Useful for back-of-envelope estimates of
1) cylinders, buffers, 2) flow-rates, 3,4,5) Radiator temperature, heat output and Area.
6) COP estimator
Please also use other sources of information (As carpenters say, measure twice, cut once!!)
Also… Pressure drops and heating simulator
No 1) Cylinders
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No 2) This one is useful for estimating flow-rates, from temperature differences and Heat (kWatts). For example, if the temperature rises by 5° as it passes through a heat pump, and the heat delivered is 5kW, then the flow-rate would be around 14 litres/minute. Also see teaching tool.
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The three Radiator calculators below are ‘type-in-the-numbers’ version of the Heating Simulator. I think that the Simulator is a good teaching tool, and do play with it first to gain an intuitive feel for what is going on. The below might however be more convenient to use.
No 3] Radiator temperature 
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No 4] Radiator heat output estimator
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No 5] Radiator size estimator
One potential problem with calculators like this is that figures are entered ‘blindly’ and the result could be meaningless. It’s always worth double checking results in some other way since results are not guaranteed. Note; the accuracy only applies to normal conditions found in heating applications. Accuracy also reduces as the dt (flow-return) increases.
For interest, the following formulas are use.
Cylinder heating minutes = dt x 4.2 x litres / kW / 60.
(e.g. kettle = 90°(c) x 4.2 x 0.5lit / 3kW / 60 = about 1 minute
Flow-rate (Litres/min) = kWatts (heat) / 4.2 / dt (Temperature change) x 60
(4.2 is the specific heat of water in kJ/kg°C)
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