Need help with Radiators cold at Bottom - not sludge

Thanks, theoretically boilers are sized based on HL and design temp, since rads are almost, always oversized.
I didn't know about HL of my propety and previous 24kw boiler worked with 8min on and 12mins off cycle (in slightly less cold weather the off time was doubled than on time), I was told that boiler is way oversized for my heat loss.
I did HL and came up with half of what boiler could output! (11 kw in my case).
As you say, flow temperature can be increased to get the required heat output (though may be not as efficiently or in condensing mode).
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Should have done the calcs with a room temp of 20C but doesn't make a huge difference.

Very good, so what size boiler is installed now and is it a system boiler (non combi)
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yes, heat only boiler 13kw for CH only, I know its tight but my essential load running most of the season is 6kw. It gets below 5 for a week and for that I have auxiliary heating arrangements; mini splits in all rooms.
No problem, forget about T50 or T30 for the moment, you do/did a calculation on the heat required to heat your house from -5C OT, you came up with ~ 22kw, you could supply that heat with all electric heating or UFH or, in your case rads, it doesn,t matter because 22kw must be supplied. All rad manufacturers now use this T50 standard when sizing their rads and the rads you have installed have been based on this because the sum of their outputs is ~ 22kw, if you now decide to run the rads at reduced temperatures to become T30 rads then they will only output 51.48% of what they would at T50 conditions, 22X51.48%, 11.33kw. You have just converted the T50 outputs to T30s which is mathematically correct but doessn't change in any way the heating requirements, you will be able to heat the house to 21C if running with T30s once the OT rises but you don't have to rigidly stick to the T30, you just increase/decrease the boiler flowtemp to suit the conditions, this is what WC is doing in effect, its changing the boiler flow temo so the rad outputs as a T50 or T45 or Twhatever to give you your required room temperature.

Thanks again for the reply its very helpful and apologies if my past replies were not clear.

So if i havent been clear enough :) -> the heat requirements for the house are 12.16570984 Kw. - This is what's needed at T50

So as an example, my office needs 2994BTU at T50, and the radiator outputs 3102BTU (0.908Kw) at T30. at T50 the Radiator outputs 6026BTU (1.765Kw)

I spec'd the rads to give out the BTU the rooms need at T30, if i were to then run them at DT50, the house would probably set on fire.. :ROFLMAO:

I really like the idea of WC, it is an option on the boiler. I just need to spend a bit of time balancing the system properly now and adjusting the pump speed to work efficiently.. But its trial and error without knowing how the settings tie into the rest of the boiler. Might have it too high or too low at the moment, as the rads do warm up, even at the bottom but not at the bottom until about 20 minutes of the heating being on.. this is my issue

thanks again
Here is a flavour of what you might expect from your system if your heat loss is fairly accurate, all the calcs are based on a fixed flowrate of 8.72LPM which IMO is the very lowest you can go to allow the boiler to fire up at ignition conditions. I have only done some numbers for fairly typical OTs.

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