Replacing a gas combination boiler with a heat pump
This is not easy!
A small gas boiler starts about 20kW and can provide water at 55 degrees or higher. More powerful system boilers are not much bigger than the smallest wall-hung unit. A small heat pump is 3kW. A 20kW heat pump is BIG. They scale up in size as they get more powerful. Also, it's best if they are outside the living space to get their source of energy.
In a small house, bungalow or flat, a 20kW gas combi is more than adequate, so fitting one is sure to satisfy demand, especially if the house has a water tank and a bath.
Designing a system to replace it with a heat pump needs a lot more knowlege and care. Heat pumps are smaller and work at lower temperatures. You have to consider insulation, solar gain, drafts, room size and whether the water tank or the bath are still needed by the occupants. Instant showers are now common and washing machines heat their own water, for example. Also there are several types of heat pump, extracting heat from different sources.
See also Heat Pumps
Some examples