Excessive Heating Savings

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Hello All,

I am having some issues with modeling the heating and cooling systems in
mid-rise multifamily buildings. I have a number of these projects and
they all use similar and fairly simple systems, a single split system
heat pump serving each dwelling unit. When modeling these projects I
have followed the guidelines from Appendix G with additional guidance
from the ENERGY STAR High Rise program's simulation guidelines. No
matter the project location, I am getting heating savings that seem very
out of whack with the upgrades made in the proposed design (higher
efficiency HVAC, better windows, upgraded insulation, etc.), the
particular model I am looking at now has heating savings over 90%.

An outline of my system inputs follows. I developed this approach based
on the guidelines discussed above, past listserv discussions, and the
input of other local energy modelers.

Proposed Model

* System type set to Residential System 2

* System capacities based on actual design

* Electric Input Ratios

o Cooling

EIR=((1/(SEER*0.778+1.4)-0.012167)/((1/3.413)+0.012167)

SEER 14 yields 0.227 CEIR

o Heating

EIR= (1/((HSPF*0.28+1.13)*3.412)-0.012167)/((1/3.412)-0.012167)

HSPF 7.7 yields 0.274 HEIR

* Fan Power set to zero given that fan power is accounted for in
SEER and HSPF efficiency ratings

Baseline Model

* System type set to PTHP, Packaged Terminal AC with heat pump
heating, per Appendix G and ENERGY STAR simulation guidelines.

* System capacities set by equest, 15% oversized for cooling and
25% for heating

* Electric Input Ratios

o Cooling

EER per 90.1: EER = 12.3-(0.213*Cap/1000)

Capacity set to 15,000 Btu/hr yields 9.1 EER

EIR= (1/EER-0.012167)/((1/3.413)+ 0.012167)

Yields 0.32 CEIR

o Heating

EIR= 3.2-(0.026*Cap/1000)

Capacity set to 15,000 Btu/hr yields 2.81 COP

EIR=1/COP

Yields 0.356 HEIR

* Fan Power set to zero given that these are packaged units

Outside air rates, infiltration levels, and supplemental heating are set
identically in both models. The only other issue of note is that the
proposed design has unit heaters (electric resistance heat) in the
stairwells, which I have modeled as PTHPs in the baseline design with
heating only. I would assume this would be a loss for the proposed
design, though negligible, given the increased efficiency of the PTHPs
versus electric resistance, but I'm not seeing that reflected in the
results.

If anyone has advice on what I might be missing I would appreciate it.
Tips on places to check myself in the .sim file would also be great.

Thanks in advance,

Robert Stephenson, LEED AP



Robert Stephenson's picture
Joined: 2011-09-30
Reputation: 0

My first guess would be to look into the residential systems further. Read
the topic on this particular system in the help file. As far as I can remember,
this system will automatically, reduce the set point temperatures, and flow
rates (or something similar) based on the occupancy. This can provide
significant savings, especially if you have a low occupancy rate. So I would
start from there. If you attach you input file, we (the online community) can
take a look at it hopefully find the true issue.

-Eddie

Eddie Corwin's picture
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Joined: 2011-09-30
Reputation: 0

Thanks Eddie,

As a trial, I ran my proposed design again with the system type set to Packaged Single Zone. This increased my heating usage and decreased the savings to ~75%. Out of curiosity I started a new project again and set my parameters in the wizard to a split system single zone heat pump (residential) and ended up with packaged variable volume variable temperature for my system type in detailed mode. Any advice on what system type I should be specifying and why the difference are there? I looked to the help file and found very little except the table outlining the different system types. Is there a layer I am missing?

Also, how would I go about backing the fan power out of my efficiencies as was suggested in another response?

I have attached my .inp and .pd2 files if anyone has some time to take a quick look.

Thanks again,

Robert Stephenson, LEED AP

Robert Stephenson's picture
Joined: 2011-09-30
Reputation: 0