What to Do with Your Micro-Manometer

Imagine this: you are pulling into your driveway from a long day at work, and you see a package waiting for you on your porch. You jump out of your truck and trip over the rose bush while running to see if this is what you have been waiting for. Upon closer inspection, you realize that the package is from TruTech Tools. This is it; the micro-manometer you ordered has finally arrived. You punch through the tape that sealed the box and rip the flaps almost completely off to inspect the magic that lies within. Before you know it, you are holding your very own micro-manometer. You feel like a 10-year-old child on Christmas morning opening a gift from Santa.

You are holding the manometer with both hands with a poop-eating grin on your face. Any competent HVAC tech would want to play with their new toy immediately. What should you test first? Well, I’m glad you asked. Whether you splurged on yourself or you woke up to a new DG-8, DG-1000, or DM32x stuffed in a stocking near the Christmas tree with a card from “Santa,” it’s time to level up your game.

For those of you who don’t already know what a micro-manometer is, let me do my best to explain. For many years, I thought a manometer was a manometer. It might be black and orange, black and yellow, or it might fit a color scheme that matches your favorite work shirt. At the end of the day, a manometer measures pressure. Most of us in HVAC have manometers that read inches of water column. Some of the new guys might be trying to wrap their heads around the concept, so let’s start with the history.

Walk Like An Egyptian

Inches of water column became a thing back in ancient Egypt when King Tutankhamun was designing the chambers in one of the great pyramids. As many of you may already know, there are tunnels and air chambers with different connection points in the pyramids. At the bottom of the pyramids, there are several “pools” or bodies of water. Two of the pools were connected underground, and both had air chambers that went straight up on each side of the pyramid, almost like skylights. What King Tut found was that when the wind blew hard from the east, it would travel down the air chamber and push down on the pool, forcing the water level lower. He also noticed that when this happened, the adjoining pool that was connected underground had an increase in its water level.  

When this phenomenon was discovered, King Tut understood that he needed to figure out how to quantify this air pressure. So he called upon one of his royal engineers, “Stanley,” to solve the problem.  Stanley presented two ideas to the king, which were different scales to measure the water level. The first was the metric system, and the second was the imperial system. The King decided that the metric system was ridiculous, rightfully so, and ended up choosing the imperial system. Stanley developed the Fat Max tape measure and used it to determine the difference between the height of pool “A” versus pool “B.” And that is how inches of water came to be. 

Unfortunately, the historical accuracy of my story is questionable at best. But there is a tool called a U-tube manometer that is shaped like a big U and has some water in the tube. With both ends open, the water level will be equal on each side, which would be the zero level. When you connect a tube to one of the open sides and add positive air or gas pressure to the U-Tube, the water level will be forced down on one side and will rise on the other. You can use King Tut’s Fat Max to measure the difference between the two water levels. The total differential measured in inches is known as “inches of water column.”  Pretty cool, right?

Dwyer U-Inclined Manometer Series 1227 (U-Tube)

Back to the meat and potatoes. So, long story short, a “regular” manometer will only measure inches of water like King Tut’s buddy at a resolution of tenths or maybe hundreds of an inch of water. It isn’t uncommon for it to have an accuracy of +/- 50% at this resolution. A micro-manometer has a much tighter resolution. It is also capable of displaying inches of water measurements but at a more accurate level. Another scale these manometers can display is Pascals. To give you an idea of how small a Pascal is, 0.1” WC is equal to 25 Pascals. 1 Pascal is equal to 0.004” WC or roughly a mouse fart.

Why would anyone find it useful to use mouse farts as a unit of pressure? I think it is important to first note all of the useful tests that can be completed using a micro-manometer and then we can dive into the flatulence of rodents. 

A micro-manometer can measure all of the same things that your grandpa’s manometer can, like:

  • External Static Pressure (In. WC)
  • Vent/ Pressure Switch Testing (In. WC)

The following are the cool tests that your Grandpa could only dream about:

  • Draft Testing (Inches WC or Pa)
  • Combustion Appliance Zone aka CAZ Testing (Pa)
  • Room Pressurization/Depressurization (Pa)
  • House Pressurization/Depressurization (Pa)
  • Dominant Duct Leakage (Pa)
  • Sub-slab depressurization (Pa)

Let’s focus on the tests that are unique to a micro-manometer.  

Draft Testing

It is probably most common for technicians to measure the draft in the flue pipe with inches of water. The following is the acceptable range of draft in common residential HVAC equipment from the BluFlame Combustion Guide. Notice how small the measurements are. Using a basic manometer with an accuracy of +/- 50% is probably not the best idea when we consider the risk of carbon monoxide poisoning.  

  • 70% Efficient Atmospheric Draft Boiler or Furnace -0.02” to -0.04”
  • 80% Efficient Boiler or Furnace -0.02” to -0.04”
  • 90% Efficient Condensing Boiler or Furnace +0.02 to +0.08”
  • Cast Iron Cone Oil -0.02” to -0.04”
  • Flame Retention Head (FRH) -0.02” to -0.04”
  • FRH Condensing +0.02” to +0.08”
  • Atmospheric Draft Water Tank -0.02” to 0-.04”
  • Forced Draft Water Tank +0.02” to +0.08”

Combustion Appliance Zone (CAZ)

CAZ testing should be performed on every home with gas appliances. Think of it as measuring the room pressure WRT (with reference to) outdoors. When I say room pressure, I am referring to the room that the gas appliance is in. 

If you have a mechanical room that has a two-pipe tankless water heater and a two-pipe high-efficiency furnace, there would be no need to perform a CAZ test. You want to measure room pressure when the appliance is relaying on the air from inside the space for its combustion. 

Think of it like this: if the appliance is burning gas, propane, or fuel oil, it obviously needs oxygen to achieve proper combustion. Now, if that oxygen can be interrupted by someone shutting a door or maybe a return air duct sucking the air out of the space, situations like this can lead to dangerous pressure issues, which can cause back drafting of the flue gases.

Image from the AccuTools BluFlame Combustion Guide

Room Pressurization/Depressurization 

Some technicians might not understand why room pressurization/depressurization is a problem and why it could lead to more serious issues. It’s as simple as one CFM in = one CFM out. If you don’t design a duct system that allows for rooms to operate at a neutral pressure, you are starting a war with Mother Nature, and let me tell you, Mother Nature will always win. And when she does, air will be pushed or pulled from somewhere that you don't want it going to or coming from, like outside.

You can measure room pressure by connecting a hose to the + port on a micro-manometer and sticking the hose under the door to the space you want to measure. Make sure you are standing outside of the space, close the door, and set the manometer to read Pascals, and you’re off to the races. Remember, the + port is what you are measuring, and the – port is with reference to.

Issues with a Positively Pressurized Room

When you close the door to a room that doesn’t have a proper return air path, the room starts to build pressure. If that pressure builds high enough, the volume of airflow coming out of the supply duct will decrease. This issue could become a comfort complaint due to the lack of airflow being delivered to the space.

Let’s say the room has an exterior wall or ceiling. If that wall or ceiling has air leakage to the outside, can you guess where that air in that room is going to go? If you said outside, you win the prize. In a case where the air is leaking from that room to outside, what do you think is happening to the pressure of the home compared to outside? If you said the house is going to depressurize, meaning it will become negative with regards to the outside, then you're on a roll. Mother Nature does not appreciate pressure imbalance. When you start pushing air from the inside of the house to the outside, the unconditioned air will leak back into the other rooms. This issue will create more comfort complaints and is not an efficient way of conditioning a home.

Issues caused by a room that has negative pressure

The opposite happens when you have too large of a return in a room. I know what you’re thinking, too large of a return, who is this guy? When I say too large of a return, I mean there is too much suck and not enough blow, or an imbalance in supply and return airflow. The room starts to depressurize, and when you have a room that has a negative pressure you start to leak air in from places that are not ideal, like attics, crawl spaces, bathrooms, or directly outdoors. If you end up sucking air into that room from outside, the house as a whole starts to become positively pressurized, which means that Mother Nature is once again irritated and insists on equalizing the pressure. The pressurized house leaks conditioned air to outside, which is not very efficient and also can lead to more serious issues.

I wrote a tech tip recently called “What airflow goes around HAS to come around”. The article compares different methods for designing return air paths in a home. The Energy Conservatory is also coming out with its version of the “Ductulator” but is geared towards solving room pressurization issues. They named this useful tool “The Roomulator®”

House Pressurization/Depressurization

House pressurization is measured in the same manner as room pressurization.  A house can go negative or positive for the reasons I spoke about above. Other things that can affect house pressurization are bath fans, kitchen exhaust, clothes dryers, water heaters, furnaces, boilers, and so on. One thing to keep an eye out for is ventilation equipment that has not been commissioned properly, like ERVS, HRVS, and make-up air systems.

Dominant Duct Leakage

This test is super easy and could be helpful in diagnosing duct leakage to the outside. The first thing you need to do is to make sure all of the HVAC equipment in the house is off and the blower motors are set to “auto”. Next, connect a hose to the – port, throw it under the front door, and then close the door. You are now standing inside of the house with nothing connected to the + port. So, we are measuring the house with reference to (WRT) outside. Turn on the HVAC equipment, make sure it is running at full capacity, and note the difference in the house pressure.

If the pressure went negative with the blower motors running, that means you have a dominant supply duct leakage to the outside, which is depressurizing the space. If the house went positive, you'd have a dominant return duct leakage to the outside.

Credit: Image is from www.menaair.com

Note: If nothing changes, you might not have duct leakage to outside. Be careful because this might also mean that you have a fairly equal supply and return duct leakage.

Keep in mind that this test is not always conclusive and depends on how leaky the shell of the house is and how leaky the ducts are to the outside. Understanding how to perform this test can give you a leg up on your competition and point you in the right direction when trying to solve comfort complaints.

Sub-slab depressurization – Radon Mitigation

Radon is a radioactive gas that is released naturally underground. It seeps up through the soil and can enter through the lowest point of the home, most commonly through the cracks in the basement or first-floor slab. Radon is a dangerous gas and a known carcinogen. Radon mitigation usually consists of a fan that is piped into the slab or a drain tile system that runs constantly, which depressurizes the area under the slab.

Credit: The image was from www.aderadonmitigation.com

When installing or testing the efficacy of a mitigation system, it is common to drill several test ports in the slab throughout the entire space. A micro-manometer is used to test the pressure field under the slab to ensure that the entire space is depressurized. If there is a portion of the slab that is not depressurized, an additional suction point and or fan may be necessary.

—Adam Mufich



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