Internal Heat load gain for a a typical condo in a 10 stories condominium building?

[genieconseil]

Hi all,
i'm a mec eng. From quebec Canada. I've a bit of a conflict with an other engineer concerning internal heat loads to condider equipment and people load.

I add a load of 2 people at 230btu/hr for a two bedroom (which i think is negligable but i add it anyway) and 600 btu/hr for various internal loads . I know that ASHRAE fundamentals ed.2001 sugests 1200btu/hr but in 2013 with the more energy efficient lighting, motors from fridges ect. My opinion is that these loads should be reduced as most of the appliances are taged energy star and thus much more efficient. Plus the use of these appliance is intermitent (random operation sequences) such that the compressor on a fridge will work +or- 25% a day same for H/W tanks ( plus they are insulated). I dissagree that these loads be taken in full consideration due to my previous comments.

The other engineer is trying to discredit my calculations with abuse on appliances loads he sugests 3200btu/hr witch also includes the heat loss of the motor (950btu/hr for a 1/2 hp motor efficient at at least 90%) of the central air handler for the central A/C system which is located in the ceiling of the bathroom??

For the rest it is fine, appart that i use HAP and they use trace and using the same data (envelope specs,orientation, shading) they come up with 19000 btu/hr sensible versus 16000 btu/hr for HAP without internal loads, i dont get that one!

Conlusion is that i come up with 24000 btu/hr total and he comes up with 32000btu/hr , again i dissagree with his results i think its over design and creat cycling and discomfort for the occupants.

I would appreciate anny constructive comments.

Thank you in advance

Genieconseil

 

[EnergyProfessional]

the differences could be because of different envelope (infiltration, insulation, solar) heat gain assumptions. what glazing, what themral mass etc? what method was used, heat balance, or which one?

both Trace and HAP should break out the gains (internal envelope etc.)

You always will have differeing results. Even if you calculate it today, and in 2 months, your own assumptions will difer a bit. You need to verify if the assumptions are valid.

Internal load for refrigerator should be the average. So if your fridge runs 2 hours a day, you average the plug load. I assume the ASHRAE data take that into account. sure people use more efficient appliances, but they also buy larger fridges, TVs etc.

Compressor heat is either disspiated outside for residential AC, or is inluded in the AC rating. This does not need to be added to the cooling load.

 

[genieconseil]

hi HerrKaLeun
Thanks for your quick response. As i mentionned in my post they have same suface values for the windows and walls and same U values ans shgc for windows, desitybof buildin, color of roof and envelope, as for infiltration this result does not figure in there 19000 btu result. I have alrzeady sent my comments to the engineer but i was curious to hear from others to see if we think the same.
As for your comment concerning heat gain from fridges or other appliances ASHRAE recommended 1200 btu/hr in 2001 it should be reduced in 2013. Even if the fridges are bigger and LED tvs are bigger they consume less energy, this is what i have experienced.

Thanks for your comments it is greatly appreciated

Genieconseil

 

[IRstuff]

I'm tempted to think that your numbers are a bit on the low side. This refrigerator:

http://www.costco.com/wcsstore/CostcoUSBCCatalogAssetStore/Attachment/761858-EnergyGuide.pdf

which is Energy Star qualifed, uses 547 kWh per year. Your 600 BTU/hr results in 1541 kWh per year, so the refrigerator is consuming 1/3 of the total energy, which is not leaving much for anything else. One issue is that while many things are more efficient, they come with their own parasitic power consumption. To wit, TVs, cable boxes, etc., burn a continuous level of power, even when they are supposedly turned off.

TTFN
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[Drazen]

your internal loads looks too low for me too.

you should never ever rely on theoretical manufacturers specs for new products which divert from current practice much. before years of observation is applied for some new products and well sounded proof available, sort of safe margin is a must.

not long ago i had clash with energy consultant during design of new low-energy-lot-of-alternative-energy homes, where i was also accused to apply too much margin.

my argument were shout at, but two years later they are found to be proven - occupant consumption profile was not realistic, now they have found that their low-energy homes consume almost exactly same amount of energy as ordinary homes, because occupants simply ignore their assumptions about usage habits.

you should be clear with yourself whether your design aims to prove theoretical assumptions or provide best compromise. applying margin is never simple story, and even when i understand that you want to focus on efficiency, you should be very careful with that.

 

[IRstuff]

Oh, BTW, I'm tempted also to think that your body loads are possibly low as well. 230 BTU/hr (67.4W) is less than the basal metabolic rate of a human body at complete rest, specifically female. Male BMR tends to be about 10% higher. If your occupants are never more than couch potatoes, then that's probably OK. I wasn't also clear on whether the number represented each person, or both persons combined. If that number is the average value over the day, then that's probably OK, but I would still think it's low.

http://www.bmi-calculator.net/bmr-calculator/harris-benedict-equation/

suggests that the average value for an average person is probably 1.25 * BMR. Assuming both people are there 14 hrs a day would result in something more like 387 BTU/hr.

TTFN
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[cry22]

Both of you are wrong,
you should use residential load calculations guidelines from ASHRAE or better yet, use Manual J calculations, it is much more suitable for residential load calcs.

HAP and Trace are for commercial load calcs. Any time these two software are used by engineers for residential, it is a nightmare. - You could use carefully Trace and HAP for high rise condo, and you should check your numbers with some established engineering checks using Manual J.

I say 1.5 ton Total load for a 2 BR apartment is today's air tight buildings with about 30% glass is adequate. more like 700 SF/TON

 

[willard3]

I will ask first if you are the Engineer of record on this project?
If you are, don't argue with anybody, physics is not debatable.
Use your calculations, you will find out in time if you were correct.
If you were short for any reason, which I think you are, your client will be in touch.

 

[EnergyProfessional]

cry22: I'm not sure why Trace and HAP shuld not work for residential loads. All that is different is different loads, and duration of loads. and yoo have multiple single-zone cosntant volume units mostly without contant ventilation. The rest of the physics is the same. Yes you can look into ASHRAE on how to estimate the loads as in residential I assume you don't have all people in all the time and the lights not on all day, and no ventilation.

Obvioulsy you need to know how to use the software, but that also applies to any software.

The only reason Trace/HAP are not used often for residential is that residential designers usually use a cheaper software, and don't have knowledge of how to properly use it.

wha tpeople ususally do wrong is use a resdiential softwre for commercial loads (like you can just add up al the singel loads

 

[genieconseil]

I am a practicing engineer for the last 24 yrs. in Montreal, Quebec. Canada
I see a lot of contradictions, but concerning the tightness of buildings,(in montreal we seal the exterior with 1½'' of urethane (insulation) I agree with that is why we have to bring in fresh air cause there is very little infiltration air available.but still adding the infiltration is required especially for the windows that are not 100% impermeable.

As for people load 2 for the first bedroom and 1 for every other bedroom , I'll go for it (not much of a factor in the total load)even if i think that we should not oversize for worst case scenario (factor in your coincidence factor in the formula) for residential applications only.

I believe that the human behavior should be taken into consideration and also the fact that people do not buy condos to raise a family.

I stand behind my theory for internal loads i disagree with everybody daytime internal loads are small again few people are home during the day and apart from an laptop or ipad or a TV there is not much going on in there (fridges operate 2hrs during a day not), the only big load to really consider is the solar heat gain. But I will allow 1200btu/hr.

Since the loads in the condo are in majority sensible heat loads when selecting the unit you must take into consideration the SHR(solar heat ratio) because in the result of the total load there is very a small portion of the total load appied to latent heat load gain (basically people and fresh air load contribution amount to usually +or- 10%)therefore you should select the unit based on the sensible heat load only. That means compare your sensible result to the sensible cap of the the unit you are selecting.
ex: 24000 btu unit will in general (depending on the manufacture) be able to treat about 18000btu sensible out of the 24000btu total.
thanks for all your comments


Genieconseil

 

[cry22]

Herr

FYI
I have been working with Trace since 1988 (when it first came out), back when it was a DOS program Trace 600. Used to use E20-II (old name of HAP when it was a DOS program).

Several times, I provided Trane with input with an engineer's wish list and they accommodated as much as they could. I also reported several bugs that Trane took care of.

I think I have as much experience with trace as most people out there.

Try Manual J, you will see the real difference. Some people say, the formula is the same, so how come? well, it takes some feel and touch in our field.

May I warn you that while trace always tends to double the numbers on cooling, it also provides half the normal numbers on heating. Do a load on your house (single family) and check your results with the actual unit installed and then get back to us.

 

[EnergyProfessional]

cry22: I'm surprised you keep working with a software that you think is wrong by a factor of 2.

I've been happy with Trace and if results were incorrect, then it turned out I entered something wrong somewhere. It has a lot of options, which alos makes it more user-error-prone.

Which software do you think is correct?

One thing to be aware of with Trace (and probablyother software) is that you need to create your won assemblies to get realistic U-values. It doesn't have built-in realsistic studframe etc. values. the base library assumes your 6" studframe has all 6"insulation and no studs with thermal bridges. As we know a steel-stud wall with R19 batt insulation only has R6 or something like that.

 

[genieconseil]

I agree with you Herr happ or trace are both good software the diff is with the input of the user. As you say it is important to input manually the thermal resistance of the envelope assembly supplied to you by the architect he is responsible of the wall assembly and consquently the R value. By the way make sure that your are supplied with the specs of the windows and that the person (client or architect supplying window specs) understands the importance of this info because often the specs given at the begining of the design process differs from the specs when they purchase the windows cause they might fall for a better deal and compromise the R and shgc of the windows without undrstanding the consquences of doing so without warning the engineer to make sure the heating and cooling calculations are updated .i personaly lived such an experience and so for that reason i make it clear by writing that they are responsible of the info they give me , and furthermore i will ask them to resubmit the widow specs before they order windows.
Cheers

Genieconseil

 

[DRWeig]

Run eQUEST for a third opinion. Trace and HAP are good, but unless you have the source code you cannot understand why the two give different answers. I have run loads using one of these three, then run the same inputs in the next version and there are differences of about 5 percent. I believe that much of the difference comes from infiltration handling.

Best to you,

Goober Dave

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[EnergyProfessional]

Also check what calculation method is used. In Trace you can check RTS, and others. Obvioulsy different methods give different results (because soem take thermal mass into account, others don't).

Regarding infiltration, in Trace you also can take into account the height (for different windspeed). so the 10th floor condo whoudl have more wall heating laod than 1st floor etc.

If something is wrong, the problem sits in front of the computer, not in the computer.

I stand by Trace, it is correct if you use it correctly.

 

[317069]

I would go with your opinion (24000 Btu/hr) as long as your unit is able to handle 100% of the latent load, but for sensible load, it is not stable like latent.
the best software is your experience and hand calculation, what software did they use when they have designed CN tower for example.

 

[genieconseil]

That is exactly my stand I'm sticking to my 24000btu syst.

Thanks all for your input

I enjoy the camaraderie and the exchange we have in this community, it is very informative I would even classify this as a form of continuing education.

Cheers

Genieconseil

 

[cry22]

Herr et al,

Standing by Trace is like believing everything you hear on TV.

I use Trace exclusively, cause in my opinion one should commit to a software and know it really well instead of working with multiple software and not master any of them.

I tried Trace and it is bogus for single family home calculations - For single family home calcs, you need "Manual J", the only reliable manual calculation method that has always resulted in "right-szing" of residential cooling and Heating systems.

Sure, you apply all sort of factors to your manual inouts to "correct the output", anyone can get somewhat descent numbers when you start overriding the software.

Take a look around any neighborhood in the US (or Canada for some in here) and check the size of split systems installed, they are always in the 600 SF/ton range on average. The same home would be at 400 SF/ton when using Trace at best.

Fortunately the contractors in the US have correctly designed AC systems for years (by down-sizing AC systems) and kept energy bills low as well as good indoor environment (cool and dry) for home owners. Had we relied on engineers with their Trace and safety factors, we would've burned the planet by now. AND.. this is an HVAC Engineer talking.

No wonder one does not need an HVAC engineer to get Mechanical permit to build a house, we are the only trade who's PE stamp is NOT required when building a home by authorities (you need a structural and Civil, architect but not MEP PE stamp to get a permit)

Well guys, instead of just sticking to your Trace guns, why don't you try a true test and compare methods.

 

[317069]

Cry22, your are the hero, 100000% right.
since software boys and all those fancy words such as LEED...etc came into the business they ruined it

 

[EnergyProfessional]

Not sure about residential buildings, but most large commercial projects use Trace and if it really would double or halve the values as you suggest, I'm sure someone would have noticed by now.....

I have reviewed TRACE calculations from projects that couldn't meet demand, and the error was not in Trace, the error was sitting in front of the computer. Many engineers don't know how to deal with thermal bridging etc. Believe it or not, an R19 metal studwall doesn't actually have R19... but this isn't a fault of Trace. Errors included not understanding ceiling and wall heights (if you only enter 1' ceiling height, your results may be off... and ignoring infiltration and floor losses entirely. not a TRACE issue, an issue of the user.

i haven't used it for residential, but don't see a reason, if applied right, it shouldn't work. you need to deal with the internal loads inc. people differently and can use Manual J or ASHRAE for guidance.