Design Recommendations
For high performance (SPF 4+) use a design flow temperature of 40C or below and ensure weather compensation settings have been set to match.
Accurate heat loss calculations make designing for 40C design flow temperatures more practical and help avoid extreme (2-3x) over-sizing of heat pump units.
When moving to more accurate heat loss calculations (e.g using blower door test results rather than pre-2000 CIBSE), carefully consider the following at the same time:
Reduce default design flow temperature from 45-50C to ideally 40C.
Add sufficient margin on heat pump capacity to cater for lower than manufacturer datasheet heat output during defrost conditions.
Adding a reasonable margin (~1.2-1.5x) on an accurate heat loss seems to work fine for heat pumps that handle mild weather cycling well and may actually be preferable, especially given max output defrost issues.
Spending time on commissioning and tuning heat pump settings to match real world heat loss makes a big difference to performance.
General recommendations
Design for open loop with no zoning, TRV’s on max or high temperature cutoff only.
Try to minimise the amount of setback on heating schedules e.g keep daytime and nightime room target temperatures fairly close e.g 18-19C setback, 20C afternoon/evening.