The fitter originally suggested a combi boiler, 38kW. After testing the water pressure and flow rates I told them I wanted to stick with a cylinder and use a pump for the showers. I trusted the fitter with their choice of the 28kW alternative instead of at least having an idea about it for myself!
Thanks for the helpful answers.
I've currently got an old indirect cylinder but was planning on replacing it with an indirect fast recovery one. I don't want an unvented cylinder as the inlet pipe to my house is small bore and whilst the pressure is high enough, flow rate is low.
I've come to realise that the boiler I have fitted is oversized for my needs. It's a Vaillant Ecotech 428. However, any sizing calculation I do comes out with a maximum heat requirement of about 18kW.
My question is, is it less efficient to run the 428 with the output dialled down than to run...