Greg wrote:

I moved into a 40-yr old home last summer. Last winter, the baseboard
heat made the common clink-clank sound as the pipes expanded every
time the heat went on. I've come to the point where I've accepted
that I'll always have that.

But this year, my upstairs unit sounds like Niagara falls when the
heat goes on. I understand that means there is air in the line. But
why? Where did the water go? Is it likely that I have a leak
somewhere that is slowly damaging my home and I just can't see it?

Furthermore, regarding solutions. I've read many posts about bleeding
air out of the valves in each section of the baseboard units. But
I've checked each piece and there are no bleed valves on the piping.
However, down near the furnace, there are two spigots...one on the
outbound line, and one on the return. Furthermore, there is a
pressure reducing valve followed by a regular manual valve that, I
suppose, allows water into the system.

So, should I open one of those spigots to flush the system? Which
one? I'm a little leary as it seems that will just drain the entire
system and I'm not convinced it will eliminate all the air.

Please help. The sound of rushing water in my walls is very
unsettling. Thanks.

Greg

First step: Visit the local library and see if they have a book
on residential Hydronic heating systems. That will help get you
up to speed. A Big Box store may have a DIY book too.

Assuming this really is air in the upper system causing the noise,
air gets in anytime the water pressure in the system goes below
that value needed to support the column of water to the highest point.
When the pressure drops, it creates a vacuum at the highest point and
that will suck in tiny bits of air anyplace it can.

The system pressure goes up and down like a YoYo as the boiler
heats and then cools off. The feed water regulator tries to
maintain some minimum pressure (if it's working) and the expansion
tank absorbs the peaks in pressure (if it's working and not water-logged).

Don't open the drain cocks at the boiler; that won't get the air out.
If your system doesn't have bleed valves, then it has an air-eliminator
device at the boiler. If the system pressure is high enough, this
should get rid of the air (if it's working).

Jim