Hot water tank AND garage door problems, not related

Since I'm having 2 different problems of similar priority, I'm making a combo thread.

Problem 1.

I have this hot water tank. When the PO moved out, they shut off the main, the toilets, and all the knobs around the water tank. This led to a puddle from the T&P drain. Any time I shut off a knob to work on something, I get water dripping and sometimes pouring from the T&P relief valve, whether it's a toilet or sink shut off, or a while "wing" of plumbing. 

I've never experienced this before with electric water heaters, even having maintained a 200+ unit apartment complex with heaters of various ages, models, and sizes. 

Before I start throwing parts like vacuum breakers, new T&P relief valves, expansion tanks, or thermostats at it, are there any diagnostics I should do? 

Problem the second.

I have this garage door opener. It worked fine until Saturday. Saturday when I went to close it, it went down 8-10", then would reverse until it hit a beam across the ceiling, well past it's standard stopping point to the point it was trying to actually bend the door.

The sensors at the bottom of the door MAY be out of alignment. One side has a green light, one has a yellow light. Internet suggests that means they're out of alignment, but nothing in the user manual about it, or the slamming past the "up stop" position. 

It's 4 years old, I feel like that's awfully young for the opener to have crapped out, but I don't know anything about modern openers. I just know the craftsman opener in my "old" house was there when the parents bought the place in 87, and worked until I foamed the door shut with no maintenance and no issues. 

Any thoughts or ideas on things to check, other than realigning the sensor until the lights are both green?

Re: water heater. That's pretty darn new. I'd hook a hose to the drain and flush it out first. Then, a new T&P valve is pretty cheap. Maybe throw parts at it. 

Garage door: Sounds like you're on the right track. Sensors.

Sorry I'm not much help. Maybe this bump will get someone with more experience to chime in. Good luck, man.

In reply to RevRico :

IIRC those Chamberlain openers were what sears sold under their Craftsman brand, and there's an upper-limit sensor internal to the unit that's know to fail. 
 

I had that happen to one of mine & despite recalibrating it, it still misbehaved about 20%-25% of the time. I think the sensor was ~$40 and a new opener was ~$120, and there was already one older sensor the PO had left behind before we bought the house, so I just bought a whole new opener. 

Are you shutting off the supply water to the water heater?

Are you shutting off the heat side of the water heater?

If the t & p valve is popping off, that should be from either high temp or high pressure.  If the tank is isolated (no water going in or going out), and it fires up, that pressure needs to go somewhere.

Well solved the hot water leak. New pressure relief valve, new everything valves, cut out a few, then slapped on a pressure tester.

155psi!!!! That was the problem. 

New pressure thing at the water main has solved that and got it to reasonable 70psi. 

Still getting nowhere with the garage door but at least the water has finally quit leaking