5000BTU=1.47kw, flow/return temps of 75C/65C would require a flowrate of 2.1LPM, even if the flowrate was only 1.0LPM (shouldnt be a problem for 10mm microbore tails) then you should get around 1.28kw with flow/returns of 75C/57C.
If you have twoX1/2" plugs/vents at both top ends, maybe just flush back from both sides (one valve shut at the time) to check for any obstructions.
Or, even as a temporary measure, run it TBOE if you could run a pipe to the top, but opposite end to the outlet. But as above post says, you may just have to live with it.
Only for interest.
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