[Equest-users] 90.1 Supply Fan Energy

[Brian Goldsmith]

I am trying to model a CAV 90.1 baseline system 4, but it won't let me input kW/cfm, just static and efficiency. The default static value is 1.25, but I'm not sure if that's correct because I obviously don't know the external static of the duct system. I would rather input the kW/cfm. Any advice?

Brian Goldsmith
Brian.Goldsmith at atce.com<mailto:Brian.Goldsmith at atce.com>
direct  408.487.1217

________________________________
From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Jeremy Poling
Sent: Wednesday, April 06, 2011 8:47 PM
To: Carol Gardner; Paul Diglio
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] 90.1 Supply Fan Energy

Carol,

Here's my general thoughts on OA in the model, for what they're worth.  The only problem with using the OA-per keywords in eQuest is that they are not additive: you can only use OA-flow-per-area or OA-flow-per-person but not both.  Since ASHRAE 62.1-2004 and later prescriptively defines both per-person and per-area requirements in an additive fashion, eQuest is incapable of self-calculating the amount of OA required to comply with ASHRAE 62.1.  I'm more often in the role of peer-reviewing models for engineers before they go to the LEED review teams and this is one thing I find myself too often receiving a response of "this way should work."  The reviewers will compare the OA amounts specified in EQp1 and EQc2 to the amounts used in the model in EAc1 and will more often than not kick out a comment if they do not match.

I usually try to minimize the time for having to respond to comments, especially knowing how many comments energy models receive.  My recommendation to modelers using eQuest is to always calculate your design CFM values (OA, supply, return, exhaust) outside of the software using appropriate adjustment factors where necessary or allowed by codes like ASHRAE 62.1 (elevation, etc.) and then use the direct specification keywords for zone-level airflows in eQuest.  As long as you have elevation zero'd out you should be able to get eQuest to match the airflow values you calculated, or else be able to easily identify where the problem is (system sizing factor >1, etc.).  An added benefit is that you do not have to run the simulation and look at the results to know what CFM values you have in the model: they are in the input file and can be accessed easily in the summary tables in the user interface.

It would be great if eQuest was able to match the ASHRAE 62.1 calcs and even provide an ASHRAE 62.1 summary report (like some other modeling software can), but I know Hirsch's team was focused on the baseline compliance engine and the soon-to-come Canadian version, so it is more than fair to be patient waiting for that feature :)  I also know that some really advanced users have ways to use the customization features in eQuest to do this already and I encourage them to share/teach...

In short - the Outside-air-flow keyword should be the preferred keyword and indeed eQuest overrides all other OA keywords at the zone level if a value is present for Outside-air-flow.

Separately, when you do calculate CFM values for the model, don't forget that ASHRAE 90.1-2007 allows credit for pressure drop due to filters ;)  Just one more thing to have to worry about in the model...

Jeremy R. Poling, PE, LEED AP+BDC
________________________________
From: equest-users-bounces at lists.onebuilding.org on behalf of Carol Gardner
Sent: Wed 4/6/2011 7:03 PM
To: Paul Diglio
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] 90.1 Supply Fan Energy
The reason someone "thinks OA might cause a problem" is because of the way that someone thinks it was entered. It sounds like it was entered using outside-air-flow, and I may be wrong because Michael hasn't really said. Using Outside-Air-Flow overrides any values eQUEST might otherwise calculate, and I would suggest that might make a difference. If OA Flow-Per was used instead, that's the one I use, it gives eQUEST the flexibility to calculate the OA cfm itself. To my mind, that a good use of an energy model that is not only interactive but uses 8760 hours of weather data.

Any other thoughts on this?
On Wed, Apr 6, 2011 at 4:07 PM, Paul Diglio <paul.diglio at sbcglobal.net<mailto:paul.diglio at sbcglobal.net>> wrote:
Michael:

The kW/CFM you used seems reasonable.

I don't know if you are using a CV or VAV system, but lets assume VAV.  90.1-2007 G3.1.2.9 shows the formula CFMs *.0013+A for baseline BHP.  A is calculated per Section 6.5.3.1.1.  Using Table 6.5.3.1.1A VAV, we have the formula BHP<= CFMs*0013+A.  The footnote defines A=sum of (PD*CFMd/4131).  So assuming you have a fully ducted return, the PD is .5 which should be multiplied by (CFMd/4131).  If there are various pressure drops throughout the system we need to estimate the CFM flow at each pressure drop.

As an example, let's take a 20,000 CFM VAV system with 18,000 CFM of return air through a ducted return.

A = (.5*(18000/4131) = 2.178

BHP =20000*.0013+2.178 = 28.17

pFan= 28.17*.746/ Fan Motor Efficiency.  Note that since we need kW not watts per CFM the 746 becomes .746.  That is probably the reason you generated a large kW number. Per Table 10.8 a 30 HP baseline motor needs to be 91% efficient.

The formula now becomes pFan =28.17*.746/.91 = 23.09 kW.  Our kW per CFM is .0011.

If I am modeling a proposed project with a defined supply and return CFM, I would expect that the baseline would be approximately the same.  The CFM/Ft2 required varies greatly between building and systems.  In the real world I see 1.2 or so CFM/Ft2 much, much more than I see .5 CFM/Ft2. Check your CFM/Ton to see if you are in the ballpark.

I don't know why someone would think that outside air would cause a problem.  Air is air.  Too much outside air would raise energy costs though.

Paul Diglio

________________________________
From: M. Shields <mshields at fstrategies.com<mailto:mshields at fstrategies.com>>
To: Paul Diglio <paul.diglio at sbcglobal.net<mailto:paul.diglio at sbcglobal.net>>; Carol Gardner <cmg750 at gmail.com<mailto:cmg750 at gmail.com>>

Cc: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Sent: Wed, April 6, 2011 6:13:53 PM
Subject: RE: [Equest-users] 90.1 Supply Fan Energy

I'll try OA as a per person number instead of a per space number and see what happens.  kW/CFM varied by space between ~ .0007 and .001, and when I look at the SV-A reports the Fan Demand (kW) matches what I calculated using 90.1.

I found G3.1.2.9 could possibly be interpreted a couple different ways (for example the denominator include X BHP instead of the whole fraction), but the only one which produced a reasonable bhp for to me was [cid:image001.gif at 01CBF4F6.FA6C6110] .  I guess this could have been where I messed up but all the other possibilities generated kW numbers which didn't make sense (quite large or negative).



From: Paul Diglio [mailto:paul.diglio at sbcglobal.net<mailto:paul.diglio at sbcglobal.net>]
Sent: Wednesday, April 06, 2011 5:51 PM
To: Carol Gardner; M. Shields
Cc: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: Re: [Equest-users] 90.1 Supply Fan Energy

Michael:

Just out of curiosity, what did you calculate for the kW/CFM?

I have modeled buildings where the ventilation fans use much more energy than the heating and cooling energy combined.

Paul Diglio


________________________________
From: Carol Gardner <cmg750 at gmail.com<mailto:cmg750 at gmail.com>>
To: M. Shields <mshields at fstrategies.com<mailto:mshields at fstrategies.com>>
Cc: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Sent: Wed, April 6, 2011 5:46:30 PM
Subject: Re: [Equest-users] 90.1 Supply Fan Energy

Yes, that is correct and obviously there isn't a large number in there. The default is 0.5 cfm/sf. I really think it's your outdoor air. Try changing it to cfm/person.
On Wed, Apr 6, 2011 at 1:47 PM, M. Shields <mshields at fstrategies.com<mailto:mshields at fstrategies.com>> wrote:
Do you mean under the Systems->Fans->Flow Parameters Min Flow cfm/ft2?  I left this blank thinking that equest would then size the flow rate based on my design parameters for supply air temp and zone temp as Appendix G requires, is that incorrect?

From: Carol Gardner [mailto:cmg750 at gmail.com<mailto:cmg750 at gmail.com>]
Sent: Wednesday, April 06, 2011 4:04 PM

To: M. Shields
Cc: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: Re: [Equest-users] 90.1 Supply Fan Energy

Under the HVAC systems tab, go to the Air Flow screen and look for the field labeled cfm/sf. It usually defaults to 0.5 cfm/sf, something I've been going on about a bit lately on this listserv. A better number is 1 or so, usually but it depends on building type, exposure, etc. For you, make sure it is not too large. I suspect, however, that this is not where your problem is. I suspect you have accidentally entered too much OA. I suggest you go back and simply put in the Standard 62 cfm/person rate and call it good. I have never specified it as a cfm rate but that sounds like not such a good idea for energy modeling. See if that helps.
On Wed, Apr 6, 2011 at 12:57 PM, M. Shields <mshields at fstrategies.com<mailto:mshields at fstrategies.com>> wrote:
I looked at the ACH based on the SV-A system CFM and the volume of the zone and it does seem high (2-5 ACH), but I let equest calculate the system CFM for me.

I have one system per zone so I specified OA in the zone tab as a overall CFM rate based on the 62.1 calculations I did.

From: Carol Gardner [mailto:cmg750 at gmail.com<mailto:cmg750 at gmail.com>]
Sent: Wednesday, April 06, 2011 3:37 PM
To: M. Shields
Cc: equest-users at lists.onebuilding.org<mailto:equest-users at lists.onebuilding.org>
Subject: Re: [Equest-users] 90.1 Supply Fan Energy

Check your cfm/sf number in your systems, that would definitely cause this if it's set too high. Also what field are you using for your OA ventilaton?

Carol
On Wed, Apr 6, 2011 at 12:22 PM, M. Shields <mshields at fstrategies.com<mailto:mshields at fstrategies.com>> wrote:
Hi all,

I am in the process of breaking out the fan energy from my HVAC system per ASHRAE 2004 G3.1.2.1, following G3.1.2.9.  The amount of energy coming from my fans seems massive and I'm not sure if that is typical, or if I've messed something up.  I started by setting the temperature difference of 20 degrees between supply and desired temp (55/75 cooling and 90/70 for heating) and specified 62.1-2004 ventilation for outside air.  I then ran equest and used the SV-A reports to get supply CFM for each system.  From that number I then used Table G3.1.2.9 to get BHP for each system, which I then converted to kW using the formula in ASHRAE.  I then took that number and divided by CFM to get kw/CFM and entered this number on the system->fan tab.  When I go to the SV-A report the system kW is exactly what I calculated it to be, but the energy required for my ventilation fans is about the same as my heating and cooling load combined, which seems outrageous.

Is this typical?  Have I entered it In the wrong way? I would think fan consumption would typically be a fraction of the actually heating and cooling energy.  Thanks for any thoughts.

____________________________
Michael Shields
Facility Strategies Group, LLC
1012 Market Street, Suite 307
Fort Mill, SC 29708
Phone: 803-493-4507
Fax: 803-548-2511
Email: mshields at fstrategies.com<mailto:mshields at fstrategies.com>


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