After a treatment system was installed (chemical,ph balancing, blending tank, and grounding, for wellwater) for whole house (2 water heaters) one water heater would only supply luke warm water, then run ice cold (no pressure drop) for a couple minutes, then return to warmer then previous temp. Never hot (it was prior to system install). I toccurs to all areas supplied by that heater, we replaced parts first, then whole heater (was12 years old). With new heater the periods of wamr water are warmer but not quite hot, and still has the period of cold water. The system installers don't see any way they could have changed anything related. Any ideas?

Tell me more about your system. Are the heaters hooked up in series, parrel or do they supply separate areas in your home?
Is a recirculating system installed?
What size are they? Are they electric or gas? Did the heaters work OK before you had the treatment installed?
Back to you, Tom