Large radiator system (60+ radiators) with one anemic zone

vadar007

I have a large radiator system (60+ rads, wood pellet furnace) across three floors. Building is located in Germany and over 300 years old. Rad system was probably installed in the 1970's. Furnace was replaced in 2021. The heated water is pumped from the basement to the attic and then distributed across 6 "zones" where it drops down to feed the rads. I have been in the process of removing the rads, flushing them and replacing both the lockshield valves and thermostat valves with new units that allow me to more precisely control the rate of flow through the rads. Everything has been balanced to the best of my abilities. The problem is that one zone, in particular, is always anemic relative to all the other zones (Figure 1 shown in yellow). If I shut off all the rads in that zone (Figure 2) but the bottom floor rads, they do heat up so I know that flow can get down there. I replaced all the insulation on the distribution pipes in the attic this summer and discovered that the pipe diameter feeding that zone is smaller and runs longer (11.6m) than the rest of the other zones. There is a small diameter run at the end of the other distribution pipe but it is 1.6m long. So I think I have two possible routes to try and address this:

1. Replace stretch of 11m pipe to the anemic zone with larger diameter pipe. Leave a 1.6m small diameter section to anemic zone to match other end of distribution pipe. This should allow more equal distribution across all the zones but will require rebalancing and maybe a bigger pump.
2. Replace present pump with a bigger pump to overcome resistance to that zone. Brute force approach.

Are there any other options and/or recommendations? Any downsides I am missing?