kstills wrote:...against a closed valve without burning it out? For extended periods of time?
As Sting would often say "it depends".
Certainly it is bad practice. It wastes energy. Granted contols and a little wiring cost money too. The most damaging possibility is that the dead headed pump will churn the water until it gets so hot it exceed the pumps rating and destroys things like seals. Hot enough to boil the water and bubbles form, then they destroy the impeller too.
It depends because the larger the pump, the more likely it will occur. Smaller, low speed (low energy) pumps may have enough heat loss to overcome the heat buildup within the housing. That old massive slow speed circulator that has enough steel in it to build 3 modern day pumps will last a lot longer than a modern, computer deigned and value engineered pump.
Just ask any fireman that is trained to operate a fire engine. Major concern to keep water moving through the impeller to keep damage from occuring. A minute or two may not matter but don't sit there and churn away.
Bottom line - bad design to dead head a pump.