The verdict was that the feeder valve to the furnace had failed, putting excessive water into the system, and overstressing the relief valve, which then caused the relief valve to fail.
This is a picture of the culprit, a valve manufactured by www.watts.com
Those of us who live in the USA are familiar with the general decline in product quality here. Whereas, in my parents' time, major appliances were expected to last 50 years -- and I do not remember any major appliance failure in my entire childhood, upgrades being done by choice rather than failure -- now appliances barely survive their warranty period, which tends to be short, generally no more than a year.
This constant appliance failure is an unnecessary drag on the typical family budget -- not to mention a big stressor. Flooding the basement causes property damage in addition to repair expenses.
Something needs to be done to return appliance quality to its former state. This widespread sale of hazardous junk needs to be halted.