circulators burning out frequently

hi there

had an oil to gas conversion done last november with new boiler , circulators etc,  4 zones ( 3 for 3 previous zones and other to hwh); hot water heating; separate gas fired hwh; home on slab.

day after conversion, 1st floor main zone crculator burned out, no heat on main floor zone I.  installer came to change circulator and bleed pipe. month later, same thing, last month, no heat on 2nd floor zone II, same thing done. Zone III is apartment on first floor side of home.

Installer came, states I may have a leak in the main zone I slab which is making the circulator burn out and have no heat. No noticeable leaks, water bill indicates essentially same water usage. he also said there were water pressure changes and it dropped when he tested. we turned off all heat and are waiting for him to do some type of test on zone I main floor to detect possibl leak this week or later today.

Never had this problem pror to conversion/installation with circulators burning out dur to pipes needing to be bled often. House is 49 years old, copper pipes, on slab,

Any thoughts? Installer was recommended by National Grid and has always been cooperative and pleasant to come to home when called without charge thus far but i dont know exactly where th problem lies; leak in pipe or with the instalation process and/or equipment

Much thanks in advance for your help