Re: How do 2-110 volt (hot) wires complete a 220 volt circuit.

You've sent in an interesting question.  First, consider the two wires as 
+ 120V and -120V.  The *difference* between these voltages is 240V, and 
that is what's powering your water heater.  The current path is +120V--
>water heater-->-120V, forming a circular path.  Normally no current flows 
through the neutral wire in this case, so it is not connected and in fact 
would be dangerous to connect, as currents *ARE* flowing through the 
neutral from other sources.  Of course, the protective ground (the green 
wire) should *ALWAYS* be connected, and the connection be inspected 
periodically to ensure that this safety measure is in place.

The actual answer to your question is a little more complex.  There are no 
+120V or -120V wires coming to your house.  What these are are two 120V 
*AC* power lines running at 60 Hz with a roughly sinusoidal waveform.  The 
critical point is that the voltage in the two 120VAC lines are of opposite 
PHASE (180 degrees apart).  In this case the two 120VAC lines can be 
connected across 
a water heater (or stove, or electric dryer, etc.) and since the two lines 
are out of phase, a 240VAC *difference* is produced across the pair.  This 
gives you the 240 VAC needed to run the appliances.  The designations 
+120VAC and -120VAC are, unfortunately, incorrect.

Why are things done this way?  Because most of the things in your house 
run off of 120VAC, not 240VAC.  You have two 120 VAC lines running into 
your house (albeit out of phase) and these 120 VAC lines power your 
electric drill, microwave, etc. which run off of 120VAC.  Here the current 
path is 120VAC-->appliance-->neutral, closing the loop.  The 120VAC 
appliances do not care at all about which phase of the incoming power that 
they run off of.  Usually some care is taken to balance the loads in your 
house, that is, to ensure that each of the 120VAC lines carries 
approximately the same current.  This is done to ensure safety in your 
house and keep the power company happy.

One thing more, and we're done:  Many people think that the neutral 
(white) wire is safe to touch since it is theoretically at zero volts.  
However, the neutral is actually carrying current and as such *CANNOT* be 
at zero voltage in the real world.  So NEVER touch the neutral (white) 
wire.  Best thing, although expensive, is to have a licenced electrician 
do ALL electrical work in your house, even such seemingly simple tasks as 
changing a light fixture or a wall switch.