Hackney plumbing
Homeowner
Hmmm ... I suppose the air in the pipe has already expanded by the time you apply the solder.
BINGO
Basic gas laws...PV=nrT, or if you move things around P=constant*T. Increase T, increase pressure. Which gas affects which constant you use, but just like a hot air balloon rises because the air expands, it will increase pressure in a pipe if it is closed. the smaller the pipe, the hotter the whole thing gets, the higher you can get the pressure. Yes, water when it turns to steam is also an issue, but even steam increases pressure as you raise it's temperature; it's just that it expands radically when it goes from liquid to vapor, which adds to the overall problem. Basic first year chemistry...
Generally, it's fairly easy to leave an end open. If you don't, the joint is at risk. Obviously, it may not get high enough all the time, but there are times when it will give you grief.
We can argue forever,you can take my word for it....or I can make you a video and prove it. Take your pick.
I'll take a piece of 1/2" about a 12" long....and solder caps on each end......Would that prove it to you? Tell me how I can prove it to you and I'll do my best.
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