An ASHP installation starts with a detailed room by room heat loss survey, a hot water demand survey and a quote that includes the hydronic design with low flow temperature (<45C) emitters, pipework flow rates, cylinder heat loss per day, fan noise level & electrical diversity calculation. This level of detailed design will maximise the heat pump’s efficiency and so minimise running costs. Your installer can then order the exact equipment to fit your home.
During the installation a team will fix the main heat pump unit outside your home. It takes air in at the back and blows it out of the front, so it needs clear space around it. Typically this is 300mm at the back and 1m at the front and sides. It will also need a condensation drip line fitted to a soakaway, as the heat pump drops moisture out of the air, as a by-product of extracting heat from it. A system of insulated pipework will then be installed along and through the wall to connect the heat pump to a hot water cylinder and to radiators or underfloor heating. Some of the radiators in the home will usually be upgraded, in order for the heat pump to work most efficiently.
The flow temperature at design outdoor temperature should be as low as possible. Ideally 45C or below, but could be higher if emitters really can't be up-sized to suffice at 45C. The flow temperature control should use weather compensation (i.e., the flow temperature should set based on the outdoor air temperature to allow lower flow temperatures in milder temperatures.
For hot water, the delta T of the water heating the tank should be minimised.
Primary Pipework Insulation
Insulation on the pipework between the main heat source and hot water storage is part of the solution to ensure an efficient system and to help reduce the risk of overheating in summer months. Where this pipework goes through the external wall or any unheated spaces, it should be insulated with weatherproof semi-rigid class 'o' insulation, of 19mm wall thickness or above. This means drilling a larger core through the wall. The pipe is fixed at regular intervals with brackets that clamp to the outside surface of the semi-rigid insulation. Any remaining metalwork should be taped and sealed with class 'o' lagging tape. Take care not to cover the outflow from any condensate or antifreeze valves.
Do you need an immersion element in the hot water cylinder?
Low temperature heat pumps will likely require an immersion heater to provide hot water legionnaires cycles. High temperature air source heat pumps can actually provide hot water up to 70C, so they are able to ramp up the temperature more efficiently, therefore not needing an immersion heater element for general operation. You may wish to still include an immersion heater as a back up and boost option. The following video explains the risks of legionnaires well, and can help when choosing a hot water temperature setting: youtube.com/watch?v=oJeyc_cGIMU