single zone system and plant

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My energy model is about done but of course I am now questioning myself somewhat.  My questions are as follows and I could really use some help!  This forum has been a great learning tool.

1. I have a baseline system type 3 with a baseline of 8 single zone systems, does each single zone system need its own heating/cooling plant? I have multiple thermal zones in each single zone system, but all the single zone systems are served by one plant.  This would increase my plants from 1 to 8, since I have 8 single zone systems in the baseline.

2. How do I properly model a mechanical room exhaust fan which turns on when room temp is above 85F?

3. Is there a way to model an instantaneous gas fired water heater?  It appears that modeling a water heater as a base utility would not show any energy savings as this models a heater as constantly heating the water vs instantaneously heating it when needed.

Thanks in advance for the help! ~Amy

Anonymous's picture

Hi Amy,

FYI - anon posts don't give you as many features as if you are logged in. You can get emailed when somebody responds with an awesome answer.

1) Yes, each plant needs to be modeled separately. This is required per "separate unloading". It is a pain, but required by GBCI

2) This can be done with a "reset and lockout schedule". Do a search on those terms and you should find what you are looking for. It's a little tricky and doesn't always work, but it's the only real way to do it.

3) You need to model an instantaneous hot water heater just like a std hot water heater. You will simply need to account for the standby losses in the std hot water heater (or make the instantaneous hot water heater more efficient by that amount). There is usually some sort of documentation on this in the mfg literature.

Bob's picture
Joined: 2010-06-30
Reputation: 475

Yes each needs to be modeled separately, but I think there's a limit as to how many cooling or heating plants you can have (21 comes to mind). Each will have it's own associated efficiency, as well with an unloading curve. I've used Bob's techniques for the reset and lockout schedule for Misc-Load unit heaters as well. As for you instantaneous water heater, there's not much I've found 'out there', but I think you will need to fine tune the best scenario for a hot water heater schedule that you can...maybe start with a schedule from NREL, then bring it down to normal, then look at a curve that better represents on/off, in lieu of warm up. Should look like a step curve (0,0), (0.05,100), (0.5,100), (1.00,100), not a linear or parabolic curve.

Be Sustainable -- Never let today use up tomorrow!.

Bobba_Fett's picture
Joined: 2010-10-01
Reputation: 229

When the Baseline calls for separate systems, ie non-predominant loads, I assign these systems to plants based on their size as that determines required minimum efficiency.  If I have multiple systems that are in the same size/efficiency range I assign them to the same plant if their schedules are the same.  Based on the responses above, it sounds like this may not be correct?

Are there any resources you can recommend that help explain how the baseline systems and plants are supposed to be modeled?

BCPratt's picture
Joined: 2012-01-05
Reputation: 1

There aren't any resources besides tons of LEED reviewer comments.

The comment says something like, "You need to show separate unloading"... meaning, you need 1 plant per unit.

However, because of TRACE's limitation, you can use the "thermodynamically similar clause" of Appendix G3.1.13. 

Separate unloading for most of this stuff is irrelevant - because the unloading curve is linear, and all of the unloading comes from ambient relief (meaning similar equipment can be combined into larger plants without affecting unloading).

In my opinion, this "requirement" came from eQUEST users, because eQUEST has "the plant" built right into the system for unitary equipment (ie rooftops, ptacs, etc).

Bob F's picture
eQUEST UserLEED EA Credit 1 TrainingTRACE 700 User
Joined: 2010-06-30
Reputation: 18

Just out of curiosity - could someone point me to the requirement that talks about needing separate unloading?  I don't recall seeing this in 90.1 anywhere - but I may have missed it.

Right now I'm finishing up an Energy Model for LEED- I've got a handful of electrical closet/server room type units, and a Kitchen area that I'm taking exception G3.1.1.b to model as system 3 since its got different usage/loads than the rest of the building.  Right now all of the kitchen zones (proposed is VAV) and the electrical/server units are sharing the same fossil fuel furnace plant.

I was about to divide these up into separate systems and plants, but ran into the limit Bobba Fett mentioned above of a maximum of 21 heating plants in a trace alternative.  I think I'll split out the small units, but leave the Kitchen zones on a single furnace plant.  Good to have G3.1.13 ready if they question it.

This seem like a lot of extra work for something that will at most have a marginal impact on the results.

Lancelot's picture
Joined: 2012-06-11
Reputation: 8


I agree with the "marginal impact". The separate unloading is just something that a large number of us have received comments on, and thus, has become the de facto "rule". There is nothing in 90.1 that I can quote, but I have seen the comment about 1 hundred times (it was a big issue that people called in about when I worked at CDS)

One of the likely reasons for this "rule" is that the baseline plant's efficiency actually changes based on size. Therefore, making large plants (ie having 1 plant for the whole baseline) actually effects your bottom line, and it works in your favor.

In my experience, just try to do something reasonable and quote G3.1.13. 

(try to do something thermodynamically similar and make a couple of plants). I often hit 20 plants, but I try to keep it under 15.

Take advantage of THERMAL BLOCKS... We hammer on this for anyone who has gone through the LEED + TRACE 700 case study.

Bob F's picture
eQUEST UserLEED EA Credit 1 TrainingTRACE 700 User
Joined: 2010-06-30
Reputation: 18