Gas and Oil Home Heating Furnaces - Boiler water valve

Normally, on my hot water boiler, should the water makeup valve be left ON, or should it be turned OFF unless I need to add water to my system? When we bought our house, I noticed it was off. It seems to me that if I leave it on, water will be admitted as needed through the pressure reducing valve. I used to work in a business with a large (100 HP) fire-tube boiler, and the water was always left on.
Thanks!

The only time the valve needs to be on is when you need to add water to it. Under normal circumstances you will not need to add water to the system. If you are losing water it could mean you have a leak. And if you had a leak there should be a low water shut off, if the water gets too low it would shut the system down until the water is replenished.
But it doesn't hurt to leave the valve on all the time either. As water leaves the system it will automatically refill the system.

If you leave it on, you ight have a leak and not know it because the reducing valve will add water when pressure drops. That can be bad and good. Sometimes, automatic water feed valves will slowly add water even when the pressure isn't low. Then eventually the relief valve opens and lets the excess water out. If yours was turned off, it may mean that someone had trouble with it. You could turn it on and watch pressure for a few days to be sure it won't build up and trip the relief valve.