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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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    • CommentAuthordelprado
    • CommentTimeJan 2nd 2018
     
    What exactly are the pros and cons of each?

    I dont understand why someone would say "I want a fast heat up time", because surely the question should be "what is the most efficient?"

    I can only see the benefit of a fast heat up time in a poorly insulated house with a very high demand - a bit like a traditional radiator but with the pleasure of UFH regardless of cost and fossil fuel wastage.

    Except I look at what look like forward thinking companies like Lithotherm in Germany and their clay tiles, boasting of fast start up times do, which is clearly aimed at the "eco market".

    Or is it just that lithotherm both heats up quickly and cools down slowly?
    • CommentAuthordelprado
    • CommentTimeJan 2nd 2018
     
    This may well answer the question:

    http://www.lithotherm-system.de/pdf/Lithotherm-English.pdf

    Whereas when people talk of fast heat up in traditional UFH systems they are talking about self levelling compound etc and loose pipes directly under floor which while they heat up fast they also lose it quickly?
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 3rd 2018
     
    A lot of it will come down to the heat source controls. If they ramp up and down at the same rate as the mass you are trying to heat, you will get, on average, a more stable temperature.
    If on the other hand they have a total mismatch, i.e. hot slab, cold air, then there is a risk of over shooting the desired air temperature.
    One way to find out is find a report that uses the same flow temperatures and timings, then look at the room air temperatures.
  1.  
    We have UFH in the bathroom mainly for barefoot comfort. It needs to come on quickly to warm the tiled floor at the time we want a shower, then be cold the rest of the day to reduce heat losses to the ground, the towel rad takes over the heating if needed. There is a layer of marmox insulation board to de couple the ufh+tiles from the slab.

    This is example of low thermal mass construction where it is efficient to alter the temperature so it is 'just warm enough' at different times of the day. In other parts of the house we have a high thermal mass (same insulation layer but positioned outside the structure) and we have to keep the same temperature all day. Neither is necessarily more efficient, just horses for courses.
    • CommentAuthordelprado
    • CommentTimeJan 3rd 2018
     
    Thanks Will, I guess bathrooms are a perfect example of where that works.

    Steamytea, my apologies but I am struggling to understand what you mean. Can you try and say it a different way?
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 3rd 2018
     
    There are basically two ways to keep a temperature stable, keep the heat source close to the temperature you want, and then hold it there.
    The other way is to let the temperature over and under shoot. The mean temperature will be the same.

    So it is really just a matter of how much variation you want and how you want to achieve that.

    You can, if you want, just limit the input temperature, but that may mean that it takes a long time to heat a room up. This is what happens when you have a large mass of concrete, or similar, to heat up before the rooms starts heating.

    You basically have 4 variables, the power of the heat source, the time it takes to start transferring energy to the room, the mass of the air in the room and the desired temperature ranges.

    Assuming that the heat losses are the same regardless of the type of heating, then you can either do short, high temperature inputs (like what radiators do) or you can, once the room is up to temperature, do small, lower temperature difference, inputs.
    Both of those methods are controlled more by the temperature source i.e. gas boiler, heat pump, electrical resistance etc, than the amount of mass the heating system has.

    Personally I think, and have no experience of comparing different types, that if you can control the flow temperatures reliably and within a small temperature range, it makes no difference, with the proviso that the floor is already up to temperature. Different matter if you want to use it like a traditional wet radiator system.
  2.  
    I would add that, in a well insulated house, there is more risk of overshooting with a fast reacting system (and it's more of a problem because the house retains the heat for longer after the heating is switched off).

    Also, potential savings from switching off heating (or set back to a lower temperature) when unoccupied for short periods (less than a day) become very small.

    The exception is perhaps a holiday home where it may not be occupied for some weeks at a time where a fast reacting, low latency system has more advantages. The one time our heating didn't work (mostly slow reacting, controlled entirely by weather comp with no internal stats) was when I've switched it off when we went on holiday in deep winter - the first time it took a couple of days to return to comfort. The second time i tried it I turned the 'target' internal temp up for a day or so to force the floors to run at a higher temp. the alternative was to have the heating off for less time while away - ie so it started preheating a few days before return.

    Given our annual heating costs are about £250 it's not really worth the effort.
    •  
      CommentAuthorSteamyTea
    • CommentTimeJan 4th 2018 edited
     
    I am not totally sold on the idea that a high mass floor with underfloor helps when there is a sudden influx of energy i.e. sunlight.
    The reason being that when the sun is high in the sky, it will not hit the floor, and when it is low in the sky, apart from having less energy (though still significant) the rays hit the walls.

    Looking at their system from Lithotherm, it seems to be 0.0.45m thick and made from clay. Clay has a SHC of 0.92 kJ/(kg.K).
    So that can hold around 0.013 kWh/(m^2.K) (assumption is that density is 1100kg/m^3).

    Timber, with a SHC of 1.7 and a density of 800 kg/m^3 and still 0.045m (45mm) thick, has a thermal storage capacity of 0.017 kWh/(m^2.K)


    So if you want to add mass, go for the clay tiles, if you want to add thermal storage, go for timber.

    (I have not done an exhaustive search for all the possible SHCs and densities, but you get the idea and can do your own sums, and I have not double checked mine).
    • CommentAuthorCWatters
    • CommentTimeJan 5th 2018
     
    I wish our UFH was slightly faster. In winter when the sun sets we seem to loose some solar gain faster than the UFH can increase its output to replace it. So on sunny days we sometimes get an hour when the temperature drops a degree or two below the set temperature.

    Noise is another possible issue. If it takes 2 hours to warm up the house, and you want it warm at 7am, then the stats will start calling for heat and waking up the boiler and creaking floors at 5am rather than say 6am.
    • CommentAuthorGreenfish
    • CommentTimeJan 5th 2018
     
    Faster heat up also allows for more variation in room temp from room to room e.g. cooler bedrooms, warmer living, and hot batroom.
    • CommentAuthorSimon Still
    • CommentTimeJan 5th 2018 edited
     
    Posted By: CWattersI wish our UFH was slightly faster. In winter when the sun sets we seem to loose some solar gain faster than the UFH can increase its output to replace it. So on sunny days we sometimes get an hour when the temperature drops a degree or two below the set temperature.


    That's not an issue we ever seem to get. I suspect that might be a benefit of the WC/constant flow approach - there isn't a warm up time as the floor is already warm

    Our kitchen tends to be a degree or so cool first thing in the morning but I'd guessed that's a result of 6 hours off overnight (and I actually find desirable). I do wonder how much 'intelligence' is built into the Viessmann weather compensator algorithm - there's little public info. I know it takes temperature over a period rather than just the instantaneous temp but it may well anticipate falling temperatures at night based on clock time.


    Posted By: CWatters
    Noise is another possible issue. If it takes 2 hours to warm up the house, and you want it warm at 7am, then the stats will start calling for heat and waking up the boiler and creaking floors at 5am rather than say 6am.


    interestingly we don't get any noticable noise from our wooden floors. However, unexpectedly you can hear the water 'rushing' in the concrete floors when its' really quiet (a quiet constant background 'white noise')
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