HVAC – Getting to know static pressure
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.
By taking measurements at various points it is possible to see the pressure drop of an entire system. The volume flow rate in a system can be measured at the entrance to the system and at the exit from the system, for example, at the supply air duct and return air duct.
There will be acceptable ranges in which a system is allowed to operate. These can be found in the installation instructions.
For the most part, in a forced air system, it should almost balance to zero. Typically, a 0.5” wg window is allowed. This difference is what can cause negative or positive pressures within a home that is heated with a forced air system or ventilated with an HRV or ERV.
For ventilation, we need to go a bit further down the rabbit hole with regards to pressure. Fan selection is typically based upon an airflow rate, measured in CFM, and static pressure.
Luckily, software designers and engineers usually do most of the math for us when working in a large commercial or industrial environment, but when thinking about a residential or small scale application, static pressure takes a back seat to CFM.
If you know how big a room is and how many CFMs a fan can move, it is usually possible to select the right sized fan for an application with little difficulty.
If we take a standard kitchen that is 10’ x 10’ with eight-foot ceilings, we know the total cubic footage is 800. If we need to do 15 air changes per hour, some simple math will help us figure out the size of fan required.
In this case, the total air to be moved in an hour is 800 x 15, or 12,000 cu. ft. Then we divide that by 60 to see how many cubic feet we need to move in one minute. This gives us a result of 200, so we need a fan that can move 200 CFM of air.
Keep in mind that we will still need to overcome the static pressure. To find that, it is best to hit the charts again.
As per the chart, if we need to move 200 CFM and are using 5” pipe, we need to select a fan that can move 200 CFM somewhere close to or above a static pressure of 0.8” wg for every 100 feet of pipe. That will create a velocity of 1,467 ft/min.
If you are concerned with noise, remember that the higher the velocity of the airflow, the louder the system will run. So if noise is an issue you may want to oversize your vent duct accordingly.
How it works
A manometer works based on two rules:
1) Pressure is equal at equal elevations in the same fluid, and
2) Pressure at the bottom of a column of fluid equals pressure at the top plus rho(g)(h), where rho is the density of the liquid, g is gravity and h is the height.
Matthew Reid is a heating technician in the HVAC and Hydronics Department at Desco Plumbing & Heating Supply Inc. He can be reached at [email protected]