No hot water after teeing to outside spigot

Question

I recently saw an outside cold and hot water spigot on This Old House. After checking out prices, I decided I'd make my own redneck warm water spigot by bringing a hot water line close to the outlet and teeing into the cold water line. It worked ok, the water out of the spigot was warm enough to wash off without gasping for air. However, the house no longer has hot water unless the spigot is running....

I believe this is due to an overall pressure reduction of the water running through the hot water heater. It must be easier for the cold water to run back through my new tee into the hot water line then for it to run through the hot water heaters...

I installed a valve on the hot water line prior to the new tee, and when closed hot water is restored to the house.

I realize I could solve this situation by either doing it right... or by installing a back-flow preventer, but I'm now curious why this doesn't occur at existing facets, showers, bath tubs, etc.? Do the valves on those components contain back flow preventers? Specifically the ones that only have a single handle, it makes sense if you have two separate valves why the water would not mix.

Here's a quick drawing of the system before the valve was installed:

Answer 1

In the normal faucets and showers in your house, the valves governing the flow of hot and cold water are before the mixing of the two. Even when these valves are wide open, the water pressure will decrease across these valves, so that where the water mixes is at a substantially lower pressure than in the source pipes. This prevents the mixed water from pushing back up the cold/hot water lines.

Here, though, your valve is after the mixing, so there's no pressure differential keeping water from backflowing. Depending on your pipe routing, the altitude change (hot water rises), and whatever's in the middle, you could get all hot, all cold, or some mixture in between. Generally not what you want.