View Full Version : auditorium too hot

08-12-2007, 07:43 PM
I run a small 60-seat theater-cafe building in Los Angeles (specs below). When we run the stage lights (say, twenty 500-watt instruments at a time) in the summer, we canít keep our little auditorium from heating up to 80 or 85 degrees or more during shows. Audience members give up and leave, sweating and complaining. The actors havenít yet passed out onstage but they sure arenít happy.

Last week we had a new 5-ton HVAC rooftop unit (compressor/heat pump) installed, but that STILL isnít enough when the show is running on a hot day like today (88 deg. F. outside temp), or even on an evening when itís only 70 outdoors. Itís the lights that are killing us! (As well as having 60 human bodies in there.)

We tried shutting all the supply vents in the rest of the rooms to focus all the AC in the auditorium Ė only a slight help. We tried cooling the aud down to 66 just before the show starts, but it still heats up very quickly when the lights come on and is close to unbearable by the end of the play.

I am annoyed that the HVAC contractor didnít calculate the extra heat generated by the lights and people when he replaced the old 4-ton, which wasnít keeping us cool enough. He says he just figured ďanother ton ought to do it for this square footage.Ē

Heís not completely off base, I suppose: When the stage lights arenít in use, the 5-ton unit seems to cool the whole building ok. (We are guessing that 5 tons will also be plenty for heating in winter. Certainly the stagelights will be working in our favor then, and not against us.)

So now he suggests selling me an extra 2Ĺ -ton unit just for cooling, to augment the other. (I asked him why he didnít install a 6- or 7-ton unit in the first place and he said that gets into a lot of engineering and complicated city permits, which he says the 5-ton is small enough to avoid. Hmmph.)

Possible fix: Another contractor said, why not try adding an exhaust vent fan or two in the auditorium ceiling. A quiet fan (under 2 sones) can move 100 cfm for example. The theory being that those lights are hanging just below the ceiling, why not vent that heated layer of air straight outside, before it circulates into the room and into the AC unit. Then perhaps the AC unit could work more effectively.
Iíd be inclined toward this low-tech, lower cost solution that seems to hit the problem right where it is generated, rather than buying another 2-ton unit just to augment the AC on these few worst days of summer.

But, do we need to worry about where the air would come in, that the exhaust fans are removing from the building? We could let it draw through the auditorium doors, from backstage rooms, which arenít so warm. . . but then I suppose those rooms would need an outside inlet vent (from the back alley for example, where itís always shady). But even then, the incoming outdoor air might be as hot as 80 on some days. I suppose thatís still better than the 100 or 120 degree air (Iím guesstimating) weíd be expelling.

What do you all think about this situation?

Thanks for any advice or suggestions.

Building ca. 1800 s.f. total.
Rooms are:
650 sf auditorium (30x25 with 12 ft ceilings - just a large rectangular room, seats are not fixed)
400 sf lobby
2 dressing rooms, 150 sf ea.
2 bathrooms, 60 sf ea
office, 75 sf.

08-12-2007, 08:40 PM
You need an engineer that can do a heat load analysis...punting won't work. One 500W lamp generates 1706BTU of heat, or nearly one ton all by itself from my handy-dandy little conversion program. You can look up how many BTU a normal person generates, too. You need some serious a/c in that kind of a situation unless you can vent some of that heat outside. Even if the lights aren't on, my guess is what you have isn't enough to keep the auditorium cool when it is filled with people.

08-12-2007, 09:40 PM
I'd be inclined to at least try the low-tech approach with exhausting the top layer of air; but rather than draw your makeup air through the conditionned space, why not a vent also in the roof, feeding outside air to the lights? That airflow would short-circuit; Maybe even, if you set up your AC return lower down, you'd get a thermocline & not much mixing of the two airflows?

Then, if that didn't work, I'd pull out the big guns (engineers).

08-12-2007, 10:34 PM
Thanks both for the tips. I've done some figuring in my table below, which seems to indicate I'd need 15 tons just to compensate for all the humans and lights, plus the 4 tons for the basic job of cooling the building in the first place. 19 tons???

Can that be right?? It would be crazy to install such a huge cooling system when it's only needed a few weeks of the year. It would probably collapse my roof! (Hmm, then ventilation would no longer be an issue...)

I will try to locate an HVAC engineer who can really calculate this. It seems exhaust/intake that deals directly with that lighting "layer" would be the way to go.

Good idea French, to add the fresh air intake right into the same airspace, to sort of isolate the upper layer of auditorium air. I see it might indeed be awkard to draw air from backstage. And to lower the AC return to avoid that top layer. Sounds logical so far.

What would happen if during hot season we block the auditorium return entirely and let the lobby return take up all the air? In grand theaters I've worked in, there's always a strong breeze from onstage to off: cool air is being dumped onto the stage and "returned" backstage. (I don't know if they also exhaust the lighting heat directly. I should track down a colleague and ask how they do it.)

Another option we are considering is only to perform plays in our birthday suits during the hot season. Of course the audience also. Well, maybe we'd let them wear swim trunks.

Item Quantity Btu/hr each Total btus/hr Tons of AC (?)
AUDITORIUM 650 sf. 1.75
People 60 2000 120,000 10.00
Lamps 20@500w 1700 34,000 2.80
BACKSTAGE 500 sf 1.25
People 10 2000 20,000 1.60
Lights 5@100w 341 1705 .14
LOBBY 500 sf 1.25
People 5 2000 10,000 .83
Lights 1000w 3410 .28
TOTAL 19.9 tons?

08-13-2007, 06:16 AM
You can see from your calculations, and you know from experience, that 5 tons isn't even close. Before making the investment, I would get someone to do a formal manual J calculation, because you haven't included climate and insulation factors.

One way to approach, after determining total BTU requirements, would be to put in multiple units, dividing the theater into zones. In cooler weather and when not in full use, you could run only one or some, keeping the energy cost down.

Bob NH
08-13-2007, 08:33 AM
Your value of 2000 BTUs per hour per person is way high.

Assume that a person eats 2400 "calories" per day (which are actually kilocalories) and dissipates it at an average rate of 100 Kcalories per hour while at rest in a seat.

1 Kilocalorie = 3.968 BTUs, so they are going to dissipate around 400 BTUs per hour on average.

One site that I looked at http://www.tombling.com/cooling/heat-load-calculations.htm suggested 600 BTU per hour per person, which probably has some margin of safety.

If you take away 1500 BTU per hour for 75 people you are getting rid of 9 tons so you are down to about 10 tons.

If you can replace the lights with LED or fluorescent lights you can cut that heat load a lot. http://www.bulbtronics.com/entertainment-lighting.html

One thing that might help is getting air circulation over the audience. That helps carry away the heat around them so they are more comfortable. They are the ones that count. The actors can sweat a bit.

08-13-2007, 08:34 AM
Can that be right?? It would be crazy to install such a huge cooling system when it's only needed a few weeks of the year. It would probably collapse my roof! (Hmm, then ventilation would no longer be an issue...)
So, why don't you just rent an ampitheater for those few weeks?

08-13-2007, 12:45 PM
I did a little googling to see what i could find, just because it's such an interesting / weird problem... not much out there. But it seems that theaters are generally set up as seperate zones, like Bob said, with the seating area cooled seperately from the rest of the room.

I came across a study comparing various duct location strategies, the difference in heat loads was pretty impressive...


the article's a bit strange / confusing - I'm not sure if it's incomplete, abridged, or just badly translated. But the upshot's pretty clear: localized supply & returns will save you about half your energy needs, compared to a mixed air strategy where you're cooling the whole space.

08-13-2007, 01:16 PM
Wow, great info you guys.

Thanks for the accurate human number, Bob. I got "2000 btu/person" off of Sylvane AC's website. Apparently they are very large people over there. But googling again found a site that elaborated: 2000, but over 2 people each additional should be figured at 600 btus. Thank gosh. I thought 2000 sounded crazy.

This does total about 10 tons, which is what an engineer I phoned today recommended off the cuff: two 5-ton units, one dedicated to the auditorium - he also said of course we'd need to really do the onsite calculation.

Thanks for the Italian study, French, it is fascinating (esp. the pictures!) and definitely somewhat helpful.
Our situation is slightly diff in that we don't have fixed seats, we rearrange them, and all our vents/returns will be in the ceiling anyway.

I will keep researching. I appreciate all the ideas!

Bob NH
08-13-2007, 01:41 PM
You can get a 32,000 BTU wall or window unit that you could put back stage where the noise might not be a problem. All you need is a hole or window and a 30 Amp 240 Volt outlet.



08-13-2007, 01:52 PM
Here's a way to do a rough version of what French's Italian friends recommend (placing the air vents closer to the audience members). Of course, they weren't concerned with huge lights generating heat - a movie theater is DARK! but the theory is good.

If such "lamp-level venting" works as in my simplistic picture, it may take some workload off the AC unit... but I am steeling myself to needing a second 5-ton anyway. (At least the 5-ton I just bought isn't "wrong" -- only does half the job. small comfort to my checkbook.)

Let me know what you guys think about this scheme.

(BTW I'm unsure how Condi Rice showed up -with briefcase!- amid the clip-art humans I inserted... I guess it's a Republican choir.)

08-13-2007, 02:22 PM
Pretty much what I meant... BTW, what is that software?

08-13-2007, 06:55 PM
You could do like David Letterman does at the Ed Sullivan Theatre in New York. They cool the room down to about 50 before they let anyone in. By the end of the one hour show, it's comfortable. :p

08-14-2007, 12:16 PM
bad news. found out our building has only 200 amps power available, which leaves nothing for a new 5-ton or even a smaller unit. For now we will have to try the exhaust fans. And lowering the vents of the AC so they are not having to push through that heat layer.

Also the Letterman idea (major cooling pre-show. we've done that already actually, down to 66. It keeps it from getting too unbearable by the end of the show.)

08-20-2007, 10:55 AM
SUCCESS, sort of. We put in 2 exhaust fans (panasonic, 240 cfm, 1.5 Sones) and we lowered 2 out of the 3 supply vents down 2 feet into the room, as well as the return, so the AC unit is not working on the hottest air.
I placed thermometer sensors on the ceiling and at audience level. At show start, the audience temp was about 74, and it rose to 78 by the end of the first 90-minute act - cooled down one degree during intermission - and stayed level during the 30-minute final act. So no-one was sweating or fanning themselves with their programs or complaining.
AND, interestingly, the ceiling temp rose a total of about 7 degrees. So we were successfully keeping the hot air away from the people. The theory worked: To stratify the hot air around the lights and not continually mix it into the people's air.
Now, we left only just barely enough cool air trickling through the backstage vents, and nothing in the lobby. Nobody seems to mind that tho.
So it may be that we need another unit by next year's hot season, because there'll be a cafe in the lobby then, and I'm sure it will need to be a little cooler. Not sure where I'll get the power for that.
Maybe we can get the air to distribute more efficiently in the ducting we have - we noticed this week that one of the three vents in the aud is a wind tunnel and the others barely a whisper. I think they need to be regulated better so they are balanced. Any hints on how to fix that?
But for now we are ok. Thanks everyone for your tips and ideas!

08-22-2007, 05:18 AM
As an added note about your original contractor. You probably couldn't put a 7 to 7-1/2 ton roof top unit on the same curb as the smaller unit. I think the break in size for the roof curb is up to 5 tons, anything over would require a larger curb....which would require structural engineering in most places.

Good luck, and as stated before, a professional heat load analysis is in order. There's a lot more info about the building itself (construction, orientation, location) that get's put into the equation.

09-01-2007, 07:29 AM
.... a professional heat load analysis is in order. There's a lot more info about the building itself (construction, orientation, location) that get's put into the equation. Start looking into radiant heat barriers. Both as building insulation and as localized reflective insulation to shield the audience from the "heat layer" in the air at the top of the room under the ceiling.

First, rewriting your numbers but using different assumptions...
Item Quantity Btu/hr each Total btus/hr Tons of AC (?)

1.) AUDITORIUM 650 sf. 1.75
People 60 400-500 24,000-30,000 2.00-2.30
Lamps 20@500w 1700 34,000 2.80
-- This assumes that seated people DO NOT generate as much heat as standing people who are moving and milling about. Reasonable guess.

Subtotal for AUDITORIUM only is about 5 tons of AC, with the current lighting left alone, and with no better insulation added.

2.) Replacing all incandescent light bulbs with Compact Fluo...
BACKSTAGE 500 sf 1.25
People 10 600 6,000 .48
Lights 5@10w 34 170 .014

LOBBY 500 sf 1.25
People 5 600 3,000 .26
Lights 100w 341 .028

subTOTAL here is a minimal amount of AC compared to the Auditorium.

3a.) Replace some of your stage lighting with expensive new alternatives that consume a small fraction of what incandescent consumes.

3b.) Isolate the air currents by adding a horizontal layer (plexi, plastic, foil-faced bubble wrap, 4'x8' foil-faced rigid foam, aluminum sheet flashing, etc) to separate the (send+return) air current completely. Rejig your vents so they shoot out air balanced among the vents.

3c.) Insulate against heat transfer as much as you insulate to prevent air mixing. Radiative, radiant, radiation, radiator - these are all the same thing with basically the same word. Heat flows through space and time, regardless of all other factors. Warm and cool air flow are only one part of a bigger phenomenon. Using a sheet of foil or sheet metal is a proven way to reflect energy (heat and cool areas) back away from each other.


09-04-2007, 09:10 AM
See post 13 in "What furnace should we get?" thread (opened by Sincraft) for an example of the temp difference you can get when you reflect a part of the sun's energy away from the building. Same thing happens to heat from other sources. You can reflect a part of it away too, sending it through walls and roofs to the great outdoors.


09-05-2007, 06:32 PM
Like genie said, there are ways to reflect heat. In your landscaping is there any way to shade the building with large bushes? I've seen this make a HUGE difference for a church with a lot of east facing glass.

09-10-2007, 06:06 PM
Thanks for the ideas.
How can I find an HVAC contractor who really knows how to figure these things out properly? The three I've dealt with so far just seem to do everything by the seat of their pants, even after I spend an hour explaining to them how I did a heat load analysis myself.

09-10-2007, 08:51 PM
figuring is mathematical modeling. Software driven, and assumption driven. Represents reality but isn't reality. Can be way way off. Works well on big buildings. Yours is small.

Read the current version of http://en.wikipedia.org/wiki/Microclimate for a few more ideas on how to cool down your surroundings.

Based on the progress you have made already, you aren't far away from where you want to be, in my opinion.