We have a four story plus cellar 1840's 2 family house with a hydronic
heating system with large radiators that was probably installed in the
1920's. The gas boiler is about 20 years old. The system has both a
conventional expansion tank (that has not been emptied in years) and a
Filltrol diaphragm expansion tank in series.

Every place I looked on the web (including the Filltrol site) says that the
system pressure should be set at 12 psi when cold and should be 20 psi when
hot and circulating. That is exactly where it is, but the radiators on the
4th floor seem to have some air left in them. When bled, water does not
come out of the valve and the top of the radiators are not very hot (the
bottom halves are).

Should the pressure be higher in a 4 story house that in a shorter one? And
if so what should it be? And how do I adjust it? The Filltrol manual does
not give instructions other than to say that it is adjusted via the intake
air valve and should be adjusted when the tank is empty.

So (if I really do need to increase the pressure) do I add air (and how
much) or remove air from the Filltrol expansion tank. And do I have to
completely drain the system to do it? Or just let out a few gallons of
water? Or not concern myself with letting any water out.

A link to a good website will suffice. As I said, my Google search was
unproductive.
--
Peace,
BobJ