Mains Pressure hot water

I look after the buildings for a local church. The hot water for the wash hand basins in the toilets is from electric heaters that store a few gallons of hot water. They are mains fed.
I'd like to replace them with some cylinders that can be heated by hot water from the central heating. If it was a gravity fed hot water system it would be easy to change to an indirect cylinder but there is no head room to put in a storage tank, if I fitted a header tank it would be no more that 2.5 meters above floor level so the flow would be pathetic.
Is there an indirect cylinder, or anything else, that can operate on mains feed and be heated by both electric and hot water from the central heating?.

Of course google unvented HW Cylinder, very common to use

ianmcd said:

Of course google unvented HW Cylinder, very common to use

Click to expand...

Thanks,
It will be easy now that I know what it is called.

Think the unit was called a "Calorifier" it is a heat exchanger, they are used a lot on narrow boats to use the heat from engine, as to how much water you will get each time you turn on tap will depend on size, and also there will be a delay in getting hot water, hotels and the like circulate the hot water so instant at the taps, but then every pipe needs insulating to stop it warming the rooms in summer.

With a large building under sink 7 litre water heaters can work out cheaper to run as less loses heating up pipes every time you want hot water, if you need to fill a bowl with cold water before you get hot water then your heating twice as much hot water as you are using.

How much use does the building get? It may not be prudent to have a cylinder full of hot water sitting idle for hours, if there are only limited hot water draw offs, at certain times. Undersink electric heaters may prove a better option if demand is limited.