Let's clear that up, there are 2 distinct cases.
1- closed loop, *no* airtrap (and no res). This is OT here so we dont consider this.
2- closed loop with an airtrap (this thread topic). Let's say the airtrap is filled at 2/3, which for a small-sized airtrap gives about 10cl of air for 20cl of water. Let's say you have 1l of water in the system (im guesstimating here, i think it's quite a big figure).
Water temps rise about 2°C in most systems, heck lets say it rises 5°C.
The 1l of water becomes 1.00138l, thats 1.38cl more in volume. So the 10cl of air become 8.62cl.
If the air was initially at atmospheric pressure, it will then be at 1.16atm (assuming a linear pressure raise, we're on a small scale). This is negligible.
The main reason one doesnt care is the water *weight* itself creates way more pressure in lower parts of the circuit. If your circuit doesnt hold a 1.16 factor of pressure rise then you've got way worse problems at hand, to begin with...
So my take is seal it, that'll prevent any leakage. One makes mistakes, and Murphy's law says they always happen at the worse time.
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