Plumbers redid everything under our house and we decided to change our water heater location. But they left the old heater full in the attic. It is still connected to pipes but they lead to nothing because they were sawed off under the house. Do I need to open a hot water tap (which i can't do!) to drain it?

Connect hose to bottom drain valve. Direct hose where you want the water to go. Drain tank.

Have beer and curse about plumber (or yourself if you didn't request drain and removal).

There should be a drain at the base of the water heater. You may need to connect a garden hose and run to an outdoor location, or you may be able to run it to a local drain if one exists there. The taps (assuming you mean fixtures in your house) are feed from the discharge at the top of the tank and would not drain it anyway.

The cursing is definitely being done don't worry! The reason I was asking is because it won't drain. Because of this, I thought it might need a different technique than I normally use since it's an unusual circumstance, so I wanted to ask the Texags experts.

https://turbotankcleaner.com/

Counterpoint said:

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The cursing is definitely being done don't worry! The reason I was asking is because it won't drain. Because of this, I thought it might need a different technique than I normally use since it's an unusual circumstance, so I wanted to ask the Texags experts.

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Just replaced mine. It was a 15 yr old tank that just wouldn't drain. All the crud at the bottom of the tank had clogged the valve.

Ended up tilting it back 10 to 15 degrees so that the valve is at the highest point. Put a small pan and towel underneath, opened the valve, blew a bit of compressed air in to clear out the crud . Some water and crud flew out. Turned the valve off quickly, left it tilted, waited a few minutes for crud to settle, connected the hose again, turned valve on...95% empty in less that 10 minutes.

Unscrew the valve all the way. Should have a plastic retainer ring . Remove retainer and stem. Hold both ready to shove back in
Stick a coat hanger in and work it back and forth til you break it up
Quickly shove stem back in and retainer ring
Reconnect hose and open valve

Tank Flush Helper DIY gizmo
All these parts & their cousins EZ to find & assemble

Photo of parts on red hose leading to a fully open WH drain cock
Open valves to drain WH naturally (since there's no water pressure, at least allow air into tank and guard against potential overflow from this open port)
If no/poor drainage, then close final water outlet valve, open valve to allow hose bibb water in
Use hose bibb water to force into tank, combat blockage in short bursts, careful to prevent tank overflow (2nd set of eyes on this best)
Close/open valves in reverse to allow natural drainage
Repeat

There will remain a lot of sludge weight inside the heavy WH, just the same but you may be able to better roll it out & down ... if not just leaving it as-is!

If you meant to write "can't open the bottom drain valve" then you might be able to set up a slow siphon from a filler pipe to outside, then leave it as-is after draining as much as possible ... even if siphon fails .... leave it after getting a drain pan w/hose under it should it rust/leak in years to come