[Equest-users] Does eQuest properly account for latent loads in infiltration & ventilation?

Hall, Brendan BHall at karpinskieng.com
Wed Mar 13 13:40:01 PDT 2013

One note about using heating and cooling degree days for that purpose is that ventilation air is usually locked out during unoccupied times so the annual total would be much lower. From a quick read of the paper you cited it seems that they used all 8760 hours. You would need to isolate the occupied hours when the economizer is not likely to be in use to get a more accurate number.

Brendan Hall

From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Z Smith
Sent: Wednesday, March 13, 2013 3:26 PM
To: equest-users at lists.onebuilding.org
Cc: Corey Squire
Subject: [Equest-users] Does eQuest properly account for latent loads in infiltration & ventilation?

We are interested in using eQuest to evaluate the impact of improved air-sealing or changed outdoor air ventilation rates.  Even with very simple test case buildings and run-of-the-mill HVAC systems, we get results that make it seem as if eQuest is not accounting for the latent component of outdoor air brought into a building.

We are using Lew Harriman et al.'s 1997 ASHRAE Journal paper on the "Ventilation Load Index" as a reality check.
The authors characterize the total load (latent and sensible) for bringing 1 cfm continuously into the building for a number of cities.  It's interesting to consider two cities with comparable CDD but very different latent loads for ventilation air:

New Orleans  CDD=2776    VLI=12.3 [latent] + 1.8 [sensible] =14.1 ton-hrs/yr
Tucson       CDD=3017    VLI= 1.5 [latent] + 3.0 [sensible] = 4.5 ton-hrs/yr

The VLI is the cumulative load to bring 1cfm from whatever the hourly condition is in the TMY2 file to 75°F, 50% RH (65gr/lb).  Its units are ton-hr/yr for convenience, converted to annual kBtu by multiplying VLI by12 kBtu/ton-h.

So, for example, if we have a building with 2,000ft2 of floor area and a volume of 20,000ft3 with a ventilation rate of 1ACH, outdoor air is being introduced to the building at 20,000/60 = 333cfm.  The annual cooling & dehumidification load associated with this airflow is 333 x 14.1 = 4,695 ton-hrs/yr in New Orleans and 1,383 ton-hrs/yr in Tucson.  If, for simplicity, we assume the air system brings this air to 75°F, 50% RH with a SEER of 13 kBtu/kWh in both cities, then the energy consumption associated with 1ACH  for this 2000ft2 building is 4,334 kWh in New Orleans and 1,383 kWh in Tucson. The impact on building site EUI will be 7.4 kBtu/sf/yr in New Orleans and 2.4 kBtu/sf/yr in Tucson - a difference of about 5kBtu/sf/yr.  If one were to consider ventilating at 10ACH, then the impact would be 10x as large-74kBtu/sf/yr in New Orleans vs 24 kBtu/sf/yr in Tucson-a difference of ~50 kBtu/sf/yr.

When we run an eQuest model of a hypothetical 2000ft2 building with no windows (to rule out solar gain differences) and no heating system, only cooling, we find that the EUI does increase with ACH at the two locations-but there is almost no difference in how the predicted EUI rises with ACH for the two cities with wildly different VLI.  The EUI rises by the same ~35 kBtu/sf/yr when going from 1ACH to 10ACH at both locations.

Results summarized below for total building EUI (kBtu/sf/yr):

                    eQuest             Simple model using VLI
  ACH     New Orleans     Tucson      New Orleans      Tucson
    1          42           40            36             30
   10         76           74           126             74
------    -----------------------     -----------------------
  1->10        34           34            90             44

(This windowless test building had R10 walls, R20 roof, R5 floor - but all we are interested in is the *difference* in total EUI associated with the increased ACH, which shouldn't depend on these choices).

Z Smith, AIA, LEED AP BD+C | Director of Sustainability & Building Performance | Eskew+Dumez+Ripple | 365 Canal Street, Suite 3150 | New Orleans, LA  70130 | 504.561.8686 | eskewdumezripple.com

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.onebuilding.org/pipermail/equest-users-onebuilding.org/attachments/20130313/f96b9195/attachment-0001.htm>

More information about the Equest-users mailing list