RTU, Plenum Return & Humidity Problem 2

[bob090470]

I'm working with an Owner that is having humidity problems in an existing office building (small - 5000sf sigle-story office in Florida). Office currently has two RTU's, one serving an interior zone and one a perimeter zone. Both units share a common plenum for return. To be specific, each unit is a Trane Model WCD075C400BC.

I work in construction (so please excuse my basic understanding of unit operation) and rely on engineers and contractors for input, but in this case I can't seem to get a straight answer. I'm really just wondering about the basic operation of units like these. It seems to me that diagnosing the humidity problem requires an understanding of how these units operate.

Is the OA damper fixed on units like this, or do we have the ability to open it during occupied mode and close it during unoccupied mode?

During occupied mode, I assume the coil shuts down when space temperature is satisfied, but does the fan continue to run to meet OA requirements (filling the space with humid outside air)? If so, would it be OK to limit fan operation to only when cooling is called for?

During unoccupied mode, since we don't have to meet OA requirement, I assume it would be OK to only run the fan during cooling? I also assume it would be OK to close OA damper (assuming it's motorized)?

If closing the damper or limiting fan operation aren't options, how the heck can you control humidity with units like these?

 

[lilliput1]

If this is a new installation, the Owner should hold the HVAC design engineer to fix the problem. You hit the nail on the head on what is causing the humidity problem. This is the big disadvantage of DX units compared to chilled water cooled systems where you can design variable volume with reheat at minimum turndown.

The OA damper should be motorized to shut down when the unit is off. Additional controls would be required to shut them down when the unit cycle on during setback (unoccupied) hours.

By code 20 CFM of outdoor air per person must be supplied for ventilation. Also the OA quantity should exceed the exhaust air requirement by approximately 0.05 CFM/SF to pressurize the building and avoid infiltration. Vestibule at entrance would be good. I am not sure if they are common in Florida which has no winter, but a vestibule would be just as effective in summer if it is sized such that only one door would be opened at a time.
Why only one unit for the perimeter? Each exposure (N,S,E,W) ideally each should be on a separate zone.
If the occupancy is variable and there is a concentrated group of people at random or infrequent schedule consider adding demand control ventilation.
The design OA CFM should be checked and also the air balancing report.
The interior whould require constant fan at occupied mode to bring in OA for ventilation. If windows are openable, the perimeter RTU may be set for intermittent fan operation with the OA damper closed when the unit is off. Openable windows are considered outdoor air ventilation source.
The engineer should verify that the units are not oversisized and that they have multiple compressor or unloading capability. Hot gas reheat may be required to provide dehimidification during low sensible loads.
Carrier units have dehumidification and demand control ventilation options in their DX rooftop units.
Last resort maybe to bypass the supply air direct to the return duct as room temperature control while controlling the cooling to maintain constant 55°F discharge 55°F air temperature and adding hot gas reheat.

 

[MPcontrol1]

well put lil but that brings me to a question though. Alot of Test and balance will set minimum requirement for the osa dampers. Why not just put a CO2 sensor in the return duct therfore the dampers wont be set on a minimum basis? I dont know if he has a current control system on the unit but instead of having the minimums cracked at 10 % at all times the co2 will bring in ventilation upon demand?

 

[lilliput1]

If the space served is mixed occupancy, you would want CO2 sensors located at each critical space the use the BAS to control for the room with the most CO2.

You would want balacing contractor to set the minimum OA to equal the exhaust airflow + pressurization of about 0.05 CFM/SF.You would have to program in control override when the system is on economizer control. Also you would have to determine how to control the return fan to allow Demand Control Ventilation. With CV fans this may involve using space pressure sensor to modulate the relief air damper while the OA dampwer respond to the DCV control.

 

[atlas06]

great answers guys,
One thing I would consider is to add a humidity sensor in the return to override discharge temp sensor and subcool so that space humidity is achieved and use hot gas reheat.
The other thing is, depending on your local code official, he may let you use ASHRAE 62.1 instead of more stringent IMC. I understand that IMC is about to add an exception to the ventilation tables that would allow ASHRAE 62.1 ventilation rates.

Another thing I would look closely at is: since this is an existing building (in humid Florida), there may be excesive infiltration to the plenum. The return air may be bringing excessive infiltration from the plenum, in addition to the OA. You could test the system in "ducted return configuration" and monitor your return air humidity levels. Use a large solid canvas duct of sort to extend the return air point down to the space and see what happens to your return air humidity.
Does syour building have a vestibule? it may be cause for excessive infiltration also.

If your engineer has sized his fan without return air plenum in his calcs, you could convert your exterior zones into a ducted return should infiltration happen to be a problem.

Good luck

 

[bob090470]

Wow, first time i've posted anything on a forum and very appreciative and overwhlemed with responses. thank you. You've provided alot of information and probably assumed I know more than I do. But let me try this:

With this equipment (during occupied mode): it sounds like the fan runs continuously to maintain OA requirements, but the coil will denergize if space temp satisfied? Sounds like overcooling (how do I sample OA humidity and make sure I'm only overcooling when necessary?) and adding a duct heater for re-heat is the only good fix here?

With this equipment (during unoccupied mode): Install a motorized damper (if unit doesn't already have one) and set controls to close damper during unoccupied times? Then run unit in normal occupied mode but with higher space temp setting?

I assume that a CO2 sensor requires a modulating damper feature? Opens and closes damper based on CO2 level?

For units like this, is a fixed damper standard equipment?

Sorry to be so ignorant....

 

[BishopMJ]

As an engineer in South Florida, I deal with this issue all the time. The hard and fast solution in our climate is to install a 100% outside air unit to operate during occupied hours, effectively seperating the space cooling and ventilation tasks. Trane, Aaon, and Addison are all frequently used locally for this application.

However, 100% OA units are expensive and generally don't make sense unless you have a high percentage of outside air such as in a school, auditorium, or locker room facility. For your application, you are correct in recognizing that the RTU's should never be set to run "fan only". That is a recipe for disaster humidity-wise. Your idea to add a duct heater for re-heat is a step in the right direction, but Florida's Energy Code does not allow simultaneous heating and cooling of the same airstream with a heat strip, unless the unit is a 100% OA unit (there are other exceptions - ref. Chap 13-407.1.ABC.2.3.1 Dehumidification).

The best solution would be to retrofit the RTU's with Trane's hot-gas reheat circuit and with a motorized damper to shut off the outside air damper when the unit is off. The motorized damper is now a code requirement (See 13-409.1.ABC.3.3 Shutoff damper controls.) Take some measurements afterwards to see if that solves the problem. Ducting the return might be the next step, or see if you have any leaks in the building envelope.

 

[torpor]

A good point - BishopMJ.

I am facing a similar situation (as a designer). Here's what complicates my scenario. I have an added economizer operation. The trouble with this is the amount of OA will then be governed by the eco. options. If the economizer control is dry-bulb type it would allow OA with all the outside RH into the space (Is that so?). If its an enthalpy type control it would have control over the humidity introduced to the space. With no reheat in either case.

The trouble is we left it as an open spec on the option of economizer control. I am not sure whats the control on the RTUs now - but the space almost starts drenching. And this is in NJ.