Ok, this prereq, in my mind, is more of a design-based item. And this is a long comment for a particularly fuzzy issue in my mind. Apologies ahead of time.
As critical zones are used for design purposes, the 62MZ calculator and this prerequisite require calculations to go down to the zone level. Therefore, critical zones would be required to be balanced and checked for ASHRAE 62.1‐2007 requirements. However, the intent of this prereq states: “Modify or maintain each outside air intake, supply air fan and/or ventilation distribution system to supply at least the outdoor air ventilation rate required by ASHRAE Standard 62.1‐2007…” and not to balance each zone to ensure the correct amount of airflow is meeting each zone’s requirements, based on occupancy count and spacetype.
And, regardless, the critical zones in question (conference rooms, office space, breakrooms,etc) all have the same requirements per ASHRAE 62.1‐2007 Table 6‐1 (5 OA CFM / Person and 0.06 OA CFM / Sq. Ft.).
So, what does this all mean, then? Why do we need to look at the critical zones if all zones served by a AHU require the same amount of OA?
Then, furthermore, lets say heat pumps are being used. These units are all served by a dedicated outside air fan. One outside air fan serves the entire building’s ventilation needs, based on ASHRAE 62.1‐2007
requirements. Also, each heat pump is not directly ducted from the OA chase. The OA chase distributes air to each floor in the RA plenum, dilutes, and mixes with the RA at each heat pump to serve the zone/space.
So, while the OA fan meets the requirements of ASHRAE, how should compliance be demonstrated for potentially critical zones (which is at the heat pump level) to determine how they affect overall system efficiency, and therefore the OA required to be brought into the AHU?
Chris Nixon
Manager, Sustainable SolutionsBRSC
36 thumbs up
May 24, 2010 - 9:04 am
On one level this could be considered a design item, but it's imperative that the building is getting enough outside air flow. Maybe the outside air damper is broken or blocked with debris. That's why it must be measured and not just taken from drawings. The way I understood the prereq is that there is enough indoor air circulation in the building through doors and return air vents that the required outdoor air makes it into the critical zones. It doesn't have to be vented directly to these zones. I assumed that the 62MZ Calculator figures all that out for us. The reason that the critical zones are critical is not due to high heating or cooling load, but quite the opposite. If the critical zones have a very low load, their HVAC dampers will remain shut and they won't get the required fresh air.
John Beeson
Green Mystic in Residence107 thumbs up
May 24, 2010 - 11:58 pm
Chris, thanks for your thoughts.
So,again though, how should compliance be demonstrated for potentially critical zones (which is at the heat pump level) to determine how they affect overall system efficiency, and therefore the OA required to be brought into the AHU?
In other words, if balancing each zone is not necessarily required to ensure the correct amount of airflow, how should project teams show compliance and what is EBOM looking for?
Roger Chang
Principal, Energy and Engineering LeaderDLR Group | Westlake Reed Leskosky
LEEDuser Expert
398 thumbs up
May 25, 2010 - 10:05 am
Achieving ASHRAE 62 compliance can be challenging for multiple-zone systems. Have you considered direct ducting of OA to each heat pump? In this case, the design becomes greatly simplified, as you don't have to worry about system ventilation efficiency (Ev). If you do supply directly to the return air plenum, you will need to calculate the ventilation fraction for each zone (ASHRAE 62 ventilation rate/supply air volume for the heat pump). The highest ventilation fraction will be used to calculate the Ev, which then determines the amount of outside air you have to bring in at your dedicated outside air unit. In buildings with interior zones with high occupant densities (e.g. conference rooms), you'll often find that the outside air fraction jumps up to 100%. We typically don't recommend introducing OA to return air plenums, because it is hard to guarantee you're not short-circuiting OA supply to certain zones.
John Beeson
Green Mystic in Residence107 thumbs up
May 25, 2010 - 11:48 am
Roger, you've hit the nail on the head for me: "...OA to return air plenums, it is hard to guarantee you're not short-circuiting OA supply to certain zones."
So while it seems outside of the scope of this credit to ensure that the correct amount of molecules of OA is properly reaching critical zones, it IS inside the scope to simply make sure that the outside air intake, supply air fan and/or ventilation distribution system is only properly maintained or modified. Any 62MZ calcs would be to ensure that the overall system supply is adequate and what is measured meets or exceeds the calcs. Is that fair to say?
Roger Chang
Principal, Energy and Engineering LeaderDLR Group | Westlake Reed Leskosky
LEEDuser Expert
398 thumbs up
May 27, 2010 - 9:04 am
John, I reviewed the ASHRAE 62 user's manual and note this following statement in 5.2.2: "it is not sufficient to deliver the correct total amount of outdoor air into the plenum and assume that it arrives in the proper quantity at each terminal." You can avoid direct ducting to each zone, but then the plenum needs to be divided up in some way. If you cannot direct duct, I would at least try to get outdoor air to each terminal unit as close as possible.
Nancy Henderson
Managing MemberArchEcology, LLC
83 thumbs up
June 9, 2010 - 3:20 pm
Roger, We also have a building with multiple heat pumps (upwards of 80). The building has a single outside air supply fan on the roof which distributes outside air through a central supply shaft which is then ducted to each heat pump. I am curious about your statement that if the outside air is ducted to the heat pump, you do not have to worry about system ventilation efficiency (Ev). Is this becuase the system is then considered a 100% outside air system? I am also confused about what the AHU would be. Is it only the main supply fan that would need to be tested is this case?
Roger Chang
Principal, Energy and Engineering LeaderDLR Group | Westlake Reed Leskosky
LEEDuser Expert
398 thumbs up
June 11, 2010 - 4:58 pm
Yes, when ducting OA directly to each terminal unit, the system in one sense is a 100% outside air system. Dedicated outdoor air systems (DOAS) are becoming the preferred system (coupled with fan coil units, chilled beams, radiant panels, etc.), because you can guarantee the right amount of OA is delivered to every zone under all loading conditions. With a VAV system, this becomes tricky. Imagine an interior conference room with a high occupant density. The room may want 100% outside air, but the main air-handling unit only supplies 30% outside air. You end up with either having to bump up the ventilation air at the main AHU or increasing your VAV box mins and using more reheat. You would only need to meter the main supply fan. Good balancing is important too.