I'm preparing a summary for the LEED reviewers regarding a model and would like your thoughts:

1.1 Introduction:
A naturally ventilated building with radiant floor heating has some definite nuances regarding the modelling of both the baseline and designcase models according to the standard 90.1. All aspects of this methodology have been pear reviewed and discussed on the LEEDonline forum and received the blessing of the experts.

1) The required, mechanically delivered, minimum outdoor air rates in both models is zero.

2) Outdoor air ventilation is achieved in both models with the same natural ventilation system.

The opening area of operable windows is maintained, as far as possible, within the bounds of the maximum window to wall ratio of 40%, equal in both baseline and designcase models. The control algorithms and setpoints that mimick user operation, such as CO2, temperature and draft, are identical in both models. However, ventilation rates via openings may differ slightly due to different occurring indoor and outdoor temperature differences occurring at different times in the baseline and designcase models. This is normal and would also apply to an economizer function.

The simulation uses natural ventilation modelling techniques far superior to what is foreseen in Standard 90.1, such as calculated wind pressure driven airflows, multinodal-bulk air flow analysis, stack effect driven airflows, user window operation based on CO2 levels, temperature of zones and outdoor air and draft control.

3) As there is no minimum mechanical outdoor air ventilation requirement, it follows that the mechanical systems are solely there for heating (and theoretically cooling). As such, the point G3.1.2.5, which requires the fans to be continuously on during occupied periods, becomes inapplicable. Fans in both baseline and designcase models will be set to cycle to meet heating or cooling loads all of the time. This way a fan simply blowing air in circles during occupied periods is avoided.

4) The baseline mechanical cooling system is also modelled in the proposed designcase model, including all the components such as the economizer and controls with setpoints. This is required by ASHRAE 90.1.

5) Increasing capacity of a radiant flooring heating system to reduce the unmet load hours has little to no effect, in that the radiative area remains the same. This limits the amount of heat transfer via convection to the room air and as such there is not (or very little) effect in increasing the hot water mass flow capacity of the system. As a consequence, it is the baseline heating sizing parameter that is reduced from 1.25 to 0.90 in order to bring the unmet load hours of the baseline system in-line with that of the design case system. Naturally, the baseline’s air system has a faster response time to meeting loads than the designcase radiative heating system. As both systems are sized whilst windows are closed, neither model’s HVAC heating system will meet the loads all of the time for when occupants open windows in winter. This is as purposed.

6) The tolerance for the heating setpoint not met hours recording is the same in both baseline and designcase models and is equal to the throttling range of the designcase radiant floor system at 1 deg C.

7) All zones have Cooling Setpoint 42ºC. The real building has no mechanical cooling, and therefore no hard defined cooling setpoints. This is an acceptable work around for satisfying ASHRAE 90.1 requirements for a building without mechanical cooling, but natural ventilation cooling. It is intended that the mechanical cooling (modelled as present in both baseline and design case models) does not jump on. As a matter of fact, because the radiant floor system is controlled via a zone air temperature sensor, some serious control issues would arise, when for example the cooler is blowing in cold air, in that the floor would continue heating more and more and the cooler would cool more and more...the two systems would fight one another. Furthermore, it has been checked that user comfort is maintainable with the natural ventilation strategy for almost all of the time...the exceptions are within acceptable limits. A last point is that the users would deploy manual shading to control solar heat gain, which we may not include in the models as per ASHRAE 90.1, because the shadings are not automatically controlled, so the zones are expected to be a little warmer than the reality.