ASHRAE 90.1 Appendix G3.1.2.2

6 posts / 0 new
Last post

hi Varun thanks for your reply! I have another question though based on your statement that the system should be based on the 'design results'; I thought the whole point of using simulations is to predict the peak capacity required and then base your design on this? (possibly by applying safety factors according to real life experience).And how can you have unmet loads when you design based on the peak load? In that case the 'peak load' is not the maximum load?Further comments anybody? best regardsMilda

milda margarin's picture
Offline
Joined: 2011-10-02
Reputation: 0

Milda,

If you did notice, HAP uses two different weather files. one for design and
other for simulation. And if you dig into it you'll find out that they both
are different. The simulation weather is the real time (actual) temperature
and other data recorded at the airports for an extended period, which cannot
be overridden, while design weather data is based on extreme or peak
weather as defined by ASHRAE which you can change in the program.

Now if you are looking at LEED unmet load hours they may not be necessarily
due to air sytem size, but may also be due to your plant insufficient
capacity. Download HAP e help 014 from here :
http://www.commercial.carrier.com/commercial/hvac/general/0,,CLI1_DIV12_ETI3906_MID1738,00.html
.
In here you will find a pdf saying understanding the LEED unmet load hours
in HAP.

Simulation results is just for us to know how the building would run in real
time when sized on the design results. In simulation , HAP takes credit of
internal heat gains, which reduces your heating load, which is OK in real
time. While in design , internal heat gain is ignored in heating load
calcualtions. This is just one of the difference in simulation and design in
HAP.

I hope this helps Milda.

varun kulkarni's picture
Offline
Joined: 2011-09-30
Reputation: 1

If there someone can define/agree upon a solution to the ?different weather data used for design and simulation? problem, we are happy to help. We have the detailed historical data from which we can calculate the design to numbers and we have both raw observations and localized current conditions/forecasts plus an engine to post process this on the fly and deliver it specific to a site. We are happy to work with anyone who wants to tackle this.

Chuck Khuen

Chuck Khuen's picture
Offline
Joined: 2011-10-02
Reputation: 0

Milda,

Just as you can use a hammer to both drive nails and extract them, all building energy simulation software is simply a tool that can be used for very different means.

If you wish to use such software to aid in design/selection of HVAC equipment, then fundamentally you are arriving at design capacities by calculating design heating/cooling loads. These use design heating/cooling weather conditions which are fundamentally different from the hourly averaged (over decades) conditions in a TMY weather file.

If you wish to use such software to evaluate system behavior over time, that?s a very different endgame. A system that is 100% properly sized for capacity at design conditions can easily have ?unmet hours? as a result of something beyond capacity. Many factors independent of capacity come into play for hourly simulations including system setback/warmup functions, economizer operation, load schedules of all sorts, part-load performance curves and sensor/thermostat settings. These all can determine whether a zone?s conditions are met or not for the hour, in spite of whether a system is properly sized for design conditions.

Some software is specific to load calculations and assisting in HVAC sizing. Some software is specifically targeted at energy modeling to simulate system behavior over time. Some software is intended/appropriate for both functions, to varying degrees. It?s up to the end-user to define what she/he is trying to accomplish, and what tool of choice is best for the job at hand.

NICK CATON, E.I.T.

PS: It appears you may have missed my response following Varun?s specific to 90.1 and ?safety factors? last week ? I?ve attached just in case ;).

Nick-Caton's picture
Offline
Joined: 2011-09-30
Reputation: 805

Design Loads programs generate weather data for a typical day of each month based on design weather conditions for summer and winter.
See http://bepan.info/engg-calcs Excel Program 3a - Design-Weather-Generator
Actual 8760 hours of weather can result underestimating or overestimating the loads.

Varkie Thomas's picture
Offline
Joined: 2011-09-30
Reputation: 0

Design Loads Programs use generated weather for each day of a typical day of month from the ASHRAE percent design conditions for summer & winter. It is a guide and not a standard that has to be followed. It is used to size equipment. ASHRAE 99% winter design -16 degF but it can go down to -40 degF. Actual weather data can result in overestimating and underestimating.
See http://bepan.info/engg-calcs Item 3a - Design-Weather-Generator
and http://bepan.info/hvac-prog/x1-apec-hccv-loads
You can down the APEC HCCV Loads program and try it (does not work on 64-bit)
HCC-Disk-1 HCC-Disk2
Varkie
Milda,
If you did notice, HAP uses two different weather files. one for design and other for simulation. And if you dig into it you'll find out that they both are different. The simulation weather is the real time (actual) temperature and other data recorded at the airports for an extended period, which cannot be overridden, while design weather data is based on extreme or peak weather as defined by ASHRAE which you can change in the program.

Now if you are looking at LEED unmet load hours they may not be necessarily due to air sytem size, but may also be due to your plant insufficient capacity. Download HAP e help 014 from here :http://www.commercial.carrier.com/commercial/hvac/general/0,,CLI1_DIV12_ETI3906_MID1738,00.html.
In here you will find a pdf saying understanding the LEED unmet load hours in HAP.
Simulation results is just for us to know how the building would run in real time when sized on the design results. In simulation , HAP takes credit of internal heat gains, which reduces your heating load, which is OK in real time. While in design , internal heat gain is ignored in heating load calcualtions. This is just one of the difference in simulation and design in HAP.
I hope this helps Milda.

Varkie Thomas's picture
Offline
Joined: 2011-09-30
Reputation: 0