A/C and Open Door — What’s the Loss?
JohnSprungX
| Posted in Energy, Heating & Insulation on
I want to convince some people here that leaving the outside door open on an air conditioned building is a big waste of energy. So, I’d like to get some numbers, like maybe it’s equal to X-many 100 Watt light bulbs.
As a start, let’s consider a small office building, say 20,000 sq ft, with an outside door 6 ft x 8 ft, single glazed all glass. Suppose it’s 95 degrees outside and 75 inside, and that the insulation and all other variables are typical for that kind of building built about 10 – 20 years ago.
Thanks —
— J.S.
Replies
It depends a lot on how much air movement there is through the door. If no air movement then the loss is relatively small.
Hint: Use a source of smoke (an incense stick, or just light matches and blow them out) to guesstimate the air velocity through the door at several spots. Eg, if air seems to move at 1 fps in the top half and 1/2 fps in the bottom half, you have 1x8x3 cfs moving through the top half and 0.5x8x3 cfs moving through the bottom half, or 1.5x8x3 total.Convert to cubic feet/minute = 1.5x8x3x60 = 2160 cfm. Now someone out there can probably give you a rough idea of the cost to cool 2160 cfm of air from 95F to 75F.(Note that it doesn't matter if the air is moving in or out -- if it's moving out some air is moving in somewhere else to make up for it.)
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Yeah, and as suggested you also want to figure the cost to dehumidify that volume of air.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Ac is very efficient at dehumidifying, a byproduct. Won't change the equation.PAHS Designer/Builder- Bury it!
Wrong. The heat of evaporation of water is fairly large compared to the specific heat of air. Don't know that it will be a major factor for such meatball calculations, but it's far from negligible.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Dan, what you're over-looking is the specific action of the appliance. When it's cooling the air, dehumidification happens. That's what byproduct means. Which event empirically takes more energy has no bearing at all. None.
It is negligible for a building with central ac. I can prove it to you here if you like. You wouldn't have any problem determining that in your neighborhood if you were inclined.PAHS Designer/Builder- Bury it!
Bullfeathers!! Physics is a strict master.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
OK, have it your way. But it seems to me you might view the larger picture.
Heat pumps and metered electricity. It really is that simple. Test, and you'll see. I once had similar questions, answered now. Makes for better design. PAHS Designer/Builder- Bury it!
"Dan, what you're over-looking is the specific action of the appliance. When it's cooling the air, dehumidification happens. "Hopefully dehumificiation does not JUST HAPPEN.It is not a random side affect. It is part of the system design.Done wrong you end of up 72 F air, but at 80% RH.And that would take lots less energy than say 76 F and 30% RH..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
I would say the temperature doesn't change that much if only one door is open, but the humidity level does. The units will have to dehumidify the space again, which takes energy and reduces comfort.
Breaktime #91528.1
Or just scroll down under this heading. VATom posted a coupled pages with formulas and such for heat transfer.
I printed it, but can't read the numbers for still air.
Dave
but can't read the numbers for still air
Still air conductivity is 0.169-0.215 (Btu-in/hr-ft²-ºF) Great book for under 5 bucks...
The door is wood? Fir conductivity is 0.8, red oak is 1.1.
That's why air is commonly used in insulated windows. Cheap, works well until the airspace gets large enough to permit convection currents, around 3/4".
Another forum went throught the Btu calcs for a guy interested in learning his heat penalty to open windows to solve his Michigan winter-time humidity problem in a high-mass house. It was negligible.
Too many unknowns to make a determination here, but I would guess that John's friend is correct. Put a Kill-A-Watt on a refrigerator for baseline, leave the door open, see what happens. Not much.
PAHS Designer/Builder- Bury it!
I can see how your numbers could work, but how are they going to achieve the "still air" conditions on either side of the door?
With an a/c running you are going to get some type of exchange, either positive or negative, across that boundry, aren't you?
In commercial buildings we are required to have a minimum of 10% outside air brought into the building at all times. This time of year our nighttime temps may go into the mid to low 60s, and our high durring the day jump to the upper 80s. Just that 10% outside air requirement can create a seriouse lag in indoor comfort level as the a/c tries to catch up to the outside temperature swings. Leaving doors open just compounds the problem for us, so all of our high traffic doors have foyer type emtrances (exterior doors/ air space/ interior doors).
What am I missing here? Even without DanH's physics, it just doesn't make sense to leave an outside door open to a conditioned environment.
Dave
OK, lets quantify what we can. Suppose we have a building of 20,000 cu.ft. air that has to be cooled from 95 to 75 F. At an average of 85 F, that works out to about 1460 lb of air. Using 0.25 BTU/lb-F heat capacity, the sensible heat for cooling that much air 20 degrees is about 7300 BTU.
If the 95 F air starts out at a 72 F dew point, and ends up at 75 F and 40% RH, the water removed works out to 13.3 lb. (about 1.6 gallons). At 85 F, the latent heat of water is 1046 BTU/lb, so the dehumidification load is 13,900 BTU, or 1.9 times the cooling load.
Now, the above numbers are not hourly rates, only absolute amounts. But they show that the dehumidification load can be twice the cooling load, so humidity can make a huge difference.
While there may be little actual transport of air from outside to inside, the movement of water vapor through the open doors by molecular diffusion likely will be significant by itself. Water is very a very mobile molecule. It has been too long since I had to do any calcs on diffusion rates, so I'll defer to others to quantify that.
I do know that in my house, if I turn on the A/C at the end of a hot humid day, after 20 minutes or so I can notice the drop in humidity, but is much longer than that before I feel the air become significantly cooler. Of course, part of that is due to the heat capacity of the walls, etc. providing more heat than the air.
Thanks, Dick -- That's very helpful.
Doing the math, a rough rule of thumb emerges that cooling and dehumidification combined can take a tad over a BTU per cubic foot of air when the temperature difference is 20 degrees F.
Now what I need to do is look up the conversion between BTU/Hr and Watts, and the efficiency of the HVAC system, and get an estimate of how much air moves thru the open door. That I can probably estimate by dropping scraps of paper and timing how long it takes them to reach the ground seeing how far they go.
-- J.S.
"Now what I need to do is look up the conversion between BTU/Hr and Watts, and the efficiency of the HVAC system, and get an estimate of how much air moves thru the open door. That I can probably estimate by dropping scraps of paper and timing how long it takes them to reach the ground seeing how far they go. "
BTU/hr * 0.293 gives watt, or 3412 BTU/hr per kW.
You can make some estimate of air leakage by convection through the door, but there still is the matter of diffusion of water vapor from humid out to drier inside. Think of a closed room, dried by A/C, then switched off and a window opened half way. How long does it take the air in the room to feel muggy again?
"a rough rule of thumb emerges that cooling and dehumidification combined can take a tad over a BTU per cubic foot of air when the temperature difference is 20 degrees"
The amount of cooling energy attributed to a volume of outside air to go from typical cooling design conditions outside (95 degF db and 74 degF wb) to typical cooling design conditions inside (75 degF db, 62,5 degF wb), is 50 btuh/cfm. Different conditions will result in different values, but this is readily decifered from a psychrometric chart.
A little off the topic, but I was watching the Arizona Diamondback - LA Dodger game a couple of nights ago, and the announcer (the GREAT Vin Scully-) said it was over 100 outside, and with the roof open and the A/C on it was about 80 in the stadium.
Does that seem crazy to anybody else? It sounds to me like they were trying to air condition all of Phoenix!
Edited 6/29/2007 6:38 pm ET by ownerbuilder
Yeah but cold=dense, which displaces the warm less dense, so as long as cold don't rise, and heat don't fall...all is well. But in te winter , stadiums better keep the lid on, if they want warmth. (G)
Have you ever heard of cold air rising? The convection from the heat of that many people will rise, they keep the doors on the lower levels closed as much as possible and cool from the top. Must have been a night game because the sun would bake the people sitting in direct light. OTOH, if it's only 100, it's really not that bad. I think they use evap coolers and they're less expensive to run, too.
"I cut this piece four times and it's still too short."
In huge buildings like that (e.g., also the case with airport terminals) they don't even bother to air condition the whole space, just where the people are. It can be 100 degrees up at the ceiling, but they don't care because nobody is 30' tall.
We do a lot of weatherizatio and energy effeciency. I can tell you that in the classes we have taken, every 1/8" of a crack will add 5 cents to your electric bill.
OK, 1/8" by how long? The other problem with working from that rule of thumb is that, say, 1/8" x 8 ft is 12 square inches, but it has a lot more surface effect to slow the air than would a hole 3" x 4". So, it could be useful as a lower bound to double check what I get from other methods.
Thanks --
-- J.S.
1/8" by 1" Most of this was pertaining to holes under sinks, windows not caulked, door sweeps not on doors, worn jambs, ceiling tiles verses sheetrock.My brain + his brawn = a perfect team!
Too many unknown and very significant variable to even perform a SWAG, including, but by no means limited to, relative pressure, stack effect, wind speed and direction, building tightness, ventilation control(s). The opening left by a well insulated, opaque door is more significant than the opening left by a frameless, single pane glass door.
Empirically, the easiest way to quantify the difference is to measure it. Run the AC for a day and measure the energy used (look at the electric meter), with the door closed, then leave the door open for a (similar weatherwise) day and see how much the difference is (reasonably asuming that the building occupancy and auxiliary power consumtption is similar). Ideally, this would be done at the expense of the individual(s) that need to be convinced of the obvious.
You really know people that need convincing of this?
> You really know people that need convincing of this?
Unfortunately, yes. And these are the same people who are so eager to change everything over to CFL's. I think the vast majority of the general public would be astonished to learn how much energy is wasted by thoughtlessly leaving doors or windows open. They worry about teaspoons, and ignore buckets.
-- J.S.
"They worry about teaspoons, and ignore buckets. "Great quote, John! I also have seen it, and totally agree with you. This sort of foolishness is much harder to countenance when we must pay for it ourselves. Good luck in your efforts to educate them.Bill
We just changed about 500 T12 to T5. Savings will be about $300 a month. Probably have a payback of 20 years minimum.
Of course, keep in mind that for every watt saved in lighting you save about two watts in cooling.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
But if my cooling load is less, my power consumption for the variable pitch 75hp fan may go up.
And then we also may have to add extra heat in the heating season.
On the upside, the ballasts are electronic, which may help clean up the power factor.
Hard to quantify actual savings.
Moral of the story.... never promise hard numbers when it comes to saving money.
T5 ?
We have an on going project like that, but are going from T12 and T10 to T8 with the switch from magnetic to electronic ballast. Energy savings was figured to be from 25 to 33%.
Commercial 277v, four foot lay in fixtures with 3 bulbs and two ballast, converted to 3 bulb, single ballast. Somewhere between 1500 and 2000 fixtures I think,
Do you have any information on the T5 bulb and ballast combination?
Dave
The ones we used allow 50% or 100% operation. They need two wall switches.
So far, because they are used in zone lighting, and the tenants have access to the switchs, they tend to be on 100%.
At 100%, they use .2 amps. T8 use .18. Our t12 used .28 All were 347v. But watts is watts.
25% would be in the ballpark, but figure in the cost of manpower to do the planning, make the switch, do the paperwork, etc. and the cost of the parts, what would your payback be?
At .08 cents kwh we saved about 300/month, worst case. Best case, 600/month.
Total costs, over 200,000.
Demo of old, all new wiring, fixtures, etc.
Near our house there is an outdoor mall and one of the stores is an art studio type place. I've never seen any customers in the place. They have a nice looking girl/woman in there behind a desk and they leave the door open.
When you walk by you can feel the cold air pouring out the door. Still no customers. Like beating a dead horse with BTU's.
Maybe the chill's coming from the woman. That would explain why no customers.
So convenient a thing it is to be a reasonable Creature, since it enables one to find or make a Reason for everything one has a mind to do. --Benjamin Franklin
Just from a casual glance she looks like she's bored out of her gourd. As far as I know art places like that need to do some sort of wine and cheese thing to promote the place. Just displaying it doesn't work. Maybe it's a write off for somebody.
I can't believe how some people just waste energy. That's not how I was raised.
John, people that stupid can be persuaded with a smack upside the head.
Joe H