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This residence went for 20 years without a problem including five years or more after they bumped the pressure up around 90lbs. All the faucets are the old fashioned washered ones. One day she started getting hammer, and two days later I get the call that the pipes have blown in the crawlspace. Found the culprit, VERY poor soldering: piece of soft copper bent too close to the fitting, only went in a little more than 1/16", and the solder was used more as glue than solder. Took out the entire mess and replaced with new. Curious to learn from a real plumber why the hammer might just start up like that. There's no arresters in the system 'cos she never had a problem until now. Any ideas? Rich.