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Heat Load in Office

01/02/2013 3:05 PM

We have a industrial office building in Ottawa Canada that had around 100 people in it and the associated computers. Presently our boilers are having issues in keeping the work area warm. My assumption is that the heat load of the workers is gone so the demand has gone up ?

What amount of heat does a average office worker generate ?

Does this make sense ?

Thanking you in advance for your assistance...

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#1

Re: Heat Load in Office

01/02/2013 3:15 PM

your problem has little to do with how much heat people generate. BUT To answer that direction question, th average adult male "burns (uses up stored heat and energy of around 2000 calories. your problem isn't that cold people are absorbing excessive heat. either your heating equipment isn't working properly or the building design is allowing for a loss that exceeds your expectations.

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#12
In reply to #1

Re: Heat Load in Office

01/03/2013 8:25 AM

Your 2000 calorie value is based on FOOD CALORIES, not thermal calories.The actually thermal value is KILO CALORIES.2000000 calories.And this is based on 24 hours of average metabolism person.An 8 hour shift of course is obviously 1/3 of this value.

Based on the BTU's posted by another user, the heat load of 100 people could be several tons of cooling or heat.

Happy New Year.

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#13
In reply to #12

Re: Heat Load in Office

01/03/2013 8:34 AM

Nouncalorie

  1. Either of two units of heat energy.
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#15
In reply to #13

Re: Heat Load in Office

01/03/2013 9:05 AM

Correct!.However, when using food calories, there is a factor of 1000 difference.Unless you are aware of this difference, it can result in large errors of load calculations.

Sometimes doctors refer to a 2000 calorie diet as a 2000 kilo calorie diet, which is more correct from a scientific standpoint.

The difference in symbology is upper case C for large calorie(The amount of energy required to raise the temperature of 1 kilogram of water 1degree C at 1 atmosphere of pressure.)

The lower case c(small calorie) is used for scientific purposes, and is the amount of energy required to raise the temperature of 1 gram of water by 1 degree C at atmospheric pressure.

The large Calorie is used normally in biological studies of metabolism.

Normally, in scientific or engineering calculations, the small calorie is used.Either will give a valid answer if the decimal point is positioned correctly.

For more info Google: Small calorie vs Large Calorie

Whooda Thunkit?

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#2

Re: Heat Load in Office

01/02/2013 3:24 PM

Wiki says 18.4 Btu/h·ft2.

Another source says about 400BTU/person.

I read somewhere that it is roughly equivalent to a single 100 watt incandescant light bulb.

Computers should be fairly simple to model.

And, Fredski, to borrow a phrase from "Oh Brother Where Art Thou" all the people's done "runn oft".

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#3

Re: Heat Load in Office

01/02/2013 3:30 PM

Interesting,

Some examples from ASHRAE

Person seated at rest, 350 Btu/Hr
Seated, Light Office Work, 420 Btu/Hr
Standing Light Work, 640 Btu/Hr
Walking 3mph, 1040 Btu/Hr
Bowling, 960 Btu/Hr
Heavy Factory Work, 1600 Btu/Hr
Heavy Athletics, 1800 Btu/Hr

It also how much appliances are running, with the older CRT screen this puts out allot more heat, but current modern offices have LCD screens.

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#25
In reply to #3

Re: Heat Load in Office

01/03/2013 4:38 PM

Exactly!!

So in most office applications a person is factored in at 500 Btu/Hr and don't forget about the 1.2 CFM per person for the OSA (outside air in and office space) requirements.

Some people call it fresh air but after a 10k cost at a law firm we now call it OSA. They literally took it as fresh air, after they made us install the Hepa filter and ionizer racks the air met fresh air standards by their independent IAQ company they hired. So we now bring in OSA for air changes. Sorry for the tangent but if the people have left the space be sure that you have shut down the OSA to meet current demands, since that could also add to your Cold feelings.

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#27
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Re: Heat Load in Office

01/03/2013 5:18 PM

A small piece of history is in order. During one of the "energy crisis" around 1970s, it was noted that the OA requirements were around 20 CFM per person. Since this was deemed as "too much OA and a waste of energy", the codes were revised down to 5 CFM per person. Unfortunately, building envelops were improved to stop air leaks and infiltration. So in the 1980s and 1990s, we developed "sick building syndrome", and IAQ became part of our vocabulary. And codes were revised to increase the OA requirements to 15 CFM. 1.2 CFM is not enough air to keep CO2 levels at an acceptable level.

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#4

Re: Heat Load in Office

01/02/2013 3:32 PM

To clarify the office was occupied and we recently moved the office staff to a new building so the office area is now vacant but we want to heat to a min of 68 degrees as we have a couple of staff members left here. I think I will need to add additional capacity to our boilers in order to replace the lost heat. Due to a expansion a couple of years back the boilers we operating at around 90% of capacity when the office area was full.

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#14
In reply to #4

Re: Heat Load in Office

01/03/2013 9:01 AM

It might be more efficient/economical if you moved those "couple" of staff members to a different location as well...or make them a smaller room with dedicated heating in it.

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#5

Re: Heat Load in Office

01/02/2013 3:44 PM

In Toronto Canada when we re-purposed a office to a factory and had 44 people working there we had to add 40,000 BTU to maintain a suitable working temperature if this helps.

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#6

Re: Heat Load in Office

01/02/2013 4:08 PM

100W per person is about right. What about the lighting as well?

If the 100 people, their computers, coffee machines and lighting have gone, why does the area have to be kept warm?

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#7

Re: Heat Load in Office

01/02/2013 5:09 PM

If your boilers were at 90% capacity when the building was fully occupied then you are short on capacity bigtime now. The air returning to your AHU was probably ciculating thru the plenum space above the ceiling or if no ceiling was there then the top of the room. Either way the heat from people, lighting, equipment, etc. was raising the return air temp considerably. The heating coil is required to raise the air, now including outside air, to the supply air temp. The return air temp being lower creates more load on the coil/boiler. If the people are no longer there, your ventilating air requirements are lower and outside air quantity should be reduced. This will help your situation, hopefully enough.

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#8
In reply to #7

Re: Heat Load in Office

01/02/2013 5:19 PM

you're making an awful lot of assumptions, the guy never mentioned ductwork or any temps

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#11
In reply to #8

Re: Heat Load in Office

01/03/2013 12:42 AM

Yah, what is it? A 10,000 square foot room with a 30 foot ceiling or a 70,000 square foot multilevel office complex with 2 foot concrete walls?

What type of air interchange are we looking at? Every time the exterior door opens you get fresh air, or a complete change of air every 20 minutes using outside air? Is there any recirculation?

Do they have a heat recovery system?

Do they have a climate control system of any kind, shape, or form? Is it computerized (I hope)?

What temp is the boiler set to?

Do you use industrial control controls for heat/air-con (DDC's)?

Do they used simple VAV reheaters for each space? How many thermostatically controlled spaces are there? Are the thermostats in crowded/overutilized spaces and the VAV's they serve in areas which are cooler than they should be?

Holy crap, man! There is a lot going on here!!!

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#16
In reply to #8

Re: Heat Load in Office

01/03/2013 9:40 AM

The assumptions are resasonable for those who work with HVAC loads and understand how the systems work.

A building with 100 people should have about 15 CFM per person of outside air. At 100 people, that is 1500 CFM that needs to be heated. With 5 people, the outside air can be resit to 75 CFM that needs heating. Based on building codes, this is a simple assumption.

A person seated at work has a sensible heat load of 250 BTU/hr sensible, and 200-250 BTU/hr latent. 100 people will provide 45,000 BTU/hr heat easily. The lighting and office equipment will probably provide another 40-50,000 BTU/hr heat. Since only 5 people work here now, most of the lights and equipment is probably turned off to save money. Again this is a reasonable assumption.

The OP should have a commercial HVAC contractor survey the building and offer a better estimate (instead of our guesses and assumptions) of what can be done based on the long range plans for the building. I would guess short term, electric heaters in limited parts of the offices will be used this year while permanent HVAC modifications can be provided for next year.

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#17
In reply to #16

Re: Heat Load in Office

01/03/2013 9:44 AM

GA,

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#20
In reply to #16

Re: Heat Load in Office

01/03/2013 10:21 AM

you can make rough assumptions all day about how much heat each person is contributing to the total heat load and you'll never come close to coming up with a useful number. the question is silly to begin with. as usual there are a ton of variables and far too little data to make any calculations. your "cfm per person is nice but these days a VOC sensor or Co2 SENSOR is used to determine how much "make up" air is needed...it isn't a fixed number. we also have no idea how much heat is being lost to a poor design or a lack of insulation, etc. so he either needs "hotter people" or to get his equipment up to speed to math his loads....I suggest the latter

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#21
In reply to #20

Re: Heat Load in Office

01/03/2013 10:24 AM

The GA is to have it analysed to determine thermal losses.

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#22
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Re: Heat Load in Office

01/03/2013 11:19 AM

I suppose you're right. However, I'd like to see an empirical approach. I'd suggest pre-warming (at the start of each shift) the few people remaining to perhaps 180 degrees or so. Assuming the building has a good security system, this could be accomplished by cranking up the wattage on the personnel X ray system (or adding a simple walk-through microwave system.) Adjust body temperature to account for any thermal losses.

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#23
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Re: Heat Load in Office

01/03/2013 11:24 AM

Thats true, there was a post that had empirical info, and if its been idle for a while, there could be dampers that are stuck or frozen.

formulas only go so far.

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#28
In reply to #20

Re: Heat Load in Office

01/03/2013 5:36 PM

I disagree. My rough assumption was useful in that in allowed me to recommend getting an HVAC professional to do an on-site survey and develop a detailed plan to deal with the problem.

VOC sensing probably is not be appropriate for this situation. If desired, CO2 sensing could work, but it might be more money than the original poster wants or needs to spend.

You are correct that required OA is not a fixed number, but code authorities do require either measuring the IAQ to vary OA supplied or supplying 15 CFM per person. The owner can decide how he wants to have the system designed and operated.

Also, the original building was expanded at some point in the past, but there is no indication whether the boilers were expanded with the addition. Original boilers might have been sufficiently over sized that some one thought this was an opportunity to save money. This happens a lot with older buildings.

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#26
In reply to #16

Re: Heat Load in Office

01/03/2013 4:43 PM

Very GA!

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#18
In reply to #7

Re: Heat Load in Office

01/03/2013 9:48 AM

Good answer. In the old days, outside air dampers were arbitrarily set at 10 or 15% for fresh air. Depending on size of the system and the quality of the dampers, they might leak enough fully closed to satisfy the fresh air needed by the 4 or 5 people left behind. If you can afford it, a CO2 monitor could be used to control the damper to maintain 400 ppm CO2 so you only heat the outside air that you need.

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#9

Re: Heat Load in Office

01/02/2013 10:45 PM

Maybe try leaving all the lights on?

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#29
In reply to #9

Re: Heat Load in Office

01/03/2013 5:39 PM

Switch to 150 watt incadescent bulbs?

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#10

Re: Heat Load in Office

01/02/2013 10:51 PM

http://www.engineeringtoolbox.com/metabolic-heat-persons-d_706.html

This has a nice table (with metric units so it's easy to use).

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#19

Re: Heat Load in Office

01/03/2013 10:19 AM

"Does this make sense ?"

No! To build a building and put heating in it depending on the number of people that will work there. Utilizing their body heat into the equation. Sounds a little off to me.

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#24

Re: Heat Load in Office

01/03/2013 11:41 AM

I assume by implication that the space is no longer occupied. To answer your specific question the average sensible heat gain from an office occupant is approximately 250 BTU per hour of sensible heat. That is the portion of a person's emitted heat that will warm the air in the space. The rate of heat emitted from their various electronic devices, computers, printers, scanners, etc. can vary all over the map. A survey of the space will reveal this value. Don't forget the heat emitted by the office lighting fixtures and ballast.

If your objective is to determine the amount of heat required to maintain say 70 to 72 in the space during the winter months you should run a simple heat loss caLculation for the space or have it done by a reputable HVAC design/buid firm. The size and operating condition of the boiler is just one part of your concerns. The design and condition of the remaining parts of the heating system, I.E, pumps, fans, piping, ductwork, temperature controls, etc will determine the comfort of each part of the building.

A question hovering over all this is why is there any concern about the space temperature if the space is not occupied? Like the old saw "if a tree falls in the forest and no one is there to hear it is there any noise?" If the room temperature is 60 but no one is there, is it uncomfortable?

Good luck and Happy New Year!

Lou Bindner

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