I know this is a slightly old thread, but seemed quite relevant to my question. Hopefully
@D_Hailsham can provide some maths genius to answer. Btw - I am going with the advice of this forum and having a LLH fitted. This is more out of curiosity than anything else.
As you know (from my previous threads! - hopefully you remember) my house has multiple zones, but this question relates just to the single upstairs zone. It has an index rad of 1.6m @ 20C (I modified the previous calculation you did and erred on the side of caution, hence the head loss has gone up from your calculated 1.09m). Pump is a Grundfos 25-80. These are the delta Ts I measured at the boiler at varying d0 settings:
24kw
16 delta T
Calculated flow rate:
0.36 l/s
1290 l/h
Boiler head loss at this flow rate (from manual): 2.4m
34kw
21 delta T
Calculated flow rate:
0.39 l/s
1392 l/h
Boiler head loss at this flow rate (from manual): 3.0m
38kw
23 delta T
Calculated flow rate:
0.39 l/s
1421 l/h
Boiler head loss at this flow rate (from manual): 3.0m
What I'm struggling with is the boiler head loss. As flow rate goes up, the boiler head loss goes up. Does the index rad head loss also go up?
Given the 2 conflicting parameters (head loss goes up, flow rate goes down, head loss goes down) and because the pump always operates on it's curve - how do you actually calculate where on the curve it is operating? Hope that made sense.
At 38KW, the minimum flow rate through the boiler is 0.45 l/s and the head loss is 4.05m.
Given the pump head far exceeds the boiler + index rad loss, why is the actual flow rate well below this (i.e. only 0.39 l/s)? Obstruction in pipe? Sludge build up in hex?
Thanks.