I want to replace my electric water heater with a gas tankless water heater. My current water heater like most electric hot water heaters takes 220 and the tankless gas water heater uses 120. The room I am installing in has no outlets. Can I use that 220 line and get a couple out 120 15 or 20a outlets out of it. Maybe with a sub panel type set up or if I can't do that if I can at least wire the new 120 water heater without running a new line. Thanks in advance.
Water – Replace electric water heater with gas tankless (electrical connection)
120-240vhot-waterreceptacle
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I've been looking into a heat pump replacement.
You said "However, when the heat pump is in heating mode, the exiting refrigerant line is actually colder, not hotter, than when it enters. As a result, you cannot heat water during the winter."
That will depend on the location of the internal connections. The HP models I have seen with a desuperheater ( hot water generator ) place the HWG after the compressor before the reversing valve. That means the pump always runs forward, the HWG always has hot refrigerant to use and it works in summer and in winter. The reversing valve does just that - in summer it sends the leftover heat to the outside and extracts heat (cools) the inside. In winter it sends the heat to the inside and extracts heat from the outside. 
I've not used a system like you have, but I'll give you some information based on what's common for heating systems in the UK.
As you've discovered, the immersion switch immediately switches the heater on. It's usually used as an override when you want water to start heating outside the timed period. (I'd hazard a guess that having the immersion on for 10 minutes won't be long enough to heat sufficient water for a shower).
You'd usually set the heating on timed, but it will take a while to heat up the full tank of water. For example, if you get up at 07:00 and have a shower, you might need to set the timer to come on at 06:00 to give you a tank of hot water ready for when you get up.
It's common to set the timer to come on twice a day, so you get hot water in the morning and again in the evening - though you'd adjust that dependent on your usage pattern. It might take a little experimentation to find out when you need to set the timer to come on to give you the hot water you need.
If the T&P valve is flowing quite an amount of water, it sounds like something is amiss with the system.
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Best Answer
In the panel, move the white wire to the neutral bar, and move the black to a 20A breaker. Fit a 15A or 20A duplex receptacle. And you're done.
You can use up to about 19.5 amps of power off that receptacle, since the water heater won't take much.
If you want two full circuits good for a total of 30A@120V, in the main panel, leave the black wire on the 30A breaker, still move white to neutral. Then fit a subpanel, of any size. Black to a hot pole, white to the neutral bar, ground to a different ground bar that is isolated from neutral. Only every other row of breaker spaces will work. Fit two 15A or 20A breakers and use each one for a different circuit. If you place the breakers in the wrong row, don't bust out more knockouts, switch the black wire to the other hot lug.