Do I still need a Boiler or Heat Pump?
Probably, unless it is a low-energy building that only requires some heating for a month or two per year
Why would I want my hot water cylinder to supplement my Heat Pump heating system?
Reducing the workload or run cycles of your heat pump, probably by far the most expensive component of your heating system, is very likely to increase its lifespan. Also, as your heat pump is likely to be weather compensated, the output or water temperature will go up and down according to prevailing weather conditions meaning any radiators or towel rails installed may not heat up when you want them to. Such as bathroom or En Suite Towel Rails in the Autumn, Spring or Summer months.
Also, heat pumps typically use the Immersion Heater for the required Pasteurisation process on a weekly basis, this means the cylinder is heated to around 65C as a minimum to kill any legionella. This excess heat or temperature can then be used to warm a towel rail for example.
Why would I want my hot water cylinder to supplement my Gas or Oil heating system?
To reduce your energy bills and carbon emission as well as minimise the risk of your appliance ‘cyling’. As well as maximising the use of any PV or cheaper flexible electrical tariffs that are likely to be cheaper than the Gas or Oil you burn. Reducing the run times of any dirty fossil fuel appliances can also reduce the need for fuel deliveries and therefore will help improve air quality. Simply put, we need to stop burning stuff.