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That pipe I think should be open to the tank, you fill the system until water starts coming out of that pipe, it is a way to observe the level in the tank from the basement.

yes, that one, look at exactly how it connects to the tank, but that was common to have an overflow in to the basement so you could tell when the system was full from the basement.

You have a very efficient gravity hot water heating system.

Your radiators will be/should be full of water and the water in the expansion tank should be half way up the sight glass.

The gauge on your boiler indicates both the system pressure and water temperature.

Please provide us with pictures more pictures of the piping connected to the open to air expansion tank as well as the top of the tank as the vent pipe is either on the top or high on the side of the tank.

If there is no riser coming off the side of the tank and passing through the roof your expansion tank drains to the basement floor drain. please Provide us with an image of that piping.

Please take more pictures of the radiators in your home and all the connecting piping as we need to determine if the radiators are top fed from a central riser that reaches the top floor and is connected to the base of the open to air expansion tank.

If the gravity hot water system is bottom fed the upper floor radiators should have/will have orifice discs in them to reduce the flow to these radiators and send the hot water to the first and second floor then the third floor and provide you with a system that has the proper balance.

That closed cell foam in that piping is not something that should be there. We need to see where this pipe leads please.

You should have a plumber that is experienced with gravity hot water systems or steam come and service the boiler and install a low water cut off to bring it up to code and clean the burners.

On your to do list you should purchase CLASSIC HYDRONICS and HOW COME from the heating help bookstore page to educate yourself about your gravity hot water heating system and how it works. your system is designed to create slow even heat in your home and provide you with warm floors as well.

We need to more pictures so we can be nosy about all your piping to enable us to tell you more about your radiators and the piping.

You should have your system serviced now by a licensed plumber "ONLY" to avoid having to wait a long time in the fall to have it done as you need a low water cut off switch to bring the boiler up to national plumbing code standards.

There may not be a check valve. The city water pressure is more than adequate to push the water up to the attic — so there has to be some way to shut it off. Otherwise that tank would be overflowing all the time, like an unattended bathtub.

So… the must be at least a shutoff valve on the city water feed, and it is remotely possible on a system that age that there might be a pressure reducing valve. Very remote. More likely is that the water level was (and still can be) controlled by the simple expedient of looking at the pressure gauge on the boiler and, if it seems low, opening the shutoff on the city water and letting water in until the tank in the attic overflows (which is why you don't want that overflow pipe plugged!)… unless you can go up to the attic from time to time and look at the level in the sight glass. Which should be about half way up the glass.