Running without expansion tank.

[Newindustar]

This is my first post. I have a Wiel McLain 250000 conventional boiler in a 1910 two story, 2400:sq foot wooden home in Minnesota with radiators
It has a conventional steel compression expansion tank, horizontal up under the joists, 10x30 inches. I've been in the house five years and have spent a lot of time learning this boiler. I had recently and always keep the tank drained and air charged. The boiler high limit is at 175 degrees. The boiler swings about four degrees from 10 to 14 psi always. The tank just started leaking from pinholes about half way up the tank, the assumed waterline. I is a simple 3/4 copper pipe off the boiler to the tank
I closes its isolation valve and so now have no expansion tank. This has made zero difference to the swing psi range. Can anyone explain how I have a very tame system with no knocks and low pressure with no expansion tank and when I did it ran the same? Thanks. I have to answer this before I decide what to do about a replacement. It's possible there may be some air in a radiator, I don't know if this could provide expansion. It's been very cold so I don't want to mess with anything till it warms up.

 

[BadgerBoilerMN]

There is direct interface in the old, long obsolete, steel expansion tank.

Air is absorbed during the regular system cycles and comes out of solution at the point of lowest pressure e.g. the top of your highest radiators.

These reason it is "working" without the benefit the expansion tank is the compressed air collected in the radiators and/or abandoned pipe.

We replace all old-fashioned expansion tanks for diaphragm expansion tanks and do the often neglected chemistry to keep things clean and efficient.