Ducted concealed indoor unit sweating! 2

[HVAC_Eng]

Hello everyone,

I'm troubleshooting a condensation issue and would appreciate any insights! Here's the situation:

-Load estimation: 10 kW (34,121 BTU/h) with an airflow requirement of 1800 m³/h (1060 CFM).
-Equipment: The customer has several concealed ducted indoor units, two of which recently started showing water droplets (sweating) on the casings.

Current Configuration:
-Outdoor Unit: 32,000 BTU/h rated capacity.
-Indoor Units (affected units): 1130 / 988 / 847 cfm (H/M/L) airflow capacity.
-Service Details: Technicians cleaned the filters, checked the condensate drain pan, and confirmed proper piping.
-Despite this maintenance, condensation still appears on the two units. I'm trying to understand if this could be due to undersizing or oversizing of the units, or if there might be other contributing factors.

Could an undersized or oversized setup be causing these condensation issues?
Might there be another underlying issue to explore?

Any advice or suggestions on what else to check would be greatly appreciated!

 

[goutam_freelance]

I presume this is a chilled water system.

Please check the chilled water inlet temperature.

If it is too low considering the indoor air dew point, then there may be condensation.

Also please check whether there is proper dehumidification of indoor air (if dehumidification is required).

 

[trashcanman]

The writer did not say it was a chilled water system. He mentioned an Outdoor Unit, which sounds like a DX split system.
Regardless there IS condensation as he stated, not maybe.
Did the technicians also check the condition of the cooling coil? If it is clogged with dirt, that would increase the air velocity across the coil. Too high and you will get blow by of the moisture condensed and will fly past the drain pan. Too low a flow rate can also cause a coil freeze up.
Is the drain pan sloped toward the drain? Does the drain actually drain water? It could be clogged also. Is the bottom of the drain pan insulated? If not condensation could appear on the bottom of the pan.
What are the air conditions of the entering and leaving air in terms of DB/WB temperatures? Have you plotted the process on the psych chart?
Always remember, the more details you provide, the more we can help.

 

[HVAC_Eng]

The writer did not say it was a chilled water system. He mentioned an Outdoor Unit, which sounds like a DX split system.
Regardless there IS condensation as he stated, not maybe.
Did the technicians also check the condition of the cooling coil? If it is clogged with dirt, that would increase the air velocity across the coil. Too high and you will get blow by of the moisture condensed and will fly past the drain pan. Too low a flow rate can also cause a coil freeze up.
Is the drain pan sloped toward the drain? Does the drain actually drain water? It could be clogged also. Is the bottom of the drain pan insulated? If not condensation could appear on the bottom of the pan.
What are the air conditions of the entering and leaving air in terms of DB/WB temperatures? Have you plotted the process on the psych chart?
Always remember, the more details you provide, the more we can help.

Hello, sorry for replying late.
It is Dx split system indeed.
Technicians did check the cooling coil and it's not clogged also they cleaned filters...

Outdoor unit : 16,030 / 32,000 / 38,000 btu/h

Indoor unit : Air Flow Rate 1130 / 988 / 847 cfm ; Max External Static Pressure ~ 0,609 inch wc

2 x Air terminals :
-Dimensions: Approximately 14.76 inches x 14.76 inches
-Airflow Rates: 294 / 406 / 559 CFM

The unit's load estimation : ~ 34100 btu/h and 1059 cfm

So the duct sizing based on 1130 cfm. Each air terminal with 565 cfm

Condensation is forming on the reduction and rectangular flexible duct connector and the space where the unit + ducting installed comes with curtain walls. The ducting installed between suspended ceiling and metal framework.


Ducting details : 2 x 45° rectangular elbow / Rectangular To Round Transition / 45° circular elbow / 90° round takeoff for one air terminal


Also the unit and the 1st reducer is located above an unconditioned space (wc).

I hope these details are enough
Attached is how the supply ducting looks like
Edit : I did not do the load estimation neither the equipement selection!

 

[goutam_freelance]

Are the fans working properly? If airflow is too less then there may be higher temperature drop across the air cooler.

In addition if the flow inside the cooler in uneven (i.e. some air is bypassed) then there may local low temperature where airflow is less.

If these lower air temperatures reach dew point then there may be condensation.

If possible please take temperature measurements for air after the cooler at different points within the cross section of cooler exit.

 

[LittleInch]

So is this sweating on the outside of the cold air casing?

What insulation does it have? - clearly not enough if this casing is exposed to warm high humidity air.

Got any photos or details of the ducting at the point where it is sweating?

What is air temp in the duct?

 

[HVAC_Eng]

So is this sweating on the outside of the cold air casing?

What insulation does it have? - clearly not enough if this casing is exposed to warm high humidity air.

Got any photos or details of the ducting at the point where it is sweating?

What is air temp in the duct?

Unfortunately i did not visit the project (located in another city) plus i joined this company recently... Sweating occurs on the reducer (from outside obv) just right after the indoor unit. Since i didn't visit the project all the details i provide i heard from the technician in charge. The drain is not clogged, filters cleaned, ducting insulated (with glass wool). But since theis happens only in one unit from 6 units in total, ig it is the location of the unit is the reason (unit and the 1st reducer is located above an unconditioned space (wc)). Also in other project (Same design , AC systems .... both gas station) same problem occur in the same unit ...........

 

[LittleInch]

More insulation?

Sealing the insulation from the unconditioned space?

 

[HVAC_Eng]

More insulation?

Thats eventually what they gonna do.

Sealing the insulation from the unconditioned space?

I will check that tomorrow.

Tyyy

 

[3DDave]

Isn't the basic cause that the surface has been cooled below the dew point?

Make sure humid air is not allowed to make contact with a surface that is below the dew point of the humid air.

If insulation is used, ensure that humid air cannot infiltrate the insulation, allowing the insulation to become wet from condensation.

 

[MintJulep]

If "it used to be fine", and now it's not then look for what changed.

For example, the sealing around the curtain wall penetration.

A degraded seal could allow outside air to contact the surface and condense.

 

[HVAC_Eng]

Hello, ty all for helping <3
We contacted the manufacturer, and they sent a technician to inspect the issue. He confirmed that the location of the unit is likely causing the problem and recommended adding insulation as well as installing shades or blinds on the curtain walls.

 

[Compositepro]

Hello, ty all for helping <3
We contacted the manufacturer, and they sent a technician to inspect the issue. He confirmed that the location of the unit is likely causing the problem and recommended adding insulation as well as installing shades or blinds on the curtain walls.

How are shades on curtain walls supposed to help with condensation? As 3DDave has said:

"Isn't the basic cause that the surface has been cooled below the dew point?

Make sure humid air is not allowed to make contact with a surface that is below the dew point of the humid air.

If insulation is used, ensure that humid air cannot infiltrate the insulation, allowing the insulation to become wet from condensation."

You have to insulate the duct with impermeable insulation, or keep the ambient humidity dewpoint below the duct temperature using adequate ventilation. These are very simple and fundamental concepts. Do not complicate and confuse the issue actions that do nothing to address these issues. It seems shades could only make the problem worse.

 

[Mark_B]

I see you have resolved the situation but wanted to contribute...

In situations like this I found it useful to start by taking surface temperatures of problem surfaces, sometimes with the help of an infrared camera for efficiency, then using a Fluke 971 or similar to check dew point of surrounding air. As others stated if something did not sweat then starts to sweat suddenly it means something changed.

Examples I have experienced are poorly applied insulation that failed/opened to air after a few months, changes in space ventilation which raised the dew point, or some airflow restriction inside the equipment that significantly lowered supply air temperature, thus lowering duct surface temperature. In your case I would be suspicious of changes in space dew point or reduction of supply air temperature, as you seem to state the ducting is not insulated.

It is common practice to insulate duct in unconditioned spaces so that is kind of a surprising design decision.