In This video we show supply air RH using the Testo 605i Hygrometers and demonstrate why the Relative humidity approaches 100%
It was December 5th, 1952, only seven years after the end of WWII in London England when the “great smog” settled across the city.
It was a meteorological anomaly combined with unchecked industrial pollution that led the great smog, but even once it settled on the city, bringing business to a screeching halt, everyone thought it pass quickly.
Four days later the smog finally lifted, and the death toll rose to 4,000. Many modern statisticians place the number of deaths caused by the smog at closer to 10,000. While the world knew the dangers of particles in the air before, there was a keener understanding of what pollution in our air can do to us since that time.
Nowadays you will hear the terms PM2.5 and PM10 thrown around and you may never stop to think about what they mean.
PM10 (Particulate Matter < 10 Microns)
Particles with a diameter less than 10 microns across. For some perspective, a human hair is about 50 – 70 microns across and a micron often called a micrometer is one-millionth of a meter. While a PM10 particle is tiny it still pretty coarse by particle standards and will often contain things like dust, dander, and pollen. These are allergens to be sure but generally, are not the most dangerous particles.
PM2.5 (Particulate Matter < 2.5 Microns)
These are particles less than 2.5 microns and often contain things like Ammonia, Carbon, Lead, and mold. Both chemically and biologically toxic particles generally fall into this smaller 2.5 microns or less size range.
In the graph shown above, you can see the results of the % of PM 2.5 sized particles that passed through some various tested filters. The study showed that not only that typical MERV 6 – 8 filters failed to do a good job of capturing a large percentage of these particles. It also showed that the efficiency of the filters varied from brand to brand as shown in MERV12(#1) and MERV 12 (#2).
Indoor / Outdoor
There is a clear link between pollution outdoors and pollution indoors. It stands to reason that if the air outside is dirty, then the air inside will also tend to more particles in it which means that a comparison of outdoor PM2.5 to indoor PM2.5 comparison can be a better way to compare one space to another.
The EPA has tested and found that indoor air can be significantly more polluted with VOCs (Volatile Organic Compounds) often 2 to 5 times more than outdoors.
But hang on…
VOCs are gaseous and are not the same as PM2.5 particles and shouldn’t be confused with them.
From a technicians perspective, you need to be aware that typical air filtration will only effectively deal with the PM10 and larger particles. For the PM2.5 you will need high MERV filtration or other types of active filtration such as PCO or Ionization strategies.
For VOC’s you can ventilate, use carbon filtration or look into products like the Air Oasis bi-polar ionizer that has been shown to reduce VOCs in studies.
Testing for what exactly is in the air is a trickier business then I initially thought and get’s tougher the smaller the particles get. Even today it is virtually impossible to “test” for specific live fungus, bacteria, and VOCs in the field. You can count particles and get in the ballpark of the issue but nailing down all the details can be challenging without sending samples off to a lab.
For most of us, a good particle counter is the best we are going to get out in the field, and we are best off following some good solid practices to help our customers.
The most dangerous stuff in the air are chemicals, tiny particles and live organisms caused by moisture issues. Knowing this will make you better tech and more capable of advising your customers well.
This tech tip is written by one of the best all-around HVAC minds out there. Neil Comparetto.
I think that we all can agree that duct leakage is not ideal. Our job is to condition the space. If we can’t control the air, that becomes difficult. On top of that anytime you are losing already paid for conditioned air. But really, how bad could it be?
I’m in Richmond Virginia, so we’ll use that as our example location. According to ACCA Manual J summer design conditions our outdoor design temperature is 92° Fahrenheit, with a moisture content of 106 grains per pound. (grains is a measurement of absolute moisture). Let’s use the indoor conditions 75° F and 50% relative humidity, which converts to 65 grains of moisture.
Our example system will be a 3-ton air conditioner moving 1200 CFM with ducts in a vented attic. For this exercise, we won’t get into duct sensible heat gain that even a 100% tight duct system will have to overcome.
This system will have a modest 10% supply duct leakage into the attic (Energy Star estimates that the typical duct system has 20-30% duct leakage). Assume 0% return leakage (which is unlikely). So we already know that 10% of our capacity is gone, never to return again into the attic.
On a 3 ton air conditioner that will be roughly 3,600 btuh. We are now delivering 1080 CFM of supply air to the living space, and returning 1200 CFM. Where does the additional 120 CFM of return air come from? You guessed it, outside. The supply duct leakage into the attic, outside of our thermal and pressure boundary, has now brought the living space into a negative pressure. No big deal, it’s only 120 CFM… but have you ever done the math!?
Stick with me, it’s not as bad as it looks. Here are the formulas for the sensible and latent heat required to bring the infiltration air back to indoor conditions (75°/ 50%RH).
Sensible BTUH = 1.08 x CFM x (Outdoor temp – indoor temp) Latent BTUH = 0.68 x CFM x (Outdoor grains – Indoor grains)
Let’s use 92° F as our outdoor air temperature number. In all likelihood, considering that the attic floor/ceiling plane is one of the leakiest parts of the house, and the attic is typically > 120° F, that in real life it will be higher than whatever outdoor temperature is.
Our example will look like this:
1.08 x 120 CFM x (92°-75°) = 2,203 btuh of sensible heat
.68 x 120 CFM x (106 grains – 65 grains) = 3,346 btuh of latent heat
2,203 + 3,346= 5,549 btuh of total heat.
That is an additional 5,549 btuh of total heat. The 3,346 btuh of latent heat is the more difficult number to deal with. Next time you are bored flip through your favorite air conditioner’s product data and see what it can produce, you may be surprised. Don’t forget about the 3,600 btuh that’s up in the attic somewhere. And just think, this is from only 10% supply duct leakage, considerably more is very possible.
As you can imagine in the heating season this problem doesn’t go away. Typically outside air is much drier than indoor air, and duct leakage will dry out the indoor space. If the heating system is a heat pump the capacity loss is corrected by electric strip heat, which is bad. That means when you seal the ducts auxiliary heat is reduced, which is good.
Leaky ducts can contribute to many more issues than just energy loss and comfort. Did you know that a one square inch hole in the duct system is equal to thirty-inch hole in the building envelope? The potential to create pressure imbalances in the building is tremendous. Pressure imbalances can cause many issues, like flues backdrafting, excess dust and allergens, uneven temperatures, and moisture issues to name a few.
Something as simple as sealing ducts can solve many issues, hopefully, you include it in your scope of work.
Download the podcast Directly HERE
This article and podcast is courtesy of Jeremy Smith, one of the most knowledgeable and helpful refrigeration techs I know.
It’s my feeling that, no matter how well explained, this topic really requires a treatment that is more in depth and one that can be absorbed slowly with the ability to continually return and re-read certain sections to allow for best understanding of the subject matter.
As discussed in the podcast, as the outdoor temperature drops, the capacity of the condenser increases dramatically causing it to be, essentially, oversized for normal operation. To counteract that, we use a valve (headmaster) or valves (ORI/ORD) to fill the condenser with liquid to effectively reduce the amount of coil that is actively rejecting heat and condensing refrigerant. This also maintains a high enough liquid pressure feeding our TEV. This prevents wild swings in TEV control because it is a pressure operated mechanical device.
This is a document I reference all the time when dealing with condenser flooding problems. If you’re tech savvy, save it on your mobile device. If you’re more of a low-tech guy, listening to a podcast and reading an internet publication on your flip phone or whatever, go ahead and print this out, laminate it and keep it in your clipboard. Heck, even if you are a high tech guy, sometimes nothing beats a hard copy of this the first few times you work through it.
If ,after the podcast, you haven’t read through this to familiarize yourself with it, take the time to do so. It seems like a really complicated procedure to work through, and the first few times that you do it on your own, it can be. With practice, however, you’ll get used to it.
We’ll work through a condenser flooding calculation here in slow time, outlining all the different calculations taken into account.
First lets lay out the basic info we need. The measurements and counts will vary, of course, depending on the equipment that you have.
If we have an R22 unit, 44 condenser passes ⅜” in diameter each are 38 ¾” long with 42 return bends. Our evaporator temperature is 20°F, current temp is 35°F and the lowest expected ambient is -20°F.
Now, that seems like a lot of information, but we’ll break it all down.
First, we need to figure the total length of the condenser tubing in feet. So, we take 44 x 38 ¾ and get 1705” of tubing. 1705 ÷ 12” per foot gives us 142.083 feet of tubing. Now, that’s just the straight tubing. We’ve got return bends to account for.
Refer to our Sporlan document. In TABLE 1, you’ll find an equivalent foot length per return bend. In the case of a ⅜” return bend, it’s. 2 feet per bend, so 42 x .2 gives us 8.4 feet more.
Add those together for total length of 150.483. Back to TABLE 1 look in the R22 section under ⅜” tubing and follow the line for -20°F across. You’ll find a density factor of 0.055. This number is how many pounds of liquid refrigerant is needed to fill one foot of tubing at that temperature. So, 150.483 x 0.055. This gives us 8.28 pounds. This is the amount required to fill the entire condenser with liquid, but we don’t really need to fill the WHOLE coil….
Back to the document..TABLE 2 this time.
Across the top, find 20° evaporating temp, now follow that down to the -20°F row. This gives us a percentage. 82% so, this unit at -20% will have 82% of its condenser filled with liquid. So let’s take 8.28 x 0.82 to get our flooding charge.
Now, what does this number really mean. This is the amount of refrigerant we need to add to a system that we’ve JUST cleared the sightglass on when the ambient temperature is 70°F or higher. If our ambient temperature were 70 degrees or warmer, we could add just that amount past a clear sight glass and walk away, satisfied in knowing that the unit will run properly no matter what the weather throws at it.
Remember, though, that our current ambient is 35°F. So, now what?
Time to stop. Get your Sharpie out and WRITE THIS NUMBER DOWN! Record it on the unit somewhere. Somewhere easy to see but somewhere that the sun doesn’t degrade the ink over time. That way, you only have to go through this one time. If you’re doing a new installation and startup, do the next guy a favor and write both this AND the total system charge down somewhere so that I don’t have to guesstimate the charge when it all leaks out.
Now, let’s go back to TABLE 2 and look at the 35°F row. We find that at 35°, we need to have 63% flooded. Well, we’ve got a clear sight glass and it’s 35° ambient so, we’re already 63% flooded.
Since the most we need is 82% flooded, 82%-63% gives 19% so, we take our total, 8.28 x 0.19 to get 1.57 pounds. At our current conditions, that’s all the flooding charge that we need to add because we’ve already got some flooding going on to have a clear sightglass because we’re under the 70 degree mark and the low ambient controls are in play and doing their job.
Some techs claim that just spraying water on the coil will flood the condenser enough to allow the use of that as a charging technique. Let’s think about it for a minute. What variables come into play with a method like that? Variables that we can’t control… for starters, what is the wet bulb temperature of the air entering the condenser? How well is the condenser wetted? With the stakes being what they are, I’m not excited about the prospect of using this because I’m probably going to be the guy who winds up on the roof when it’s -20 and the wind is howling and this unit is low on gas because someone tried to use this method to figure a flooding charge, didn’t get enough gas in the unit and now it’s short. I’ve still got to my due diligence as a service tech, do a full leak check, not find anything, and walk away wondering if I missed a leak somewhere all because someone else didn’t take a couple minutes to do a little work to do the job properly. This is a totally preventable service call.
What about TABLE 3, you ask? Very astute and that tells me that you’re reading ahead. Excellent. I have never had to use it.
It gives a different flooding percentage for units with an unloader and low ambient controls where they’ll be running in low ambient conditions. With the unloader, remember that we’re really moving less heat, changing the condenser dynamic and making it even MORE oversized than it would be if there weren’t an unloader, so more refrigerant needs to be added to properly flood the condenser.
— Jeremy Smith
P.S. – You can checkout the Testo 770-3 multimeter we mentioned in the middle by going here
We talk with Trevor Matthews with Emerson about causes of air conditioning and refrigeration compressor failure and the causes Verifying System Operation Sheet from Emerson https://hvacrschool.com/EmersonVerify Diagnosing Compressor Failures from Emerson https://hvacrschool.com/CompFailures
Part 2 of the discussion with Trevor Matthews with Emerson about causes of air conditioning and refrigeration compressor failure and the causes Verifying System Operation Sheet from Emerson https://hvacrschool.com/EmersonVerify Diagnosing Compressor Failures from Emerson https://hvacrschool.com/CompFailuresWe talk with Trevor Matthews with Emerson about causes of air conditioning and refrigeration compressor failure and the causes Verifying System Operation Sheet from Emerson https://hvacrschool.com/EmersonVerify Diagnosing Compressor Failures from Emerson https://hvacrschool.com/CompFailures
So what do you think of when you hear an “ideal gas”? R22, R12 maybe… Natural? Take a look at the F-18 above… It is breaking the sound barrier and that cloud is a shockwave… This has nothing to do with this article but I think it’s pretty darn cool!
An ideal gas is a gas that obeys the ideal gas law, it’s ideal because it’s good at following rules. These ideal gasses walk in a straight line, they don’t run on the playground and they never fish without a proper permit. More like an ideal gas behaves in a predictable way with changes in volume, pressure, temperature, and mass.
The problem is, a truly “ideal” gas really doesn’t exist.
While many gasses behave close to ideal at normal temperatures there is no gas that obeys the ideal gas laws in all conditions.
The ideal gas law is –
P= Absolute Pressure (gauge pressure + atmospheric pressure)
V = Volume (How much space the gas occupies)
n = Mass measured in “moles” (the number of molecules)
R = The universal gas constant (varies depending on the units of measure being used Example: [lbf ft/(lb mol oR)]= 8.3145 )
T = Absolute Temperature (temperature in a scale that starts at absolute zero like Kelvin or Rankine)
The ideal gas law is really a combination of several different laws into one.
The result is that many gasses that we work with behave in about the same way with changes in mass, volume, temperature & pressure. This is the case because the primary force at play in a nearly ideal gas like nitrogen or CO2 is simply the velocity of the molecules bouncing around in the container and against one another like tiny little ping pong balls.
If the molecules react, or interact with one another through attraction or repulsion due to their intermolecular forces then they can cease to behave as an ideal gas. A perfect example is when a gas is in contact with its liquid form (saturation) it no longer obeys the gas laws. This is why most gasses behave more and more like an ideal gas the hotter they get (within a range) because the hotter they are the greater the force of molecular velocity (temperature) will be relative to the intermolecular interaction of the molecules.
Once the gas gets to the “supercritical” state all bets are off once again. So like most good kids, even the most ideal gasses have their limits where if pushed they become little molecular rebels.
Many ductless systems and some high-efficiency unitary systems have electronic expansion valves.
If you find one that is stuck closed you may be able to get it open temporarily by putting a strong magnet like the one shown above on the valve body and turning it counter clockwise.
This is likely only temporary so valve replacement is still needed but it can help get the customer running while you wait to get a new valve.
Pretty cool Ulises.