I was wondering if anyone had experience modeling DX systems in their proposed systems. I received a USGBC comment that the actual airflow had to be modeled, auto-sizing was not allowed. My baseline system was already modeled correctly with both the cooling and heating systems oversized by 15% and 25% per Appendix G. My main question is how does Appendix G account for the fact that the fan airflows for constant volume DX systems, like packaged and split heat pumps, is set at 350-400 cfm/ton for them to work properly. My baseline system is a packaged DX system, and even with the 15% and 25% oversizing of the equipment, the baseline system is still below 350 cfm/ton. Yet, I must model my actual airflow rate of my equipment, which will increase my fan power and thus increase energy over the baseline becuase i'm using a real world piece of equipment that does not operate or dehumidify properly below 350 cfm/ton. Is Appendix G pentalizing me for using a DX system (by requiring me to model the actual airflow), when it does not require "real-world" airflows in the baseline DX system?
Any guidance on the matter would be appreciated!
Barbara McCrary
Mechanical Engineer, PE, LEED AP BD&CHHB Engineers, P.C., Prattville, AL
119 thumbs up
October 18, 2010 - 2:47 pm
Let me correct myself, App G makes you both oversize equipment capacities in baseline AND size air flow rates based on a 20F supply air to room temp difference. So still not a "real-world" cfm based on the heat pump capacity!
Michael Tillou
Director of Energy ServicesCannon Design
52 thumbs up
October 18, 2010 - 3:11 pm
Barbara,
Appendix G is not penalizing you as you describe.
If your baseline DX systems work out to less than 350 CFM/ton than the baseline supply air CFM has not been calculated correctly. Based on a 20F design delta T, required by Appendix G, the supply air CFM should be approx. 555.5 SCFM/ton which is higher than the 350/400 CFM/ton you describe. I would confirm that you are performing the sizing runs correctly.
The proper method for doing a sizing run for an Appendix G baseline energy model is:
1. Set all internal load and envelope parameters to the baseline conditions.
2. Set design day criteria to the required ASHRAE conditions specified in Appendix G. Remember that programs like eQUEST require the creation of special heating and cooling design days otherwise the peak heating and cooling is based on the weather file.
3. Simulate the model and find the peak heating and cooling loads for each system.
4. Increase the peak heating loads by 25% and the peak cooling loads by 15%.
5. Calulate the supply air flow rate for each system using a 20F delta-T.
6. Enter in the calculated heating and cooling capacities and the supply air flow rates for each baseline system.
7. Simulate the final baseline HVAC energy model.
If your energy modeling program is not doing this correctly you may have to do it manually to get the correct answers.
Barbara McCrary
Mechanical Engineer, PE, LEED AP BD&CHHB Engineers, P.C., Prattville, AL
119 thumbs up
October 18, 2010 - 3:17 pm
Michael, thanks for your quick reply. I also just performed the calc you did and you are right, a 20F delta T works out to 555 cfm/ton. I'm using Trane Trace and will double check what it is doing, because it is calculating less than 350 cfm/ton on the baseline system.
Barbara McCrary
Mechanical Engineer, PE, LEED AP BD&CHHB Engineers, P.C., Prattville, AL
119 thumbs up
October 18, 2010 - 4:02 pm
Michael - actually, correct me if i'm wrong. Did you use the formula Q=1.08*CFM*DeltaT to come up with 555 cfm/ton? The Delta T in this formula is the different in entering air and leaving air across the coil, which will not be 20 if you have untempered outside air mixing with your return air. So by assuming a 20 different in supply air and room air temperature, it doesn't automatically mean you will have 555 cfm/ton. For high outside air quantities it can be much lower. So i guess i'm back to my original question on setting cfm/ton on DX equipment to 350. My Trane Trace sensible capacites on my coils (which is seems to be setting my airflows), do seems a bit off, so i'm still going to have to take a look at them.
Michael Tillou
Director of Energy ServicesCannon Design
52 thumbs up
October 19, 2010 - 11:49 am
I did. That formula applies to any sensible load whether its a load on a coil or a load in a space. Appendix G assumes that ventilation loads are handled by the system prior to calculating the baseline supply airflow rate. That is why it specifically says you use a supply air temperature to room air temperature delta-T of 20F, it assumes a 55F SAT and a 75F room temp setpoint. So you are correct that if you add in the ventilation load the CFM/ton of a specific unit would be less than the 555 CFM/ton I mention.
The baseline does not assume you are using a "real world" piece of equipment.