Hello everyone,
I am currently working on the energy model of an apartments building in Miami. The building has rooftop units to provide OA (which provides 30% more than the 62.1 requirement) and control the humidity, and water-cooled packages in the units to control the temperature.
I have doubts about modeling the humidity control system in the baseline model, which is a system type 2. I have found this note in the LEED v4 reference guide (table 3, minimum energy performance): “Humidification must be modeled identically in the baseline and the proposed models, since it is not addressed in Appendix G. Use the exceptional calculation method if claiming savings.
If the proposed design includes dehumidification controls, they must be modeled as designed. Dehumidification controls may be modeled in the baseline only if one of the exceptions to Section 6.5.2.3 applies. Exception (d) for process dehumidification does not apply to computer rooms.
Table G3.1.4 requires that identical schedules be used in both models, and this includes humidity setpoints. A problem may arise if the proposed building has a dedicated outdoor air system (DOAS) that maintains proper humidity. PTAC or small DX systems in the baseline design may not be able to maintain both temperature and humidity simultaneously in the same way that the proposed system can. The project team may then incur a penalty for higher humidity levels in the baseline building.
In this situation, model a DOAS in the baseline design using the same volume of outdoor air as for the proposed design, but with the same efficiency and efficiency curves as the baseline HVAC systems. Additionally, the baseline fan power allowance would be separated between the DOAS and the baseline system using the same ratio as the proposed system.”
My questions modeling the baseline are:
- Should I model the same OA in the DOAS for the baseline model? I mean, with 30% excess over 62.1 requirement.
- Should I autosize the cooling and heating coils in the baseline DOAS or put the proposed capacities?
- Do I calculate the cooling coil and heating coil DOAS efficiencies with the same table I used to calculate efficiencies for system 2? or from table 6.81A (as a rooftop unit)?
- I don’t understand how to split the fan power in the baseline, between the DOAS and the internal units. Any advice?
- The rooftop unit in the proposed model has filters and heat recovery. How can I adjust the pressure drop in the baseline DOAS?
Thank you,
Tyler Thumma
7GroupLEEDuser Expert
61 thumbs up
August 3, 2021 - 12:42 pm
The note you reference in the LEED v4 Reference Guide refers to humidification, not dehumidification. Dehumidification control would not be modeled in the Baseline case, and there would not be any Baseline DOAS. Note that even ASHRAE 90.1-2016, which added Section G3.1.3.18 to address dehumidification controls, does not include those controls for Systems 1-2, and only allows dehumificiation control for other systems if the Proposed design includes humidistatic controls. This should eliminate your questions 2-4.
For question 1, the Baseline should be modeled using the 62.1 minimum requirement, not including any 30% increase.
For question 5, Systems 1-2 do not include pressure drop adjustments, so you would not be able to account for the filters and heat recovery for these systems. If you have any nonresidential systems in the Baseline case by applying G3.1.1 exception (a), you can apply the appropriate pressure drop for the filtration to the outside airflow of the Baseline system (since the design filters are used for outside air only), but a pressure drop for heat recovery may only be applied if heat recovery is required for the Baseline system.
Harini Goteti
September 11, 2023 - 12:01 pm
Hello All,
I have electric humidifiers on a DOAS system that is providing ventilation for a historic offices/museum/atriums style building with district heating and cooling.
I am wondering what the baseline humidifiers shall be like. ASHRAE 90.1-2010 doesn't have a guideline, however, 2016 recommends using adiabatic humidifiers for the baseline systems. My question is what fuel source are we expected to use? Electric (same as proposed) or District loop? I'm assuming that it might be a hot water loop, need some guidance on this. Also, the discussion about humidifiers being process loads that need to be identical, might not be possible, since system 7 bunches everything together into larger systems per floor, and assigning a humidifier electric or otherwise will result in huge savings.
Also, using eQuest for this exercise.
Thanks all in advance,
Harini
Tyler Thumma
7GroupLEEDuser Expert
61 thumbs up
September 29, 2023 - 9:50 am
The Baseline humidifiers should be modeled using the same fuel source (electricity) as the design. It is possible that the spaces requiring humidity control would qualify for a secondary Baseline system using the exceptions of G3.1.1, and then only these secondary systems would need to be modeled with the humidifiers.