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Dear Chris and all

I had mentioned to Chris outside the forum (sorry should have copied all in) that it might be useful carrying out a tracer gas decay test. As many of you note the hot wire anenometer will not sense direction and the use of pitot tubes or a flow grid may not be suitable for such low pressure ranges. This is the reason that I suggest a decay method may be the best way to analyse the overall flow rate through the room. This is the way I measured flow rates in both model environments and test rooms when I did my PhD. The main reason for this is the unsteady nature of ventilation flows themselves. Ventilation by its nature is not unidirectional across an opening it is an unsteady I.e. varying and fluctuating pressure across the opening that if you measure it with a transducer will have positive and negative amplitude all the time. It is the differences in the combination of buoyancy and wind driven pressures around the building that cause a variance in the mean pressure differences that drive the flow in one or other direction overall. Obviously this combines with other factors such as opening position, size, etc. The tracer gas test will get around this unsteady issue as you are then looking at a time decay method that will give you the decay rate and therefore a direct relationship to the flow rate into a room. Fairly easy to test, a relevant gas sensor, datalogger (though many sensors now have them built in) and a can of trace gas like SF6 or preferably something less ozone depleting but easy to test for.

I recommend that anyone studying ventilation flow reads the book on ventilation theory and measurement by David Etheridge and Matts Sandberg. It is an invaluable resource and will answer many questions people may have regarding these sorts of studies.


Dr Paul Carey

Paul Carey's picture
Joined: 2011-10-02
Reputation: 0

Hey all,

I am looking at an energy plus based sim program and one of the major selling points would be doing the load calcs in the same program as the energy modeling. From what I have read, the heat balance method that E+ uses should be more accurate than the transfer function method that a widely used load calc program like HAP would use. However the results are different so I wanted to get people?s thoughts on the subject.

I did a test case, a 20 x 20 office space (384 ft conditioned area) with one southern facing window.

- 10 ft tall

- 4 People

- 1 W/ft2 Lighting

- 0.5 W/ft2 Equipment

- ASHRAE Office default schedules

- Basic Walls and Roof, ASHRAE minimum windows (U=.55,SHGC=.4)

- Design Weather ? 95 db / 75 wb

I?m attaching some of the results, but overall the E+ calc has a lower peak load. Occupant, lighting and plug loads seem to follow their schedules. HAP looks like it very heavily weights the delayed load effect (TFM) even though I used a medium weight (70lb/ft2) wall (fyi - changing it to a lightweight wall has some but not a huge effect). HAP also ignores the slab heat loss in its cooling calculations. This shows up most clearly in the unoccupied zone temperature. In HAP the residual loads drive the temperature up to 82 where in the E+ calc the slab loss dominates, driving the temperature into the 60s. I could see leaving it out of the peak to be conservative but then allowing the unocc temperature to jump up overnight and having to deal with that load seems a bit too unrealistic. HAP also seems to over predict the peak roof conduction gain, probably due to the use of the sol-air temperatures, which I have read can over predict gains.

I tend to want to believe the E+ analysis because I know that it?s calculation methods are in general more rigorous but I am very interested what others may think, sim engines are known for being poor design load predictors (I?m looking at you eQuest) and HAP is a very established and trusted program.

Thanks in advance for anyone that feels like diving into this with me.

Brendan Hall

Hall, Brendan's picture
Joined: 2011-09-30
Reputation: 1

Dear Brendan,

Caveat emptor: I have not attempted this type of comparison, since my ?day job? is energy modeling with E+, not load calculations.

In no particular order:

1. I suspect you are correct to assume that E+ has a more rigorous load calculation routine, but do not know much about HAP

2. I applaud your effort to ?right size? equipment. There is a lot of anecdotal and some published information which suggests up to 50% oversizing for cooling systems, and everyone wins if less capacity can be installed and still work properly.

3. I have the feeling that HAP and E+ define various load differently.

a. There?s no other reason why HAP would show a solar load in the middle of the night; that?s plainly wrong if we?re talking about true solar impact.

b. Similarly for roof conduction, ASHRAE and thermodynamics tell us that a roof will probably radiate energy to the night sky at least in the pre-dawn hours (and thus give a negative load to the zone). HAP is showing low, declining, but still positive values at all hours. Not knowing your roof construction, however, I am not sure what to expect.

c. Lighting load for E+ reflects your lighting input energy, but HAP appears to be showing something like ?net load to the cooling system?

d. Ditto for Occupant loads

e. If it is true that E+ and HAP define things differently, I don?t think you can directly compare loads as you are doing in the ?E+ loads? and ?HAP loads? charts. HAP seems to miss the lunchtime load dip altogether, for example. Did you check the Output:Variable, zonename, Zone/Sys Sensible Load Predicted, Timestep? Or some other variable? (or check with OpenStudio or DesignBuilder to see what their graph data represents?)

4. Floor conduction was a discussion thread on the EnergyPlus Yahoo forum about a month ago and, apparently, is a big factor in some zones. This is a two-edged sword; if you ignore it, a major load influence may be missing. If you include it, better define it properly!

5. With properly or autosized systems, E+ always displays the same exactly-at-setpoint as you are showing. I am not sure why HAP shows that the room is 1-2F away from setpoint.

6. In the E+ Loads chart, there appears to be NO load from ?Computer + Equip? and ?Lighting? appears only during mid-day.

7. In the ?HAP loads? chart, Window Transmission is always a positive load; I suspect that the outdoor temp falls below zone setpoint at some time during the night, so this should be a negative load.

Executive summary: Load calculations are the stuff of which lawsuits are made if done poorly, so you want to be well informed about how your software is responding to your inputs. HAP was, I think, primarily a load calc program that evolved to do energy calculations. EnergyPlus was an energy calculation program that, with proper inputs can be used for load calculations. You may want to nose around or ask a question on the E+ forum (http://tech.groups.yahoo.com/group/EnergyPlus_Support/ ) or on the E+ Help Desk site (http://energyplus.helpserve.com/ ) for more on this topic.

James V Dirkes II, PE's picture
Joined: 2011-10-02
Reputation: 203

If someone from your company attended the recent ASHRAE conference there was a great presentation on using simulation software to calculate loads. Results using a software package that calculates loads and also performs simulations showed a pretty good match, assuming thte assumptions for the space were the same. (i.e. worst case gains/schedules vs typical daily gains/schedules)

As you mention, heat balance method is the most fundamental procedure to use. There is certainly the potential for the results to be smaller numbers than what people at your company are accostomed to seeing. Your results show a pretty big difference though, for a relatively simplified case.

One item in your pilot results, for your test case it doesn?t look like the same temperatures were maintained, to get a true apples-to-apples comparison of the two methods (and to determine the load in your space for which the VAV terminal would be sized) the temperature should be maintained at the same level. Syncing those temperatures is only going to enhance the difference though, as e+ is currently a lower zone temp.

Also at the end of the occupied period the HAP temperature goes way up, and the e+ temperature drifts down (you mention heat loss through the slab? Seems like a pretty large ?T over the course of two hours from the slab), and it doesn?t look like the schedule inputs are an exact match. (e+ zone control starting at 7:00 a.m. and HAP starting at 5:00 a.m., etc., e+ run seems to have been pre-cooled while HAP was setback.)

Overall I think there might need to be a further review of what the e+ and HAP inputs/outputs are, and be sure that these are consistent with the load calculation assumptions in use at your company. As Jim mentioned, the space temperatures shouldn?t be diverging under a consistent set of assumptions.

David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, BEAP, HBDP

David Eldridge's picture
Joined: 2012-05-08
Reputation: 1

Thanks for the response, do you happen to know what the name of the presentation was or who it was that did it? It sounds like just what I?m looking for.

To address the couple points you brought up, someone else had noticed that the HAP run was running at the high end of the throttle range (75 + 1.5?F default). I think since the space I used is very small , the floor slab has a larger than usual influence (high P/A). It seemed like in the unoccupied time HAP was still applying a small load through the transfer functions but ignoring the slab loss, hence why HAP goes up but E+ goes down. The schedules are the same, I had noticed that also and I think it is because the 75? setpoint is not hit until 7am even though cooling is available and the HAP run cooling is need as soon as it is available. It just seemed odd that they could be so different for such a simple case.


Hall, Brendan's picture
Joined: 2011-09-30
Reputation: 1


A couple thoughts on your comparison. Having done comparisons like this with other tools and having worked with teams doing these comparisons with multiple tools, my experience is there are many pitfalls that its easy to fall into which can lead to a comparison of results that are not apples-to-apples. That makes it difficult to draw meaningful conclusions from the comparison. The pitfalls that may be in play for your comparison:

1. Heat Gain vs Load ? I suspect the component data from your EnergyPlus sim tool is ?heat gain? data while the component data from HAP is ?cooling load? data. Therefore the comparison of results is heat gains versus loads and is not apples-to-apples. Heat gain, of course, is heat from the individual sources like solar, occupants, lights. The Load is heat transferred to air after going through the conversion process in the room involving conduction, radiation and convection. I suspect the EnergyPlus results are heat gain for two reasons:

a. In your ?Component Comparison.PDF? your EnergyPlus plots look much more like heat gain profiles than load profiles. For example, the solar profile only has values when the sun is shining. Solar values are zero at night If this is a load profile that means the building mass has zero heat storage capacity. The lighting and occupancy profiles are completely flat during mid-day which also looks like a heat gain profile, or else a load profile in a zero mass building..

b. As you noted EnergyPlus uses the ?Heat Balance? load calculation method. This method is a truer representation of building physics than other methods the industry has used like Transfer Functions and TA/TETD. True building physics says there is no such thing as a component load like a ?wall load? or a ?people load? or a ?solar load?. This is because once a heat gain enters a room, it gets mixed together with all the other heat gains through radiation and convection processes. All you get from Heat Balance is THE LOAD ? total heat convected to the room air for the hour. In that Load it is impossible to determine how much came from each individual heat gain source, so component loads cannot be defined. Therefore an EnergyPlus calculation should not be able to produce a report of ?component loads?. But it could produce a report with ?component heat gains?.

The Transfer Function Method (TFM) uses three key simplifying principles to speed up calculations. One of these, the Principle of Superposition, results in loads being computed component by component. That allows tools based on TFM to report component loads. That?s part of the compromise of TFM ? you get speed and component loads in exchange for some amount of accuracy and inability to see component loads.

2. Apples to Apples Input Data ? A common pitfall is not completely matching input data between the two programs. Often this is caused by programs requiring different sets of inputs, using inputs in slightly different ways, or using similar terms with different meanings. Often there is more than meets the eye to inputs and that can have adverse results in a comparison. There isn?t enough information provided to say whether this happening in your comparison, but a couple things that would be worth considering:

a. Weather Data ? You mention you used 95/75 as the design condition. More important is whether the complete 24-hour design day DB and WB profiles match. And also what solar data was used. HAP is using an ASHRAE clear sky profile for solar radiation for the 21st day of each month. Is EnergyPlus using something similar?

b. Steady-Periodic ? Calculations using a single design day each month are aimed at reporting steady-periodic behavior. Because heat transfer in the building is transient, you inevitably have to start with assumed initial conditions and then run the 24-hour cycle multiple times to damp out the effect of those initial conditions to get to steady periodic. This is what HAP is doing. If EnergyPlus does not automatically do this, it is important to force calculation of possibly 4-6 consecutive days of the same weather and heat gain conditions to get to an equivalent steady periodic set of results for comparison.

c. Internal Loads ? Not all internal heat gains like lighting, equipment and occupants are created equal. 1 W/sqft lighting can have different load results, for example, depending on the fixture type because that will cause a different split between the radiative and convective fractions of the heat gain. For example, load behavior of a 100 W heat gain from free hanging lights will be different from that for recessed unvented fixtures. Equipment and occupant heat gains also have radiative and convective splits depending on the type of equipment or the type of occupant activity. Its important to match the assumptions about radiative / convective split between the two tools being compared to get apples-to-apples load results.

d. Envelope ? Wall and Roof assemblies need to have exactly the same sequence of material layers outside to inside, with each layer having a consistent set of thermal properties ? thickness, conductivity, density, specific heat.

3. Floor Conduction Heat Loss ? Just based on intuition it looks to me like something strange is going on with your floor conduction heat loss. Its odd that the largest heat loss is occurring in the middle of the day when it is warmest outdoors and that heat loss magnitude is a significant percentage of the heat gain magnitude from other sources. For this behavior to occur my guess is the surrounding soil temperature would have to be significantly colder than the indoor air and surface temperatures in the room. Since this is a slab floor I wouldn?t think the near-surface temperature of the soil would be that cold on a 95 F day. In addition, if detailed ground heat transfer calculations are being used in EnergyPlus, the heat transfer has very long transients on the order of weeks or months. Could the huge heat losses represent the effect of initial conditions that assumed a cold ground temperature or a deep ground temperature?

Hope this helped in some way. Would be interested to hear how your comparison turns out.

Best Regards,

Jim Pegues

Pegues, James F           CAR's picture
Joined: 2011-09-30
Reputation: 0

Steve Brunning from Newcomb & Boyd gave the presentation during Seminar 21 on Monday morning.

I would definitely re-run your simplified case without a slab to eliminate the effect of this variable, and circle back to it once you are otherwise satisfied with the results.

Right now you are trying to validate the concept, but may be getting tripped up with a model component that is difficult to model.

Here are some additional papers that might be useful for you on a similar topic regarding heat balance method for load calculations:




Definitely look at the other two presentations from that ASHRAE seminar as well.


David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, BEAP, HBDP

David Eldridge's picture
Joined: 2012-05-08
Reputation: 1


The ASHRAE Seminar was Seminar 21 presented on Monday morning (8:00 am session). The presentation was by Steve Bruning ? 2nd of the 3 speakers.

I think the differences you note in the 2nd paragraph may still involve comparing heat gains with loads. Because of building heat storage, a heat gain that occurs during one hour will result in loads spread over a series of following hours. Late afternoon solar gains, for example, will spread their loads over a series of hours which can run into the evening, after the sun has set. If you turn the equipment off at, say 6pm, the residual heat from those occupied operating period serve to raise the air temperature in the space overnight.

To Mr. Dirkes? point that there should not be solar loads at night. I?d agree there should not be solar heat gains at night. But there usually are solar loads after sunset because heat gain from the daylight hours is still going through the process of storage and heat release after the sun sets. In a room with significant mass this can continue thru the nighttime hours. As a qualitative check, Table 20 in Chapter 18 of the 2009 ASHRAE Handbook (Solar RTS factors) shows the decay period for a solar heat gain in a medium weight room varies from 19 to 21 hours in length. A solar heat gain at 6pm would therefore still be converting to load at 1pm the next day. Any heat gain with a radiative component has this type of behavior to one extent or another.

Jim Pegues

Pegues, James F           CAR's picture
Joined: 2011-09-30
Reputation: 0

Energyplus uses the same heat balance method as the RTS design calcs in the ASHRAE Fundamentals.

But the design calcs are different.. read up on the methods. I.E., no sun, no internal loads for heating. And full occupancy, full loads, etc for cooling.

Jim nailed it... Caveat emptor.

----- Reply message -----

Dru.Crawley at bentley.com's picture
Joined: 2011-10-02
Reputation: 0

Thanks for all the info, the OK State papers address a lot of what I had been looking for. I?m going to try and track down that presentation sometime today. I know what I was doing was about equivalent to back of the envelope but I was looking to understand where the differences were as opposed to doing a thorough validation. My understanding of the design calc that E+ does was that it used the heat balance method but made certain assumptions that were typical of a load calc (ex no sun or interior loads in winter). The point that was made of being careful not to use loads and gains interchangeably was a very good one. I think some of my confusion was stemming from that. It appears that HAP reports component loads while E+ was reporting component gains, and would therefore look much different. I re-ran my E+ run with a adiabatic floor and it corrected much of the overnight heating/cooling disparity that I was seeing. Apparently, ASHRAE recommends excluding the floor loss in cooling design calculations, which is why it was correctly excluded in HAP. It looks like all of the difference in the peak cooling load is from the roof (yes they are defined the same), which could be explained by the difference between using HB and the sol-air temp method. Thanks everyone for all the comments, I think I?m slowly getting a more complete picture of this process.

Brendan Hall

Hall, Brendan's picture
Joined: 2011-09-30
Reputation: 1

I have also been testing differences between EnergyPlus and other software that use the transfer function method i.e. eQUEST. My goal was to compare the energy consumption differences between the two software with different system types. My first step was to make sure to check that the loads being calculated by both programs were consistent. Overall the results were in agreement when using the ideal air loads system in EnergyPlus. However, once I confirmed that the annual loads calculated by both programs were in agreement I changed the system in EnergyPlus to a VAV reheat system and the annual loads (not design day peak loads) were quite a bit different even though they maintained a similar profile. I have not figured out what might be causing this difference, could it be the iterative nature between loads and systems in EnergyPlus?

I agree with Jim that what you might be comparing are loads in one program with gains in another, I did the same when I first looked at the results of the test case I was running and was trying to figure out what was causing such a huge difference. Its also very important to maintain the exact inputs in both programs, for example for all envelope parameters, keeping the thermal properties (heat capacity, density) and thickness the same and not just the overall U-value.

See below and attached for some of these results.


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[cid:image010.png at 01CD5B5E.A71420A0]


[cid:image011.png at 01CD5B5E.A71420A0]

[cid:image012.png at 01CD5B5E.A71420A0]

Mirza Sajjal

Mirza Sajjal3's picture
Joined: 2011-11-16
Reputation: 0

On 07/06/2012 10:11 AM, Hall, Brendan wrote:


ASHRAE recommends excluding the floor heat loss for commercial cooling
design calculations, not for residential. If you look at the residential
calculation example in ASHRAE HoF, you can see that the floor heat loss
is a significant portion of the total load. What I mean is that if you
exclude the floor loss, and then apply your safety factor, you will end
up with an oversized AC.

In fact, we have tried the same exercise as what you have done. We
compared EPlus with whatever the mechanical engineer was using for the
sizing (in an existing building), and then go to the actual unit itself
on a design day (i.e. a hot day with max temp higher than the design
temperature) and ensure that the internal gains are as assumed in the
calculation. Then we look at the compressor operation during that day,
and estimate how much oversized the compressor is. We practically nailed
down everything else, except the floor loss. When I tried using
adiabatic floor like you did, I got exactly the same sizing result as
what the mechanical engineer had (I cannot remember what they used, but
it was not HAP), of course with other assumptions being equal.

Can anybody enlighten me on why there should be any difference between
commercial and residential design calculation with respect to the floor
loss? The small commercial spaces that we have worked on are just like
residential spaces, only with different profiles of occupancy, lighting
and equipment, plus ventilation requirements of course. For multifloor
commercial buildings, yes floor heat loss is tiny. But for small single
story commercial buildings, floor heat loss is significant.


Ery Djunaedy2's picture
Joined: 2011-09-30
Reputation: 200

On 07/05/2012 02:21 PM, Jim Dirkes wrote:

Jim, shouldn't you do this on every project?

Our lab does not do design, but we end up doing this type of comparison
on virtually every project. There is no point of energy savings if we
are still designing an energy hog. Like you said, load calc is not our
job. But its our job to educate the owner that their building must have
an optimal load, and that has to be reflected in the system and plant
sizes. ONLY when the load is optimized, then we can talk about saving
some energy.

Of course there is ALWAYS some resistance (in various degrees) from the
mechanical engineers, but we usually succeed in streamlining our inputs
with the load calc inputs (read: to make their assumptions less
conservative and reduce their safety factor, that we end up with an
optimum peak loads).

I believe that you have a similar experience. Care to share with the
forum about your experience on this?


Ery Djunaedy2's picture
Joined: 2011-09-30
Reputation: 200

It appears that the peak loads compare closely between the two programs.
Energy, however, does not compare well; with eQuest consistently and significantly projecting higher energy.
My guess is that, assuming you have diligently compared all of the major input values, the two programs use different partial loading curves for fan and cooling system.

James V Dirkes II, PE's picture
Joined: 2011-10-02
Reputation: 203

Is there any data out there comparing the results from: load calcs, energy models loads, and measured loads (maybe 3 years of data as not every year has a design day)?

That would be the only way to be sure to say which approach is better. Of course the assumption would have to be made that the load calcs and energy model were completed correctly.

Compiling a statistically significant data set would probably be quite the undertaking, but would make a nice master?s thesis. If we?re lucky, someone has already done the work.


Fred Betz's picture
Joined: 2011-09-30
Reputation: 0

Dear Ery,
Your point is well made! Several things come to mind in response:

* I was not aware previously and do not have enough data from this thread to be convinced that something like E+ is significantly better than the "originally a load calculation" software like HAP or Trace.

* If I did have that conviction, I think it would be hard to convey that to the design engineer. It's a hard sell to convince one of us stodgy engineers to use something different than our ASHRAE-compliant load software and take more risk (by sizing smaller) when we're in a big hurry and very design-budget-constrained.

o On one project when I was barely out of the University, I sized a heating system here in Michigan USA with no "safety factor" because I was very confident of my calculations. After its installation, the office area would not get above 20C / 68F when winter temperatures were near the design condition. I checked every aspect of operation and triple-checked my calculations in order to fix the problem and save face - and failed. My company didn't fire me when we had to pay for heating upgrades, but they were not happy! After that, I resolved to "size generously with excellent part load operation". I still think that is a reasonable approach, but I am smarter now and might still be willing to specify the smaller size instead of a larger size.

* I do use E+ for load calculations when performing a detailed energy audit. My conviction and my experience say that most equipment is oversized and if a piece of equipment needs replacing, we may as well recommend replacing with the proper size. One recent project replaced the existing boiler with one 1/3 the original size! (Another project kept the same size boiler.)

* This brings up another major reason for oversizing. If I need a 320 ton capacity chiller per my load calculations, I must choose between a 300 ton or a 350-400 ton machine. Which will I choose? Will internal loads from PCs go up or down? If they go up, will my company be blamed for the cooling system inability to meet the load? Heck, the 350 ton chiller doesn't cost that much more; I'm likely to play it safe! It's worse on a percentage basis when my load is 5.5 tons and I must choose between a 5 ton and 7.5 ton system.

* Finally, (can you tell I've thought about this before :)?) most of us Americans love to be comfortable ALL the time - at home, in the car, at work, while shopping and at the restaurant - no exceptions (We're all comfort wimps). Even though the peak heating and cooling conditions only occur a few hours every year, few are tolerant of a little discomfort for those few hours.

* Although all of the above is true, I know there are abuses in the design community that could be gently corrected. As part of a recent commissioning project, I reviewed load calculations and found that the engineer was using 5W / sq.ft (>50W / sq.m) for internal loads ... in a standard office! On top of that, he refused to make any changes when I pointed it out. "Too much work to change and it will work OK as is."

* All in all, the bottom line is probably something like "encourage excellence in the engineering community" and promote the benefits of excellence. One of my mottos is "Excellence pays" - that is, excellence does not cost more, it pays you back.

p.s., In case readers are not aware of ASHRAE Standard 140, it is used to compare results of energy analysis programs for a set of standardized systems to a reference calculation. I don't think it focusses on peak loads, however.

James V Dirkes II, PE's picture
Joined: 2011-10-02
Reputation: 203