I should have said that it depends on the size of rad. A 2kW rad will need the LS valve open further than a 500W one as the flow rate is four times faster.
Never the case in real world LPHW hydronic heating systems.............
Obviously the flow rate through a 2kW rad will only be four times that of a 500W rad if both are giving of the rated output at the specified flow and return temperatures.
to get 80% of the design heat output (80f/70r) we need only about 25% of the design flow rate (non mixing)
That will depend on
how you reduce the rad output to 80% of the quoted output.
If you maintain the same flow temperature and reduce the return temperature, by closing down the LS valve, then the flow rate will not be proportional as you will now have a higher temperature differential. But if you reduce the flow temperature and maintain the same differential, then the flow rate will be 80% of the full output flow rate.
if the temperature difference between design flow & return is increased the response becomes closer to linear ie: 80f/60r would require around 48% of design flow rate for 80% of design heat output,
Why do you use non-standard temperatures?
A 1kW rad has a flow rate of 0.024 litres/sec at standard temperatures (75/55/20). If you want to reduce the output to 800W, there are several, related, ways you can do this: reduce the flow temperature and /or reduce the return temperature.
If you maintain the same flow temperature (75C), the return temperature will have to reduce to 51C and the flow rate will then 0.009l/sec, which is 37.5% of the flow at 1kw output. But if you reduce the flow temp to approx 67C and maintain the 10C differential, the flow rate will reduce to 80% of 0.024 l/sec.
this implies that a larger design temperature difference between flow & returns is preferred as control of emitter heat output would be improved
I don't follow your logic.
take a look @ the drayton TRV , orifice entry size is around 8mm
, 'technically' speaking these would be no good fitted to any convector larger than around 1.5 kw (velocity not exceeding 1m/s).
Drayton say that you shouldn't exceed 200mB differential pressure across the valve, which works out at 3kW. That would give a velocity of 0.45m/s in 15mm pipe.
Have you ever looked at the size of the opening in a TRV4 body if you alter it using the special key (the setting numbers are on the black ring)?
This shows the opening when the setting is on 1 or 2 (can't remember which).
View media item 18380
Setting 1 is suitable for a 500W radiator and 2 for a 750W.