CH flow temperature on a Greenstar 18i

[Robbox]

I started to go down the heatpump rabbit hole last month and on my journey in the warren I came across this website www.heatpunk.co.uk. Woopee I thought this is right up my street, measuring my house layout, window sizes and what not. I got it all done and felt very pleased with myself. Then I looked at the result! So to get a flow temperature of say 45C I will need to add several extra rads, in most of the downstairs rooms, not to mention the HP and assorted bits to go with.
Do I really want all the mess of extra rads, where to site the HP (best location is on the side of the house, 15ft up!) etc etc.
But then I started to play around with flow temperatures to see at what temperature I could get away with doing no rad upgrades. Turns out only one room is unbalanced for heat input vs heat loss at 55C. With this one room, it would be a fairly simple and not expensive job to add one more rad.
So I loooked at my Greenstar 18 i, fitted in 2014, and thought how do I set the CH temperature to match the heatloss? There is a dial with numbers on it but no temperature indicator.
So that's the question here, what number on this system boiler equates to 55C flow temperature?

 

[MNW67]

Is it like this knob?

 

[Johntheo5]

I started to go down the heatpump rabbit hole last month and on my journey in the warren I came across this website www.heatpunk.co.uk. Woopee I thought this is right up my street, measuring my house layout, window sizes and what not. I got it all done and felt very pleased with myself. Then I looked at the result! So to get a flow temperature of say 45C I will need to add several extra rads, in most of the downstairs rooms, not to mention the HP and assorted bits to go with.
Do I really want all the mess of extra rads, where to site the HP (best location is on the side of the house, 15ft up!) etc etc.
But then I started to play around with flow temperatures to see at what temperature I could get away with doing no rad upgrades. Turns out only one room is unbalanced for heat input vs heat loss at 55C. With this one room, it would be a fairly simple and not expensive job to add one more rad.
So I loooked at my Greenstar 18 i, fitted in 2014, and thought how do I set the CH temperature to match the heatloss? There is a dial with numbers on it but no temperature indicator.
So that's the question here, what number on this system boiler equates to 55C flow temperature?

A flow temp of 55C will give a rad output of 56% of its rated output (T50) but a heat pump (if/when installed) obviously needs to run at the lowest possiblt temperature to get a maximum COP, if the HP is run at 45C then you might get DHW at 38C/40C with boosting to 60C only required once/week for legionella protection, the rad output then will be 37% of its rated output, a flow temp of 50C will give a rad output of 46% of rated output.

 

[Mister Banks]

A flow temp of 55C will give a rad output of 56% of its rated output (T50) but a heat pump (if/when installed) obviously needs to run at the lowest possiblt temperature to get a maximum COP, if the HP is run at 45C then you might get DHW at 38C/40C with boosting to 60C only required once/week for legionella protection, the rad output then will be 37% of its rated output, a flow temp of 50C will give a rad output of 46% of rated output.

So based on those figures will the OP need radiators roughly twice the output of the existing ones?

 

[Johntheo5]

Correct, T30s, the attached will give some idea of what might be achieved with rads that were originally sized for -5C outside temperature, with a required room temp(s) of 21C, of course in lots of cases the existing rads are oversized anyway but hardly by a factor of 2?.