ECO switch is more common on electric heaters, but I guess some gas ones have them too? Not sure about yours.
Yeah, I’ve run into a couple gas units with ECOs, but it’s not the norm. Usually it’s just a thermocouple or flame sensor acting up on those. Had one job where the customer swore it was a bad thermostat, but after poking around, turned out to be a clogged vent pipe—bird nest halfway up. Burner kept shutting down for “safety,” but the tank looked fine otherwise.
Sediment’s a real pain if you’re in an old house or somewhere with hard water. I’ve seen tanks where you could hear the thing popping and crackling from all the crud at the bottom. Sometimes flushing helps, sometimes you’re just rearranging the mess. If you’re still getting cold showers after all that, I’d double-check the burner flame—should be blue, not yellow or lazy looking. Anything weird there and you’re probably not getting enough heat transfer.
Dip tube’s another sneaky one. Had a call where hot water would run out in like two minutes—sure enough, dip tube was half gone and bits were clogging up every faucet in the house. Fun times...