[Equest-users] Low Heating Consumption

Bishop, Bill bbishop at pathfinder-ea.com
Fri Oct 24 08:11:31 PDT 2014

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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From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of David Eldridge
Sent: Thursday, October 23, 2014 6:18 PM
To: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Low Heating Consumption

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

From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Daniel Caporizzo
Sent: Thursday, October 23, 2014 12:19 PM
To: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: [Equest-users] Low Heating Consumption

I have been working on an energy model for a 5-story, 400,000 SF office building. The ground floor is slightly different in terms of layout (including a kitchen and parking garage) but the other floors are mainly office space with some conference rooms. Currently the system is VAV but I have been toying with doing it as chilled beams, which still has a low heating consumption. The project is in schematic so the zoning does not incorporate the proposed room layout, but rather just perimeter and core zones.

My problem is that the building has very low heating energy consumption. I believe it is due to the core interior zones on 2F-4F have 0 heating load according to my LS-A report (the top and bottom floors only have minimal load due to roof and slab, respectively). The exterior zones have higher heating loads that look in line. This would cause a disproportionate amount of the building is receive no heading, drastically reducing my heating load. Even for a newly constructed building these results look low compared to standard rule-of-thumb numbers.

I noticed that my internal loads (particularly the misc equipment) have a big effect, as I originally had 1 W/sf but had to change it to 0.75 W/sf to get a higher heating load in the building. I have it set to 25 CFM OA/pers at the moment, where in reality it would need much less, but I needed to get the heating load up somehow. The building is a large rectangle for the most part so I don't know if my building just has a low heating load. Currently the boilers are hot water boilers w/ draft, but an option in the project exists to utilize condensing boilers instead. I tried switching them (as well as the loop supply temp) but the heating energy consumption barely changed. Most of the equipment is auto-sized, except for AHU static & CFM, Chiller size, boiler size & efficiency, and pump head, which were calculated separately by the design team.

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