I have an Xtramax II combi boiler - 4 bed detatched house, trvs on all but 3 rads, total 15 rads in house. temp on 18-19 day night. using aprox £6-7 gas per day... this seems excessive to me. boiler set to 21kw and hot water switched off at night(it has integral h/w tank) can there be a fault/setting causing it to use to much gas. ? thanks.