler, the cost increment is generally negligible, and it would allow you to refill the HW cylinder quicker if there are 6 of you in residence.

    The calculations outlined above using SAP (or the also free MCS Heat Pump Calculator spreadsheet) are more critical for air source heat pumps where the incremental costs of higher capacity heaters/boilers are much greater. There are also a number of other online calculators for this type of calculations, but they are often crude.
    • CommentAuthorkates
    • CommentTimeAug 22nd 2017
     
    Thanks AP, that's really helpful. Our heat loss parameter is slightly higher than your example at 310 W/K so our max heat requirement will also be slightly higher. And I don't think we're going to be so super efficient as to be more concerned about HW requirements.

    Could oversizing the boiler by too much have a detrimental effect on boiler efficiency? Especially as we've got low volume radiators (admittedly oversized ones) that won't lose as much heat as normal ones?
  3.  
    You are right, oversizing the boiler might impact the efficiency, depends on the total output of your emitters (radiators), flow rates, and what the capacity boiler can modulate down to. You sometimes find that within a range of boilers the minimum modulation power is the same despite the boilers have different maximum outputs. You really need to look at the specs, but boilers generally modulate down to 15% to 25% of max power, and some will adjust flow rates, cycle to try to ensure low enough return temperatures for efficient condensing to occur. Unfortunately I don't have enough knowledge to be able to help with this detail, perhaps someone on the forum can help, or perhaps better most manufacturers technical departments would be able to help you decide?
    • CommentAuthorkates
    • CommentTimeAug 22nd 2017
     
    It's very helpful to know that we're in the right ballpark.

    I've spoken to two boiler companies today, and during one conversation I was told that "you can't have a thermal store connected to a boiler... it has to be connected to a heat pump"...!!
    •  
      CommentAuthordjh
    • CommentTimeAug 22nd 2017
     
    Something's a bit fishy with the figures. 2065 kWh for the worst month gives an average power for that month of about 2065/(30*24) = 2.86 kW whilst 310 W/K gives 7.75 kW for a 25°delta T or 9.3 kW for a 30° delta T. You don't say where you are but -10°C is perhaps more realistic for a minimum temperature, IMHO. Anyway, an 18 kW boiler sounds plenty big enough. Lots of insulation on the thermal store is a good idea, and buying from somebody who understands that the world is not flat will also help :devil:
    • CommentAuthorkates
    • CommentTimeAug 22nd 2017
     
    We're in South Yorkshire so -10 is a possibility. I'll try calling again tomorrow. Thanks.
    •  
      CommentAuthordjh
    • CommentTimeAug 22nd 2017
     
    One point I forgot to mention. Our house is all-electric and our thermal store has two immersions, one at the bottom and one half way up. The lower one is connected to my solar diverter and the immersion's thermostat is set to 90°C to maximise stored PV energy. The centre immersion has its thermostat left at the default of 65°C and is connected to a mains timeswitch. During summer, the centre immersion is never switched on. During winter, it's programmed to come on for a couple of hours in the E7 cheap period. So we generally always have hot water available and the PV always has somewhere to add at least some heat, whilst we minimise standing losses from paid-for electricity. If we ever seem likely to run out, I just press boost on the timeswitch to make some more.

    In your situation, I'd suggest you make sure you have an immersion at the bottom for your PV input and try to ensure the heating coil for the boiler (or boiler flow and return connections) isn't quite at the bottom
    • CommentAuthorkates
    • CommentTimeAug 23rd 2017
     
    Thank you very much for your valuable advice. We'll bear it in mind.
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 23rd 2017 edited
     
    Posted By: katesOur SAP predicts the house will need a total of 17660 kWh/yr input
    That seems high to me, try working it out again.

    Space heating at DHW do different things, and different temperatures and at different times, the 3Ds. It is best to split them up totally in my opinion.

    Water usage could be worked out from your current usage, just needs a thermometer, a jug and a stopwatch, then a bit of time logging the data and working out what you are really using.
    Reduce it down to a person/year usage and then multiply by the number of people, then you have the expected range of usage.
    If your 'guests' use it all up, tell them to stop!

    Any storage of thermal energy will have losses, the current methodology assumes that the store is heated up, then the stored water is used immediately. This is far from most peoples usage patterns.
    You can calculate the real losses from the thickness of the store insulation, the temperature difference between water and air and the surface area of the store, just like you work it out for the house walls.

    Posted By: katesOne of us has a medical condition that requires plenty of mains pressure HW for decent showers.
    As you mention a vented system, you will be using a pumped shower, so mains pressure (and flow), is irrelevant here.
    I like vented systems, they are generally easy to install, are reliable and cheap. Others may disagree.
    Depending on your water hardness you may want to fit a water softener.

    Realistically, using PV to heat the water is the best local usage for it. You can do a calculation as to the expected proportion that the PV (from PVGIS) will contribute and then decide if you really need to plumb the store into the gas system.
    So say that each person uses 750 kWh of hot water a year and your PV can supply 80% of that for 8 months of the year and 20% for the remaining 4 months.
    If electricity costs 15p/kWh then:
    750 [kWh/year] x 0.66 [fraction of year] x 0.2 [electricity from mains] x 0.15 [p/kWh import) = £14.85
    750 [kWh/year] x 0.33 [fraction of year] x 0.8 [electricity from mains] x 0.15 [p/kWh import) = £29.7
    Total per person £44.55

    If my arithmetic is right (and it is often wrong, so needs checking), is it really worth plumbing into the gas boiler?
    •  
      CommentAuthordjh
    • CommentTimeAug 23rd 2017
     
    Posted By: SteamyTeaAs you mention a vented system, you will be using a pumped shower

    No, she mentioned a vented thermal store, which supplies mains pressure hot water directly.

    Given your numbers, I would still plumb the boiler into the store, myself. It'll be a fairly minimum cost at install time and provides more flexibility.
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 23rd 2017
     
    It all comes down to the numbers in the end, so until they are sorted, it is all guess work.
    •  
      CommentAuthordjh
    • CommentTimeAug 23rd 2017
     
    No, it doesn't come down to the numbers because at the end of the day they're all guesswork (pronounced 'estimates') anyway. The numbers just provide a one-sided limit on system size. Kate has a choice, IMHO, of a 12 kW, 15 kW or 18 kW boiler but the price difference from the smallest to the largest is only £100 or so and they're most probably all of a similar efficiency so I wouldn't argue strongly for the smallest. Whichever she gets I would plumb it in to the thermal store regardless of 'the numbers'. I would also make sure the thermal store does have two immersions, even if I didn't use the top one. That way I would keep the options open.
    • CommentAuthortony
    • CommentTimeAug 24th 2017 edited
     
    +1
    •  
      CommentAuthorSteamyTea
    • CommentTimeAug 25th 2017 edited
     
    Posted By: djhNo, it doesn't come down to the numbers
    You are right, we should just accept what the heating engineer suggests and trust their judgement, no need for any thought, science or mistrust in the industry at all.
    I don't know what I was thinking, they are engineers after all.
    • CommentAuthorTriassic
    • CommentTimeAug 27th 2017
     
    Yes SteamyTea, what were you thinking, you must always trust the supplier of goods and services, they know best!
  4.  
    And of course don't forget if you tell them what to do that is in any way different to what they want to do - then if anything goes pear-shaped then they have their 'get out of jail free' card
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