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.
Christopher Schaffner
CEO & FounderThe Green Engineer
LEEDuser Expert
963 thumbs up
July 15, 2013 - 3:15 pm
My guess is that this will give the reviewer a headache. Too much irrelevant info. Questions I'd want to see answered if I were the reviewer:
1) "The simulation uses natural ventilation modelling techniques far superior to what is foreseen in Standard 90.1" - really? like what? tell me about the software.
2) is your baseline also passively conditioned? Why? Baseline should still be one of the eight Appendix G systems.
3) I don't agree with the assessment that "G3.1.2.5, which requires the fans to be continuously on during occupied periods, becomes inapplicable." On multiple projects with natural ventilation through windows, we've modeled the baseline fans to run continuously, while the proposed case fans cycle. This feels like cheating, but we've specifically gotten this approach approved.
4) I'm confused about the radiant floor. If I understand, you are purposely undersizing the base case so that it has as many unmet load hours as the proposed case. If you are expecting lots of unmet hours, why not just reduce the heating setpoint instead? Also, I'm not buying that you meet comfort criteria in EQc7.1 if you have too many unmet hours.
My two cents.
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
July 16, 2013 - 3:40 am
Yes! More of this! In principle all easy to explain questions...I just want to pre-empt them. Usually, it's just Marcus that agrees to everything, ha, ha.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5911 thumbs up
July 16, 2013 - 12:19 pm
I disagree with your last statement! OK you asked for it! :-)
Title - There is a major difference between engineered natural ventilation systems (CIBSE) and operable windows (ASHRAE 62.1 paragraph 5.1). One is generally designed to induce air flows and the other just allows them when outdoor conditions are right. I would call what you are doing Window Ventilation or ASHRAE 62.1 paragraph 5.1 or some other way of making sure it is clear that we are just dealing with operable windows. Technically they are both "natural" ventilation but calling it natural ventilation raises many flags in a LEED review. Be clear about this up front.
Introduction - the LEED Reviewer does not care that you have posted issues in LEED User. GBCI even has a stock language comment that LEED User is not a valid way to get a formal response on issues. I would not make reference to these discussions as they hld no weight with the reviewer. (peer review, not pear review unless you consider us fruit!)
2) Be clear about the fact that the natural ventilation system is via operable windows. You should not compare the software to 90.1 as that is an apples and oranges comparison. Mention the software (assuming Energy Plus) and simply highlight its features relative to modeling the strategy in question.
4) Not sure how you can model a baseline economizer and claim that there is no baseline mechanical ventilation but maybe you can?
7) Make sure to read the methodology for modeling natural ventilation system in the Advanced Energy Modeling Guide for LEED in the Appendix. It does require a thermal comfort comparison and it sounds like you have completed this analysis. If so you should submit the results. Finally you can model manual shades in the proposed if they are identical in the baseline. You just cannot claim savings for manual shades.
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
July 16, 2013 - 3:29 pm
A bit of fruit and veg is good for project health. These comments are worth their weight in gold. I will try and work them all in.
A point that I'm scared to try on, is the shading. The first review asked me to eliminate quite a lot of research and effort in modelling the shading of a specific species of trees and neibouring building structures, even though I was not taking any credit as they were in both models (as is now available in ASHRAE 90.1 - 2010).
Your last comment is kind of a dilema for me. I had the credit for increased ventilation ticked off in the review, but in the mean time I have reworked the entire LEED project so that the same achitectural plans are used consistantly across all the credits (due to the Acoustics prerequisite for schools where I was supplied with a plan with the acoustic panels just before submission (let that be a warning!) on a newer layout). So I need to re-submit pretty much everything.
So for thermal comfort I'm a bit stuck. The windows have no tilt function which means the low angle solar heat gains in winter are difficalt to catch without accepting too many unmet heating hours from using the windows in winter. Consiquently, the zones are often uncomfortably hot as seen below.
** Warning ** More than 4% of time (350.4 hours) uncomfortable in one or more zones
** ~~~ ** Based on ASHRAE 55-2004 graph (Section 5.2.1.1)
** ~~~ ** During Environment [01/01 - 12/31]: KITA
** ~~~ ** 563.7 hours were uncomfortable in zone: 118371
** ~~~ ** 483.9 hours were uncomfortable in zone: 118394
** ~~~ ** 403.7 hours were uncomfortable in zone: 118410
** ~~~ ** 565.7 hours were uncomfortable in zone: 118311
** ~~~ ** 1110.9 hours were uncomfortable in zone: 118271
** ~~~ ** 466.2 hours were uncomfortable in zone: 118145
** ~~~ ** 487.8 hours were uncomfortable in zone: 118250
** ~~~ ** 1501.2 hours were uncomfortable in zone: 118212
** ~~~ ** 658.9 hours were uncomfortable in zone: 118220
** ~~~ ** 730.1 hours were uncomfortable in zone: 118228
** ~~~ ** 936.5 hours were uncomfortable in zone: 118288
** ~~~ ** 772.2 hours were uncomfortable in zone: 118238
** ~~~ ** 742.3 hours were uncomfortable in zone: 118320
** ~~~ ** 2515.6 hours were uncomfortable in zone: 118129
** ~~~ ** 1915.8 hours were uncomfortable in zone: 118434
** ~~~ ** 632.6 hours were uncomfortable in zone: 118443
** ~~~ ** 545.4 hours were uncomfortable in zone: 118364
A possible solution would be to drop the cooling setpoint from 42ºC (where it never jump on) to 26ºC (in both models). This will bring control problems for the radiant floor system where the systems may start fighting each other.
Sorry for taking up so much space on the forum.
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
July 17, 2013 - 4:22 am
By modelling the window ventilation in both models, I am trying to avoid the exceptional calculation. Why?...
Because, I have trouble defining how, when the windows remained closed in the baseline and the outdoor air requirements stay zero...what would happen? I find myself moving further away from a valid comparison (I start comparing a mechnically cooled building with a window cooled building).
Secondly, the design case still has a implied minimum outdoor air requirement in that the window ventilation also has the implied function to keep the CO2 level acceptable. If this is to be applied on the baseline system, one would have to create a CO2 DCV setup for the baseline model.
Thirdly, I have a bit of a bad consious with the definition of "hybrid". If a cooling system was retrofit, because user comfort is compromised too often (as is the thinking behind this), then would a "code conform" retrofit necessarily require window contact sensors to turn off mechanical conditioning equipment if windows are utilized?
Is the idea that a window ventilated project is always modeled as if a retrofit has already taken place? What about simply letting the mechanical systems work in parallel with the window operations? This is not truely hybrid, but could still be seen as hybrid.
Bottom line is that I can model any scenario...I just need to get the right way without bothering my consious...starting with modeling the proposed case as designed.
Christopher Schaffner
CEO & FounderThe Green Engineer
LEEDuser Expert
963 thumbs up
July 18, 2013 - 11:36 am
1) ventilation rates need to be equal in both base and proposed, regardless of ventilation method - Unless you have DCV.
2) agree with Marcus's point about terminology. We use "natural ventilation" to mean two different things - Passive conditioning or operable windows.
3) we generally model "operable windows ventilation projects" as though the windows are closed. We ignore the window ventilation in both base and design.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5911 thumbs up
July 18, 2013 - 12:24 pm
Not sure why the reviewer would question the shading from the surroundings, especially since 2010 will require it.
I think it is better to have the high heating set point. Uncomfortably hot in the winter does not result in unmet load hours so you should be fine.
I agree with modeling window ventilation in both models. Without opening the windows are your unmet cooling load hours too high? If not then maybe what Chris suggests would work.
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
July 19, 2013 - 6:32 am
Thanks to both Chris and Marcus for the continued support.
To pick up on Chris's first point: Mechanical ventilation minimum outdoor air requirement and window ventilation CO2 upper level and zone temperature setpoints for opening and closing are set equal in both models.
This does not mean that the outdoor air that comes into the zone will be identical...this is a dynamic simulation after all and depends on time dependant zone temperatures (which also relay effects of occupant CO2 production) for the window ventilation and I'm betting the economizer / oda mixing damper for the mechanical side.
No, the cooling load not met hours won't be too high if the windows are kept shut, but as the design case has more window area than the baseline, it will use more cooling energy so I'll loose an advantage there (this house is almost behaving like a passive house). I have tried the window shut approach, and although the minimum savings for prerequisite are achieved...it's not good enough for the project as a whole. This is exasperated by the fact that the floor heating system starts to battle with the cooling system (which must be as the baseline) due to the low air supply temperatures that the system brings into the zone.
So without gaming, I'm trying to get the best out of the comparison and avoid possible control conflicts.
I actually called the head of the nursery (that's the building that we're certifying) today, and she confirmed many of the issues the model predicts, as well as the high temperatures on the south facing zones in summer. But no winter overheating.
Another interesting point that came out was that the external south facing schalousie blinds are manually lowered via motor, but there exists an automatic reset that is controlled via the rooftop weather control module. That is half-automated if you ask me.
There is one more tip that she gave me which I've tested and which has improved, but not eliminated winter overheating which is largly due to solar gains on the south side where I am currently modelling no shading.
My heating unmet load hours are 90 for the proposed model (not bad for a window ventilated building with radiant floor heating, in my opinion).
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
August 6, 2013 - 10:37 am
After some updates with feedback from the building operator as to how the buildings windows are used, the results from the simulation are GREAT.
- 37% saving
- unmetloadhrs less than 50
- baseline sizing parameters reset at 1.25 and 1.15
- Only the kitchen and two other problem rooms exceeded 4% time uncomfortable per ASHRAE 55 simple method.
But the rooms still get very hot in summer (2 deg C more than ODA temp). So I did a test and dropped the clg setpoint to 26 deg C (and adjusted other parameters to suit) so that the clg will work as in a retrofitet building, which is probably what the standard 90.1 intended. Savings only dropped by 2%...an acceptable hit.
Q) Which is the better variant...the one with "retrofited clg", i.e. clg setpoint 26 deg C, OR the one with "no cooling", i.e. clg setpoint set to 43 deg C.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5911 thumbs up
August 6, 2013 - 5:01 pm
Personally I tend to always favor modeling it as designed rather than fudging something to meet an ASHRAE rule that does not always make sense.
The rational for having to include cooling that I heard was that eventually the people in the space will complain enough that air conditioning will be installed. If you think that is the case then model it with the cooling set point. If not then model it with the "natural ventilation" set point.
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
August 6, 2013 - 5:10 pm
Thanks. That makes sense. The reality is that this is a school and as such by default has "no money" as I'm always told (hence this is a charity project for us). So retrofits won't happen. More likely, blinds will be fitted and used manually. Okay. Thanks. Wish me luck.
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
November 1, 2013 - 1:12 pm
Many thanks to Marcus and Christopher for all the help and suggestions. I have had a successful final review regarding this facility. We modelled the window ventilation identically in both baseline and design case models with cooling systems that never turn on.
Additionally, we got a positive review on the increased ventilation credit by explicitly examining the thermal comfort aspects using the model (thermal comfort was shown to be "acceptable" if not to standard required for the thermal comfort design credits whilst maintaining "sufficient" ventilation levels, by investigating CO2 levels and comparing them to both statistical values for schools in Germany as well as values listed in Standard 62.1. For this I created a copy of the first design model and added the shading and lighting with "operator mimicked control".
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5911 thumbs up
November 1, 2013 - 2:11 pm
No problem, happy to help. Appreciate your contribution to these forums as well.