I have a 7 year old home in which I have had 4 cases of leaks developing in the elbows over the last 5 years, 2 in the last 3 months. These leaks have all been on the hot water side and occur in the copper elbow not in the sweated joint. I do have a circulator in the system. Anyone aware of what may be causing this?

Simple enough: Water velocity

Copper Pinhole Leaks

The circulator doesn't need to run all of the time, more importantly, it's probably oversized.

Take a look at some of the stuff on this webpage: Advanced Conservation Technologies: D'MAND Hot Water Systems

The system can be used with a return loop.

Cavitation is a leading cause of pin holes in my part of the country. This is usually associated with circulating hot water systems or extremely high water pressures.

Water velocity in undersized copper tubes. For tubing sizes normally installed in home plumbing, the design water velocity should be targeted toward 4 fps. The greatest effect of velocity occurs where the water is forced to change flow direction, such as at elbows and tees, but excessive water flow rates can be damaging to the entire plumbing system. When copper tubing is installed that is too small in diameter for the pressure and flow available, the resulting high flow rates can erode the protective coating creating areas of bare, unprotected copper. This effect can result in a high rate of corrosion wherever the protective coating is eroded. A White Paper Review:

As reported by:
History Of Use And Performance Of Copper Tube For Potable Water Service, Richard O. Lewis, P.E. Lewis Engineering and Consulting, Inc. Washington Suburban Sanitary Commission Home Page
Copperpipe/letters/copperpipewp.cfm

One other thing that caused many pin hole leaks is the fact that back in the '50's,'"60's and 1970's the US got a lot of inferior copper imported from foreign countries but I doubt that is your problem since you house is only seven years old.

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One other thing that caused many pin hole leaks is the fact that back in the '50's,'"60's and 1970's the US got a lot of inferior copper imported from foreign countries but I doubt that is your problem since you house is only seven years old.

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We've had a number of problems recently with copper tubing imported from Mexico.

It's pretty rare that we use copper tubing these days, but when we do, we pump it up with air pressure in excess of 150psi and let it sit overnight, if there is even a slight drop in pressure, we wipe all of the piping down with a sopping, soapy terry towel and look for bubbles -- Even a pin hole leak will show up at that pressure.