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)
Hi John
What’s the best U-Value calculator or radiator sizing calculator for a room?
Seems to be a lot around all giving differing figures
Hi gareth, yes, there seem to be a lot about. I am no expert on them, and the ones i have tried seem faffy/limited. Heatpunk is free, but has limitations, so you have to use it creatively. Of course, for many buildings, the actual U value is unknown, so the heat loss excercise is sometimes just an approximation, but many seem over trust the accuracy. Furthermore, a house is rarely in steady-state, and the thermal mass of all the fabric (heating or cooling) affects the actual heat needed at any one point. That said, nobody would try to estimate that, and heat loss calculations are the best indicator of how many kWs might be needed
Great thank you. Are we okay to use 10mm pipe to supply radiators when using an ASHP fed off a 22mm flow and return?
Is there a limit to kwh 10mm pipe can support?
Hi Gareth,
The length of pipe is just as important as the diameter. Yes, an apprximate rule-of-thumb could be made, but i feel a little reluctant to attempt that since it could easily be used ‘blindly’ without thinking it through. I would suggest to look at the free online calculator at https://www.pressure-drop.com/Online-Calculator/ and try a few sample sizes > I think the bore of 10mm copper is about 8.6mm. You can try diffrent lenghts, and assume you might need 3 litres/min per kW of heat output.
Re velocity.. many people seem to worry about velocity. I have not worried too much in the past. In fact, the velocity to worry about could be that on the seat of a part-closed TRV valve. The velocity there could be extremely high. Now that pumps are more energy-efficient, then arguably we can cope with higher pressure drops, but risk of some noise. Most of that could be from closing valves. Anyhow, noise is very subjective
Had a look on that site but I can’t really seem to work out the heat capacity of a pipe.
I’m just wondering if flow and return is 22mm for main branches, can I just use 15mm to feed rads and if so how many?
Or do I need to have separate 22mm for upstairs and 22mm for downstairs
Is there a table or guide?
Aiming for a 40c flow and also have UFH in another area of the house all controlled on one zone if possible?
Its a little complicated with pipe runs to radiators since as the pipe run goes on, and we branch off to radiators, then the flowrate reduces. You can use a rule-of-thumb that says a flowrate of about 3 litres/minute for every kW (this gives a dt (flowT-returnT) of about 5). So, if you have a radiator emitting 1/2kW, it needs about 1.5lit/min. The typical arrangement for most systems is as you say. 22mm pipes for each floor, branching with 15mm for each radiator. You could estimate how many kW’s the radiators are likely to emit at 40c. from that, estimate the flow rate for each branch. Allow a bit extra in case. The longest pipe run and biggest radiator is the one to worry about. Short runs will no doubt need a radiator valve(lockshield) closing, so pressure calcs for small rads may be irelevant. Generally, the flowrate around UH needs to be a bit faster so the floor has more even temerature. A mixing valve and manifold pump might help. However, I have done systems where its just one pump on the whole system. This takes time with pipe thermometers to balance up. A home owner may be happy to spend time and learn the system. For an installer, they may need a more ‘solid’ design that will work without too much tinkering. e.g. low loss header etc.
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