HPAC Magazine

How Much Is Enough?

Making sense of relative humidity, wet bulb and grains of moisture.

August 1, 2015   By Ian McTeer

A caller once said to me on a frosty winter’s morning “The humidifier your dealer installed is not working.” I was expecting to hear a litany of complaints about how long it runs and how much expensive water it was flushing down her expensive municipal drain, and how the static shocks were upsetting the cat.
“What’s the matter with it?” I asked.
“I had hardwood flooring installed during the summer and the installer told me I must have 70 per cent RH during the winter or the floor will be ruined. Right now I only have 35 per cent!” she responded.
Wow, that was a new one for me. It was -11F (-24C) that morning. I mentioned to the caller that the use and care guide for the humidifier recommended that the RH setting for such a low outdoor temperature was 20 per cent.
“It’s working too well, you must adjust the setting to 20 per cent,” I said. Unfazed, the caller insisted on having the product removed. Apparently the hardwood flooring installer’s opinion carries a lot of weight.
I checked with Armstrong, the flooring products manufacturer. They recommend keeping the RH in the range of 35 per cent to 55 per cent for their hardwood flooring products. Mattamy Homes recommends 35 per cent to 45 per cent for hardwood flooring.
I have had hardwood flooring for 20 years and even though I allow the RH to drop into the 20 per cent range for short periods of time on the coldest days, my flooring has not shrunk or cracked. A good quality humidifier equipped with an automatic controller with access to outdoor air temperature that has been set-up properly should be able to do an adequate job of keeping occupants comfortable.  

Water can be anything from a nasty chemical (dihydrogen oxide) to the mainstay of all life. Water exists as a solid (ice), liquid and vapour. We all know the perils of ice in certain circumstances. Liquid water is essential to all life but too much can result in flooding and other issues. Water in the vapour phase can be extremely dangerous. Despite the fact that steam is enormously useful for a variety of tasks, exposure to steam that is not even superheated can ruin your day. So we live, carefully, with water every day.
Dry air is a mixture of gases: nitrogen (78 per cent), oxygen (20.9 per cent), aragon (1 per cent) and other gases (.1 per cent). Rarely do we ever encounter 100 per cent dry air; thus, moist air contains all these gases and water vapour. Humidity, then, is the amount of water vapour in the air. Moist air reaches its saturation point when it contains all the moisture it can hold.

When it comes to determining human comfort, relative humidity is probably one of the most misunderstood terms in the HVAC lexicon. When you set out to measure relative humidity, you are measuring the amount of moisture in the air relative to (or compared to) the amount of moisture the air could hold at the time you are taking your measurement. This is where people get confused. If the air temperature is currently 85F and the RH is 50 per cent, then the air could hold 50 per cent more moisture. However, if the air temperature is 50F and the RH measurement is 50 per cent, the air could also hold 50 per cent more moisture. But how much moisture is 50 per cent more in each case? It is time to think about grains.
In order to sell, install or service humidification equipment, you must also measure wet bulb temperature. Sensible heat is the amount of heat added to air without affecting moisture content. Since there is almost always moisture in the air, it takes a certain amount of heat energy to keep water in its vapour state in the air. We refer to this hidden heat as Latent Heat. Thus, Sensible Heat + Latent Heat = Enthalpy. Enthalpy can be measured using a wet bulb thermometer. The wet bulb temperature (enthalpy) is the measurement of the total heat in one pound of dry air.

Once the wet bulb and dry bulb temperatures are known, plotting those values on a psychrometric chart will reveal a lot of detail about the current condition of the air in any given application. The most useful measurement for the “humidity technician” is Specific Humidity or the grains of moisture per pound of air. There are 7000 grains of moisture per pound of water. A psychrometric chart (or several good apps) will tell the technician how many grains of moisture are present. This is much more useful than knowing just the relative humidity.
Technicians should appreciate that heating air is a sensible process: the moisture content of the air is not affected by adding heat. However, the relative humidity changes dramatically. Consider that air entering a furnace at 70F with 42 grains of moisture per pound will have a relative humidity of 39 per cent. When heated to 125F, the air will leave the heat exchanger with 42 grains of moisture, but the RH will now be at seven per cent.
It is not so much that the air has been dried out. It has more to do with the fact that the leaving air can now hold a lot more moisture. This heated air will rapidly mix with all the other air in the building, and, depending on the conditions of mixture, the grains per pound will likely drop while the RH will increase somewhat. This air will be “looking” to increase its moisture content and it is going to get it from anything containing moisture in the building: occupants, furniture, hardwood flooring, pets, etc.
Just like in a poor cooling performance investigation, a technician needs to know the conditions of the air entering and leaving the system, especially when the tech is trying to determine if a humidifier is doing anything. During the winter, outdoor air cannot hold as much moisture and when it infiltrates a building, the RH and grains will drop. Infiltration caused by air exhausting appliances such as bathroom fans, a kitchen fan, water heater, clothes dryer or a central vacuum system, must be considered. Building construction problems like air leakage around fixtures, improper sealing, and damaged or missing door and window seals should be checked.


Moisture, like heat, can migrate depending on the difference between indoor and outdoor conditions. Consider the examples shown in Figure 1: Moisture moves outdoors because the outdoor grains are lower than the indoor grains.
In the depths of winter, moisture will rapidly escape the loosely constructed or “well ventilated” house as shown in Figure 2. This house will require a properly-sized humidifier to offset excessive moisture migration.
As shown in Figure 3, moisture can migrate into the building.Windy conditions create considerable leakage (see Figure 4).

Forced warm-air heating systems do a good job of providing an adequate level of human comfort and that includes humidity control. There are several types of humidifiers available: bypass, fan forced and steam humidifiers that can be installed on a central heating system. As long as the unit is properly specified/installed/maintained (PSIM), all of them are capable of doing a decent job. It is better to install an automatic controller, which has an outside temperature reference that helps to prevent over-humidification.  
Each kind of humidifier will have specific characteristics that need to be addressed or poor performance is guaranteed. For example, one humidifier manufacturer has the following installation requirements (among others):
1. Do not install humidifier or bypass connection on the furnace jacket.
2. Do not install humidifier or bypass connection on a plenum face where the blanked off ends of the cooling coil will restrict air movement through the humidifier.
3. Do not install humidifier on systems with greater than 0.4 inch w.g. pressure differential between supply and return plenums. (Note:  0.4 inch differential is still way to
o high).
4. Humidifier body must be level.
Steam humidifiers have critical installation and maintenance requirements. One brand I am familiar with has an installation, operation and maintenance manual (IOM) that runs to 22 pages. The manufacturer says, “90 per cent of all operation problems are created by improper steam piping from the humidifier unit to the duct distribution pipes.”  A great tip there, installers.  Remember that steam humidifiers need a separate electrical circuit (some use 230 VAC) so an electrical permit and inspection will be required too.

Because a bypass humidifier “steals” a small amount of heated air from the supply plenum to evaporate water trickling through the wetted panel, it is possible that under some conditions too much air will bypass. Typically, the culprit is a non-PSIM air handling system. Why wouldn’t air take the path of least resistance through the bypass duct when given the chance?
If heated air bypasses in significant quantity, the return air temperature will climb beyond 80F putting the furnace at risk of tripping on the limit control. The constant volume effect of an ECM motor ramping up to maintain temperature rise only makes the problem worse.
The pressure difference between Point “R” and Point “S” should not exceed 0.15 inch w.g. as shown in Figure 5. While a PSC blower is capable of creating excessive bypass, probably the best thing about a PSC blower is its characteristic tendency to unload when airflow is partially restricted by a dirty filter, for example.
On the other hand, the ECM blower will understand that it is unloaded and ramp up in order to keep to its programmed temperature rise.
Figure 6 shows an ECM blower that when set to the factory “normal” position will deliver 1400 cfm of air at 0.5 inch w.g, total ESP on a call for second stage heating. Temperature rise should be 49F and the motor will use 500 watts of power. A non-PSIM system will cause the blower to ramp up in an attempt to maintain a constant volume.
As Figure 6 shows, this ECM does its best work at 0.5 inch (normal). In a nightmare duct system expect the pressure differential to reach 0.5 inch in the bypass duct. By then, close to 260 cfm of air will bypass at high velocity approaching 1300 fpm causing water to blow-off the evaporation panel. It might be noisy as well. It is important to know when not to use a bypass humidifier.

Be sure to read and to follow the manufacturer’s instructions about PSIM. It is not a good idea to offer a “free” humidifier as a sales hook. If it does not work properly, as it probably will not since no one will bother doing a study of the situation, you will have an irritated customer on your hands. As a contractor, sales person, or technician, endeavor to become a humidity expert because it is always a potential problem. Summer or winter, there can be too much or too little–only a professional can make it just right.  <>

Ian McTeer is an HVAC consultant with 35 years experience in the industry. Most recently he was a Field Rep for Trane Canada DSO. McTeer is a refrigeration mechanic and Class 1 Gas technician.

House #1: 73F DB and 57F WB
House #2: 70F DB and 62F WB
House #1 = 43.8 Grains @ 36.7 per cent RH
House #2 = 71.1 Grains @ 64.1 per cent RH
Thus, House #2 has considerably more moisture in the air than House #1. Under any winter condition, House #2 requires serious remediation; an RH this high means trouble.


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