fourth year, MOST of your statements are correct:
wrote on 21 October 2000 at 02:40 PM:
The pump is a water moving device, it does not change t[he]o total pressure of the system. … Air, being compressible, is used as a moderating medium in heating systems to keep the pressure from making large changes as the water is heated. Your pressure reducing feed valve may have lost its adjustment, in which case you can back the adjusting screw out a little, or it may need to be replaced.
The following statement was incomplete and misleading:
If the pressure is normal when the system is cool and high when it is hot, then you do not have enough air in the system, notably in the compression tank as opposed to a comment that you have too much air.
The intended idea was correct; but the wording made the statement incorrect. A properly sized and filled expansion tank will permit a low (usually the 12 psig fill valve setting) when the system is cool, and a higher pressure (less than 25 psig to prevent the relief valve from popping or weeping) when the system is at its hottest.
Adding more air to the expansion tank would raise the pressure and cause the relief valve to open, so letting air out might help by lowering the pressure.
Adding more air by adding another or larger expansion tank might solve a problem related to the improper sizing of an expansion tank.
Sylvan’s reference to Dan Holohan’s pump deadheading proposes the problem of raising the temperature and pressure of water at the pump by heating water trapped between closed isolating valves. This is an isolated situation where the pump does raise the local operating pressure within the system.
The small difference in wording is important when writing technical articles or refining code language.