Understanding Dew Level To Be A Higher HVAC Technician

Turning into A Higher Practitioner

The journey to turning into an amazing HVAC technician is a group of small steps.

To be a greater diagnostician, it’s essential to grasp smaller expertise first.

These might embody taking temperature and strain readings, calculating superheat and subcooling.

These are the foundations of being an excellent technician.

To be a higher technician, you’ll have to have a deeper understanding of refrigeration and psychrometric measurements. 

One among these is dew level.

Dew Level

Dew level is without doubt one of the most underrated readings a technician can take when diagnosing consolation issues. 

Most of us are accustomed to the psychrometric chart, however most instantly get turned off by the strains getting in each potential course.

The dew level line merely goes from proper to left on the chart. 

When it intersects with the dry bulb line, which runs up and down, this varieties a “cross hair”, like a rifle scope. 

And within the crosshair lies the present situation of the air you might be studying.

However What’s Dew Level?

Merely, it’s a measurement of the quantity of water within the air. 

What it’s truly telling us, although, is at what temperature the moisture will start to fall out of the air within the type of condensation. 

Consider air as a sponge, which may maintain a most quantity of a precise quantity of water.

Should you squeeze the sponge, it can’t maintain as a lot, and the surplus water will fall out.

At a given temperature, air can maintain a precise quantity of water earlier than it’s fully saturated.

If we start to chill the air, it’s like squeezing the sponge. 

If we squeeze it onerous sufficient, we make the sponge smaller and ultimately, water falls out. 

After we cool air, we make it “smaller,” and it will definitely reaches saturation or its dew level, and condensation varieties.

Years in the past, it was troublesome to measure dew level as a technician. 

The most typical technique was to make use of a sling psychrometer, by which you give your dry bulb and moist bulb, then you definately needed to plot dew level on the psychrometric chart.

It was almost inconceivable to take these readings inside a duct. 

However now, handheld digital hygrometers (additionally known as psychrometers) can be found, reasonably priced, and moveable.

 They even work with Bluetooth and sync as much as highly effective apps like Measurequick. 

Taking an outside dew level measurement or realizing what the ASHRAE outside design dew level is will allow you to make good suggestions and design selections.

Outside Dew Level

Let’s first perceive what outside dew level tells us.

The upper this quantity is, the extra moisture is within the air.

Humid climates just like the Southern U.S. have prolonged durations of excessive dew level over 63°F (17.°C). 

It’s not unusual for coastal areas to expertise durations of extraordinarily excessive dew factors of 80° (27°C)! 

Realizing what your outside dew level is can assist you perceive why condensation varieties inside a duct, a wall, or one other place the place moisture droplets shouldn’t type. 

The truth is, the ONLY place we wish to see condensation type is on an evaporator coil.

Wherever else is undesirable.

Let’s say that your buyer is noticing mildew of their house throughout humid climate. 

Organic development varieties on account of condensation.

You understand that the outside dew level generally will get above 70°F (21°C), and humid air travels proper by means of porous supplies like wooden and insulation.

In case your buyer likes to set the thermostat under 70°, when the humid air hits a wall floor under its dew level temperature, condensation will type, resulting in this development.

This could occur inside a wall the place it might go unnoticed for a very long time. 

Is the reply a dehumidifier? 

A dehumidifier will assist however solely treats the signs, not the trigger, by drying the inside of the constructing. 

The issue is excessive dew level air from exterior is getting inside.

The home must be air sealed.

If it by no means had an efficient water vapor barrier, comparable to home wrap, put in, this may very well be a significant undertaking.

As an HVAC technician, that is most likely exterior your scope of companies.

However understanding outside dew level will allow you to diagnose the issue accurately and level your buyer in the proper course.

It’ll additionally arm you with a scientific cause why your buyer mustn’t set their thermostat so low as a result of it invitations condensation to type.

Indoor Dew Level

Identical to outside dew level that’s above 63°F (17°C) is taken into account excessive, the identical applies to indoor dew level.

The truth is, just a few years in the past, ASHRAE revised its Commonplace 55 Thermal Environmental Situations for Human Consolation, which now states that indoor dew level shouldn’t be larger than 62.2°F (16.7°C) to stop mildew.

Previous to that, it solely used relative humidity as a metric.

In Measurequick, you may change the Firm Vast Settings “Air Moisture Indicators” from the default Moist bulb to the dew level.

I counsel making this alteration when you have the authority to take action.

What can inside and return air dew level let you know?

It’ll let you know how humid it’s in the home.

If you’re on a service name the place the system will not be working, you’ll most likely discover a excessive inside dew level, particularly on a moist day with rain.

As soon as the system is repaired, indoor dew level ought to return to regular.

However if you’re on a upkeep, or a consolation session, taking an inside dew level measurement can determine a humidity downside that the occupant might not even concentrate on.

It’ll clarify why there’s condensation forming on the provision and even why mildew and organic development are forming on surfaces round plumbing, duct, and electrical penetrations that aren’t sealed.

For the rest of this text, I’ll go over numerous eventualities when checking dew level in 3 locations:

1. Return Grille.
2. Return plenum
3. Provide Plenum.

Return Plenum Dew Level

The return grille dew level and return plenum dew level usually are not all the time the identical. 

And when they’re drastically completely different, it is a big pink flag.

In lots of locations, the ducts run by means of unconditioned areas like crawlspaces and attics, which typically have larger dew factors.

Taking an preliminary indoor dew level studying at a return grille, you make discover a regular dew level of 55°F (13°C). 

Let’s say the return ducts run by means of an unconditioned attic to an air handler additionally within the attic.

Should you take a second dew level studying contained in the return plenum on the air handler, chances are you’ll discover a a lot larger dew level, maybe 65°F (18°C). 

That tells you that the return duct is selecting up moisture!

Keep in mind that dew level is an indicator of the precise moisture content material within the air.

How would a return duct decide up moisture? Via duct leakage!

You might say: “I might’ve come to the identical conclusion by measuring temperature as a substitute of dew level.”

But when the ducts run by means of a very popular attic, the air is more likely to decide up warmth conducting by means of the partitions of the duct, even when they’re insulated, thus not proving there’s leakage. 

Conversely, if the duct ran by means of a cool however humid crawlspace, you most likely wouldn’t learn a temperature rise (you would possibly even learn a temperature drop), however you undoubtedly would see a dew level distinction.

Keep in mind, if recent air is being launched into the return plenum you’ll learn a dew level distinction on the return plenum.

Perceive that duct leakage is a large supply of indoor humidity issues.

Provide Plenum Dew Level

I don’t usually use the phrase “minutia,” however once I do, I usually discuss issues like provide air dew level.

As heat air passes throughout the chilly evaporator coil, the air molecules come into contact with the coil fins, and the moisture that the air incorporates begins falling out.

Theoretically, the air is “saturated” as a result of it’s cooled under its dew level. 

When Dry Bulb and dew level temperatures are each the identical the air is saturated.

In actuality, not all of the air comes into direct contact with the coil. 

Among the air molecules go by means of or across the coil unaffected. 

That is known as “coil bypass”.

When that unaffected dry air then mixes again with the saturated air, the precise Dry Bulb could be 3-5° hotter than the dew level.

The air is near saturation however not fairly saturated.

Let’s take an instance with an air supply warmth pump in cooling mode.

Reference the image under. 

If Provide Dry Bulb is 54°F (12°C) and the Provide dew level is 52°F (11°C) this tells us that the evaporator is chilly and there’s little or no coil bypass.

The air is near saturation which is what we would like.

What if Provide is 59°F (14°C) however the provide air dew level is 52°F (11°C)?

What would trigger such a big separation between Dry Bulb and dew level?

There could also be a warmth strip financial institution caught on, reheating the air. 

Or there could also be air bypassing the evaporator coil, mixing saturated air with unconditioned air. 

This could occur if the blower pace is ready too excessive.

Typically the Provide air Dry bulb and dew level each learn excessive whereas nonetheless being inside just a few levels of one another. 

For instance, Dry Bulb could also be 58°F (14°C), and dew level is 56°F (13°C). 

This often signifies a excessive load on the evaporator, the place coil temperature is larger than regular however leaving air continues to be near saturation. 

The TXV is reacting to the excessive load.  However measuring the dew level can alert a technician that there’s a efficiency downside.

How To Get Began

The simplest technique to get began is to get a pair of Bluetooth hygrometers that connect with Measurequick.  Testo and Fieldpiece make some nice merchandise.

Discover a system cooling that’s cooling correctly and begin a Non- Invasive take a look at.

Then, word the temperature and dew level on the return grille, return plenum, and provide plenum.

Assume it by means of and be capable to clarify to your self why you see these variations.

Quickly, you’ll get to the purpose the place by taking the three dew level measurements alone, you’ll be capable to rapidly perceive how the system is performing.

Conclusion

Checking techniques utilizing dew level is fast and straightforward as soon as mastered. 

It’s non-invasive and doesn’t require the usage of gauges and even pipe temperature clamps. 

However it’s not an alternative choice to correct commissioning and benchmarking system efficiency. 

Consider it as a fast efficiency screening.

Should you see one thing irregular, examine additional. 

Understanding dewpoint is a key step to turning into a greater technician. 

Remember to use it and grow to be the perfect practitioner you will be.

Take a look at our dialogue with Tim DeStasio on Constructing Consolation