It`s the boiler obviously.
Your old boilers used a set amount of gas whether they were heating the H/W or C/H.
A Combi uses a LOT more gas when heating hot water,this is why a new larger gas supply had to be run to it.
In your old systems,your tennants would probably use the hot water till it was finished.
In your new system they can in theory stay in the shower for longer and shower more often.
My advice,get a coin operated shower(like in caravan parks)that`ll cut down gas usage .
Your old boilers used a set amount of gas whether they were heating the H/W or C/H.
A Combi uses a LOT more gas when heating hot water,this is why a new larger gas supply had to be run to it.
In your old systems,your tennants would probably use the hot water till it was finished.
In your new system they can in theory stay in the shower for longer and shower more often.
My advice,get a coin operated shower(like in caravan parks)that`ll cut down gas usage .