Low Heating Consumption

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Note that the LS loads report shows the heating load of the zone, this is different than the energy consumed to provide heating by the system. A zone that has no heating load in LS-A still consumes reheat energy if the VAV system is supplying more air than is required for cooling. Or the zone will be too cold.

1) Check the SS-O reports and the unmet hours tabulation for your zones to make sure that the two cases are meeting the temperature requirements.

2) Have you set the minimum airflow requirements for the zones, if these aren't zero minimum spaces? Reheat energy will be underreported if you allow the VAV to modulate lower than the design's minimum for the zone.

The chilled beam case may have less reheat than the VAV system since the configuration allows for less air to be cooled, and then heated up in the non-critical zones. The amount of heat gain required to the zone to satisfy LS-A will still be the same in both cases, even though the systems use more or less energy to deliver that heat to the zone in usable form.

Your other point is correct, internal loads are dissipated at heat, which would offset your need for reheat...your observation is on target that if your modeled internal loads are too high, the heating will be underestimated. (And electricity use overestimated.)


David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, BEAP, HBDP
Grumman/Butkus Associates

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Joined: 2012-05-08
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I have seen times when the annual heating energy use is relatively low if there is significant internal load most of the time. However, some items to consider:

* Are you applying a profile to the plug load that is accurate (i.e. not constant 0.75 W/SF constant)? Is the plug load too high? Most times it is not as high as past "rules of thumb".

* Are you accounting for heating setback during unoccupied times?

* Are you applying a schedule to the lighting that is reasonable? Does lighting control align with actual design for lighting control (e.g. are there occupancy sensors that would keep lights off part/most of the time in some spaces; e.g. conference rooms).

* Check the building envelope wall/window performance for U-value to make sure it is correct.

Beyond the points above, I would say that core spaces don't really have a heating load (except for roof/slab as you mentioned). An interior conference room does not know if it January or July; it has the same cooling load year-round.

Scott Parker PE

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I suspect your heating and cooling energy would both increase if your zones weren't so simple. Your current model doesn't have any high-load zones, so the simulation can keep zone and air handler air flow to a minimum year-round. (I'm guessing on this - I didn't run a custom report to verify.) More zone diversity would require more air flow, so more cooling, and more reheat. Also, I know you said airflow was calculated by the design team, but based on SV-A numbers, you have 0.44 and 0.57 cfm/ft2 for the two air handlers, which works out to 2.6 and 3.4 ACH, while the ASHRAE Pocket Guide shows general design conditions for offices are minimum .75 cfm/ft2 and 4 air changes per hour. Perhaps someone else with VAV system design experience can chime in? If these numbers have worked for your design team before, it would be good to know for updating our rules-of-thumb.

William Bishop, PE, BEMP, BEAP, CEM, LEED AP | Pathfinder Engineers & Architects LLP
Senior Energy Engineer

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