hi could anyone please advise , i have a worscester 40 he combi , its oversised as i wanted a good water flow rate i could manage with a 30 kw , what water flow rate would this give me compared to a 40 , im only thinking of changing as i think the 40 uses more gas than a 30 to heat up a 4 bed semi , will changing to a 30 save me 25% on gas usage or does it not work like that , is ther a straight forward way of finding out how much gas each model uses per hour , thanks rob