electric heating issues

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Hello all,

I currently modeling a project for LEED NC v2009 submission for a corporate office building that's roughly 125,000 SF. The proposed design's primary HVAC system are AHUs + VAVs w/Electric Reheat supplemented with electric baseboard along hot water preheat generated from gas boilers. This system also uses chilled water with very efficient chillers from a DES. The baseboard is suppose to take care of the envelope loads and the VAV electric reheat coils will take care of the rest. Based on the size of the building, the baseline HVAC system shall be System #6: Packaged VAV with PFP Boxes.

I'm pretty down the road on the models and for some reason cannot get savings between the 2 models. The heating energy use on electrical consumption overrides any savings attributed from lighting, site lighting, cooling energy use, etc. Also, I know that in 90.1-2007, preheat must be model identical on the proposed and baseline models. I am having a hard time modeling both to be the same (inputs for preheat capacity are the same but the energy use is different). The gas consumption is significantly higher on the baseline model.

I double checked the loads from both models and the heating/cooling requirements are roughly the same so there's nothing wrong with the building shell/internal load inputs. Any guidance on how to reduce the consumption on the heating use for electrical consumption would be great! I am confused why the baseline model wouldn't require similar, if not more, heating requirements and isn't using more heating use for electrical consumption. Also, guidance to ensure that the preheat is model identical would help too!

Thanks!

Cliffnotes:

Proposed System: AHUs w/VAV Electric Reheat + Electric Baseboard//Chilled Water from DES//Hot Water Preheat

Baseline System: Packaged VAV with PFP Boxes

Issue #1: Heating electrical consumption is super high on proposed model despite similar loads for both models.

Issue #2: Preheat is not being model identical as required from 90.1-2007. Capacity and design inputs are same for building but baseline building is using significantly more gas.

William Mak, LEED Green Associate

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Will,

The first thing I would do is reconsider your selection of System 6 for
the Baseline. Since you have gas boilers for preheat, I would choose
System 5. System 6 is all electric, but you say your baseline gas
consumption is high, so you are currently modeling gas boiler(s) in the
baseline. Unfortunately, if you go with System 5, your baseline reheat
will be hot water which will save cost versus your proposed design.

Since both your baseboards and your reheat are electric in the proposed
design, your high electric use is probably not a baseboard control issue
(provided you are not showing many unmet load hours). One thing to
consider is your preheat leaving temperature - making it higher will
favor the gas boilers and reduce the zone electric heating. I'm guessing
that the requirement for identical preheat coil control in G3.1.2.3
means that you use the same preheat leaving temperature in the baseline
and proposed models.

Make sure you are modeling your ventilation identically between the
baseline and proposed models, unless you have DCV.

Check your reports to make sure you don't have a lot of simultaneous
heating and cooling going on. Maybe you can increase the cooling supply
temperature (and convince the HVAC designer to do it) to the highest
allowable temperature that meets cooling and dehumidification loads.

Your baseline may have similar, or even less, heating load depending on
the proposed envelope, due to the reduced lighting load in the proposed
design. With electric baseboard and electric reheat, it will be
difficult to show much heating savings unless you have a much better
envelope or ventilation heat recovery. We should all be red-flagging
electric reheat and baseboards whenever we see them on projects.

Regards,
Bill

William Bishop, PE, BEMP, LEED(r) AP

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See comments below, thanks!

William Mak, LEED Green Associate

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as per the baseline system discussion, i would use System 5. If you read
the 90.1 manual, it says if you use a fossil/electric hybrid heating system
to implement System 5 in the baseline.

Rob

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I thought the heating system only referred to the heating source itself? (not including preheat fuel source??)

William Mak, LEED Green Associate

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If the only fossil fuel type heating system is a preheat coil, there is a CIR that says you can use a Baseline system of the "Electric and Other" heating type.

Note that in this case, the Baseline will also have a preheat coil of the same fossil fuel type, as per Appendix G G3.1.2.3.

_____________
Demba NDIAYE

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Hi Will,

Sorry to say but I agree with Rob and Bill. If you are providing heated hot
water to the baseboards then you are using a hybrid system. Maybe I am
missing something, but it seems pretty clear.

Carol

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Sorry, I thought I put down that the baseboard and VAV reheat coils were electrical, not hot water. The hot water is only for the preheat.

William Mak, LEED Green Associate

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Neat discussion - looks like my position on this is going to go against the grain a bit =)!

First, the CIR Demba referenced looks like something of prime interest... I understand CIR's don't exactly hold water with the current LEED reviewer-ship, but it would sure help at times know whether history is on your side...

I can offer some recent directly related experience: Without having been aware of that particular CIR, I recently wrapped up a LEED project with HVAC gas furnace heating used only for morning warmup/preheat at the central AHU. The proposed system otherwise was effectively well-described by baseline system #6, with electric reheat accomplishing the vast majority of conditioned air heating (substantial heat recovery was a part of this), so I used baseline system #6. The reviewer on first pass stated the baseline needed to be system #5 due to the presence of gas heating, and I composed a comment response substantiating why the electrical heating was the "predominant condition," referencing the corresponding subnote to Table G3.1.1A... Had I been aware of a CIR to the same effect I would have included a reference. The model has since been approved and all resubmitted credits have been awarded =).

I can't guarantee your reviewer will consider the issue at hand in the same fashion, but in these sorts of cases applying an "all gas" baseline system against a "99% electric" proposed system can make for a truly unfair comparison when the utility rates are quite different.

For those not keeping score, I don't use a term like "unfair" lightly or often.

A completely literal reading of 90.1-2007 Table G3.1.1A may lead you to use system #5 over #6, and for that reason it's not wrong to say that's a "right" answer. On the other hand a case can be made, particularly if you can demonstrate that the HW preconditioning is a small fraction of the total space heating energy, that the intent of the "predominant condition" subnote speaks directly to this instance and can present another "right" answer. Without using a word like "unfair," be prepared to intelligently discuss why system #6 is fundamentally a better baseline for the purposes of establishing a performance rating against your proposed systems.

Finding and referencing that CIR Demba mentioned wouldn't hurt either... =)

~Nick

PS: On a related tangent... how do you guys find past CIR's? I can only ever get so far using google - is there some kind of archive currently within LEED online I've been missing?

NICK CATON, E.I.T.

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Nick,

Try this link. You may have to have a USGBC account.

https://www.usgbc.org/leedinterpretations/lilanding.aspx

Luke Wilson

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