Need help with Radiators cold at Bottom - not sludge

Cant post the actual spreadsheet but you can see the formulas (calcs) I used, you only have to put them in for the first rows and then just copy and paste to create as many rows as you like, these reflect your numbers but you can play around with them, have fun.
Only change the values in the yellow cells.

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Cant post the actual spreadsheet but you can see the formulas (calcs) I used, you only have to put them in for the first rows and then just copy and paste to create as many rows as you like, these reflect your numbers but you can play around with them, have fun.
Only change the values in the yellow cells.

View attachment 319272

View attachment 319271
Legend i will have a play!

So this is an accurate reflection of the current setup, i'm trying to get that extra 4 degrees out of the DT

One thing not on the sheet that i need to know is the pump speed for the CH (c) and Modulating Pump (c.)

I guess i can add an extra couple of columns in and play with the formulas unless you know?

thanks!

 Room Rad Rad(S) Rated Flo Temp Temp Ret Temp Flow Flow Delta T Output KW "Deg Rad" Deg C Deg C Deg C LPM M3/hr Deg C Output KW 22 22​ 50​ 20​ 34​ 6.78​ 0.41​ 16.00​ 34.39%​ 7.57​

Sorry also assuming that Deg Rad is the radiator temperature

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Sorry also assuming that Deg Rad is the radiator temperature
No, the deg rad is the T rating to give you the required output, a 50 deg rad is a T50 etc to give you 100% output, the 20 deg is now a T20 because of the reduced flow/return temperatures and gives 30.4% of the output if it was running in T50 conditions.
The required flowrates are shown above so you will have to adjust the pump speed to give you that bearing in mind that a higher pump speed may be required on boiler start up to prevent tripping on overtemp before the boiler modulates down from its ignition conditions.
I will post another set of readings later showing the nearest outputs as above but with the same flowrate as that required for T50 (100%) conditions. Some boilers now like Vaillant will automatically control the pump speed to maintain a settable dT of, I think, between 10C and 20C, others, will change the pump speed between 100% at full boiler output and something like 70% at minimum boiler output.

Just to clarify the T50, T20 etc
The "50" or the "20" etc is always the mean rad temperature minus the required room temperature.
So the T50 is (75+67)/2 minus 21 = 50
and the T20 is (48+34)/2 minus 21 = 20

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Below shows the data if the flowrate was kept at the T50 rate of 39.42 LPM BUT in practice you would need a enormous pump head to drive this through the boiler heat exchanger, the pressure loss through most boilers is based on a dT of 20C to give a reasonaby low pressure loss through the heat exchanger, a 22kw boiler should be limited to a max flowrate of 15.767LPM, (22*860/60/20).
The second set of data, below, gives the realistic numbers and even though the dT is very low with the T19 & T20 outputs, the return temp at 37C/38C still gives excellent condensing with very high boiler efficiencies.

Can't see any mention of what types of rads you have , t11's t21's t22's or designer rads etc?
How many metres of 22mm out of the boiler before it reduces to 15mm?

Can't see any mention of what types of rads you have , t11's t21's t22's or designer rads etc?
How many metres of 22mm out of the boiler before it reduces to 15mm?
 Name KW dt50 KW dt30 FLOMASTA TYPE 22 DOUBLE-PANEL DOUBLE CONVECTOR RADIATOR 600 X 600MM WHITE 1.025207​ 0.52778​ FLOMASTA TYPE 22 DOUBLE-PANEL DOUBLE CONVECTOR RADIATOR 600 X 1200MM WHITE 2.050414​ 1.05555​ FLOMASTA TYPE 22 DOUBLE-PANEL DOUBLE CONVECTOR RADIATOR 600 X 1200MM WHITE 2.050414​ 1.05555​ FLOMASTA TYPE 21 DOUBLE-PANEL PLUS SINGLE CONVECTOR RADIATOR 700MM X 700MM WHITE 1.063883​ 0.54769​ Capri - Anthracite Towel Radiator 1420 x 500 0.610905​ 0.31449​ FLOMASTA TYPE 22 DOUBLE-PANEL DOUBLE CONVECTOR RADIATOR 500 X 1200MM WHITE 1.765618​ 0.90894​ FLOMASTA TYPE 22 DOUBLE-PANEL DOUBLE CONVECTOR RADIATOR 600 X 1200MM WHITE 2.050414​ 1.05555​ FLOMASTA TYPE 22 DOUBLE-PANEL DOUBLE CONVECTOR RADIATOR 500 X 1400MM WHITE 2.05979​ 1.06038​ FLOMASTA TYPE 21 DOUBLE-PANEL PLUS SINGLE CONVECTOR RADIATOR 500MM X 1000MM WHITE 1.14856​ 0.59128​ Milano Arno - White Bar on Bar Heated Towel Rail - 600 X 1738 1.074724​ 0.55327​ FLOMASTA TYPE 21 DOUBLE-PANEL PLUS SINGLE CONVECTOR RADIATOR 500MM X 1400MM WHITE 1.608277​ 0.82794​ FLOMASTA TYPE 21 DOUBLE-PANEL PLUS SINGLE CONVECTOR RADIATOR 600 X 400MM WHITE 0.534432​ 0.27513​ Milano Bow - Anthracite D Bar Central Connection Heated Towel Rail 1533mm x 500mm 1.193389​ 0.61436​ FLOMASTA TYPE 21 DOUBLE-PANEL PLUS SINGLE CONVECTOR RADIATOR 500MM X 1400MM WHITE 1.608277​ 0.82794​ FLOMASTA TYPE 21 DOUBLE-PANEL PLUS SINGLE CONVECTOR RADIATOR 500 X 1200MM WHITE 1.378565​ 0.70969​ FLOMASTA TYPE 21 DOUBLE-PANEL PLUS SINGLE CONVECTOR RADIATOR 500 X 1200MM WHITE 1.378565​ 0.70969​

Meters of 22mm to 15mm copper and 10mm plastic?(PEX? builders installed) = about 5m initially, 3x15mm come off a run of about 8m

Furthest 15mm is probably about 12-15m
Furthest plastic 10mm is probably about 15-20m

Below shows the data if the flowrate was kept at the T50 rate of 39.42 LPM BUT in practice you would need a enormous pump head to drive this through the boiler heat exchanger, the pressure loss through most boilers is based on a dT of 20C to give a reasonaby low pressure loss through the heat exchanger, a 22kw boiler should be limited to a max flowrate of 15.767LPM, (22*860/60/20).
The second set of data, below, gives the realistic numbers and even though the dT is very low with the T19 & T20 outputs, the return temp at 37C/38C still gives excellent condensing with very high boiler efficiencies.

View attachment 319324

View attachment 319323
Thanks very much i think this makes sense to me now

The boiler should do 24kw, so 17.2LPM and that should mean that i can run down to 44 on the flow temp, return of 38 - 17.15LPM.. but i think from a "beat the crap out of the pump" perspective, it would be better at 45 degrees with T20 and flow LPM of 12.86

Amazing that 1 degree difference does to the flow..

Let me know what you think of that, i'll have to play with he settings

thanks very much for the help much appreciated

Yes that calculation above is correct, I think the 12.86LPM is a good choice as well.
what is the norm for the UK in choosing the lowest ambient temp, do you size for say from -5C outside temperature or what??. is there a standard figure?.

One thing to note here, though I would need to confirm - @ factory set, the 28/24 HRE will run @ 70% of 23.7Kw = ~18kw - there is a radiator load of 22.7Kw nominal with a ~10% add for pipework = 25Kw @Delta50. @ full rate on the 28/24 it can only achieve 85% of full power = 22.8Kw approx .... the boiler will not be able to satisfy the full load of the rads + pipework at nominal Delta 50

If the nominal delta is dropped, given the rads will have been sized (heat loss) for the standard Delta 50, lowering that to what is proposed then outputs may now not cover each rooms specific heat loss and rooms will be slow to heat or may not even reach desired temps
what is the norm for the UK in choosing the lowest ambient temp
Depends on location really - e.g London WD is -2 - Glasgow's WD is -5

One thing to note here, though I would need to confirm - @ factory set, the 28/24 HRE will run @ 70% of 23.7Kw = ~18kw - there is a radiator load of 22.7Kw nominal with a ~10% add for pipework = 25Kw @Delta50. @ full rate on the 28/24 it can only achieve 85% of full power = 22.8Kw approx .... the boiler will not be able to satisfy the full load of the rads + pipework at nominal Delta 50

If the nominal delta is dropped, given the rads will have been sized (heat loss) for the standard Delta 50, lowering that to what is proposed then outputs may now not cover each rooms specific heat loss and rooms will be slow to heat or may not even reach desired temps

Depends on location really - e.g London WD is -2 - Glasgow's WD is -5
The rads have been sized for T30 conditions so the max expected heat demand is 11.7kw?

The rads have been sized for T30 conditions so the max expected heat demand is 11.7kw?
Have they? I may have missed something in the original posts, wouldn't be the first time

heads back to the OP - DOH!!

(edit) I think this was the part that confused me - "I have set the Boiler to 100% for (3) - 23~24~kw and the rads are all summing up to about 22kw" - as this boiler can't be set to 100% - max is 85% on the 28/24

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The T50 rad list, above, sums to ~ 22.6kw, and the conversion factor to T30, seems OK (just calculating for one or two) at (30/50)^1.3, 51.48% = 11.6kw, even, as sometimes happens, the T figure is directly (incorrectly calculated, in this case, 30/50, or 60%, then the required demand might still only be, 22.6*60%, 13.6kw.

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