Office humidity complaints

[carlosgw]

North east - hot fairly dry summer
Existing building newly renovated last few months. Office space with rooftop units with gas heat.

Occupants complaining about musty smell and humidity in in the 60% range.
Space maintained at about 74 deg.

RTU discharge temperatures are low 48-49. So they should be dehumidifying.

Ideas to consider or look for?

 

[itsmoked]

1) Look for actual condensate discharge at the evaporator drain.

2) Then look at the actual pan while the system is running. Is there water? Is it entirely draining or is the pan mis-angled and there's a large standing lake?

3) Is what's dripping into the pan in line with what you see dripping from the drain?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Compositepro]

This can be caused by having too much A/C capacity and as a result the units only run for a short time. The temperature reaches setpoint before much humidity can be removed.
Running a negative pressure in the building can cause humid air to infiltrate the walls and condense water.

 

[itsmoked]

Excellent points Comp. All the water condensed on the coils can just sit there still on the coils and return to the air if the coils don't reach a saturated point allowing steady-state dripping and a condensate drainage flow.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[willard3]

What's the air velocity of the coil? Are you entraining condensate with high velocity air?

Goes back to what is coming from the condensate drain.

 

[GT-EGR]

How big is the unit? How big is the office?

Is it a single zone or does the unit serve VAVs or reheat coils?

It doesn’t sound like the condensate, if you measured it coming out of the unit at 49 degrees then that air has successfully been dried, and it likely isn’t carrying those water droplets all the way to the space. Is the ductwork at the beginning of your supply rusted or wet?

Likely candidate is that the unit is oversized. Sure the unit got tested and discharges at 48, which is successfully dehumidified, but when and for long does it do that - was this during normal operation or was this during a test? To further pick this as the theory what you’d want to do is trend the supply air temp over an entire day, and also trend run time/duration.

The other item also mentioned here that makes sense is Your building could be under negative pressure. Do you have a lot of exhaust systems, and was this accounted for when the rooftop unit minimum outside air was balanced? Does your unit have a relief fan controlling the space pressure? Or do you have any building pressure control at all? These past couple weeks have been real humid.

Another question is the placement of your thermostat. Does it represent your typical zone condition, or is it in a location that gets cooled easy or that doesn’t see the cooling loads the way the rest of the zone does.

60% RH is not bad this time of year, so I’m not sure you can directly blame the musty smell and complaints on that alone - although it would make a smell more noticeable. Has the thermostat been checked/calibrated?

 

[dbill74]

During the renovation, were changes made to the HVAC system? Has the space usage or occupant density changed?

I agree with GT-EGR that the musty smell is not a direct result of the humidity. Was the space empty prior to the renovation? If so, there could be mold in the walls or flooring causing the smell.

Checking the drain pan and drain piping will tell you if the RTUs are adequately removing moisture from the air.

What is the outside air temperature and humidity like?

Check the controls, make sure OA dampers are closing when the building is unoccupied. Is the supply fan running continuously and the compressor cycling? This will bring in unconditioned OA. Does the unit have an economizer sequence? Is economizer working correctly?

I was just investigating a similar situation in SC last week. In my case, a walk through of the buildings made the cause very clear, oversized units. There is a catch, the spaces weren't being utilized as the designer was told they would be. In one case, the space was designed for two dozen people and lots of computers and a server rack in the middle. A year after turnover, they had less than 10 people working in the space and less than 20% of the computers installed.

In my building, the occupants would kill for 60% RH versus the 70%+ they've been getting.

 

[itsmoked]

Earth to carlosgw.. Are we just talking to ourselves?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[trashcanman]

First of all, has a complete Test and Balance procedure been done?
If not, have it done and reviewed by a competent HVAC engineer. Then, address the complaints.