Selgas
10 years life out of a water heater is about the average although many make it to 15 or so years. As a recommendation only – unles the other cylinders are showing signs of water leakage I would suggest you inform the owner and introduce a replacement programme that would ensure the heaters do not exceed 12 years of working life.
You do not say how many cylinders are installed in the apartment block so it is difficult to forsee what likely capitol costs would be involved with such a proposal. In any case the capitol expenditure is tax deductable to the owner. 
Selgas Services Ltd
Craftsman Gasfitters, Plumbers, Electrical Service Technicians
