Be the first to ask a question want gb points? just write a review! A conventional boiler has a separate expansion tank to heat water – usually in a house’s loft – and is connected to the boiler using an external feed, which takes up quite a lot of space. Our expert staff will gladly assist you. Even though you can see that the running costs of electric combi boilers are higher, there are more. With easily interchangeable drums, the. Cooling systems, air conditioners, geothermal, indoor air.