In the designed AHU a cooling coil operates also in order to control the humidity ratio. During the summer season, because of the required dehumidification after the cooling coil the air temperature can become uncomfortable (16°C or even less). To avoid discomfort for the occupants, after the cooling coil there is a reheat coil. The reheat coil controls the supply temperature, which should be about 20°C.
The cooling loads are balanced by radiant floors.
Since the building is going to be used for healthcare purposes (in particular for seriously ill people), avoiding discomfort conditions is very important.
I think that in the proposed model I shall consider such a control strategy.
Since reheating causes an energy demand increase, I am wondering whether I could consider it in the baseline model. Shall dehumidification and reheating be modeled identically in the proposed model and in the baseline model?
The baseline system is System 3 – PSZ-AC.
Best Regards
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Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5916 thumbs up
September 22, 2015 - 2:00 pm
ASHRAE 90.1 Appendix G is unclear in how to appropriately account for humidity (both humidification and de-humidification). As stated in ASHRAE 90.1 Table G3.1.1 (b), all conditioned spaces shall be modeled with temperature and humidity control setpoints the same for the Proposed and Baseline Case, however there is no other guidance/requirement for modeling humidity. Humidification should therefore be modeled the same in the Baseline and Proposed Case unless an Exceptional Calculation Method is used. The Exceptional Calculation should justify the use of the Baseline (including evidence for what is standard practice in the project location for the type of application).
It is not always possible to model identical forms of humidity control. In that case, it is recommended that the same fuel source for humidity control be modeled in the Baseline and Proposed Case. For example, if electric reheat is used, that would be modeled in both cases. If fossil fuel is used for humidity control, that would be modeled in both cases. You should provide additional justification if including reheat in the baseline.
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
September 23, 2015 - 2:56 am
This is a great thought provoking question. Comfort vs. energy savings. Which is more important?! I think the incentive to save energy is already well established. If ASHRAE in this case would refuse to make the baseline reheat, it is saying that user comfort should be degraded to save energy! I don't think that is a good approach.
In the LEED system we will punish the design for bad comfort, but do we punish the baseline also?
In the spirit of LEED I would make the baseline reheat and make my case to the reviewers. Perhaps, this is something for the 90.1 commitee to consider that as much as you may want to seperate 55 and 90.1 to do their respective jobs, sometimes you just can't.
Francesco Passerini
engineer90 thumbs up
September 23, 2015 - 5:45 am
Thank you for your answers. I have considered another aspect: including reheat in the baseline could be impossible because of control problems. I mean, System 3 is CAV, i.e. the zone air temperature is controlled only through the supply air temperature (there is no possibility to modulate the air flow rate). If I reheated the supply air, the setpoint zone air temperature could be unreachable.
I'm going to see how much the reheat is important (from the point of view of the energy requirements) in the proposed model.
Best Regards
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
September 23, 2015 - 5:52 am
Yip. Try and see if you can "cut it off" onto a seperate system using one of the exceptions. So like if a zone sees very different occupation schedules, but hangs onto the same system, you could have the baseline serve it with a different system. Similary, there are exceptions for schedules regarding loads, outdoor air requirements, and so on.