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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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    • CommentAuthorgoodevans
    • CommentTimeDec 12th 2018
    Pilgrim, don't worry too much about the efficiency.

    You will see on this forum the idea of using low temperature water in UFH and radiators as a efficiency issue. This is true - particularly for heat pumps as the higher the flow temperature is the lower the efficiency of the heat pump is.

    It is partially true of condensing boilers - unless the return flow to the boiler is below some temperature (I think around 45 degC or so) then energy is lost in water vapour escaping out the flue and not condensing and releasing the heat.

    In your case you have a WBS - you need to burn hot for a clean efficient burn etc but in effect all the heat from the WBS stays inside the fabric of the house with the exception of the flue gasses.

    The only additional energy being consumed is the pumps. A small pump going full wack may run at 40 watts - but in your case it will be much lower than that - prob at the 10 to 15W range. Your best bet is to ensure that the heat generated goes to all the places in the house where you want it - without it ending up where you either don't want it or over heating some area (causing windows to be opened etc).

    Running the floor at a lower temperature near continuously may give a better heat control rather than running it on/off at a higher temperature (and also helps to prevent overshooting the target temperature). Getting mixer valves down to the desired temperature is not easy - so it may be better to consider a floor thermostat or connect the stat the return temp of the UFH zone.

    And be careful with delta T - for radiator specs delta T is the difference between the room temperature and the average radiator temperature, For boilers and heat output for pumps delta T is the difference between the flow and return temperatures.

    Room temperature control is the key now - size the radiators in proportion to the heat loss of each room/area. locate the thermostat for the zone in an area 'typical' for the zone (wireless thermostats are good for this). Balance the radiators to fine tune the relative output of each radiator.

    As we said at the beginning 2500kW is not a large heat load to pump. Its all about releasing the heat to the right places.
    Thank you very much once again @goodevans. I assume that your original calculation will be enough guidance to buy a pump which has sufficient capacity. I'll have a look at the Wilo pump.
    I thought I'd post a quick update to this thread. I've installed a single pump, connected to the ground floor manifold. I went for the smallest Wilo pump, and it seems to cope fine with the demand so far. With all circuits open, the flow meters are showing about half flow on the next to minimum pump setting for UFH. I'm going to connect the next manifold for the radiators and see if it can cope with this.

    Thanks again to everybody for all the help.
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