Locating and turning off hot water valve?

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I am no expert, but if you turn on the hot water and let it run a bit, the pipes it runs through will get hot. Find the valve on the hot line, and you should be in good shape.

Oh man it is hard to tell from your picture. I see you have a tank and all tanks have an input and an output. Most installations have two valves one going into the tank and one coming out. If the valve has a round handle on it, turn it clockwise to close.
If the valve has a straight handle that only moves 90 degrees, you close it by turning the handle perpendicular to the water pipe.

Some valves have no visible handle. They generally have a slotted screw that you can turn with a large screw driver. To turn off the water, turn the slot sideways, perpendicular to the pipe. You might need to loosen a large nut that is often around the outside of the slotted screw. You will need an adjustable wrench to loosen it. You will only need to turn it ½ of a turn.

Interesting. The only time that I haven’t found supply shut-off valves under a sink is in old manufactured homes.

Is the boiler for your heating system, or your potable hot water? If the former, somewhere you have either a water heater tank, or an on-demand water heater; look for it. A third option found in newer homes is a manifold off a water heater that controls each line to each fixture.

If the boiler is your potable water heater, it’s different than any I’ve seen. The real old ones had the cold supply line entering at the bottom; these days they go in the top. I don’t know what the middle line is. All heaters that I’ve seen have the hot water exiting at the top.

@snowberry and @LuckyGuy have good responses, I think. I’m with @LuckyGuy in that it’s hard to tell just from the photos which is the outlet and which may be the pressure relief outlet (that’s generally a third outlet, located at the very top of the water heater).

Generally with electric water heaters I think the inlet will be at the bottom, and it should be fairly easy to track that pipe back to the cold water mains (which will be pretty easy to tell, since it should branch fairly close to the main shutoff for the whole system) that is, the shutoff for the whole house / apartment / living unit.

Once you know where the cold water inlet is, you may only have two other outlets: the safety relief (and the valve for that should not have a shutoff), with an outlet that goes to a floor drain or some other “open” way to relieve pressure. The safety relief has to be “to atmosphere”, so that it won’t be trying to vent into another closed system. When you know what / where that is, then the remaining outlet from the tank will be your hot water main.

(Oh, except for the drain that’s likely to be at the very bottom of the tank, and which may not be piped at all, or if it is, it’s directly to a drain.)

But I also liked the part of @bossob‘s response about the shut-off valve under the sink. Most good plumbing has a way to isolate each fixture in the dwelling, so that you can shut off any tap, any toilet or any hose bibb (if you have them) without having to shut down the entire system. If you don’t have them now, you could start to add them as you replace the individual water lines (or fixtures) as they need replacement or repair from time to time.

sorry i can’t help :(

Sometimes it’s best to just get out your VISA card, call a reliable plumber, then put your feet up and wait for him to arrive. This sounds like one of those times.

It’s kind of like that old joke, which ends like this…..

Plant Manager: ”$900.00 ?!?!......You just pushed a button !!

Emergency Repair Technician: “Well, yes sir…..but I knew which button to push…..”