Dumb dc question

[Bob Marino]

Ok, really basic here. For all the guys using a central dc system and venting to the outside. In all but the nicest weather conditions where there is neither heat nor a/c running, wouldn't such a system almost immediately exhaust all the warm or air conditioned air to the outside, negating the heat and a/c?

Bob

[J. Scott Chambers]

What you remove, you must replace, one way or another. You can do it in a controlled manner and condition it, which would be quite expensive, or you can let the system balance itself and pull air from where it can, usually the outside, unconditioned, humid air ( or cold, depending on season).

So yes, exhausting to the outdoors in a conditioned shop is removing that conditioned air.

[Andy Fox]

And, make sure the replacement air isn't coming from down a furnace, water heater, or fireplace flue/chimney. :eek:

[Bob Marino]

And, make sure the replacement air isn't coming from down a furnace, water heater, or fireplace flue/chimney. :eek:


Nope, it's in a stand alone garage. I pretty much knew the answer that all warm or conditioned air would be exhausted, but I was wondering about the folks that do vent outside and how they managed to deal with that issue.

Bob

[Bill Pentz]

Ok, really basic here. For all the guys using a central dc system and venting to the outside. In all but the nicest weather conditions where there is neither heat nor a/c running, wouldn't such a system almost immediately exhaust all the warm or air conditioned air to the outside, negating the heat and a/c?

Bob

Bob,

Heating Commercial shops have been blowing the air outside for decades. They use radiant heater like our heat dishes because the heat is radiated instead of being carried by the air. We can do the same and be in pretty good shape until the weather outside drops below freezing.

Air Conditioning This one is lots tougher because the cool is in the air and pumping it outside gets rid of it.

You can use a wye coming out of your blower with gates to blow outside when the temperature is moderate and into your filters otherwise.

When blowing outside, make sure you provide an open door, window, or otherwise to keep the air from being sucked backward through flues which can fill your shop with deadly carbon monoxide. I also recommend if you have any fired appliances in your shop that you include both a smoke detector and a carbon monoxide detector.

bill

[Steve Clardy]

No a/c in my large shop.
I heat with a humongus wood stove, so never really notice heat loss in the winter months.

Mine is all pushed outside. Unit sits outside, not much noise, except for the normal air movement noise when a blast gate or two is open.
I do use my remote, and try not to let it run much in the winter if not used.

[Frankie Hunt]

Ok, really basic here. For all the guys using a central dc system and venting to the outside. In all but the nicest weather conditions where there is neither heat nor a/c running, wouldn't such a system almost immediately exhaust all the warm or air conditioned air to the outside, negating the heat and a/c?
Bob

I do NOT dump my air outside so I have NO experience here. But here are my caculations for my shop.

24' x 26' = 624 sq ft
8' ceilings = 4992 cubic ft space
dust collector = 1200 cfm

so.... 4992 / 1200 = 4.16 minutes
real life is probably closer to 4922 / 750 = 6.6 minutes

so.... I would call that almost immediately. Especially considering my little AC unit takes HOURS to cool the shop down.

[Bill Pentz]

I do NOT dump my air outside so I have NO experience here. But here are my caculations for my shop.

24' x 26' = 624 sq ft
8' ceilings = 4992 cubic ft space
dust collector = 1200 cfm

so.... 4992 / 1200 = 4.16 minutes
real life is probably closer to 4922 / 750 = 6.6 minutes

so.... I would call that almost immediately. Especially considering my little AC unit takes HOURS to cool the shop down.

Frankie,

Good point! Your math is good, but experience shows that what really happens is a little different. Like so many other things about dust collection and low pressure airflow, this is another that is not obvious.

Many make a very similar assumption in terms of using a shop air cleaner and figuring that their 1200 CFM air cleaner should clean the shop in about 6.6 minutes. In reality, it will probably take more like four to six hours.

The fine dust spreads faster than you are cleaning the dust out. After one full air change in your shop, you will be lucky to have reduced the overall airborne dust level much at all. The math is a series with the reduction being so low per cycle that it takes about 20+ full air changes before a 15 mg per cubic meter of air dust load drops below 5 mg per cubic meter. Most commercial air engineers long ago gave up on using air filters and exhaust fans. It just takes so long to bring that airborne dust level down that in the meanwhile their customer shops will fail an OSHA air quality inspection. They instead found we have to move enough air and use the right tool ports and hoods to capture the dust as it is made then get rid of it.

What happens to our heated air is very similar to what goes on when using an air cleaner. We have this same issue with heated air dispersing quickly like the fine dust, plus the incoming air rapidly gets warmed by the existing air. As long as you are adding heat at about the same rate of the loss, you shop will stay comfortable. This is why Steve can heat his shop with a big wood stove while his big blower is pumping all outside on even cold days. With a radiant heater passing the heat through radiation instead of the air, we are even more comfortable down to lower temperatures.

When dealing with air conditioned air, we have a very different problem as the hot air coming in warms the already cool air, plus we are blowing the cool air away outside. Instead of blowing away a little heat on cold days, we are taking in and keeping almost all of the heated air we bring in. This makes even large air conditioners unable to keep up.

bill

[Russ Massery]

I just set my cyclone up with a wye at the plenum with two blast gates. One goes into the the plenum. (with has the filters attached) and the other going outside.I plan to blow most of it outside. Only using the fiters in extreme tempertures. Like now it's 94 degrees outside.

[Matt Meiser]

I've been running my blower output outside for about a year now. I've noticed no perceptable difference in how much the AC (large window unit) or furnace (large Hot Dawg type unit) run when I'm running the blower. My furnace has a forced draft flue which shuts the furnace down in the absence of a proper draft and I have a carbon monoxide detector neither of which noticed any problems with this configuration. My blower is the Jet 1200 blower with 6" duct work and my shop is about about 11,520 ft^3 (30x32x12)

I usually shut my blower off when I'm not using it for more than a couple minutes.

I used slightly more gas in my shop this year vs. last year, but we used about the same percentage more in the house. I also heated the shop to 40 degrees any time I wasn't in the shop this year vs. shutting it off if I wasn't going to be in the shop for more than a day or two last year.

So while the science seems to make sense, my experience has shown otherwise and I plan to replace the temporary outlet I set up with one that exhausts the fine dust further from the shop so it doesn't cling to the siding as much.

[J. Scott Chambers]

I usually shut my blower off when I'm not using it for more than a couple minutes.



Using your DC intermittently probably won't impact your A/C or heating system all that much. If you were to run it continuously while you were in the shop, during peak conditions, I'd bet you'd see a marked difference in the ability of your system to maintain conditions in the space. This is also dependent on your system size and it's ability to handle the outside air.

[Andy Fox]

Does anyone know how to calculate the size of the opening needed to supply makeup air when venting outside?

[Mark Singer]

Bob,
I exhaust to the outside with my Onieda cyclone....In So. Calif. it is not a problem ....my shop doors are open and I let the weather in and out...

[Frankie Hunt]

Real life experience wins out just about every time. This is why I really like to see questions and answers show up at Sawmill Creek.

I do understand about the dilution of the existing environment with the take up air. As an extreme example if my 4992 cubic ft of shop space were a 1 ft x 1 ft x 4992 ft with the dust collector on one end and the replacement air entering on the other end then I would have almost complete air turn over in just a few minutes. The same size shop with the dust collector on one end and the replacement air coming in a window about 1 ft down from the collector then it would take an extremely long time for total air replacement! So....The location of the entrance of the replacement air would make a difference too. My shop is a stand alone 2 car garage and I put two windows on the rear wall (opposite the two garage doors in the front) so that I would maximize my airflow through the shop. This has worked out pretty well.


What really gets me about all this is Mark sneaking in a gloat about that California weather.:D:D

[Bill Pentz]

Snip..What really gets me about all this is Mark sneaking in a gloat about that California weather.:D:D
Good point on practical experience being a big help..

Hummm it says California on my driver's license and home address too, but it was 107 here yesterday and even my dogs thought I was crazy when I wanted them to go outside and do their jobs.. People forget that this is a huge state with just about every possible climate you could find elsewhere in the U.S. *S*

[Jeffrey Makiel]

I quickly ran some calculations to determine what it would cost to exhaust a 2hp dust collector to the outside during the summer and winter. These costs are in addition to the normal costs to heat and cool the shop.

Assumptions:
1. 2hp dust collection system with ductwork performing around 700cfm.
2. Shop temperature: winter=65 degrees F; summer=75 degrees F at 55% indoor relative humidity (RH).
3. Shop is heated via electric heaters at 11 cents/KWhr, or 80% efficient oil/gas heat at $2.50/gal.
4. Shop is air conditioned via electric a/c unit with a SEER of 10 or better at 11 cents/KWhr.

Winter heating cost per hour for a given outside temperature....
Outside temp...BTU/hr...Electric......Gas/oil
60 ................3,780 ......0.12 ..........0.06
50 ...............11,340 .....0.36 ..........0.16
40 ...............18,900 .....0.60 ..........0.27
30 ...............26,460 .....0.84 ..........0.37
20 ...............34,020 .....1.08 ..........0.48

Summer cooling cost per hour which includes dehumidification...
Outside temp......BTU/hr....Electric
90F at 75% RH...76,545.....2.40
90F at 50% RH...32,319.....1.29

[Doug Edwards]

Jeff. Are you sure about those numbers? Creating 32,000 BTU of cooling on an air conditioner should use way less energy than creating 34,000 BTU of heat with resistance heaters. I think they are high by a factor of ten and should be $0.24 and $0.13 per hour

[Jeffrey Makiel]

It's the other way around. The ease of transfering heat (hot to cold, or cold to hot) depends greatly on which way the heat is going because the devices performing the transfer are very different. Electric resistive heaters at 32,000 BTU/hr is 100% efficient while an air conditioner that uses a compressor to transfer 32,000 BTU/hr may require up to 40,000 BTU/hr of electrical input to overcome the cost to run the compressor. This is reflected in the calculation.

However, I'm not sure how I'm off by a factor of ten, but it's possible as it often happens when balancing my check book! Also, my engineering skills ain't what they use to be, and they were not that great back then either.

Jeff:)

[Matt Meiser]

Through the magic of electronics, I know my blower has run for 3.9 hours since July 1. So that's cost me less than $10 according to Jeffery's chart.

[Jeffrey Makiel]

Matt...It could be even less were you live because I based the estimate on electrical costs in New Jersey which is a whopping 11 cents/KWhr. Before electricity was deregulated in NJ about 2 years ago, it used to be about 8 to 9 cents.

There is a strong argument to be made about discharging to the outside instead of recirculating in a residential (non-commercial) shop. In addition to removing more fine particulate matter from your shop, you also help refresh the air from the off gassing of engineered woods, glues/epoxies and finishes.

But there are some downsides to exhausting outside in addition to the energy costs. The most appreciable downside is for the folks that have a shop in their basement. The heating system (radiators or heating duct) will most likely not be large enough serving the shop, or the shop does not have its own thermostat zone. If the dust collector is ran for a long time, you will certainly chill out in the winter. You would also need some type of relief damper to let air in when running the DC, and close when off. Otherwise, an open window or door will be required which is an energy and comfort killer.

-Jeff

[Frank Hagan]

I've been running my blower output outside for about a year now. I've noticed no perceptable difference in how much the AC (large window unit) or furnace (large Hot Dawg type unit) run when I'm running the blower. My furnace has a forced draft flue which shuts the furnace down in the absence of a proper draft and I have a carbon monoxide detector neither of which noticed any problems with this configuration. My blower is the Jet 1200 blower with 6" duct work and my shop is about about 11,520 ft^3 (30x32x12)

I usually shut my blower off when I'm not using it for more than a couple minutes.

I used slightly more gas in my shop this year vs. last year, but we used about the same percentage more in the house. I also heated the shop to 40 degrees any time I wasn't in the shop this year vs. shutting it off if I wasn't going to be in the shop for more than a day or two last year.

So while the science seems to make sense, my experience has shown otherwise and I plan to replace the temporary outlet I set up with one that exhausts the fine dust further from the shop so it doesn't cling to the siding as much.

Bill's the man when it comes to these things, and I know what he says about radiant heat is right. However, forced air heat does heat the objects in the room. The heat you retain when running your DC is the heat that has been absorbed by your equipment, the floor, etc. There is some energy loss in transferring the heat from the fire to the air, then another loss in transferring the heat in the air to the objects in the room. So a radiant heat system would probably be better. But as Bill points out in the example given for clearing dust using the CF of air in a room and the CF of air filtered, the math isn't as simple as you would think.

I have completely re-thought my stance on DC now that I've read more about it. I almost bought a DG from Onieda, but now I'm seriously considering an outdoor cyclone. I never heat or air condition my shop, so the movement of air is not a problem (except for the water heater that shares the shop ... I do have to make sure I have more-than-adequate make up air to prevent pulling air in through its vent pipe).

[Jeffrey Makiel]

I find radiant heaters to be very uncomfortable. They are mostly used in commercial settings where they provide localized warmth within a large building like a warehouse, or an outdoor lobby of a fancy hotel. In a small shop (like a two car garage), one side of your body facing the heater will be warm, and the other side will be very cold. You need to rotate like a hot dog at the 7 Eleven to gain some degree of comfort.

cheers, Jeff

[Frank Hagan]

I find radiant heaters to be very uncomfortable. They are mostly used in commercial settings where they provide localized warmth within a large building like a warehouse, or an outdoor lobby of a fancy hotel. In a small shop (like a two car garage), one side of your body facing the heater will be warm, and the other side will be very cold. You need to rotate like a hot dog at the 7 Eleven to gain some degree of comfort.

cheers, Jeff

There is some truth to that! The radiant heating that is installed in the slab of a floor is better, in that you get warm, even heat radiating from the floor. But that's not an easy retrofit (unless you go electric).

[J. Scott Chambers]

Overhead radiant heating is very effective, IF you provide proper coverage of the space to maintain comfort. One heater at your back most likely won't do it.