LsFarm wrote:mw cougar, I'm not sure I understand why you need boiler pressure to circulate water?? I have only about 10-15 psi in my system ever, and I circulate fine from the dungeon up to the second floor with regular cartridge circulator pumps. ??
Do you think the pumps will cavitate without applied pressure? Educate me please.
Boiler water pressure is related to the weight of the water. Think of a column of water as tall as a two story house, the pressure will be the greatest in the basement, less on the first floor and the least on the second story. You don't need water pressure for the circulation pumps to work. What you need is NO air in the system. Air, or more correctly gases, comes from three sources (1) the air trapped in the system when it is first filled with water, (2) the dissolved gases in the water and (3) air that leaks in. Most boiler systems have a combination water temperature and pressure gage on the boiler. So it measures the greatest pressure in the system because it is in the basement. When water is heated it expands. It has to go somewhere. In a closed system like most residential systems there is an expansion tank. In any system installed in the last 5-60 years it will be a bladder tank. An enclosed tank with a rubber membrane separating the tank into two parts. The air side of the tank has a tire valve like stub. Before the tank is installed it is pressurize to 12 psi. The other side of the tank is piped to the boiler water. When the water is heated in expands into the tank raising the system pressure above 12 psi. If the tank is properly sized the total system pressure will never rise to the safety valve pop off pressure of 30 psi. Initially the system must be bled to get No. 1 out. This was done long ago. No. 2 is removed by an air scoop, something that catches the air bubbles as they are circulated. They are vented by a small cylinder looking thing that has a tire valve like stub on it. An internal float falls when it fills up with air. Closes when additional water is feed into the boiler. The additional water comes from the automatic fill valve and regulator. It's connected to the house water supply and keeps a regulated pressure of 15 psi on the system. Now all piping systems will have leaks. Many you will not see, a little drop leaks out and evaporates because it is hot. That leak is a two way leak. When the boiler is off and the water cools it creates a internal vacuum, sucking air in through the leak. That is the source of No. 3.
If you are having problems with no heat in a baseboard or radiator unit particularly on a unit on the highest floor the problem is likely because one or more of the above components has deteriorated. You can add water via the manual fill valve. It's usually a lever on top of the pressure regular. If you constantly need to add water your boiler components need servicing, usually removing the components and cleaning the tire valves will fix it. Heating service companies will not clean it, they will just replace it.