Hydronic Manometer Calibration
HVAC – Regardless of the type of system at play, if you are heating or cooling a building you are moving some sort of medium through a conduit to either give off heat or absorb heat.
In a hydronic system, when you move the heating fluid you are overcoming head pressure. This is measured in pounds per square inch (psi) or feet of head. When moving air (a gas)
through a forced air or ventilation system, it is static pressure that is being overcome, and
this is measured in inches of water column (wg). In theory, there are very few differences between the two but it is important to know the differences, similarities and the importance of both. Someone designing a hydronic system would be abhorred if they did not take proper steps to ensure the head pressure of the system was calculated and designed around to ensure proper operation of the system.
Ask someone installing a forced air system what the total static pressure of the duct system was and they probably won’t be able to answer that question. However if you were to speak to someone designing a ventilation system they would stress the massive importance of knowing static pressure.
The pressure of heating
On the heating side of things, hydronic and forced air are somewhat similar in that they both move a medium (either fluid or air) through a series of conduits (pipes or ductwork) to deliver heat to designated areas. Both systems have to overcome resistance through their respective distribution systems.
In a hydronic system, every foot of pipe, elbow and T creates resistance to flow. These fittings are classified with Cv ratings, and the pressure drop due to friction in the pipe is also measured. A standard 1/2” copper 90° elbow has a Cv rating of 2.5. This means that when 2.5 gpm of fluid passes through the fitting it creates a one psi pressure drop.
Pipe will have a measurable resistance based on flow and size. For instance, 1/2” plastic pipe carrying one gallon per minute of fluid will have a pressure drop of somewhere around 1.1 ft H2O per 100 feet.
A designer takes these numbers and adds them up to make sure that the pump selected for the project will be large enough to overcome the total pressure drop of the whole system. If the pump is undersized it won’t be able to move the fluid through the pipe, and will dead head.
When moving air, the “pipes” are usually much larger and made of sheet metal. Since ductwork is generally much larger than pipes carrying fluid, the resistance to flow is much lower. This could be a fraction of a pound per square inch, so rather than measure in psi, this resistance is measured in inches of water column (wg).
Although the resistance is lower, it is still there and is still important to consider. To measure these pressure drops we usually use a manometer. A tube placed in a duct facing into the direction of the flow will measure the total pressure in the duct.
This is why it is important to have your hydronic manometer calibrated regularly.