Is it on the top floor? Could it be a tank to increase water pressure?

Storage tank for an upstairs radiator

It's an "overflow" tank with a "sight gauge".

In a closed hot-water heating system (as opposed to a steam system), you have to have a space for the water to expand when it warms up. The tank has to be the highest part of the loop.

They used to have those gauges on them so you could tell if the system had enough or too much extra water.

Thanks Everyone! lazygardens - that makes perfect sense, it does have hot water radiators and it is positioned just higher than the highest radiator.

And just for the record..a tank does not increase pressure

Pup, if a tank is on an upper floor, it does increase pressure for showers and flushing toilets, etc. Kind of the same principle as the "mushroom" water towers in midwestern towns. In fact, just like the old high-tank toilets. Get the water up higher, and gravity gives you greater pressure. I've seen tanks like this in older homes.

chibimi,

Believe me, I am a retired master plumber and I can tell you that regardless of how big the tank is or how high you put it in your house it will not increase the pressure.

Pressure is measured in "Pounds per square inch" and a 1 square inch column of water weighs 0.434pounds per vertical foot.

The water coming into your house is between 45 & 80psi, which means the internal pressure in your water lines is equal to a one square inch column of water between 103.6 & 184' high, which explains why they build those big municipal water towers.

On the one hand, your house is simply not tall enough to increase the water pressure,

On the other hand,even if your house was tall enough, a tank does not increase pressure. The pressure is equal to the downward force exherted upon one square inch at the bottom of the standing column, and it doesn't matter if that standing column is a 3/4" pipe or a 50' diameter tank, you are still only measuring 1sq.in.

My municipal water pressure is 21 psi, according to the gauge in my basement. I'm not one of the unfortunate ones at the top of the hill, either. I have never measured the pressure on my second floor, I know it must be lower.
I know for true that early water delivery systems consisted of nothing more than a manually-filled attic tank feeding the taps by gravity. The attic tank was not pressurized.
On a hydronic (hot water) heating system, the expansion tank can now be anywhere in the system, not just the highest point, because the expansion tanks have air bladders and cannot become waterlogged.
Casey

Pup, I'm not a plumber or a physicist, but I am pretty sure the weight of the water in an elevated tank will cause greater water pressure in an outlet below it. A few hundred pounds of water pusing down from the attic will send water through that one-inch pipe faster and harder than if the water isn't there.

It's like a large coffee urn or water filter with the spigot at the bottom. When the urn is full, the liquid always comes out faster and more powerfully than when the urn is nearly empty.

It's not how high the tank is, it's the fact that it is above the outlet and therefore has the weight of the water working for it.

I'm not saying it's a good system or a modern one, but I have seen this in a few old houses, although most )if not all) were overseas.

I'm sorry if I confused things by saying "gravity," but to me (again, not a physicist!) weight is caused by gravity.

I apologize for my mistake if I am wrong, and defer to your experience,

Of course, this is moot, since the tank in question is for a different purpose.

Regardless of how high or how large the containers are, the pressure at the outlet is equal to the differential in height between the top surface of the liquid and the elevation of the outlet...for water the resultant pressure is 0.434psi per vertical foot of differential.

Or 27" per psi in the water column. I'm with pup here, although hydrostatic pressure can increase flow pressure remains constant.