Before we start, we have to remind ourselves of the following
Heat Output = Specific heat capacity x flow rate (l/s) x (flow temperature - return temperature)
Specific heat capacity (SHC) of water is 4.2 and it’s 3.8 for Gylcol.
Flow Rate conversion m3/h to l/s = (m3h * 0.277)
Flow Rate conversion: l/hr to l/s = (l/hr / 3600)
So 860l/hr = 860 / 3600 = 0.238 l/s
3.8 x 0.238 x 5DT = 4.5kW
No if we go back to our ‘gold’ tech document, whilst it doesn’t list the 3.5kW model, it lists a 3kW one.
Which suggests that even that model can kick out between 4kW and 6kW of heat depending on outside temp and flow temp combinations.
During my hot water run, i’ve got my flow rate fixed at 550l/hr and I still output almost 8kW of heat cos my DT is around 10 on hot water runs.
So yes, I have much to play with there too.
But that makes me wondering why your installer has put a secondary pump in.
Is the cylinder a long way away from the outdoor unit? Are they trying to extend the distance?
Because as shown, you don’t need super high flow rates to get high output.
Do you track your flow and return temps? During heating and hot water?