We've got an 11,000 square foot maintenance garage for trucks, modeling it under LEED and ASHRAE 90.1-2007. This is a tricky space.
It has more than 15 BTUH/SFT of heat so according to Table 3.1 it is not a "semiheated" space - it is a "heated" space. In the real world, it is heat only, no cooling. They use radiant floor heat.
According to Appendix G Table 3.1 1 B we have to simulate the space as both heated and cooled. We get in a lot of trouble real fast after that.
An automotive repair garage requires a huge (41,000 cfm in this case) exhaust fan. This is a gigantic cooling load. The program (Trace) wants to simulate the exhaust as if it entered through the air conditioning unit.
This is heading down the path of simulating a 41,000 CFM 100% outside air rooftop unit, in a space that in the real world has no air conditioning. Yow!
Now, if I say this is a "process" load, and just add the power for the fan as a process electrical load, without adding the airflow in the energy model, is that the correct way to do this? I see another related question below hinting at the same idea. Not sure I am approaching this correctly.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
April 20, 2015 - 10:47 am
I'll stop you at the cooling. In a heated only space you have a couple of options. In some cases you could possibly model it as a system 9 or 10 (heated only). In cases where the space is not covered by a system 9/10 then you can set the cooling temperature set points so that the cooling never operates. Quite often if you just tell the reviewer you are aware of this work around related to temperatures and do not model the cooling system it should be accepted.
Lawrence Lile
Chief EngineerLile Engineering, LLC
76 thumbs up
April 20, 2015 - 11:10 am
The workaround of setting the cooling setpoint to a very high temperature makes sense. That'll be my first approach. I think some conversations with the review team before turning this in will be critical to success.
Marcus, I forgot to mention this is LEED 2009 under 90.1 2007. The original version of this standard doesn't have systems 9 and 10, although I have just become aware of a July 24 2010 addendum which apparently addresses some of these issues. A brief scan of it shows we have a system 10 and 11, (no 9, apparently) and it has a building type called "Heated Only Storage" . System 11 would be a warm air gas fired furnace, appropriate to this situation. I don't know if we can apply this addendum (this is a heated only shop, not storage) or if it helps the situation at all.
Lawrence Lile
Chief EngineerLile Engineering, LLC
76 thumbs up
April 20, 2015 - 11:28 am
The other part of the question - how to model a fan that is a process load? These fans can move a whopping amount of air, but there are gas detection sensors that shut them off unless turned on manually, or various gases are detected - combustible gases, diesel combustion products, not just CO2. I expect they won't run much. As you mention in a post below they are "not interlocked with the HVAC".
If I am understanding correctly, 90.1 doesn't regulate these exhaust fans, so we model the motor's energy use but don't try to condition this whopping airflow in the energy model - is that correct? I'm still a little confused on this point.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
April 20, 2015 - 11:23 am
Yes the addendum incorrectly labeled the system number. If you look in 90.1-2010 they use 9/10. These system technically can only be used in the spaces listed. In your case a system 9 would probably be accepted since it is conservative relative to a system 3.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
April 20, 2015 - 11:25 am
An exhaust fan is process if it serves an unconditioned space or if it is separately controlled from the HVAC system serving that space.
If it is process then it gets modeled identically in both models. If you are conditioning the airflow then the fan is not process.
Lawrence Lile
Chief EngineerLile Engineering, LLC
76 thumbs up
April 20, 2015 - 11:33 am
I see. So the energy model only includes a load for the motor power, but ignores the airflow - both because it is not part of the HVAC, and because safety controls will probably prevent it from running most of the time.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
April 20, 2015 - 11:42 am
In the model the schedule you put the fan on will dictate its run time. The fan power and schedule is modeled identically.