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ASHP Radiator Sizing Calculator

  • 08-05-2020 2:25pm
    Registered Users Posts: 7,484 ✭✭✭ Blisterman

    Is there anywhere online where you can calculate the size of radiator required for a room for an ASHP system, where the water is 45 deg. None of the calculators I've found online have a parameter for water temperature. Which is of course key.

    Secondly, my thinking at the moment is to err on the sides of oversizing radiators, since they'll have thermostatic valves. In theory, they'd only output as much heat as needed for the room, so it shouldn't make a significant difference to energy usage. Am I off the mark on this one?


  • Registered Users Posts: 237 ✭✭ ShanE90

    Hi have a read of this document, pages 7,8 & 9 outlines radiator sizing, the bigger the heat emitter the lower the flow temperature required to maintain the space temperature will be, which in turn is where the efficiency lies.

  • Registered Users Posts: 2,958 ✭✭✭ John.G

    Above very interesting, maybe to answer Blisterman's query more directly....
    radiator outputs are based on a "50 Deg" rating which is the mean radiator temperature - the required room temperature, so, for example if one had a radiator flow temperature of 80C and a return of 60C with a required room temperature of 20C then this will result in a 50 deg rad, (80+60)/2)-20), a 45 deg rad will output (45/50)^1.3, 87.2% of the 50 deg rad, so theoretically, the 45 deg rad should be oversized by a factor of 1/0.872, 1.15, (15% "bigger")

    However, ASHPs work best (most efficiently) at low temperatures so if one just uses a example of a radiator flow temp of 40C and a return temp of 32C with a required room temp of 20C then this results in a 16 deg rad,(40+32)/2)-20), a 16 deg rad will output (16/50)^1.3, 22.7% of the 50 deg rad, so theoretically, the 16 deg rad should be oversized by a factor of 1/0.227, 4.4, (340% "bigger"). If the system is run 24/7 this of course will result in a far smaller oversizing requirement as one is only maintaining the room temperature.