@aliceinwonderland_id Thank you! But how could the vertical washing machine pipes retain water? I disconnected the main at the meter to drain the system.

aliceinwonderland is 100% correct on your first 3 questions and the answer to your 4th question is,,,,,,. After disconnecting main at meter, a backflow preventer and/or pressure regulator valve prevented drain-back,,,and/or you failed to open ALL facuets in and outside house (hoses must also be removed from washing machine facuets)

Scott,there's another possibility. If the meter is up hill from house,water wouldn't drain regardless of conditions mentioned above.

To better understand necessity of opening facuets after opening a low point drain,do this. Stick a clear drinking straw into a cup up liquid,place index finger on tip of straw and withdraw from cup. Liquid remains' in straw as long as finger remains' on top.

@klem1 Thank you! That straw analogy makes sense. I thought I had turned on the washer to drain those pipes, but maybe I forgot to do it this time.

That doesn't explain the radiant heat pipe though. I drained the boiler with a hose and drained the pipes under the house at the lowest point.

Frustrating to return home and unable to breath a sigh of relief and relax. I hope you were able to get repairs done quickly.

@klem1 Indeed! I managed to get it mostly repaired. I still need to replace the radiant heat pipe which is a little beyond my limited skillset.