Help with Boiler Selection

Older (frankly, obselete) designs of boiler used to operate and a fixed heating power or gas rate as it called. If you wanted less than full power as was often the case the boiler had to switch off/on/off to adjust to the requirement. It turns out that less gas is used if the boiler can be made to adjust its gas rate to suit the load.

Modern non-condensing designs can adjust their gas rates this is known as modulation and typically they can adjust down to about 40% of maximum.

Non-condensing boilers which use forced premix burners can modulate down to around 30% or even less of full power.

Your figure of 3.5kW for the HW in summer is rather low. A HW cylinder which is Part L compliant will take 10kW or more when it is operation and then the demand will cut out.

So a 20kW (or a little more) boiler will be satisfactory. A lot of the time in Autumn & Spring the heating load may only be a few kW so a boiler which can go that low maybe helpful. However so long as a suitable sized boiler is chosen you should be OK.

You will need to select a model which fits your requirements. Most manufacturers will let you either down load the full instructions from the web site or will let you download a technical spec containing casing sizes, cupboard requirements, and flue positions.

Now that is unfair.

You know full well that IMM/DrE would be of the "two hands are better than one" school.

Owain

ITYM 'two fingers'.

The information written by this lunatic in this post *is* not worth using by anyone human. So, best

um.

oops that should be Condensing boilers...

Self awareness, and improvement.

You're not the brightest spark, are you...

Nah, sorry about that. He has not improved. Sad but true.