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Thermostat Mixing Valve

  • 15-02-2019 2:39pm
    #1
    John.G
    Registered Users, Registered Users 2 Posts: 3,562 ✭✭✭


    Query re above and a 25kw oil fired boiler with a flow temperature of 65C fitted with a 3 way mixing valve installed to give a mixed flow temperature of 40C to radiators with rad (and boiler) return temperature of 35C. Above required by someone who wants to see if rads are suitable for use with ASHP running 24/7.
    Has anyone on here done this or similar for UFH with an oil fired boiler? and if so any advice.
    The mixed flow rate to boiler return flow rate at the above conditions are 6:1, (my calc), if the heat demand is say 20 kw then the mixed flow rate to the rads is 57 LPM so the boiler flow rate is 9.5 LPM, shouldn't be a problem, but if the heat demand is say only 5 kw (zone valves closing etc) then the mixed flow rate required is 14 LPM and the boiler flow rate is only slightly more than 2 LPM.
    This is obviously a problem so how to deal with it? ABV between boiler flow and return maybe?.


Comments

  • #2
    Dtp1979
    Registered Users, Registered Users 2 Posts: 12,840 ✭✭✭✭Dtp1979


    I wish I was good at maths like you John.


  • #3
    Sleeper12
    Registered Users, Registered Users 2 Posts: 17,156 ✭✭✭✭Sleeper12


    Dtp1979 wrote:
    I wish I was good at maths like you John.


    That's both of us so. My mind refuses to work like John's. He alw leaves me in awe


  • #4
    DGOBS
    Hosted Moderators Posts: 3,496 ✭✭✭DGOBS


    You shouldn't have a return temperature to an oil boiler of less than 55ºC, unless it's a totally stainless heat exchanger.
    The reason is that you will promote condensing in the ferrous portion of the boiler which will rust through in short order.


  • #5
    gctest50
    Registered Users, Registered Users 2 Posts: 9,605 ✭✭✭gctest50


    John.G wrote: »
    ..........

    This is obviously a problem so how to deal with it? .......

    An accumulator tank maybe ?

    Boiler heats tank ( put a loading valve on it so it's in the happy condensing but not too cold zone )

    House draws heat heat from tank when needed

    Boiler / ASHP failure in winter ? just use the immersions in the tank - beats freezing to death and burst pipes

    Expensive but much happiness


  • #6
    John.G
    Registered Users, Registered Users 2 Posts: 3,562 ✭✭✭John.G


    Thanks boys.


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