Sudden increase in usage after electrical work

[UNG]

I agree it's Combi boiler, so water cylinder no longer required.
But did the installers of the new boiler remove the cylinder ?
Has the old immersion heater been powered by the CU replacers ?

Not sure why your pushing the immersion heater route even if one had been left connected I would think it would have burnt out by now as the water evaporates from the cylinder also there would likely be a noticeable increased level of humidity / condensation in the property

 

[mainline]

I agree it's Combi boiler, so water cylinder no longer required.
But did the installers of the new boiler remove the cylinder ?
Has the old immersion heater been powered by the CU replacers ?

Op said : There are no immersion heaters or anything like that on the property.

 

[michael8554]

The water feed to the tank would keep the tank full.
But we'll see what the OP discovers at some point.

 

[Richard-the-Ninth]

What tank, they have a combi.

 

[UNG]

The water feed to the tank would keep the tank full.
But we'll see what the OP discovers at some point.

Clearly you have little understanding of plumbing, when changing over to a combi it is normal to isolate the water feed to the tank so you can divert the hot water pipe from the tank to the combi boiler