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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.

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    • CommentAuthordelprado
    • CommentTimeMay 26th 2018 edited
    In the next few weeks I will be installing UFH but I am conscious 100w per sqm might be overkill for my house. How do you reduce the output? Boiler temp, more mixing, thinner pipe, wider spacing? These reductions can’t all be linear in their Efficiency so which do I do?

    Thinner pipe to me seems like it might be the best idea as it saves cost
    • CommentAuthortony
    • CommentTimeMay 26th 2018
    What U-values are we talking about for well insulated

    In my house used 200mm spacing only ever used in lounge with 22C water for my warming system

    I would say bigger spacing on pipes and lower temperature water
    • CommentTimeMay 26th 2018
    You can reduce output as much as you like by reducing the temperature of the water in the pipes.

    Reducing the area of pipe will reduce the maximum power that you can produce to heat the house. You won't normally want that, but there may be occasions when you do want to heat the house more than normal - from cold after a long power cut, when the weather is very cold and somebody is ill etc.

    So work out what maximum emergency power you are happy with and size the system for that. Then control the actual output by reducing water temperature - boiler temp & mixing.
    • CommentAuthorbhommels
    • CommentTimeMay 26th 2018
    I am no UFH expert, but I would not go for thinner pipes. They make the pump work harder, and are less suitable for heatpump based systems if you ever wanted to go that way.
    Standard size pipe, close spacing, very low temperature.

    Weather compensation controller to varies flow temp based on external temp. Run it continuously (ours shuts off 1pm to 4am only.). Circulation pumps run slow to mimimise power consumption. Minimal number of zones (downstairs/upstairs?) based on floor type. No need for room thermostats or motorised zone valves which saves a load of cost.

    I don't know where you're looking to buy from but at /www.wundatrade.co.uk the 16mm pipe is cheaper than the 12mm. I suspect because 16mm is more common that also means all the other fitments you need are cheaper for 16mm as wll.
    • CommentAuthorsmileypete
    • CommentTimeMay 27th 2018

    The maximum power of the system is normally specified in Watts per square metre. If your floor is well insulated and you have a reasonably modern home, the power of an underfloor heating system usually needs to be between 65-85W/m² to give the required output. When it comes to choosing underfloor heating, a 150-200W/m² system is usually specified to reduce heat up times as the system will not be “on” continuously. When the system is only “on” half of the time the room is used, the power provided is half of the Wattage of the system. That is, a 150W/m² system usually provides 65-85W/m² per hour."

    • CommentAuthordelprado
    • CommentTimeMay 28th 2018 edited
    Thanks everyone. All understood, however I can never quite get my head around mixing down, because it means you boiler is producing water pointlessly hot, then reducing the temp at the manifold. Isnt the lowest a boiler can output be 45 degrees?

    Simon what weather comp do you use?

    Also what manufacturer pipe do I use? I was looking at uponors alu lined one and its 2 quid a metre, and I need 700m, which is more than I wanted to spend...
    • CommentTimeMay 28th 2018
    Posted By: delpradobecause it means you boiler is producing water pointlessly hot, then reducing the temp at the manifold

    But what's the problem with that?
    I presume the assumption would be that there would be a loss of efficiency (more losses at boiler and in pipework) in producing water hotter than needed and then cooling it down.
    Mine is a Viessmann boilers and WC controller/mixing kit. Seems it will deliver down into the low 20s for the primary circuit when necessary.

    The Wunda pipe i linked is 60p a metre or less.
    • CommentAuthorTimSmall
    • CommentTimeMay 29th 2018 edited
    My >10yr old Vaillant will set-point down to 40C (but you can effectively run it lower with external controls), I don't use a mixing valve, just run the whole system with a design flow temp of 34C (for -5 outdoor temp). Also "range-rate" the boiler down to its minimum output capacity to maximise efficiency.

    Use tight pipe spacings too, 100 mm is good. The extra pipe costs very little, but it's tricky to add extra in if you need it later! It'll give the system fast reaction times, and also make it suitable for a heat-pump upgrade in the future.

    • CommentAuthorCWatters
    • CommentTimeMay 30th 2018
    You want a fully modulating gas boiler. If it's got to be a oil boiler (these don't modulate) consider a thermal store or buffer tank to reduce cycling.
    • CommentAuthordelprado
    • CommentTimeJun 5th 2018
    Thanks all. Tim that article is great - but it will still double the cost to halve the spacing.

    Does anyone have any wisdom on what pattern to use? Meandering or bifilar?
    • CommentAuthorSprocket
    • CommentTimeJun 6th 2018
    For the sake of efficiency we went with tightest (100mm) spacing of UFH pipes and lowest temperature we could get away with from our heat pump.

    If you have a condensing boiler then it will be more efficient with a lower output temperature so you should turn the boiler down as low as you can.

    I would not fit narrower pipes.

    If you need to save cost on underfloor pipe install then IF your heat delivery capability is significantly greater than required you could drop to 200mm spacing on the UF pipe. This may require you to run things slightly hotter but unless you are at the limit of what the system can do (and floor overly warm) then it is probably not a big deal.

    One other factor that may or may not be an issue is that using higher floor temperatures is more likely to crack floor tiles (or worse, the screed). Might be relevant when considering floor finish, tile adhesives, expansion joints etc.
    • CommentAuthorgyrogear
    • CommentTimeJun 6th 2018 edited
    You could economise on pipe cost by not laying it in front of south- and west-facing windows for, say, 2 meters or however far the sun gets into the room.

    • CommentAuthorgravelld
    • CommentTimeJun 6th 2018
    Not so effective for spreading heat then.
    • CommentAuthorgyrogear
    • CommentTimeJun 6th 2018
    Well, the idea is that one benefits maximally from solar gain: if the floor is being warmed overnight, it will absorb less gain the next day...

    (My electric radiant stops 2 meters before my large sun-windows) -- I had to go hunting to find out why :devil: )
    • CommentAuthorgravelld
    • CommentTimeJun 6th 2018
    I think it's a marginal benefit, so I don't want to labour the point, but if the UFH is taking heat to other areas of the house, it is also cooled by those areas, increasing the delta at the window and thus maximising heat input further.
    • CommentAuthorgyrogear
    • CommentTimeJun 6th 2018
    I'm afraid I don't understand any of that !

    • CommentAuthordelprado
    • CommentTimeJun 8th 2018
    Meandering or bifilar?
    • CommentAuthorbhommels
    • CommentTimeJun 8th 2018
    Bifiliar with the incoming/hot loop nearer to the heat sinks (walls). I have yet to understand why anybody would want a meandering over bifiliar pattern.
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