This problem started recently. I am in a condo of six units. Our hot water is derived from two big hot water heaters, working in tandem. Recently the overall hot water pressure seems lower then before and now when someone else either yourself or someone else in the building also uses hot water, the flow is reduced to a near trickle. Needless to say this is a problem when taking a shower. A plumber has visited the scene. The water inlet pipe to the heaters is only 1/2" while the heater outlet pipes is 1 1/2". To evidently compensate, there is an electric pump on the inlet. The plumber thought the pump was defective and had it replaced although the new one seems smaller. However the problem still exists with no change in the symptoms at all. The plumber was called again and he says it must be for some reason the overall pressure of the water entering the building has been reduced for whatever reason as that can be the only explanation. The thing is unlike the hot I can run multiple sources of cold water without a noticeable drop in pressure although the overall cold water pressure is greater then the hot to begin with.

My thought is perhaps it's something to do with one of the heaters but they merely just heat water as it's passed thru don't they, meaning it wouldn't factor in to any pressure problems?

If there are any ideas, I would appreciate them. I know very little about plumbing and this is bewildering.