Tag: dehumidification

This is the second article in a three-part series, where Advanced Psychrometrics are explored. The source material for each of the articles in this series is ACCA Manual P Sections 3, 4, and 5. This article is based on information found in Section 4.

If you followed the previous Advanced Psychrometrics article, you now know how to use a Psych Chart to plot a Room Sensible Heat Ratio (RSHR) Line, and how to calculate Design Room CFM. However, if you followed that exercise, you will note the absence of real-world variables, such as ventilation and bypass factors. Equipment Sensible Heat Ratios are almost never an exact match to the RSHR. This exercise will account for these variables, and walk you through how to plot these properties on a Psychrometric Chart. 

It is worth reminding you that this is an exercise to help illustrate the complexities of psychrometry in the real world. This may not always be a practical method utilized in the design process. 

When outdoor ventilation air is mixed with return air before the equipment coil, the equipment is exposed to latent and sensible loads beyond that of just the conditioned space. This characteristic causes the Coil Sensible Heat Ratio (CSHR) to alter from the RSHR. Remember, the Room Design Conditions will be met only when the supply air properties fall on the RSHR Line. With two different SHRs, we no longer have the luxury of choosing any supply condition we wish. The supply air must be able to cool and dehumidify the space. It also must now compensate for the additional load introduced by the ventilation air. Therefore, the only supply condition that will satisfy the Room Design Condition is the point at which both the RSHR Line and CSHR Line meet on the Psych Chart.

To plot the RSHR Line should be a breeze at this point. For a review on that process, and the first part of this article series, CLICK HERE.

The construction of the CSHR Line, however, is a bit more involved. There is a little trial and error in the construction of the CSHR Line. It’s not impossible, of course, and with practice, you get pretty good at nailing it on the first try. Here’s why a trial and error process is required in order to plot the CSHR Line:

  • The location of the CSHR Line is determined by the Mixed Air Condition (MAT) and CSHR
  • The CSHR and the MAT can’t be plotted without knowing the percentage of Outdoor Air (OA)
  • The percentage of OA can be calculated only when the supply CFM is known.
  • The supply CFM can be calculated only when the ΔT between the room return and supply is known, which is determined by the intersection of the CSHR and RSHR Lines
  • The CSHR Line is the line we are solving for; therefore, it is unavailable.

This is why a trial and error process is required. Simply put, we’re going to use an estimated guess as to what we think our supply air condition will be, then follow the process until we can determine if our selection actually results in the intersection of the CSHR and RSHR Lines. To help aid in the accuracy of your guess, keep in mind that, on average, a Direct Exchange Fan Coil can provide supply air temperatures which may fall between 14-25 degrees below the space temperature at typical relative humidities between 80% and 95%.

To begin this exercise, let’s start with some basic information, which will ALWAYS be available to you from a quality load calculation. This information can be plotted on the Psych Chart with complete certainty:

Room Sensible Heat: 21,700 BTUh

Room Latent Heat: 2,300 BTUh

Room Total Heat: 24,000 BTUh

RSHR: 0.90

Room Design Condition: 75℉ db / 50% RH

Outdoor Design Condition: 95℉ db / 75℉ wb

Ventilation Required: 245 CFM

In this scenario, a Room-to-Room load calculation has been done on a home. The RSHRs have all been averaged together for a mean room sensible heat ratio. We can go ahead and plot what we can on the chart:

Let’s select a 57℉ supply temperature at about 90% RH. Now we can determine the Supply CFM. Since the RSHR is the average of the entire home, the Supply CFM will equal the total system CFM.

CFM = Room Sensible Load ÷ (1.08 x ΔT)

21,700 ÷ (1.08 x 18) = 1,116 CFM

Now that we know the Supply CFM, we can calculate the percentage of ventilation air.

Ventilation =  245 CFM ÷ 1,116 CFM 

Ventilation = 22%

We have a good bit of information here now, but the math starts to get a little confusing without explanation. We now know that 22% of Outdoor Air (at 95℉ db / 75℉ wb) will be mixing with the remaining 78% Return Air (at 75℉ db / 50% RH). To calculate the Mixed Air Condition, complete the following equation:

MAT = (0.22 x 95℉) + (0.78 x 75℉)

MAT = 20.9℉ + 58.5℉

MAT = 79.4℉

We can now plot the Mixed Air Condition on the Psych Chart.

At this point, we have everything we need to construct the Coil Sensible Heat Ratio Line. If you notice on the Psych Chart, there is a list of helpful formulas to the left of the page. We need to solve for Total Coil Heat Load (Qt) if we are to determine Coil Sensible Heat Load (Qs) and CSHR. To do that, we need to figure out the change in enthalpy (ΔH). Enthalpy is heat energy in BTUs per pound of dry air.

ΔH = 30.6 – 23.4

ΔH = 7.2

Now let’s plug our ΔH into the Total Coil Heat Load calculation. (4.5 here is Air Density x Run Time in minutes. 0.075 x 60 = 4.5)

Qt = 4.5 x CFM x ΔH

Qt = 4.5 x 1,116 x 7.2

Qt = 36,158 BTUh

Solve for Coil Sensible Heat Load. To do this, make sure you are using the entering air condition the equipment will actually see: MAT.

Qs = 1.08 x CFM x ΔT

Qs = 1.08 x 1,116 x 22.5

Qs = 27,119 BTUh

We can finally solve for Coil Sensible Heat Ratio at this point:

CSHR = Coil Sensible Load ÷ Total Coil Load

CSHR = 27,119 BTUh ÷ 36,158 BTUh

CSHR = 0.75

We can now plot the CSHR Line on the Psych Chart.

If you look closely, you may be thinking, “Wait a second, the CSHR Line does not intersect with the RSHR Line.” You would be absolutely correct. This is why the trial and error solution is necessary. However, if you notice, the CSHR Line is extremely close to our selected supply temperature. The CSHR Line is just slightly above the RSHR Line.

What does this mean?

We can still use the design CFM and supply condition, and the equipment will satisfy the sensible load, but will maintain a slightly higher humidity level in the space than what was designed. Take a look at the actual grains of moisture for the Mixed Air Condition in comparison to the Supply Air off the coil at 57℉.

The equipment will be able to dehumidify from 73 grains of moisture/lb of dry air down to 63 grains of moisture, rather than the ideal 62 grains. We’re talking about a difference of 1 grain of moisture. This can be acceptable, and the difference likely unnoticeable. In cases where a coil selection will not match the latent load requirements of a space, a viable option would be to add supplemental dehumidification to deal with the remaining latent load. Ultra-Aire Ventilating Dehumidifiers are an excellent option, and will also help lessen the additional latent load from the ventilation air.

Lastly, let’s talk about Bypass Factor. Remember, the ideal supply temperature would be the apparatus (equipment) dew point. However, there is a small percentage of air that will bypass the coil and not transfer its heat to the coil. This can be calculated using the known apparatus dew point. The Bypass Factor formula is as follows:

 Bypass Factor = (Supply Air Temperature – Apparatus Dew Point) ÷ (Mixed Air Temperature – Apparatus Dew Point)

Bypass Factor = (57 – 53.5) ÷  (79.4 – 53.5)

Bypass Factor = 3.5 ÷ 25.9

Bypass Factor = 0.14

At this point, you would need to look up a manufacturer’s extended performance data for their equipment to ensure that the coil you select will meet a sensible capacity of 27,119 BTUh and a total capacity of 36,158 BTUh at 1,116 CFM, with an entering condition of 79.4℉ db / 65.6℉ wb and an outdoor condition of 95℉ db / 75℉ wb. Let me translate that to something you might actually see on a Performance Table:

Entering Air Condition: 80℉ db / 67℉ wb

Outdoor Air Conditions: 95℉ / 75℉ wb

Total Capacity: 36,000 BTUh

Sensible Capacity: 27,000 BTUh

Airflow: 1,100 CFM

If you can select a coil that will match these criteria, you will be able to maintain an indoor air condition that is nominally close to your design.

To see how this chart would look in another scenario (without going through the step-by-step process), here is a psych chart based on my house and ASHRAE Design Conditions:

Room Sensible Heat: 16,800 BTUh

Room Latent Heat: 7,200 BTUh

Room Total Heat: 24,000 BTUh

RSHR: 0.70

Room Design Conditions: 75℉ db / 50% RH

Outdoor Design Conditions: 90℉ db / 80℉ wb

Ventilation Requirement: 46 CFM

In this case, my selected supply air condition happened to fall perfectly at the intersection of the CSHR and RSHR Lines. The tricky part, however, is finding a coil that will meet the sensible and latent heat requirements under the design conditions. I would need to look for a coil with 18,000 BTUh sensible capacity and 29,000 BTUh total capacity. I’d have to settle for a 2.5 ton (30,000 BTUh) coil with a close CSHR under design conditions, and potentially add supplemental dehumidification. (A Carrier FB4C–030 would actually fit the bill quite nicely.) Remember, the equipment selection performance table will have actual capacities that differ from the nominal rating; thus, care must be taken when using manufacturer performance tables to select equipment.

If you’ve made it to the end of this exercise, congratulations: you are as nerdy as they come! I hope this helps illustrate the complexities of psychrometrics. If nothing else, the take away should be a new-found respect for psychrometrics, and its integration into a technician’s daily diagnostic toolbag.

Stay tuned for Part 3, where we will dive into ACCA Manual P, Section 5. There we will learn how to account for duct gains, and how reheat dehumidification looks on a Psych Chart.

 

–Kaleb Saleeby


I’ve been reading a book called “Cool, How Air Conditioning Changed Everything” and it got me interested once again in the history of air conditioning and refrigeration. Like many things the people who are credited with “inventing” are the ones dogged enough to make an idea commercially successful, not the idealists forever tucked away in the lab.

I bought a 1921 version of the periodical “Ice and Refrigeration” and mixed in with the ads for absorption ice machines and “mineral wool” insulation was the advertisement shown above. Willis Carrier understood how to connect ideas and make sense of emerging technology, first to keep paper dry in a factory and later to cool the world with “Manufactured Weather”. Look carefully at the ad, you will notice it mentions many things… but not cooling, the ad is in ICE AND REFRIGERATION and the ad doesn’t mention COOLING.

Many of you know that in 1906 Willis Carrier patented what is now referred to as the “First Air Conditioning System” but do you know what it was that he actually invented?

You may be led to believe that Willis Carrier invented compression refrigeration? Nope, the first commercial attempts at compression refrigeration began in the 1830’s and the patent above actually has no compression refrigeration in it whatsoever. Many will say that he was the first to dehumidify the air, this is also false, there had been compression refrigerated cooling coils in use that dehumidified the air before Willis came along they just didn’t do it on purpose.

What Willis Carrier understood better than anyone else in his day was the RELATIONSHIP between humidity, temperature and saturated air or “dew point” and how to manipulate water temperature, water volume and air volume to produce a CONTROLLED humidity environment first and later a controlled temperature, humidity, and ventilation environment.

The Carrier “Air Washer” was nothing more than water pumped through nozzles that produced a mist of water. The air would blow through the water mist and it would clean the air, drop it to dew point (100% RH) and then continue to sensibly cool the air. Willis worked in northern states with cold groundwater at a time before water use restrictions so the cold water would serve to cool AND dehumidify the air. At the time it seemed like black magic that running air over water could REMOVE water from the air, but so long as the water temperature was below the dew point temperature of the air that is exactly what would happen. All Willis had to do to change the dehumidifier to humidifier was to increase the water temperature or change the dehumidifier to a sensible cooling machine was to use cold water and give the air more dwell time or passes through the water to decrease the sensible temperature.

In the process Carrier and his team made many discoveries about air and in 1911 Carrier presented possibly his greatest work which he called the “psychrometric formulae” which is the founding document on which all of current understanding of psychrometrics is built. Carrier took a VERY SIMPLE idea and pursued it and understood better than the others around him and because of that, we remember him today. He thought about cooling, heating, ventilation, humidity and air cleanliness and combined them together into one machine that controlled it all.

Later on, Carrier would begin actively “cooling” the air with compression refrigeration and replaced water sprays with refrigerant evaporator coils to leverage the latent capacity of refrigerants, but it all started with a mist of water an understanding of dewpoint, some dogged determination and some clever marketing for his “manufactured weather”.

— Bryan

To find the catalog where I found some of this information I created a link to the national archives at hvacrschool.com/willis

 

 

 

 

Sometimes you find yourself in a position where you are going to replace a fancy thermostat with a simple one. It may be because the customer got fed up with all the options or because you are there on a weekend service call and all you have is a basic stat.

No matter the reason you need to make sure the new thermostat can do the job the old one did before you quote, an option that gets overlooked in matching up is dehumidification.

Most manufacturers of residential variable speed air handlers have a terminal that will drop the blower speed when de-energized. It may be marked DH or D or dehum or something else. From the factory, they generally have that terminal connected to R using a pin or jumper so that the blower will run up to full speed. When one of these special thermostats get installed the tech is supposed to remove that jumper or pin and connect a wire from that terminal to the thermostat dehumidification terminal so that the thermostat can energize the terminal for full blower speed or drop 24v to the terminal to go into dehumidification.

If we install a new thermostat and forget to reconnect that pin or jumper then the system will ALWAYS run in dehumidification mode because there will never be any power on that terminal at the air handler board.

The lesson is to pay attention to whether or not a system is wired for dehumidification. If you do need to replace it with a basic thermostat make sure to replace the pin or jumper (J1 on the board example above).

If you forget to do so the system will run less efficiently with lower airflow, suction pressure and coil temperature.

— Bryan

 

 

Dehumidification features are common on residential systems ever since the introduction of variable speed blower motors. The system is set up so that the blower can produce less CFM per ton when latent (humidity) load in the space is higher than the setpoint relative humidity. Slowing the blower increases moisture removal by reducing the sensible load on the evaporator coil and therefore dropping the coil temperature and surface dewpoint.

Most variable speed fan coils and furnaces have a terminal designated for Dehumidification and it can be called D, dehum, DH or something else depending on the manufacturer.  In all cases I am aware of, this dehumidification terminal must be energized for the blower to go to full speed and when that terminal is de-energized the blower speed (usually) drops to 80% of full speed.

For years we have seen thermostats with designated dehumidification terminals to match up with the fan coil/furnace terminal, so it was just a matter of disconnecting a jumper from the dehumidification terminal to the R terminal in the unit and connecting a wire from the designated thermostat terminal to that dehumidification terminal in the unit. The diagram below is an example of this on an old Carrier Thermidistat with a variable speed Carrier fan coil.

We now have 24v control smart thermostats like Ecobee, Cor, Nest and Lyric with a lot more flexibility in how they can be set up rather than having a single, designated dehumidification terminal.

I am a big fan of EcoBee for many reasons including their Alexa integration, remote wireless sensors and application flexibility… but you need to be really careful with how you set them up, ESPECIALLY when setting up dehumidification.

The image above is a GIF and should show you the first part of the dehumidification setup. I am setting it up for a single speed compressor heat pump with a variable speed fan coil. EcoBee has contacts labeled acc+ and acc- that can be set up to do a wide variety of functions. For this typical dehumidification function using the system you would select Menu>Installation Settings>Dehumidifer >1 Wire ACC+>Open contact state to activate dehumidifier.

This setup uses 24V power from the R terminal to energize the acc+ terminal and therefore the dehumidification terminal in the fan coil/furnace when there is NO call for dehumidification.

Now for a controversial part. Go to the equipment menu and select Dehumidifier to “dehumidify with fan”= no. We have seen several occasions where the blower continues running with no cooling call if this setting is set to yes when there is a dehumidification demand and no cooling demand.. According to the EcoBee website HERE it appears to say the opposite, but we have confirmed on a few occasions that this occurs and there appears to be no adverse effects from setting it to off becasue the blower is still controlled by the thermostat for cooling operation and dehumidification without cooling is not possible without an external dehumidifier.

In order for the system to over cool below the temperature setpoint to dehumidify you need to go into the thresholds menu and set up AC over cool Max to the maximum temperature below setpoint that would be allowed during dehumidification by the equipment.

— Bryan

 

 

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