Thanks for introducing the idea of Delta ... wondered what the triangle sign mean't for a while there. I will pursue this. We have 15 radiators.I was in a similar position last year. I’m a homeower not a gas engineer.
I tried several free heat loss calculators and preferred the MYSON Heat Loss Manager because it lists the heat loss in each room for each element: Glazing, External Wall, Floor, Roof Glazing, Roof and Ventilation.
I also measured all 13 radiators and looked up the heat output of similar radiators to determine the total heat output of my radiators with a 50 K Δt.
A 25 kW boiler was replaced by a 15 kW boiler. It was more than enough last winter.
Behind the front panel the Vaillant ecoFIT pure 625 appears the same as the Glow Worm Energy7 25S. The installation and maintenance instructions are largely identical.
I think they are suggesting a Vaillant Pure but now I have realised they didn't say 412 or 612 so I need to follow that up with them.
Not even clear what the difference is from my perusal just now.
Other plumbers reckoned big boilers need the fatter pipe as I said so I need to keep lower than 20kw ( think my Glowworm 70ff is 20 kw)