I wouldn't replace the water heater for fun trouble shoot it first. Buying a spare isn't a terrible idea though. When its running does it crackle like the water is boiling? Im assuming its gas.

There really isnt much complex about a standard water heater. Two pipes a flue and the gas. Thats % 75 of the complexity of the water softener.

Also. You should find where the sprinklers are fed and be sure to install after that. Also leave the hose spigots hard water and or add a seerate soft water spigot. If for example you're washing the car its nice to use soft water but watering the roses is a waste and possibly damaging