Hi All,

I am doing an energy simulation model in eQuest. I noticed that the

load through roof conduction in LS-C is zero where as in LS-B (for

plenum space) it is showing some load.

As per my understanding this load should appear in LS-C also.

Also, if I reduce the U value of the roof, the load through roof

reduces and can be accounted in LS-B only (plenum space).

Refer attached LS-C and LS-B reports.

Any help will be appreciated.

Thanks & Regards

Bhartendu Awasthi

Hi,

This happens because the load calculation is made with a fixed temperature

(see picture). All the spaces, include plenum zones, have the same

temperature by default (70?F). So, all roof load appears in the plenum

spaces. To considerate the roof load, you have to change this temperature in

plenum spaces and a part of roof load will appear like "internal surface

conduction". But I'm not sure if this is a good practice. Maybe someone else

has an opinion about that.

Rodrigo Cerqueira

De : equest-users-bounces at lists.onebuilding.org

[mailto:equest-users-bounces at lists.onebuilding.org] De la part de Bhartendu

Awasthi

Envoy? : 8 ao?t 2013 06:35

? : equest-users at lists.onebuilding.org; bldg-sim at lists.onebuilding.org

Objet : [Equest-users] Roof Conduction Load

Hi All,

I am doing an energy simulation model in eQuest. I noticed that the load

through roof conduction in LS-C is zero where as in LS-B (for plenum space)

it is showing some load.

As per my understanding this load should appear in LS-C also.

Also, if I reduce the U value of the roof, the load through roof reduces and

can be accounted in LS-B only (plenum space).

Refer attached LS-C and LS-B reports.

Any help will be appreciated.

Thanks & Regards

Bhartendu Awasthi

In the space screen for the plenum zones, there is a check box "include in reports" (or something like that. Because it is unchecked, neither its conduction loads, nor its area are included in the building level reports. Checking this box for all plenums will add the roof and plenum wall conduction load to the LS-C report but the area reported will include plenum area.

Brian Fountain

Hi,

This happens because the load calculation is made with a fixed temperature

(see picture). All the spaces, include plenum zones, have the same

temperature by default (70?F). So, all roof load appears in the plenum

spaces. To considerate the roof load, you have to change this temperature in

plenum spaces and a part of roof load will appear like "internal surface

conduction". But I'm not sure if this is a good practice. Maybe someone else

has an opinion about that.

Rodrigo Cerqueira

De : equest-users-bounces at lists.onebuilding.org

[mailto:equest-users-bounces at lists.onebuilding.org] De la part de Bhartendu

Awasthi

Envoy? : 8 ao?t 2013 06:35

? : equest-users at lists.onebuilding.org; bldg-sim at lists.onebuilding.org

Objet : [Equest-users] Roof Conduction Load

Hi All,

I am doing an energy simulation model in eQuest. I noticed that the load

through roof conduction in LS-C is zero where as in LS-B (for plenum space)

it is showing some load.

As per my understanding this load should appear in LS-C also.

Also, if I reduce the U value of the roof, the load through roof reduces and

can be accounted in LS-B only (plenum space).

Refer attached LS-C and LS-B reports.

Any help will be appreciated.

Thanks & Regards

Bhartendu Awasthi

Hi All,

The LS-C report comes from the LOADS simulation portion of DOE 2.2. The

plenum reference temperature used in LOADS is adjusted in the SYSTEMS

portion of the program and the loads in the spaces adjacent to the plenum

are corrected

correspondingly.

Dave Wood

Just to repeat what David and Rodrigo have already sad, DOE-2 calculates conduction loads

in a two step process. In LOADS, a constant reference temperature is specified, which

DOE-2 uses to calculate the amount of heat flow through the surfaces. This is the LOADS

loads summarized in the LS-E and LS-F reports. In SYSTEMS, DOE-2 calculates the true

temperature of the zone based on the loads passed from LOADS, as well as the actions of

the HVAC system. In the course of this calculation, the heat flows ("loads") from LOADS

are adjusted, but this is not done surface by surface, but in total using the so-called

"Zone Conductance" times the difference between the actual zone temperature and the

reference temperature used in LOADS. At this point, heat flows through internal walls are

also considered, using the difference between this hour's zone temperature and the

previous hour's zone temperature of the adjacent space, e.g., the Attic or Plenum. If you

are interested in this number, I believe it can be extracted in an hourly report.

In reference to Rodrigo's question, I don't see any benefit in forcing a heat flow between

the room and the attic by using different reference temperatures. You will simply get a

constant heat flow at that temperature difference throughout the simulation. In reality,

attics are generally colder than the room in the winter and warmer than the room in the

summer, so the derived heat flow with this temperature difference doesn't get you any

closer to reality, in my opinion.

Joe Huang

Joe, Just wondering, given your expertise how would you compare this to

the heat balance method.., and what do you think is more realistic?

*Jeremiah D. Crossett*

DOE-2 does not simulate radiation.

Just to repeat what David and Rodrigo have already sad, DOE-2 calculates conduction loads

in a two step process. In LOADS, a constant reference temperature is specified, which

DOE-2 uses to calculate the amount of heat flow through the surfaces. This is the LOADS

loads summarized in the LS-E and LS-F reports. In SYSTEMS, DOE-2 calculates the true

temperature of the zone based on the loads passed from LOADS, as well as the actions of

the HVAC system. In the course of this calculation, the heat flows ("loads") from LOADS

are adjusted, but this is not done surface by surface, but in total using the so-called

"Zone Conductance" times the difference between the actual zone temperature and the

reference temperature used in LOADS. At this point, heat flows through internal walls are

also considered, using the difference between this hour's zone temperature and the

previous hour's zone temperature of the adjacent space, e.g., the Attic or Plenum. If you

are interested in this number, I believe it can be extracted in an hourly report.

In reference to Rodrigo's question, I don't see any benefit in forcing a heat flow between

the room and the attic by using different reference temperatures. You will simply get a

constant heat flow at that temperature difference throughout the simulation. In reality,

attics are generally colder than the room in the winter and warmer than the room in the

summer, so the derived heat flow with this temperature difference doesn't get you any

closer to reality, in my opinion.

Joe Huang

Jeremiah,

Your question is not quite on the mark because what I've described is downstream from the

calculation of room loads where techniques such as the Weighting Factor or the Heat

Balance methods come into play. Before I get to that issue, though, I want to point out

that the "Adjust Loads" procedure I described is done automatically by DOE-2 and totally

invisible to the User; I did so only to explain whyBhartendu (the original poster) was

not seeing any conduction loads through the ceiling.

Now, back to this "Adjust Loads" procedure, there is no simulation program I know of that

solves simultaneously for the heat flows and the zone temperature. DOE-2 does it in the

two-step way that others and I have just described. When you said "compared to the Heat

Balance Method", I assume you're thinking of EnergyPlus. So, how does EnergyPlus meet

this problem? Whereas DOE-2 does one year of LOADS and then one year of SYSTEMS,

EnergyPlus does them in the same time step, but also sequentially. Therefore, EnergyPlus

calculates the heat flows using the zone temperature from the previous time step which are

then used in its HVAC simulation without adjustment. This should be okay when zone

temperatures don't change, but what about at morning start-up when temperatures could

change by 5-10 F within one hour? I've always wondered whether that's why EnergyPlus

gives much lower heating loads in mild California climates than does DOE-2? As far as

which procedure is "more realistic", I can't say.

Joe Huang

Jeremiah,

Your question is not quite on the mark because what I've described is downstream from the

calculation of room loads where techniques such as the Weighting Factor or the Heat

Balance methods come into play. Before I get to that issue, though, I want to point out

that the "Adjust Loads" procedure I described is done automatically by DOE-2 and totally

invisible to the User; I did so only to explain whyBhartendu (the original poster) was

not seeing any conduction loads through the ceiling.

Now, back to this "Adjust Loads" procedure, there is no simulation program I know of that

solves simultaneously for the heat flows and the zone temperature. DOE-2 does it in the

two-step way that others and I have just described. When you said "compared to the Heat

Balance Method", I assume you're thinking of EnergyPlus. So, how does EnergyPlus meet

this problem? Whereas DOE-2 does one year of LOADS and then one year of SYSTEMS,

EnergyPlus does them in the same time step, but also sequentially. Therefore, EnergyPlus

calculates the heat flows using the zone temperature from the previous time step which are

then used in its HVAC simulation without adjustment. This should be okay when zone

temperatures don't change, but what about at morning start-up when temperatures could

change by 5-10 F within one hour? I've always wondered whether that's why EnergyPlus

gives much lower heating loads in mild California climates than does DOE-2? As far as

which procedure is "more realistic", I can't say.

Joe Huang

To try to add to that, the sequential calculation of load then system

reaction is more a result of the calculation engines preferring to use only

linear solvers than the fundamentals of the procedure used. The main issue

arises because the load generally depends on the action of the HVAC system

(e.g. surface temperatures depend on convection coefficients) and the HVAC

system action depends on the load, which is a nonlinear relationship.

The Heat Balance Method (weird name because there is no such thing as a

conservation of heat) simply enforces an energy conservation equation at

each discrete temperature/enthalpy node. For N nodal temperatures, this

gives a set of N simultaneous equations to solve at each time step. If all

of these equations are linear, the calculation engine can use more

efficient linear solvers to resolve the temperatures. Nonlinear solvers

would require some sub-timestep iterations, which adds calculation time,

and there's also the problem that nonlinear solvers do not always converge

and can be sensitive to initial guesses. The preference for linear solvers

is also the reason that radiation is usually linearized.

Strictly speaking, Energy Plus is an "integrative" modeling tool and does

solve the loads and system response "simultaneously". The use of previous

time step temperatures are used because the energy balance equations

general involve transience which requires a finite difference approximation

of heat storage. As Joe pointed out, this can lead to errors especially

when large time steps are used in the vicinity of large temperature swings.

The integrative technique of Energy Plus is to first assume the thermostat

setpoint is exactly met, then calculate the system response to meet this

load, then to recalculate the zone temperature caused by this system

action. I'm not sure how many of these iterations are made each timestep,

but there is some information passing back from the system reaction to the

zonal temperatures. Similarly, this method (they call it Predictor

Corrector method) is used to model system controllers. Again, I'm not sure

if the information loop stops with one iteration or continues until a

convergence criterion is met.

The weight factor method used by DOE-2 relies on the convolution principle

of linear systems, so there is no way to obtain a simultaneous solution

(unless there is some way to first deconvolve the weight factors). On the

other hand, network or quasi-network solutions like the Heat Balance Method

have the ability to solve for space temperature and HVAC reactions

simultaneously, but this is not a requirement of the method, and true

simultaneous HBM solvers are either rare or do not exist because of the

computational expense.

Aaron

Jeremiah,

Your question is not quite on the mark because what I've described is

downstream from the calculation of room loads where techniques such as the

Weighting Factor or the Heat Balance methods come into play. Before I get

to that issue, though, I want to point out that the "Adjust Loads"

procedure I described is done automatically by DOE-2 and totally invisible

to the User; I did so only to explain why Bhartendu (the original poster)

was not seeing any conduction loads through the ceiling.

Now, back to this "Adjust Loads" procedure, there is no simulation program

I know of that solves simultaneously for the heat flows and the zone

temperature. DOE-2 does it in the two-step way that others and I have just

described. When you said "compared to the Heat Balance Method", I assume

you're thinking of EnergyPlus. So, how does EnergyPlus meet this problem?

Whereas DOE-2 does one year of LOADS and then one year of SYSTEMS,

EnergyPlus does them in the same time step, but also sequentially.

Therefore, EnergyPlus calculates the heat flows using the zone temperature

from the previous time step which are then used in its HVAC simulation

without adjustment. This should be okay when zone temperatures don't

change, but what about at morning start-up when temperatures could change

by 5-10 F within one hour? I've always wondered whether that's why

EnergyPlus gives much lower heating loads in mild California climates than

does DOE-2? As far as which procedure is "more realistic", I can't say.

Joe Huang