What is an A and B Dimension

What is an "A" and "B" Dimension?

"A" Dimension

When installing a replacement steam boiler with a gravity return there is one dimension that is normally overlooked which is called the “A” dimension. The “A” dimension is the distance from the boiler manufacturer's normal water level (NWL) to the lowest steam carrying main. The distance can exceed the 28” minimum vertical distance but should not be less than then 28".

"A" Dimension Wet Return

"A" Dimension Dry Return

Returning Water to Boiler

The idea of the 28" measurement is a column of water 28" high equals 1 pound per sq. inch pressure. Let's assume the steam boilers operating pressure is a maximum of 2 psi. To get the return water back into the boiler through the Hartford loop, we must exceed 2 psi minimum in the return piping. When the boiler is operating at 2 psi, the steam is going out to the system condensing back to water and encountering flow resistance in the piping. As steam moves through the system we can drop about 8 oz. We do not have enough pressure to overcome the 2 psi of steam pressure in the boiler. To clarify, we have about 1.5 pounds pushing down on the return water and 2 psi at the boiler. Now let's add a 28" column of water as condensate stacks in the vertical return at the end of the steam main. The steam in the steam main is condensing and stacking in the vertical return. We will get enough condensate return at 2.31" per 1 psi to raise the pressure above 2 psi pressure at the boiler. Since high pressure always goes to low pressure the return, water pushes back into the boiler until the pressure equalizes as the stacked condensate drops.

Improper "A" Dimension

If you don't have the proper 28" "A" dimension condensate water will back up into the horizontal steam carrying main and cause water hammer. This is a very loud knocking sound from the steam system. It actually sounds like someone hitting the pipes with a large hammer.

”B” Dimension

The "B" dimension is similar to the "A" dimension except is used when you have systems with steam traps and/or zone valves. The major difference is the "A" dimension is 28" the "B" dimension is 30" per pound of steam pressure. If the system is operating at 2 psi the "B" dimension would have to be 60" from the normal water level to the lowest steam carring main or the bottom of the steam trap. This means the ceiling height may not be high enough. Systems with "B" dimensions not high enough will use condensate pumps or the preferred boiler feed tanks. You can learn more about these tanks here.

Disclaimer: The information found on this website is for informational purposes only. All preventive maintenance, service, installations should be reviewed on a per job situation. Any work performed on your heating system should be performed by qualified and experienced personnel only. Comfort-Calc or its personnel accepts no responsibility for improper information, application, damage to property or bodily injury from applied information found on this website as it should be reviewed by a professional.