Dumb dc question

[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.