hi. thinking of replacing ancient heat only boiler for a worcester green star heatslave combi oil boiler. the house heated nicely with existing size rads, i will problably replace them with same size newer ones. there is bathroom, with bath, basin, toilet, elec shower. 1 kithen sink. total btu`s of rads: 49458 ish. do you think a heatslave 12/18 will be big enough for system, will a 18/25 or a 25/32 be too big? thanks.