If the city water valve is not on all the way will it affect water pressure in the home if there is no pressure regulator in the house or outside the home

If the amount of water you are trying to use would normally excede the amount that can pass through the partially open city water valve you would see a drop in pressure at that point... however unles that happens you will see the full system pressure at all other times..

In short... its not a subsitute for a pressure regulator if needed.

Actually, the answer is NO, it will NOT affect the water pressure. Instead, it will affect the VOLUME of water available to the home. Here, you would actually see a reduction in water flow and that is commonly mistaken as a drop in pressure when it is in fact a reduction in water volume as stated.

It may not necessarily be noticeable using one fixture, but as more fixures get used, the volume reduction becomes obvious!

Why do you ask this?

Mark