It doesn't matter if the longest one is fully open if the shorter ones are open too much, the water will shortcircuit through the shorter loops.
The only way to balance the thing properly is to use a thermometer. Even if they used a thermometer they may not have let the system run long enough to come up to temperature and if it as balanced at the height of summer the readings may not have any meaning - the floor in our living area gets enough sunshine in august to be well over 20 degrees.
Maplin sell a multimeter that comes with a K-type thermocouple for 20-25 quid, you'll find a million other uses for it too over the years.
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Read Heinbolds post as suggested:
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Good luck
