Kw of system boilers

Scott_D · Mar 27, 2021

10 rad system and 210l unvented with a 16kw coil
Do you size to cope with full heating load and full coil kw which may be needed in a rare occasion (25-28kw)
Or size for usual demand (15-18kw)

I tend to go for the bigger value as there is less chance you will get a call but it’s going to cycle more as it won’t modulate down as low.

mfgs · Mar 28, 2021

For something like that you can’t really justify sizing for full coil kw unless there is massive hot water demand. I would be looking at approx 18kw and advise to time the hot water to come on at different time to the heating.

Chalked · Mar 28, 2021

An overworked small condensing boiler is more efficient than an oversized.

ShaunCorbs · Mar 28, 2021

I would size it at 18kw and stagger the hot water times you will find it will take around 40-55 mins to heat up the cylinder from cold

king of pipes · Mar 28, 2021

Depends on the property a well insulated thermally efficient home you can go with a lower kw boiler, traditionally brick and block built I find you need the extra to get you to a comfortable temperature quicker I'd go larger, on Valliant system boilers you can alter the cycling time and down rate the kw so not really a issue . Kop