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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    I was told today by my Code Assessor and Architect today that I am failing to meet the Ene 1 criteria of a 25% in carbon emissions over the Part L 2010 Target Emissions Rate.

    We've specified insulation well above currently building regs and triple glazing throughout, but it apparently isn't enough without any renewable technology. (I am not sure if it is a common problem or because we have quite a bit of glazing.)

    It seems I am now faced with a choice of either PV, an air source or ground source heat pump. My build is in London so connecting to mains gas, electricity etc.. isn't a problem.

    My initial thoughts are to go for PV. Although I don't have a lot of south facing roof, the systems seem cheaper to install, payback periods are shorter and the electricity has flexible uses (e.g. to power my computer when I'm on the forum or to heat water for me if I have no other need for it.)

    Heat pumps seem expensive to install (ground) or can be noisy (air). I am going to install underfloor heating downstairs, where I believe they are effective, but radiators upstairs, where I think they are less good. I am also not sure how useful they are for domestic hot water?

    Does anyone have any advice on the way to go? Or factors I should be considering?

    • CommentAuthorEd Davies
    • CommentTimeJun 24th 2014
    What's your main heating going to be (or would have been without this)? If it's gas then the received wisdom is that heat pumps more-or-less break even with it for carbon (33% efficient gas power stations vs COP of 3 for the heat pump) so from a real-world (as opposed to box ticking & other market distortions) perspective there's no gain for the higher install cost of a heat pump. HPs only really make any sense off the gas grid.

    If you can put in a realistic PV system, ie, one with enough area and lack of shading at least in summer, then that seems the much better option to me.
    • CommentAuthorstones
    • CommentTimeJun 24th 2014
    On the question of radiators combined with heat pumps, there is no reason why they will not deliver the heat you need provided they are sized correctly. We have a low temp radiator/HP system and it comfortably heats the house - our flow temp at -5C is 39C so not that far off UFH temps. However, since you have mains gas available, I think i would be inclined to use that, assuming connection costs were reasonable.

    I would agree with Ed that a Solar PV system would be a better use of money compared to a HP. As you rightly say, you can use the output and heat your DHW with it. Bear in mind that if you are struggling for south facing roof, you could always have a split east/west system.
    • CommentTimeJun 24th 2014
    Ask yourself the question 'Why would I not have PV' :wink:
    • CommentAuthorEd Davies
    • CommentTimeJun 24th 2014
    The only answer I can think of to Steamy's question is “…I don't have a lot of south facing roof” in which case solar thermal might make sense. Normally I tend to argue towards PV even for DHW but the exception is when the roof space is limited, as the OP indicated here.
    • CommentAuthorDantenz
    • CommentTimeJun 24th 2014
    Would a CHP gas boiler do the trick?
    Thanks for all your comments.

    Ed - the main source of space heating and DHW was planned to be gas

    The architects are going to model what size system I can get on the south facing roof that I do have. It is a small area but shouldn't have too much shading.

    I also have a small area of south facing flat roof as well, I could have some panels on stands there too. Shading might be a bit of an issue, but it is quite heard to quantify when the building doesn't exist yet! :)

    My current thought is that if you are going to install a system the incremental cost of extra panels is quite low and therefore I might put some on my east facing roof as well to get me up to a 3/4Kwh system in total. Morning generation would have some benefits for my lifestyle.
    • CommentAuthorBeau
    • CommentTimeJun 24th 2014
    PV seems like a no brainer.
    No need for south facing as east west can still give you 80-85% of a perfect south facing system. Also the power generated is more likely to coincide with domestic use being morning or evening.
    • CommentAuthorSeret
    • CommentTimeJun 24th 2014
    I agree with Beau.The useful range of orientation for PV is actually pretty wide, shifting towards the east and west horizon mostly just shifts the production earlier or later in the day without affecting overall production too much. My array points east southeast and I get plenty out of it.
    As it looks like I need to go down the PV route, do people have any advice on what I need to look out for when selecting a system?
    • CommentAuthorSeret
    • CommentTimeJun 25th 2014
    Posted By: richardelliotAs it looks like I need to go down the PV route, do people have any advice on what I need to look out for when selecting a system?

    Not really, assuming you've got an uncomplicated rooftop install they're pretty much an off-the-peg system. You can get an estimate of what you'd produce using PVGIS so you don't have to rely on installers' estimates:


    Make sure you're getting an inverter from a reputable brand, looking for an installer that's well established could be handy should you require any after-sales support. Avoid the Big 6 power companies for handling your FITs, I use Good Energy and they're excellent. My installer actually did the setup admin for me and said dealing with them was like a breath of fresh air.
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