I have a two 40-gallon tank gas water heater system plumbed in parallel. Only one of the tanks has or has ever had gas supplied to it. (the other tank is plumbed for gas but has no igniter switch installed and the pilot light has never been lit). Our house and water heaters are 6 years old. Recently we have experienced a strong drop off in available hot water. Last year we noticed a slight decrease in available hot water so I turned up the temperature on the one active unit. This year the problem is much worse. We cannot even fill a tub before we run out of hot water. I observed that when the temperature in the active tank decreases due to water usage, the active tank burner comes on and shortly thereafter the passive tank begins to slowly vent very hot water from the over pressure over temperature valve and continues to do so while the active tank burns. How does it work with only one tank having a heat source but both being fed by the same cold water pipe and both supplying the same hot water pipe and what could be causing our lack of available hot water? Thanks.

If you have two water heaters plumbed in parallel as you describe, and only one is active, the inactive one is just a water storage tank. When you open the faucet, half of the water (hot) comes from the active heater and half (cold) comes from the inactive one. When you turned the temperature up on the active one, the heating of the water increases the pressure in both. If your water pressure is very high, that causes the T&P valve to release. The T&P on the inactive heater is either defective or the spring in the inactive heater is just not quite as strong as the T&P on the active one.

Close the cold water stop valve on the inactive heater or light the burner.