Hotel guest room using natural ventilation

The fans in the Proposed HVAC system would be allowed to cycle since outdoor air is supplied by natural ventilation, while the Baseline HVAC system would supply the outdoor air mechanically and therefore require continuous fan operation during regularly occupied periods. The volume of outdoor air should remain identical and in both cases would still introduce an identical load on the system, but the method of delivery would be different. The baseline would be through the mechanical system and the Proposed through constant infiltration. This methodology is currently allowed within the LEED energy modeling protocol

Is the methodology that you proposed supported by some statements of ASHRAE 90.1, Marcus?
In the past I modeled natural ventilation in the same way in the proposed model and in the baseline model. I modeled it as a "special infiltration", i.e. the ventilation doesn't occur when the outdoor conditions are extreme or when the building isn't occupied. As flow rates I considered ASHRAE 62.1, althought natural ventilation is obviously much less regular than mechanical ventilation.

I think the best way to think of window ventilation is as a DVC system. As such, I believe the baseline should be modeled with a mechanical delivery of ODA as a DVC such as to take account of heat recovery, which windows can't. The window ventilation should be modeled as a DVC system that responds such as to represent how the occupants would react and consider at a minimum thermal comfort and CO2 concentration. The infiltration should modelled identically.

In Francesco's way both models must condition the same amount of ODA, but baseline can't take advantage of heat recovery to precondition outdoor air for free which is the one of the big reasons that window ventilation sucks...the other being the thermal comfort degredation.

The energy credit should keep its nose out of the comfort credit and the comfort credit should include air quality as a prereq and not just thermal comfort. If you have NatVent, you should be required to demonstrate this.

No one opens windows enough in winter.

With natural ventilation you don't have heat recovery but you save the energy that is needed by the fans. Advantages and disadvantage depend on the climate.

Although true that you save fan energy and maintenance, in the balance I find mechanical ventilation with heat recovery more sustainable...for most climates and most projects (especially those in CZ 4 and greater). I find you save sooo much energy in the two months of cold winter that it is simply difficalt to "make up" that amount of energy over the rest of the year. Take fan energy, as an example: as we move into the future EC motors become common place (already required by legislation in the EU), frequency converters as well, i.e. VSDs, and the recovery rates of heat recovery devices for larger commercial applications are pretty darn good. Energy saving aside, people have been kidding themselves about the IAQ of window ventilated spaces for a long time. And occupant comfort is seldom comparable.

Last but not least, I would point out that NatVent only works when properly designed...very few know how to do this IMO, and getting it done cheap and fast is not possible, so its not ordered that often. When was the last time you saw a NatVent "properly designed" building? Painting pretty arrows on how you expect the flows to go based on perfect (unpredictable) conditions does not count. End of rant. Sorry.

Your statements are correct and interesting, Jean. But if we consider the difference between theory and practice, I saw some pictures relative to some AHUs that were extremely dirty (actually the images were pretty terrifying...). If AHUs are not cleaned properly they don't guarantee air quality.
Anyway, I would like to understand whether the approach that I used for the baseline model is wrong according to ASHRAE.
Best Regards

The methodology is allowed by LEED as the adopting authority. I am not aware of it directly mentioned in 90.1.

Thank you, Marcus.
What do you think of the following approach?

In the past I modeled natural ventilation in the same way in the proposed model and in the baseline model. I modeled it as a "special infiltration", i.e. the ventilation doesn't occur when the outdoor conditions are extreme or when the building isn't occupied. As flow rates I considered ASHRAE 62.1, althought natural ventilation is obviously much less regular than mechanical ventilation.

As long as you model the special infiltration the same in both models there should be no problem with the approach described.