The science of fluid mechanics has found that in our heating applications, a plumber needs to provide for more than 2 feet per second and ideally less than 4 feet per second flow rate in pipes, in other words our hydronic Design Flow Velocity target is 3 FPS. (close to 3 is better) The flow should be 2 FPS in a manifold where multiple pipes connect, and can be up to 8 FPS in a Heat Exchanger, avoid 10 FPS, it is not wise to try to get 10-12 GPM through a 3/4" pipe without a lot of power and resistance.
TIP: Allow for 2.5 GPM per 12,000 BTU heating capacity is the normal expectation. So a 14 kW SunPump is 48,000 BTU and needs 10 GPM flow rate, ideally on the middle speed and middle part of the pump curve chart.
TIP: Plan the distribution system to operate at 100 F. water supply temperature or less. The lower the better. Do not try to operate a heating design at 110-120 because a heat pump buffer tank will not deliver that high for very long. The key is large surface areas - do not scrimp on pex pipe and expect a low-temp design.
By staying close to the target flow rate the heating system with perform the best, it will conserve energy, last longer, be quieter, and avoid problems. At 3 FPS the Reynolds number is in the Turbulent Flow over 4000 Rn category that is needed to transfer heat efficiently and avoid Laminar flow. This is important with Heat Exchangers that perform best at the 4000+ Rn and 4-8 FPS design numbers.
The chart shows some Head Pressure rises sharply at higher flow rates.
A good example for 1/2 inch PEX is a 1 gpm flow rate would only have a low 1.70 PSI Head over 100 foot length (or equivalent with fitting restrictions).
The 10 meter x 22 mm OD (3/4" Nominal), copper immersion heat exchanger inside a SP 3.0 is 33 feet, and 50 feet on the 14 kW model.
The pressure drop on the 22 mm x 10 m coil at 5 GPM is 5.26/3 = 1.75 PSI. and for the 15 m coil is 5.26/2 = 2.63 PSI
Pressure drop should be kept at 10 PSI or lower
The 2 page reference is attached below