Applies to

HES consumer

Hes pro

Home Energy Scoring Tool

Multifamily v1



X 

Thermal Distribution Efficiency
As documented in a companion
report (Warner 2005), the Home Energy Saver uses the hourly DOE2 thermal
simulation model to estimate heating and cooling consumption. The treatment of
air distribution duct losses in DOE2 has been revised in 2013 such that duct
losses are computed each hour throughout the year based on the circumstances
around the duct system and TMY weather data for the location being modeled.
The method employed estimates
the effect of duct materials and the type of space in which the majority of
their duct system is located, since duct losses differ significantly depending
on these factors. We used the ASHRAE 152 duct model to estimate duct losses for
use as an input to DOE2 (ASHRAE 1997a). Although this model is intended to
calculate seasonal or design duct efficiencies based on detailed diagnostic
testing, we assumed typical values for most of the inputs (such as duct surface
area, air film resistances, unsealed leakage rates, , and number of return
ducts) so that the number of inputs required of the user is reasonable. In the
implementation of the model, the duct efficiencies are calculated hourly,
depending on duct characteristics, location, buffering of unconditioned spaces
and potential regain of heat exchange (for instance, duct losses can moderate
the temperatures in buffered spaces such as crawlspaces, attics, and basements
such that thermal losses are reduced through surfaces enclosing the primary
conditioned space).
Users are able to specify
whether or not the ducts are insulated and/or sealed, and the duct location.
Insulated ducts are assumed to have R6 insulation. Based on the work of Lauvray
(1978) uninsulated ducts are assigned an insulation value of R2 (to account for
the thermal resistance of the external air film on the ducts which are
approximately 1.5 hr/sqft/F/Btu). It is important to note that the model is
quite sensitive to duct insulation level and only uninsulated metal ducts
should be input as uninsulated.
Unsealed ducts are assumed to
have a leakage of 15% of the total air handler flow. Because concerted duct
sealing efforts can typically reduce leakage by a significant amount, we assume
that sealed ducts have a leakage rate of 3%. As this is a critical input, users
are required to specify the duct location. If duct location is not input, its
location is inferred based on foundation types and typical building practices.
The ASHRAE 152 model
generates duct efficiencies for both the heating and cooling seasons, which are
computed hourly. An average computed duct efficiency is passed to the DOE2
model as an input to the thermal simulation for each hour. This hourly duct
efficiency is determined based on the type of heating and cooling equipment in
the house, the temperatures and enthalpy of the air surrounding the duct system
and the insulation and leakage characteristics of the duct system.
There are a number of
calculation steps in the estimates in the ASHRAE 152 model. First, the duct
location is defined. Within the calculations, this, in turn, sets how the
hourly environmental temperature around the ducts will vary and how much of the
losses from the ducts will be regained to the conditioned space. The location
data is summarized from the original 152 source, although some of the location
parameters are simplified in the HES calculation. The simplified regain
fractions used in the HES duct efficiency calculation are based on Walker, 1998
(LBNL40588), Tables 810. T_{in }is the inside of the home’s conditioned
zone. T_{design} in the case of our calculations is actually the hourly
air temperature on the TMY record. Regain fraction is the portion of the
computed duct losses that actually moderate unconditioned spaces and reduce
heat lost through adjacent building components such as floor of the ceiling to
attic interface.
Table 1
Heating System Location Temperatures

Duct Location

Conditions

Regain


Temperature, ^{o}F

Fraction

Attic

t_{design}+
7

0.10

Garage

t_{design}+
11

0.10

Crawl Space, Unvented, Uninsulated

(2t_{design}
+ 3t_{in})/5

0.60

Crawl Space, Unvented, Insulated Building Floor and Crawl
Space Walls

(t_{in}+3t_{design})/4

0.60

Crawl Space, Unvented, Insulated Floor Only

(t_{in}+5t_{design})/6

0.30

Crawl Space, Vented, Uninsulated

(t_{in}+t_{design})/2

0.60

Crawl Space, Vented, Insulated Building Floor and Crawl
Space Walls

(t_{in}+5t_{design})/6

0.63

Crawl Space, Vented, Insulated Floor Only

(t_{in}+8t_{design})/9

0.30

Basement, Uninsulated

(5t_{ground}
+2t_{design} +3t_{in})/10

0.50

Basement, Insulated Walls

(t_{ground}
+ t_{in})/2

0.60

Underslab

t_{ground}

0.20

Table 2
Cooling System Location Temperatures (IP)

Duct Location

Design Conditions

Regain


Temperature, °F

Fraction

Attic

t_{design}+16

0.10

Garage

t_{design}+
7

0.10

Crawl Space, Unvented, Uninsulated

(2t_{design}
+ 3t_{in})/5

0.60

Crawl Space, Unvented, Insulated Building Floor and Crawl
Space Walls

(t_{in}+3t_{design})/4

0.30

Crawl Space, Unvented, Insulated Floor Only

(t_{in}+5t_{design})/6

0.30

Crawl Space, Vented, Uninsulated

(t_{in}+t_{design})/2

0.55

Crawl Space, Vented, Insulated Building Floor and Crawl
Space Walls

(t_{in}+5t_{design})/6

0.60

Crawl Space, Vented, Insulated Floor Only

(t_{in}+8t_{design})/9

0.30

Basement, Uninsulated

(5t_{ground}
+2t_{design} +3t_{in})/10

0.50

Basement, Insulated Walls

(t_{ground}
+ t_{in})/2

0.60

Underslab

t_{ground}

0.20

Similarly, according to the
original ASHRAE standard, modified values of the absolute moisture content of
air outside versus inside are estimated for cooling influence from leaking duct
return air leakage. Below, we show the relevant portions of the ASHRAE Standard
152 calculation with the original equations in engineering units indexed to the
original standard equations.
Duct Areas
Surface
areas of supply and return ducts outside the conditioned space shall be
determined using Equations 63a and 63b, respectively:
A_{s}=0.27 A_{floor} [Eq.
63a]
A_{r}= 0.05 Stories A_{floor }[Eq.
63b]
Delivery Effectiveness (DE) for Heating
Systems
DE
is calculated using high capacity (and airhandler fan flow) for design
calculations.
The
supply and return conduction fractions, B_{s} and B_{r}, are calculated
with the following equations:
[Eq. 616]
where
r_{in}
is the density of indoor air [use 1.2 kg/m^{3} (0.075 lb/ft^{3})
at sea level], and Cp is the specific heat of air (0.24 Btu/(lb×°F). The duct leakage factors for the supply and
return sides shall be calculated using Equations 618:
[Eq. 618]
The
temperature rise across the furnace, Dt_{e}, is
calculated based on the equipment capacity (either input or estimated by DOE2),
E_{cap}, and the system air flow, Q_{e}.
For IP:
[Eq.
620]
The difference between the building and the ambient
temperature surrounding the supply, Dt_{s, }and return, Dt_{r}, shall be
calculated with Equations 621 and 622:
Dt_{r}
= t_{in}  t_{amb,r} [Eq. 622]
The heating delivery effectiveness shall be calculated using
Equation 623:
[Eq. 623]
Delivery Effectiveness (DE) for Cooling
Systems
The
delivery effectiveness is calculated using the system flow rate during cooling,
B_{s}
and B_{r} are determined using Equations 613 to 616; a_{s}
and a_{r} shall be determined using Equation 618. Dt_{r} shall be determined using Equation
622. t_{sp} is the supply plenum drybulb temperature,
which depends on latent load and shall be calculated using ACCA Manual S 2^{nd}
Edition (1997)^{5} or assumed to be 55°F. The equipment capacity (E_{cap}) for
cooling systems must be negative in Equation 625.
[Eq. 625]
6.3.3.3 Enthalpy
Calculations for Cooling Systems
The indoor air enthalpy shall be taken from prevailing indoor
conditions during the simulation. Enthalpies shall be calculated for return
duct locations using Equation 62b from the original Standard 152 calculations.
The drybulb temperature for those locations shall be determined from Table??a
or Table ?? The humidity ratio is the
difference between the indoor or outdoor air humidity ratio from that inside
the building and on the TMY weather tape.
h= 0.240t + w(1061 + 0.444t) [Btu/lb] [Eq. 62b]
It should be noted that the
calculated duct leakage moisture enthalpy effects in the HES model are modified
to account for the fact that only a part of the latent loads from additional
added returnside moisture will be removed, in ratio of the sensible heat
fraction of the air conditioning equipment (assumed to have an sensible heat
ratio or SHR of 0.75). The added moisture not removed, causes the space
relative humidity to rise, rather than additional machine power to be expended.
Solar Heat Gain Reduction
If the building simulated has
a radiant barrier, lowabsorptance roof or tile roof, it is assumed to benefit
for cooling duct efficiency calculations if the ducts are located in the attic.
The governing equation to calculate t _{amb,s}—the temperature of air
surrounding the duct from ASHRAE 152 procedure is:
t_{amb,s} = 0.7(t_{attic})+0.3(t_{in})
6.5.4 Seasonal Distribution System
Efficiency
The
delivery effectiveness corrected for regain, DE_{corr}, shall be
calculated using Equation 638:
[Eq. 638]
The losses for nonducted
systems and systems with multiple duct locations in HES are described by the
following logic:
 Unducted systems such as room air conditioners and minisplit heat
pumps are not assumed to have any losses to unconditioned areas.
 Ducted system with the ducts located inside the conditioned space
are not assumed to have meaningful losses.
 Based on the fact that hydronic distribution system do not leak fluid
and have much lower heat transfer surface areas, they systems are assumed
to have losses of 5% if the piping is uninsulated and 2.5% if insulated.
 If ducts are located in multiple space types, the DOE2 simulation
is run for each location and the areaweighted result is used to compute
heating and cooling loads. Any portion of the ducts located inside the
conditioned space (including interior returns) is assumed to have no meaningful
losses.
Efficiency of Boiler Pipes
Boiler pipes are assumed to have a baseline efficiency of
95%, Users are able to indicate whether their pipes are
insulated. For insulated pipes we stipulate efficiency of 97.5%.