fossil hybrid or electric

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Hi - I've got an appendix G baseline system question. Here's the
scenario, let me know what you think.

Proposed Building:

1. Multi-family residential, amenity/common space < 20,000 square
feet

2. Electric baseboard heat in the units

3. Condensing gas furnaces in the corridors and back of house spaces

4. VRF in amenity common spaces

My understanding is that because my non-predominant condition
(common/amenity space) is less than 20,000 square feet, I use a single
system for the whole baseline building (G3.1.1a). I've got a mix of gas
and electric heat in the baseline building, does this mean that my base
system is PTAC with a hot water boiler?

Thanks!

Morgan Heater, P.E.

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Morgan,

My vote is for PTAC with hot-water boiler.

Regards,
Bill

[Senior Energy Engineer 28Jun2012]

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Morgan,

I agree with Bill. I find this LEED CIR (see below) to be helpful in
determining how fossil/electric hybrid systems should be handled. Your
scenario sounds similar to item 5 under the examples of systems that would
be modeled with fossil fuel heating.

LEED Interpretations
11/1/2011 *ID#10132*

- MPR/Prerequisite/Credit: *EAp2: Minimum Energy Performance*
- Primary Rating System: *Core & Shell v2.0*

Email Ruling

Clarification is requested regarding when a building heat source in Table
G3.1.1A should be identified as "Fossil/Electric Hybrid" versus "Electric".

The ASHRAE 90.1-2007 User's Manual states that a fossil/electric hybrid
source "refers to a system with any combination of fossil and electric
heat, and the baseline system for this is a fossil fuel system". Therefore,
the heating source for the proposed building would be considered "Fossil
Fuel" or "Fossil/Electric Hybrid" if the building uses any fossil fuel
source for space heating (including backup heating or preheating), and the
baseline building heat source would be fossil fuel.

Exception: ASHRAE 90.1 Section G3.1.1 Exception (a) stipulates additional
system type(s) for non-predominant conditions (i.e.
residential/non-residential or heating source) if those conditions apply to
more than 20,000 square feet of conditioned floor area.

EXAMPLES OF BASELINE HEATING SOURCE DETERMINATION:

The Baseline heat source from Table G3.1.1A for the following Proposed Case
system types would be fossil fuel since the proposed system design includes
a combination of fossil and electric heat:

1. Variable air volume system with gas furnace preheat and electric reheat
2. Packaged terminal heat pumps with outside air tempered by fossil fuel
furnace
3. Water source heat pumps with fossil fuel boiler
4. Ground source heat pumps with backup fossil fuel boiler
5. 90,000 square feet is conditioned by a variable air volume system with
electric reheat, and 10,000 square feet is conditioned with fossil fuel
furnaces

The following buildings would be modeled with an additional system type
with a different Baseline heating source in accordance with Section G3.1.1
Exception (a):

1. 90,000 square feet is conditioned by a variable air volume system with
electric reheat, and 20,000 square feet is conditioned with Packaged DX
systems with fossil fuel furnaces. In this case, the 90,000 square feet of
area would be modeled with an electric heat source in the Baseline Case
(System Type #6 - Packaged VAV with Electric PFP Boxes), and the 20,000
square feet of area would be modeled with a fossil fuel heat source in the
Baseline Case (System Type #3 - Packaged Single Zone AC with fossil fuel
furnace).
2. 50,000 square feet is conditioned by water source heat pumps with a
fossil fuel boiler, and 25,000 square feet is conditioned by electric heat
pumps. In this case, the 50,000 square feet of area would be modeled with a
fossil fuel heat source in the Baseline Case (System Type #5 - Packaged VAV
with hot water reheat), and the 25,000 square feet of area would be modeled
with an electric heat source in the Baseline Case (System Type #4 -
Packaged Single Zone Heat Pump).

Applicable Internationally.
Formal Inquiry

Table G3.1.1A lists two possible categories for the building heating
source:
(1) Fossil fuel, fossil/electric hybrid, & purchased heat;
(2) Electric and other.

In cases where the proposed building design includes both a natural gas
heating source and an electric heating source, when should the heat source
in Table G3.1.1A be identified as "Fossil/Electric Hybrid" versus
"Electric"?

Robby Oylear, PE, LEED AP

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I think that in Morgan's case using PTHP baseline is also justified. From
what I understand, the project is a residential buildings with electrically
heated living units and gas-heated common spaces. Applying G3.1.1 to the
project, predominant conditions are residential with electricity as heat
source; non-predominant conditions are non-residential with fossil fuel
heat. Based on G3.1.1 exception (a), non-predominant conditions accounting
for less than 20,000 SF may be ignored when selecting the baseline system
from Table G3.1.1A, thus the baseline for the project is PTHP. If
residential units were served by WSHP, then predominant conditions would be
residential with Fossil/Electric Hybrid heating source, and the baseline
system would by PTAC.

Maria

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The interpretation issued by the ASHRAE Standard 90.1 Committee agrees with Maria.

http://www.ashrae.org/File%20Library/docLib/StdsInterpretations/IC-90-1-2007-11.pdf

Michael Rosenberg, CEM, LEED AP

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Thanks for sharing this Michael!

Robby, I don't believe LEED CIRs are precedent-setting. In addition, many
Appendix G models are developed for programs other than LEED (e.g. incentive
programs) where LEED rulings don't matter. I would use PTHP on a LEED
project for the situation described by Morgan, referencing G3.1.1 exception
(a), footnote to Table G3.1.1A, and the ASHRAE interpretation that Michael
brought up. I don't think CIR meant to change ASHRAE rules, it just
misinterpreted how they apply to this example . For Vinay's project, I would
use PTAC in the baseline since apartments use both gas and electricity as
heating source, so fall under Fossil/Electric Hybrid category.

Maria

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Well that 90.1 Interpretation makes everything clear as mud. There is no formal definition for "hybrid" system in the Standard, and it is only explained in the User's manual. A "predominantly" electric-heated system with some fossil fuel is still a hybrid. Otherwise, what is a hybrid system?

[Senior Energy Engineer 28Jun2012]

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Bill,

I did not dig for an official definition, but thought that hybrid means that
the same area is heated with both electricity and fossil fuel, such as with
WSHP or the system in Vinay's example. If some areas are heated only with
electricity, and others only with fossil fuels, I would apply the
non-predominant condition rule to decide on the baseline HVAC.

Maria

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In the ASHRAE interpretation that Mr. Rosenberg highlighted, the interesting thing that is missing is ventilation and exhaust. What if the questioner had said 'exhaust fans in the guestroom toilets, no ventilation at the PTAC, transferring air under the hallway door, outside air in rooftop unit determined based on needed guestroom toilet make up'? Would that have made it a "hybrid"?

I suspect the design did that and thus heated outside air at the rooftop with natural gas. And the interpretation let the project use the entirely electrically heated baseline with a good portion of electric resistance heating. While USBGC/GBCI anticipated that (#2 in their list) and said "hybrid".

It is increasingly tougher problem to avoid crediting fuel switching. We will all just keep doing the best we can.

Paul Riemer, PE, LEED AP BD+C

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Thanks for all the discussion. I think a literal interpretation of the
code would actually define the baseline as PTAC with a hot water boiler
for heating. The ASHRAE interpretation actually appears to be a
misinterpretation to me, but as the "misinterpretation" makes more sense
than the code, I'm going to go with it. It is a little quirky, as the 80%
electric vrs 20% gas seems pretty arbitrary. Can we use this on non-res
projects too?

The ventilation scheme described by Mr. Riemer doesn't meet the IBC,
you're not allowed to use a corridor as a duct. Dwelling unit or guest
room doors are supposed to be weather-stripped and make-up air is supposed
to come from the exterior through trickle vents or some other sort of
make-up opening. This doesn't mean that many buildings aren't built that
way of course.

Morgan Heater, P.E.

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