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Thread: Venting Outside Heated Shop

  1. #16
    As a thought experiment...I'm in Canada, it hit -35 C in February. Hypothetically speaking, as a worst-case if I'm pushing 800CFM out of the building at 15 C then I would need to warm up 800CFM of air by 50 C. The specific heat of air is 1006 J/(Kg*K). The density of air at -15 C is about 1.35 kg/m^3. That works out to 25.6 KW of heat energy, or 87000 BTU/hr. (Thanks to Google for doing all the unit conversions!)

    I pay about 14 cents per KWhr, so it would cost at most $3.60 per hour of running the dust collector in extra electric heating energy if I vented outside. I actually have gas heat, so that would drop to more like $1.20 per hour.

    That's the absolute worst-case. If you can mix the air less, or the temperature difference between inside and outside is less then the numbers go down.

  2. #17
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    Quote Originally Posted by Chris Friesen View Post
    As a thought experiment...I'm in Canada, it hit -35 C in February. Hypothetically speaking, as a worst-case if I'm pushing 800CFM out of the building at 15 C then I would need to warm up 800CFM of air by 50 C. The specific heat of air is 1006 J/(Kg*K). The density of air at -15 C is about 1.35 kg/m^3. That works out to 25.6 KW of heat energy, or 87000 BTU/hr. (Thanks to Google for doing all the unit conversions!)

    I pay about 14 cents per KWhr, so it would cost at most $3.60 per hour of running the dust collector in extra electric heating energy if I vented outside. I actually have gas heat, so that would drop to more like $1.20 per hour.

    That's the absolute worst-case. If you can mix the air less, or the temperature difference between inside and outside is less then the numbers go down.
    Thanks for the math Chris. I am definitely going to vent outside. I am also using gas, in floor hot water so it should be less.
    When working I had more money than time. In retirement I have more time than money. Love the time, miss the money.

  3. #18
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    Quote Originally Posted by William C Rogers View Post
    Thanks for the math Chris. I am definitely going to vent outside. I am also using gas, in floor hot water so it should be less.
    If you are using gas for heat, you definitely want to insure that you have a good sized air inlet for make up air. The flue for your gas heater will act as a make up air return if there is not an easier opening for the air to get back into your shop and you can suck combustion products back into your shop.
    Lee Schierer
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  4. #19
    Lee, is that still correct if you have one of the high efficiency furnaces with the plastic vent. Makes sense with a standard furnace, but the high efficiency furnaces use a blower to push the exhaust out.

  5. #20
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    Quote Originally Posted by Lee Schierer View Post
    If you are using gas for heat, you definitely want to insure that you have a good sized air inlet for make up air. The flue for your gas heater will act as a make up air return if there is not an easier opening for the air to get back into your shop and you can suck combustion products back into your shop.
    This effect can kill you and burn the shop down too. I had a several restaurants once, the make up system (cold air return) for the hood over the grill, got shut by a clueless employee. It makes the whole building negative pressure, which as Lee says, can use chimneys as air intakes. This caused the water heater in our situation to fill the basement with exhaust and blow lots of scary looking flames out the fire chamber. This happened in both locations at different times. The exhaust alone is scary enough, but add the fire and it's a whole different animal. Be careful venting your dc outside, make sure you are not going to get these results.

    If you are using a direct vent (burns outside air) it will be fine. Check your water heater too. Lots of us have the utility room in the garage space.

  6. #21
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    Quote Originally Posted by Jim Andrew View Post
    Lee, is that still correct if you have one of the high efficiency furnaces with the plastic vent. Makes sense with a standard furnace, but the high efficiency furnaces use a blower to push the exhaust out.
    If they installed your furnace with its own PVC air intake you are probably okay, but if the furnace gets its air from inside the house you can pull exhaust gases back into the house when you use your outside exhausting DC or dryer, or, kitchen exhaust or bathroom fans if your house is pretty tight. Some furnaces with fans will sense if there is enough outgoing air flow and shut down if there is a problem.

    Many contractors will save a few dollars and not install outside air inlets on high efficiency furnaces to save a few bucks and tell you it is okay to do so that the outside vent is not required. Thy do this without checking to see how tight your house is or how many and what size exhaust fans are in the house.

    Try this test. On a calm day with all the doors and windows shut, open an outside door and close it without latching it. Then run around the house and turn on every exhaust fan including the clothes dryer. Once everything is running go back to the door you left closed but not latched. If it has come open, you are like pushing more air out than the leaks in the house can provide make up for. Then close and latch the door and go to your furnace and feel if there is air coming out of the "air intake" on the furnace. A candle or match will flicker or go out if it is held near the air intake on the furnace.
    Last edited by Lee Schierer; 03-05-2015 at 4:58 PM.
    Lee Schierer
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  7. #22
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    I am good. I have in floor hot water heat. The water heater/boiler gets combustion air from the outside.
    When working I had more money than time. In retirement I have more time than money. Love the time, miss the money.

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