Tricky CH balancing problem

I have a problem with balancing the largest radiator in my house
(which is in the coldest room). It is a 1.8kW radiator. The other 15
radiators are between 0.3kW and 0.8kW. When the system was installed
the drop was 25C (and the room was very cold). The best I can get is a
16C drop (with its lockshield valve fully open). However:

1. The lockshield valves on many of the other radiators are now almost
closed (open 1/8th turn or less) which causes them to whistle.
2. The pump is on maximum.

The 1.8kW radiator is probably the furthest from the boiler. The
designs show it is connected by a 15mm pipe, but my suspicions are
that a 10mm pipe was used (I can't check properly as the plastic pipes
are behind plasterboard). It is about 6 metres from radiator to the
22mm pipe.

Question 1: should I really be having any problems balancing a large
radiator? Does a 10mm pipe sound too small? Would a 16C drop be
considered OK?

Question 2: should I try to balance the one radiator without a TRV, or
is this going to cause problems. I'm concerned the system is going
into safety mode when the TRVs on most of the radiators are shut.

I'd appreciate advice as I've spent 2-3 days studying this problem and
trying to balance the system. My NHBC warranty finishes in 2 weeks and
I need more evidence as the system designers are being evasive.

Thanks in advance,

Steve

System details: Glow worm Micron 60FF boiler. CH+hot water. Sealed
system (Megaflow). Grundfos UPS 15/50 pump. DeLonghi radiators,
plastic pipes.

You might consider having a bypass valve fitted if there is a possibility that all your TRVs will shut down before the room thermostat reaches its set point.

Try setting the TRV in the room where your thermostat is located such that the TRV is still slightly open when the thermostat has reached its set point.

niggle said:

You might consider having a bypass valve fitted if there is a possibility that all your TRVs will shut down before the room thermostat reaches its set point.

Try setting the TRV in the room where your thermostat is located such that the TRV is still slightly open when the thermostat has reached its set point.

Click to expand...

Problem is the thermostat is near the front door! I'm also getting system resets as soon as the boiler comes on even when two or three of the radiators are still on. Can these be caused by the pump being overloaded?