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

Joe Huang yjhuang at whiteboxtechnologies.com
Wed Mar 13 17:30:35 PDT 2013

I have a hard time believing that eQUEST /DOE-2 has been missing this important cooling 
load component (latent load from outside air) all these decades.
How are you modeling this outside air flow?  In LOADS as infiltration or in SYSTEMS as 
mechanical ventilation?  If it's the latter, do you have the FAN-SCHEDULE
set to always on?   If you're simply trying to see the change in loads from a constant 
change in outdoor air flow, why don't you model it as INFILTRATION, and that
way you can see the impact on sensible and latent loads due to INFILTRATION as well as 
circumvent any of the interactions between this outside air
flow and the system operations. I would be quite surprised if indeed DOE-2 has been 
missing this latent component, and that you're not seeing some other
unexpected or unaccounted for effect.


Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 108D
Moraga CA 94556
yjhuang at whiteboxtechnologies.com
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"

On 3/13/2013 3:16 PM, Z Smith wrote:
> Thanks to both David and Bruce, whom I'll answer together:
> The beauty of the VLI metric is that it doesn't depend on how a particular HVAC system 
> will get the ventilation air to the comfortable condition (75°F / 50%RH)---it just 
> quantifies the enthalpy difference between the outdoor condition and the desired 
> endpoint inside the building.  It could be that an Outdoor Air unit is cooling warm 
> moist outside air from to 55°F and then doing re-heat, or it could be that the re-heat 
> is being provided from internal gains or solar gain---but the VLI doesn't care.
> In our first message, we turned off heat for clarity.  When we have also run cases where 
> we use DX cooling + heat pump, and also DX cooling and electric resistance heat---but we 
> always find that eQuest predicts that New Orleans and Tucson (which have similar HDD and 
> CDD but very different VLI) should have very similar EUI, and the very high Outdoor Air 
> rates (at which we would expect New Orleans to show off a much higher EUI) do not show a 
> different predicted EUI for the two cities.  They should have different EUI vs ACH, but 
> they don't---they match in slope and value.
> To David's point about ACH and fan power: The ACH in our discussions are not 
> recirculating air rates but the rate of Outdoor Air. We plot up EUI vs OA ACH; you would 
> expect fan energy for handling skin loads to fall away as an effect once we get to high 
> ACH.   Our results are very similar whether we introduce this outdoor air as deliberate 
> ventilation or as infiltration---so the fan energy associated with actually introducing 
> the outdoor air is not a big effect compared to the energy to condition this air.
> To the question of what we're really trying to do here:  When we work in hot-humid 
> climates, we are concerned that eQuest will show the correct benefit for either 
> decreased infiltration or improved energy recapture from systems like Enthalpy Recovery 
> Ventilators.  We have set up the Tucson / New Orleans comparison as a thought-experiment 
> where common sense tells us that we should see a big difference but eQuest shows none.
> *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 <http://www.eskewdumezripple.com/>
> *From:*David Eldridge [mailto:DEldridge at grummanbutkus.com]
> *Sent:* Wednesday, March 13, 2013 4:51 PM
> *To:* 'Hall, Brendan'; Z Smith; equest-users at lists.onebuilding.org
> *Cc:* Corey Squire
> *Subject:* RE: [Equest-users] Does eQuest properly account for latent loads in 
> infiltration & ventilation?
> The VLI is meant as a method of comparing different climates and informing the designer 
> about suitable technologies when looking at design loads, as opposed to being used as a 
> direct predictor of energy usage.
> The VLI approach used for energy calculations is also a little bit of an 
> oversimplification to only supply room neutral air -- the system in eQUEST (or any other 
> software) would typically be supplying cooler air at the cooling coil, and then some 
> combination of the load in the space and reheat brings the total quantity of air back to 
> the room conditions -- obviously not the same calculation as the VLI method.
> If the goal is to recreate the VLI calculation, then you probably want to have a room 
> that doesn't have ANY exterior walls, as well as removing internal gains from occupants 
> and lights, etc. You may also want to zero out the fan power, which may be included in 
> the SEER rating. For instance in your 1 ACH case the room may not be in control if the 
> thermal load requires more than 1 ACH at the peak times. Another issue is the VLI is 
> operating in terms of load, while the modeled building is using equipment efficiencies 
> which are going to vary over the range of annual conditions. The SEER approach tries to 
> account for that, but the modeled buildings would have varying tempeature and humidity 
> ranges which affect the system efficiencies in different proportions -- but mainly to 
> say that using the same SEER in the hand calc won't necessarily give the same results as 
> an energy model, unless you had also replaced all of the equipment efficiency curves.
> Anyway, not knowing what the purpose of the effort is, to recreate the VLI you'll need 
> to strip the model of many other factors, and there is likely to be a combination of 
> environmental conditions and system efficiency causing the Arizona / Louisiana results 
> not to behave as you expected. Start with verifying the AHU configuration and setpoints, 
> and whether the modeled spaces are maintained at the same temperature and humidity 
> between the two cases, secondly verify if the conditions were the temp/humity that were 
> desired. From your description of no heating system I'd expect some variation from the 
> constant conditions used in the VLI calculation.
> David
> David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, BEAP, HBDP
> *Grumman/Butkus Associates*
> *From:*Bruce Easterbrook [mailto:bruce5 at bellnet.ca]
> *Sent:* Wednesday, March 13, 2013 4:22 PM
> *To:* Z Smith
> *Cc:* Hall, Brendan; equest-users at lists.onebuilding.org; Corey Squire
> *Subject:* Re: [Equest-users] Does eQuest properly account for latent loads in 
> infiltration & ventilation?
> I would think without the ability to heat or reheat you aren't controlling your humidity 
> to 50% RH in the New Orleans building.
> Bruce Easterbrook P.Eng
> Abode Engineering
> *From:*equest-users-bounces at lists.onebuilding.org 
> <mailto:equest-users-bounces at lists.onebuilding.org> 
> [mailto:equest-users-bounces at lists.onebuilding.org] *On Behalf Of *Hall, Brendan
> *Sent:* Wednesday, March 13, 2013 3:40 PM
> *To:* Z Smith; equest-users at lists.onebuilding.org 
> <mailto:equest-users at lists.onebuilding.org>
> *Cc:* Corey Squire
> *Subject:* Re: [Equest-users] Does eQuest properly account for latent loads in 
> infiltration & ventilation?
> 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> 
> [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 <mailto: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.
> http://masongrant.com/pdf_2008/Ventilation_Loads.pdf
> 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
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